Harris DX 25U Technical Manual

TECHNICAL MANUAL

DX 25U MEDIUM WAVE TRANSMITTER

994 9168 001

I Introduction/Specifications
II Installation
III Operation
IV Theory
V Maintenance
VI Troubleshooting
VII Parts List

Subsections

VIII RF

Oscillator
Buffer Amplifier
RF Amplifier Modules
Driver Combiner/Motherboard
Driver Supply Regulator
RF Multimeter
RF Combiners
Output Sample Board &
Output Monitor

IX Audio

Analog Input Board
Analog to Digital Converter
Modulation Encoders
DC Regulator
External Interface

X Control

Controller
LED Board
Switch Board/Meter Panel
Driver Encoder/Temp Sense Board

T.M. No. 888-2297-002
© Copyright HARRIS CORPOR ATION

1994, 1995, 1996, 1997, 1998, 1999, 2000 All rights reserved

Printed: August, 2000

Rev. AA: 8-9-2000



TABLE OF CONTENTS

Section I

Introduction/Specifications

Introduction......................................1-1

ScopeandPurpose..............................1-1

EquipmentDescription.............................1-1

BlockDiagram ...................................1-1

Specifications....................................1-1

Section II

Installation/Initial Turn On

Introduction......................................2-1

DeliveryAndStorage..............................2-1

ReturnsAndExchanges............................ 2-1

Unpacking.......................................2-1

FactoryTestData.................................2-2

Transmitter Placement .............................2-2

RemovingtheCombinerCrossoverBar .............2-2

CabinetPositioning..............................2-2

CabinetBolts...................................2-3

CabinetGroundStrap............................2-3

CabinetGroundingPlate..........................2-3

AirSystemAndCooling ...........................2-3

ElectricalInstallation ..............................2-3

StepStartPanel.................................2-3

ACPower .....................................2-3

InterconnectionWiring...........................2-4

Transformer Tap Settings .........................2-4

Component Installation.............................2-4

PACombinerOutputBar.........................2-4

Transmitter Grounding ............................. 2-4

RFOutputConnection............................. 2-5

DummyAntennaInformation......................2-5

Customer Interface Connections......................2-5

AudioInput....................................2-5

Modulation Monitor Connection ...................2-6

FrequencyMonitorConnection.....................2-6

ExternalRFInput............................... 2-6

ExternalInterlock(Failsafe).......................2-6

Pre-TurnOnChecks(Mechanical).................... 2-7

Pre-TurnOnChecks(Electrical).....................2-7

InitialTurnOnProcedure........................... 2-7

Power Supply Test ..............................2-7

UnderdriveOverloadTest ........................ 2-8

PAPowerSupplyCheck .........................2-8

FanRotationCheck .............................2-8

DriverOperationCheck.......................... 2-9

Raise/LowerFunctionCheck...................... 2-9

PowerAmplifierstageTurnOnCheck..............2-9

Setting Modulation Monitor Sample Levels .........2-10

Modulation Check.............................. 2-12

Setting Audio Input Level .......................2-12

AudioInputPhasingCheck(Optional)..............2-12

ControllerBatteryBackup .......................2-12

RecordingNormalMeterReadings................2-13

AMStereoInstallationandTuningHints.............2-13

InterfacingForStereo...........................2-13

BandpassTuning2C1...........................2-13

RemoteControlConnections.......................2-13

ControlInputs ................................ 2-13

AnalogMonitoringOutputs ..................... 2-13

StatusMonitoringOutputs....................... 2-13

Section III

Operators Guide

Introduction ..................................... 3-1

OperatingProcedures.............................. 3-1

Daily Preoperational Checkout ...................... 3-1

Transmitter Turn-On Procedure ..................... 3-1

Transmitter Turn-Off Procedure ..................... 3-1

Transmitter Operations ............................ 3-2

RoutineMeterReadings......................... 3-2

Fault Conditions................................ 3-2

Section IIIA

Controls and Indicators

Section IV

Overall System Theory

Introduction ..................................... 4-1

BlockDiagramDescription......................... 4-1

RFSection.................................... 4-1

AudioandModulationSection.................... 4-2

ControllerSection.............................. 4-2

PowerSupplies ................................ 4-2

LowVoltagePowerSupply:CircuitDescription........ 4-2

PowerDistributionBoard,A39.................... 4-4

PAPowerSupply:CircuitDescription................ 4-4

StepStartPanel................................ 4-4

PA Supply Discharge Circuit (Crowbar) ............ 4-4

PowerSupplyShortingSwitches .................. 4-4

PA Power Supply Transformer T1 ................. 4-4

InterphaseTransformerT3 ....................... 4-4

+230 VDC Supply .............................. 4-4

+115 VDC and +60 VDC Supplies ................ 4-4

SupplyCurrentMeter,M2 ....................... 4-5

Secondary Winding Fuses........................ 4-5

SupplyFuses .................................. 4-5

FanMotorFuses ............................... 4-5

A24andA25FuseBoardSampleCircuits........... 4-5

ACPowerProtectionCircuits..................... 4-5

OvervoltageandUndervoltageProtection........... 4-5

LossofPhaseand“Brown-Out”Protection.......... 4-5

AirSystemandSensingCircuits .................... 4-5

Fans ......................................... 4-6

AirFlowandTemperatureSensing ................ 4-6

Interlocks....................................... 4-6

DoorInterlockCircuit........................... 4-6

ExternalInterlockCircuit ........................ 4-6

RFCircuits...................................... 4-6

RF Drive Splitter, A15 .......................... 4-6

RFDriveCables ............................... 4-6

PowerAmplifierDescription...................... 4-7

CombinerDescription........................... 4-7

RFSamplesfromtheOutputCombiner............. 4-8

OutputNetworkDescription........................ 4-8

Bandpass Filter................................. 4-8

PIMatchingNetwork ........................... 4-8

iv 888-2297-002 8/9/2000

WARNING: Disconnect primary power prior to servicing.

SparkGap,E2 ................................. 4-8

Digital Modulation Principles ....................... 4-8

DigitalTermsandConcepts ...................... 4-8

Analog to Digital Conversion ..................... 4-9

RFAmplifierControl............................ 4-9

Amplitude Modulation in the DX-25U ............. 4-10

PowerAmplifierStage.......................... 4-10

Summary:DigitalModulator..................... 4-10

Section V

Maintenance/Alignments

Introduction ..................................... 5-1

Maintenance..................................... 5-1

Maintenance Logbook ........................... 5-1

PreventiveMaintenance.......................... 5-1

MaintenanceOfComponents ..................... 5-1

ModuleReplacement/Alignment..................... 5-3

Modules Which Can Be Replaced With No Adjust-

ments.......................................... 5-3

BufferAmplifier(A16).......................... 5-3

Predriver...................................... 5-3

PAModules................................... 5-3

RF Drive Splitter Removal ....................... 5-4

RF Drive Splitter Replacement .................... 5-4

Boards Which Require Preset Switch Settings or

JumperPlugPositions............................. 5-4

Modulation Encoder (A37) ....................... 5-4

ControllerA38................................. 5-5

Binary Combiner/Motherboard, Main Com-

biner/Motherboards(A5-A8)...................... 5-5

DriverCombiner/MotherboardA14 ................ 5-5

PrintedCircuitBoardsWhichRequireAdjustments ..... 5-5

Analog to Digital Converter (A34) ................. 5-6

AnalogInputBoard(A35)........................ 5-6

Oscillator(A17)................................ 5-7

DriverSupplyRegulator(A22).................... 5-8

DCRegulator(A30)............................. 5-8

OutputMonitor(A27)........................... 5-9

LEDBoard(A32).............................. 5-11

DriverEncoder/TempSenseBoard(A19).......... 5-13

SwitchBoard/MeterPanel(A31) ................. 5-14

FrequencyChangeProcedure...................... 5-15

TestEquipmentRequiredforFrequencyChange..... 5-15

Frequency Determined Components ............... 5-15

FrequencyDeterminedJumpersandSwitches....... 5-15

OutputNetworkDryTune....................... 5-15

RFCircuitsCheckout........................... 5-16

InitialDriverTuningandSetup................... 5-17

InitialTuningAtLowPower..................... 5-18

TuningAtHighPower.......................... 5-19

Completion of Basic Frequency Change of Transmit-

ter.......................................... 5-19

OtherAdjustments............................. 5-20

Section VI

Troubleshooting

Introduction ..................................... 6-1

Symptom: Transmitter will not turn ON - No Color-

Stat™panelindicatorsareilluminated................ 6-6

PossibleCauses ................................ 6-6

Symptom: Transmitter will not turn on - all Color-

Stat™panelindicatorsareilluminatedGreen........... 6-6

PossibleCauses................................. 6-6

Symptom: Transmitter will not turn on - one or more

ColorStat™ panel indicators are illuminated RED. ...... 6-7

PossibleCauses................................. 6-7

Symptom: Transmitter will turn ON but immediately

turns OFF - one or more ColorStat™ panel indica­tors illuminate RED. The transmitter may try to turn
on twice and a fault indicator illuminates AMBER

thenRED. ...................................... 6-7

PossibleCauses................................. 6-7

Symptom: Transmitter turns On (LOW, MEDIUM or

HIGH buttons illuminate) but there is no power out­put and no PA current is indicated. Supply voltage is

indicated on the multimeter. ........................ 6-7

PossibleCauses................................. 6-7

Symptom: Transmitter is running, but power is lower

thannormal...................................... 6-7

PossibleCauses................................. 6-7

Symptom: Unable to raise power past a certain point.

ColorStat™ panel ANT and/or FILTER LED indi-

cateRED........................................ 6-8

Possiblecause.................................. 6-8

Symptom: Unable to raise power past a certain point.

NoColorStat™panelindicatorsIlluminatedRED....... 6-8

Possiblecause.................................. 6-8

Symptom: Transmitter turns ON (Low, Medium, or

HighIndicatorsIlluminate)butwillnotmodulate....... 6-8

PossibleCauses................................. 6-8

ColorStat™panelOvercurrentFaultIndication......... 6-8

Random Faults With Program Audio. Possible

Causes:.......................................6-8

Faults With Tone Modulation. Possible Causes:....... 6-9

OverloadsonTurnOn.PossibleCauses:............. 6-9

ColorStat™panelOvervoltageFault.................. 6-9

PossibleCauses................................. 6-9

ColorStat™panelSupplyFault..................... 6-10

PossibleCauses................................ 6-10

ColorStat™panelUnderdriveFault ................. 6-10

PossibleCauses................................ 6-10

ColorStat™panelOverdriveFault................... 6-11

PossibleCauses................................ 6-11

ColorStat™panelDoorInterlock ...................6-12

PossibleCauses................................ 6-12

ColorStat™panelExternalInterlock................. 6-12

PossibleCauses................................ 6-12

ColorStat™panelAirInterlock.....................6-12

Possiblecauses................................ 6-12

ColorStat™panelOscillatorFault................... 6-12

ColorStat™panelBufferFault ..................... 6-13

ColorStat™panelPredriverFault................... 6-13

ColorStat™panelRFAmpFault.................... 6-13

ColorStat™ panel Analog Input Board: +15V and -

15VSupplyFaults............................... 6-13

ColorStat™ panel Analog to Digital Converter ........ 6-13

+15V, -15V, and +5V Supply Faults............... 6-13

ConversionErrorFault..........................6-13

ColorStat™ panel Modulation Encoder: Cable Inter-

lockFault......................................6-13

8/9/2000 888-2297-002 v

WARNING: Disconnect primary power prior to servicing.

PossibleCauses................................6-14

ColorStat™ panel DC Regulator B+ and B- Supply

Faults .........................................6-14

ColorStat™panelOutputMonitorFaults.............6-14

+5Vand-5VSupplyFaults......................6-14

VSWRFaults .................................6-14

Symptom: Loss Of Positive Peak Capability...........6-16

PossibleCauses................................6-16

Symptom:HigherThanNormalAudioDistortion ......6-16

PossibleCauses................................6-16

Symptom:ConsistentLossofRFAmplifiers..........6-17

Repeated Loss of Same RF Amplifier In Any Position 6-17
Consistent Loss Of An RF Amplifier In One Particu-

lar Position ...................................6-17

Consistent Loss Of Modules In Random Positions ....6-17

Other Troubleshooting Techniques .................. 6-18

HandlingMOSFET’s ...........................6-18

TestingMOSFET’s.............................6-18

FindingAMissingStep.........................6-18

Using FlexPatch™ for Bypassing a Failed PA Mod-

ule..........................................6-18

Using FlexPatch™ for Isolating Modulation En-

coder/RFAmplifierProblems....................6-19

MeasuringDriveLevel..........................6-19

MeasuringDrivePhasing........................6-19

RFAmplifierDrainPhasing......................6-20

Section VII

Parts List

Section VIII

Diagrams

Introduction......................................8-1

Section A

Oscillator (A17)

Introduction..................................... A-1

Location........................................ A-1

PrinciplesofOperation............................ A-1

CircuitDescription ............................... A-1

Supply Voltages and Voltage Regulators ........... A-1

OscillatorStage................................ A-1

Buffer/SquaringAmplifier....................... A-1

FrequencyDivider.............................. A-1

ExternalInput................................. A-1

Normal or Combined Transmitter Operation ......... A-1

FrequencyMonitorOutput....................... A-1

OscillatorSync................................ A-1

OscillatorOutput(Buffer-Driver).................. A-2

“RFPresent”Output............................ A-2

Troubleshooting ................................. A-2

Symptom: Oscillator LED on ColorStat™ panel is

Red,transmitterwillnotoperate.................. A-2

Symptom: No RF Output, External Oscillator Used . . . A-3

Symptom:FrequencyInstability................... A-3

Symptom:OutputAtIncorrectFrequency........... A-3

Symptom: Oscillator LED on ColorStat™ panel is

Redbuttransmitteroperationisnormal............. A-3

Section B

Buffer Amplifier (A16)

Introduction..................................... B-1

PrinciplesofOperation.......................... B-1

CircuitDescription................................ B-1

BufferAmplifierSupplyVoltage.................. B-1

FirstRFAmplifierStage(U1) .................... B-1

SecondRFAmplifierStage(Q1andQ2)............ B-1

ThirdRFAmplifierStage(Q3andQ4)............. B-1

Output Coupling Network........................ B-1

BufferAmplifierRFSense....................... B-1

Predriver Supply ............................... B-1

Troubleshooting.................................. B-1

Symptom: Buffer Amplifier LED on ColorStat™

panel is Red, transmitter will not operate............ B-1

Symptom: Buffer Amplifier LED on ColorStat™

panel is Red, transmitter will operate............... B-2

Section C

RF Amplifier

Introduction ..................................... C-1

PrinciplesofOperation............................ C-1

RFAmplifier:BasicTheoryOfOperation........... C-1

RFAmplifier:HalfQuadConfiguration............. C-1

RFAmplifier:FullQuadConfiguration............. C-1

RF Amplifier Module On/Off Control Circuit ........ C-1

RFTransformerPrimaryCurrent:AmplifierOff...... C-3

OscillatorSyncSignal........................... C-3

CircuitDescription................................ C-3

SupplyVoltage ................................ C-3

LEDIndicators................................. C-4

CableInterlock................................. C-4

RFDrive...................................... C-4

ControlSection ................................ C-4

RFOutput..................................... C-5

Troubleshooting.................................. C-5

Symptom:BlownFuseIndicatorIlluminated......... C-5

Section D

Driver Combiner/Motherboard (A14)

Introduction ..................................... D-1

CircuitDescription................................ D-1

BufferAmplifierConnections..................... D-1

Predriver...................................... D-1

DriverStage................................... D-1

ControlSignals................................. D-1

RFDriveCombiner............................. D-1

DriverTuning.................................. D-1

Section E

Driver Supply Regulator (A22)

Introduction ..................................... E-1

Location........................................ E-1

PrinciplesofOperation............................ E-1

CircuitDescription................................ E-1

+15VoltRegulator............................. E-1

Control+VDCReference........................ E-1

PowerMOSFETOperation(AShortReview)........ E-1

RegulatorSectionCircuitDescription .............. E-2

DCAmplifierStage(Q2) ........................ E-2

Series Pass Transistors Q3, Q4, and Q7 (For Section

D8A Supply Voltage)........................... E-2

Series Pass Transistors Q5, Q6, and Q8 (For Section

D8BSupplyVoltage)........................... E-2

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WARNING: Disconnect primary power prior to servicing.

MeteringCircuits............................... E-3

Troubleshooting The Driver Supply Regulator.......... E-3

Symptom: Driver Sect D8A +VDC and Sect D8B

+VDCBothHigh.............................. E-4

Symptom: One Output Voltage Is +100 To +110

Volts,OtherCanBeAdjusted..................... E-4

Symptom: Both Driver Supply Regulator Output

VoltagesAreZero.............................. E-4

Symptom: One Driver Supply Output Voltage is

Zero,theOtherCanBeAdjusted.................. E-4

Symptom: Section D8B Voltage Increases Before

SectionD8AVoltageReaches+100Volts........... E-4

Symptom: Open Loop Operation is Correct, Closed

LoopOperationisFaulty......................... E-4

Section F

RF Multimeter (A23)

Introduction ..................................... F-1

CircuitDescription................................ F-1

MeteringDriverSectionParameters................ F-1

MultimeterProbe............................... F-1

Section G

RF Combiners:

Binary Combiner/Motherboard (A1)

and Main Combiner/Motherboards (A2 through A4)

Introduction .....................................G-1

PrinciplesofOperation ..........................G-1

CircuitDescription................................G-1

MainCombiner/Motherboards(A2throughA4)...... G-1

BinaryCombiner/Motherboard(A1)................G-1

Section H

Output Sample Board (A26) and

Output Monitor (A27)

Introduction .....................................H-1

CircuitDescription................................H-1

OutputSampleBoard............................H-1

OutputMonitor.................................H-1

Section J

Analog Input Board (A35)

Introduction ..................................... J-1

CircuitDescription................................ J-1

Audio Input.................................... J-1

BesselFilter................................... J-1

InstrumentationAmplifier(U6,U9)................ J-1

BufferAmplifier(U7)........................... J-1

MaximumPowerAdjust(U7,R27) ................ J-1

AnalogDivider(U10)........................... J-2

DigitallyControlledPotentiometer(U8)............. J-2

AnalogBuffer(U4)............................. J-2

DifferentialAmplifier/InverterU4.................. J-3

-(Audio+DC)SampleToDCRegulator............ J-3

AnalogInputBoardPowerSupplies................ J-3

BCD(BinaryCodedDecimal)Coding................ J-4

Troubleshooting .................................. J-4

Symptom: Normal Signal At TP4, No Signal At TP7

(DigitallyControlledPotentiometerOutput). ........ J-4

Symptom: Power Increases or Decreases in Steps,

NotContinuously.............................. J-4

Symptom:+15VDCor-15VDCfaults............. J-4

Section K

Analog To Digital Converter (A34)

Introduction..................................... K-1

PrinciplesofOperation............................ K-1

CircuitDescription............................... K-1

Converting a PA Sample to the A/D ENCODE Pulse

(T1,U29,Q9) ................................ K-1

FrequencyDivider(U29,Q9).................... K-1

ENCODESignalPulseWidth(Q9)................ K-1

Analog to Digital Converter Circuit................ K-1

ErrorDetectingCircuits......................... K-2

One-ShotOperation(U13,U14) .................. K-2

Big-StepSyncCircuit........................... K-3

ReconstructedAudioCircuit ..................... K-3

Maintenance .................................... K-4

PrintedCircuitboardMaintenance................. K-4

Adjustments................................... K-4

Troubleshooting ................................. K-4

Symptom: ColorStat™ panel CONVERSION ER-

ROR Indicator is RED, transmitter operates nor-

mally........................................ K-4

Symptom: ColorStat™ panel CONVERSION ER-

ROR indicator is RED, transmitter can be turned

ON.NoRFout................................ K-4

TechnicalAssistance.............................. K-5

ReplaceablePartsService.......................... K-5

Section L

Modulation Encoder (A37)

Introduction......................................L-1

PrinciplesOfOperation ............................L-1

Modulation Encoding: Explanation and Example......L-1

CircuitDescription................................L-1

SupplyVoltages................................L-1

DigitalAudioDataCircuits.......................L-2

Inverter/DriverCircuits...........................L-2

CableInterlockCircuit...........................L-2

PAOffCircuit..................................L-3

BlownFuseCircuits.............................L-3

ClipCircuit....................................L-4

FlexPatch™......................................L-4

Example.......................................L-4

SingleRFAmpMomentaryTest...................L-4

Troubleshooting ..................................L-5

Symptom:SuspectedFaultyModulationEncoding ....L-5

Symptom:Red“PAOff”IndicatorIlluminated .......L-5

Symptom:CableInterlockIndication ...............L-5

SECTION M

DC REGULATOR (A30)

Introduction..................................... M-1

PrinciplesOfOperation ........................... M-1

Circuit Grounds on the DC Regulator .............. M-1

UC3834IntegratedCircuitLinearRegulator......... M-1

B+ (+5.75 VDC) Regulated Supplies (DC Regulator) . M-2

Modulated B- Supplies.......................... M-3

Modulated B- Supplies: Circuit Description ......... M-3

Other Negative Regulated Supplies in the Transmitter. M-3

RegulatorOutputInhibitCircuit(U2).............. M-3

+5VDCSupply ............................... M-4

-5VDCSupply................................ M-4

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WARNING: Disconnect primary power prior to servicing.

FaultDetectionCircuits ......................... M-4

B+FaultCircuit ............................... M-4

B-FaultCircuit................................ M-4

InterlockStatusCircuit.......................... M-4

LCDMultimeter............................... M-4

Troubleshooting ................................. M-5

Symptom:B+Fault............................. M-5

Symptom:B-Fault............................. M-5

Section N

External Interface (A28)

Introduction..................................... N-1

PrinciplesOfOperation ........................... N-1

CircuitDescription ............................... N-1

StatusOutputs(TypeA)......................... N-1

Control Inputs (Type B) ......................... N-1

MonitorVoltageOutputs ........................ N-3

ExternalInterlock.............................. N-3

AudioInput................................... N-3

CombinerInterconnect.......................... N-3

“PATURNOFF”and“OFFCONTROL” .......... N-3

ExternalInterfacePowerSupplies................. N-4

Troubleshooting ................................. N-4

Symptom:NoRemoteControlInputsOperate....... N-4

Symptom: Some Remote Control Inputs Do Not Op-

erate ........................................ N-4

Symptom: Remote Status Outputs Do Not Operate . . . N-4
Symptom: Some Status Outputs Operate, But One Or

More Does Not (fault Indication On Transmitter Col-

orStat™panelButNoRemoteStatusIndication).... N-4

Symptom: One Or More Remote Status Indications

Remain “ON”; Transmitter Status Indication is Off

(Green)...................................... N-5

Symptom: No Monitor Outputs (Analog Signal Out-

puts)........................................ N-5

Section P

Controller (A38)

Introduction......................................P-1

Function.........................................P-1

Location.........................................P-1

BlockDiagramDescription .........................P-1

TransmitterTurn-on/Turn-offControlLogic..........P-1

TransmitterPowerControlLogic...................P-4

Turn-on/Turn-offControlLogic:CircuitDescriptions....P-5

“K1 Turn-on One-Shot” (Monostable U56A-13) ......P-5

One-Shot Trigger and Operation During Transmitter

Turn-on.......................................P-6

DelayCircuits:Description........................P-6

PAOffAndOverdriveInhibitGateU52-6...........P-6

PATurn-off(U53-3,U52-6,andS2)................P-7

PowerControlLogic:CircuitDescription..............P-7

CommandInputs................................P-7

SwitchDe-bounce(U45) .........................P-7

PriorityEncode/Decode..........................P-7

“PowerLevelChange”Pulse......................P-8

PowerLevelLatch(U40).........................P-8

Latched“Off”Command.........................P-8

PowerLevelLatchOutputs .......................P-9

InhibitGatesU39-3,U39-6ANDU39-8 ............P-9

Turn-onRequestGate(U52)andInverter(U55) ......P-9

Up-Down Counters: Setting and Storing Digital

PowerData................................... P-9

Up-Down Counter Control Gates (U1, U2, U14, U26) P-11

DataStrobeGates(U1-3/8/11andU13-8).......... P-11

Up-DownCounter“Inhibit”Circuits.............. P-11

Multiplex and Output Buffers For BCD Power Data

(U9-U11,U21-U23,andU33-U35)............... P-12

“Data Strobe” Output and Delay (U13-8, U62-10) . . . P-13
Power Control Status Indicator Drivers (U46, U47) . . P-13

ClockInhibitGate(U1-6)....................... P-13

Clock Frequency Divider and Delay (U24, U38, and

U50-4/10)................................... P-13

InterlockStatusLogic:CircuitDescription ........... P-14

Interlock Status Logic: Inputs .................... P-14

InterlockStatusLogic:Outputs................... P-16

InterlockStatusLogic:BasicCircuitDescription .... P-16

ExternalInterlock:NormalOperation.............. P-16

DoorInterlock:NormalOperation................ P-16

External Interlock: Fault Condition................ P-16

Door Interlock: Fault Condition .................. P-16

InterlockStringOperation....................... P-17

+5BResetCircuit ............................... P-17

+5BReset-LOutput(U37-1) .................... P-17

+5BReset-HOutput(U37-14)................... P-17

PowerSupplies ................................. P-18

VoltageRegulatorIntegratedCircuits.............. P-18

+5B(Backup)Supply.......................... P-18

BatteryBack-up............................... P-18

Supply Fault Circuits............................. P-18

Regulator“FaultAlerts”........................ P-18

“RegulatorFaultSummary”Indicator,DS1......... P-18

“FastOn-SlowOff”DelayCircuit(U49-4)......... P-18

“DataClear-L”signal .......................... P-19

“Supply Fault-L” signal to LED Board ............ P-19

AnalogMeteringBuffer/Drivers.................... P-19

ForwardandReflectedPowerMetering............ P-19

VSWRDetectorNullMetering(U64)............. P-19

SupplyVoltsMetering(U63C)................... P-19

AC Power Recycle (Recycle “On” After Power Fail-

ure) .......................................... P-19

GeneratesTurn-onRequest...................... P-19

InhibitedOFFcommand........................ P-19

SupplyFault-L,Fivesecond“Off”delay........... P-19

Section Q

LED Board (A32)

Introduction ..................................... Q-1

Location........................................ Q-1

Transmitter Fault Types............................ Q-1

Type1Fault:TurnsTransmitterOff ............... Q-1

Type 2 Fault: Recycles Transmitter Off/On One Time . Q-1

Type 3 Fault: Lowers Transmitter Power............ Q-1

Type4Fault:AppliesPATurnoff................. Q-1

Type5Fault:ClearsModulatorData............... Q-1

Type6Fault:PAModuleBlownFuseIndication..... Q-1

BlockDiagramDescription......................... Q-1

Type1Faults.................................. Q-2

Type2Faults.................................. Q-4

Type3Faults.................................. Q-4

Type4Faults.................................. Q-4

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WARNING: Disconnect primary power prior to servicing.

Type5Faults..................................Q-4

Type6Faults..................................Q-4

RFSenseCircuits...............................Q-4

ResetCircuit...................................Q-4

CircuitDescriptions...............................Q-5

ResetCircuit...................................Q-7

Type1FaultCircuits............................Q-7

Type2FaultCircuits...........................Q-10

Type3FaultCircuits...........................Q-12

Type4FaultCircuits...........................Q-16

Type5FaultsCircuit...........................Q-18

Type6FaultCircuit............................Q-18

“RFSense”CircuitsontheLEDBoard............Q-18

OSCTESTswitchS1 .......................... Q-20

Maintenance....................................Q-20

PrintedCircuitBoardMaintenance................Q-20

Adjustments..................................Q-20

Section R

Switch Board/meter Panel (A31)

Introduction .....................................R-1

PowerControl.................................. R-1

MultimeterCircuit(M1) ......................... R-1

SupplyCurrentMeterCircuit(M2)................. R-1

PowerMeterCircuit(M3)........................ R-1

Maintenance .....................................R-2

Adjustments....................................R-2

Troubleshooting ..................................R-2

Symptom:IncorrectMeterIndications...............R-2

Section S

Driver Encoder/Temp Sense Board (A19)

GeneralDescription ...............................S-1

TheoryOfOperation ..............................S-1

DriverTurn-OnControl..........................S-1

TemperatureSensor/PowerFoldback ............... S-3

AirFlowDetection..............................S-4

CableInterlockMonitor.......................... S-5

PowerSupplies................................. S-5

Maintenance .....................................S-5

Adjustment ......................................S-5

Appendix A

Lightning Protection Recommendation

Introduction...................................... a-1

EnviornmentalHazards............................. a-1

WhatCanBeDone?............................... a-1

ACServiceProtection ............................. a-2

Conclusion....................................... a-3

8/9/2000 888-2297-002 ix

WARNING: Disconnect primary power prior to servicing.

MANUAL REVISION HISTORY

994 9168 001

888-2297-XXX
Rev. Date ECN Pages Affected
002-A 09-23-94 39409 Title Page and pages 7-7 to 7-9. Added MRH-1/MRH-2
002-A1 12-12-94 Errata Misc pages - reference to 839 7855 068 & 839 7855 122 changed to 839 7855 151
002-B 01-09-95 39470 Title Page, MRH-1/MRH-2, and pages 7-4 to 7-7
002-C 04-21-95 39791 Title Page, MRH-1/MRH-2, and pages 7-20 to 7-23
002-D 08-17-95 40081 Title Page, MRH-1/MRH-2, pages 7-28 to 7-32 and 7-41
002-E 10-27-95 39917 Title Page, MRH-1/MRH-2, and all of Section VII
002-F 11-15-95 40244 Title Page, MRH-1/MRH-2, and pages 2-4, 2-5, 4-8 to 4-11 & 7-26 to 7-28
002-G 11-29-95 40170R Title Page, MRH-1/MRH-2, and page 7-42
002-H 12-04-95 40153R Title Page, MRH-1/MRH-2, and page 7-20 to 7-23
002-J 03-01-96 41095R Title Page, MRH-1/MRH-2, and pages 6-2, 6-12, 7-2 & 7-3
002-K 03-19-96 TPD Title Page, MRH-1/MRH-2, and page 2-3
002-L 05-02-96 41154 Title Page, MRH-1/MRH-2, and all of Section VII
002-M 06-13-96 41280A Title Page, MRH-1/MRH-2, and all of Section VII
002-N 07-08-96 41326 Title Page, MRH-1/MRH-2, and pages 7-38 & 7-39
002-P 07-31-96 41404 &

41390
002-P1 10-09-96 40242 Title Page, MRH-1/MRH-2, and pages 7-24 to 7-26
002-P2 10-14-96 41282 Title Page, MRH-1/MRH-2, and pages 7-2 to 7-3
002-R 11-11-96 41384 Title Page, MRH-1/MRH-2, and pages iv to ix, 1-1, all of the following sections 2, 4, 5, 6,

002-S 05-02-97 41732 Title Page, MRH-1/MRH-2, P-14 and all of Section VII
002-S1 05-29-97 41726 Title Page, MRH-1/MRH-2, P-14 and all of Section VII
002-T 06-13-97 TBD Title Page, MRH-1/MRH-2, and page 5-22
002-T1 06-25-97 41845 Title Page, MRH-1/MRH-2, and pages 7-2 & 7-3
002-T2 06-27-97 41809 Title Page, MRH-1/MRH-2, and pages 7-24 to 7-26 & L-7
002-U 08-05-97 41856 Title Page, MRH-1/MRH-2, and page K-1
002-U1 01-23-98 41937 Title Page, MRH-1/MRH-2, P-14 and all of Section VII
002-X 05-13-98 42020 Title Page, MRH-1/MRH-2, and pages 5-6 and K-5
002-X1 11-23-98 42239A Title Page, MRH-1/MRH-2, and all of Section VII
002-Y 02-10-99 42629 Title Page, MRH-1/MRH-2, and page 2-5
002-Z 05-22-00 46105 Title Page, MRH-1/MRH-2, and page 5-15
002-Z1 06-02-00 Errata Title Page, MRH-1/MRH-2, and page 5-16
002-AA 08-09-00 46332 Title Page, MRH-1/MRH-2, chapter II, pages 3A-13 and 4-4

Title Page, MRH-1/MRH-2, and pages 7-15 to 7-18 & 7-21 to 7-23

7, 8, H, P, Q, & R

WARNING: Disconnect primary power prior to servicing.

888-2297-XXX MRH-1/MRH-2

Section I

Introduction/Specifications

1.1 Introduction

The DX-25U Medium Wave Transmitter is a 25 kW transmitter
that is designedto beable to beupgraded quickly toa 50kW unit
(thus becoming a DX-50 Transmitter). This upgrade requires
very little time and the newly configured system would be back
in operation at the new power level in a minimum of time.

1.1.1

Scope and Purpose

This technical manual contains the information necessary to
install and maintain the DX-25U MEDIUM WAVE TRANS­MITTER. The various sections of this technical manual provide
the following types of information.

a. SectionI, Introduction/Specifications, provides introduction to

technical manual contents.

b. Section II, Installation/Initial Turn-on, provides detailed

installation procedures and initial turn on instructions.

c. Section III, Operators Guide, provides identification and

functions of panel and board mounted controls and indica­tors as well as of components located in the interior of the
transmitter.

d. Section IV, Overall System Theory, provides a block dia-

gram description of the overall transmitter and operation
of the various sections of the transmitter not covered in
later sections.

e. Section V, Maintenance, provides preventive and corrective

maintenance information as well as alignment procedures.

f. Section VI, Troubl eshooting, provides a listing of the pro-

tection devicesin the transmitter as well as low power and
high power troubleshooting procedures.

g. Section VII, Parts List, provides an itemized parts list for

individual transmitter modules and the overall transmitter.
h. SECTION VIII, Diagrams
i. The following sections provide principles of operation for

boards and modules in the DX-25U TRANSMITTER:

• Section A, Oscillator (A17)

• Section B, Buffer Amplifier(A16)

• Section C, RF Amplifier Modules (A101-A244)

• Section D, Driver Combiner/Motherboard (A14)

• Section E, Driver Supply Regulator (A22)

• Section F, RF Multimeter (A23)

• Section G, RF Combiners: Binary Combiner/Motherboard

(A19) and Main Combiner/Motherboard (A2 through A4)

• Section H, Output Sample Board (A26) and Output Moni-

tor (A27)

• Section J, Analog Input Board (A35)

• Section K, Analog To Digital Converter(A34)

• Section L, Modulation Encoder (A37)

• Section M, DC Regulator (A30)

• Section N, External Interface (A28)

• Section P, Controller (A38)

• Section Q, LED Board (A32)

• Section R, Switch Board/Meter Panel (A31)

• Section S,DriverEncoder/TemperatureSense Board(A19)

j. Appendix A,Lightning ProtectionRecommendation, pro-

vides information on environmental hazards and possible
steps to minimize their effects.

1.2 Equipment Description

The DX-25U isa solid state medium wave amplitude modulated
transmitter with a rated output power level of 25 kW. The
transmitter is capable of 30 kW and has three user-set power
levels. The frequency range is from 531 kHz to 1605 kHz.

1.3 Block Diagram

A BlockDiagram description is contained in Section IV, Overall
System Theory.

1.4 Specifications

The brochure at the end of the manual gives specifications for
the DX-25U transmitter.

NOTE

Harris maintains a policy of continuous improvements on its
equipment and therefore reserves the right to change specifica­tions without notice.

Rev. R: 11-11-96 888-2297-002 1-1

WARNING: Disconnect primary power prior to servicing.

DX-25U

Figure1-1. DX 25U Front View with Doors Closed

Figure1-2. DX 25U Rear Viewwith Doors Closed

1-2 888-2297-002 Rev. R: 11-11-96

WARNING: Disconnectprimary power prior to servicing.

Section II

Installation/Initial Turn On

2.1 Introduction

This sectionprovides informationand instructions necessary for
the installation and initial turn on of the HARRIS DX-25U
MEDIUM WAVE TRANSMITTER, including AM Stereo In­stallation and Remote Control connections. The instructions are
given to minimize the installation time required. Care and pre­cautionarymeasures are given to preventproblems orinjuryfrom
occurring during installation.

Planning and Preparation are the most important factors in a
successful, efficient, and safe installation of a new transmitter.
This section should be read thoroughly prior to installation for a
basicunderstanding of theoperation,circuitry,andnomenclature
of the transmitter.

The transmitter equipment installation PHASES or STAGES
should be planned before the equipment arrives and a detailed
plan worked out and written down. Determine what installation
equipment and materials are supplied with the transmitter and
what equipment the station mustsupply.In general, a transmitter
installation requires that the following areas be addressed:

• In a new installation, WILL THE BUILDING/TRANS-

MITTER ROOM BE COMPLETED? Electronic equip­ment can be damaged or made inoperable by dust and dirt.
Interior walls should be in place, ceiling work should be
complete, concrete floors should be aged and well sealed.
Even a plastic covering placed over the transmitter rarely
keeps out concrete dust and plaster dust created from dry­wall installation.

• In a new installation, WILL ELECTRICAL POWER BE

AVAILABLE WHEN NEEDED? Often transmitter instal­lationand checkoutis heldup becauseprimarypoweris not
available for the transmitter.

• In an existingfacility,MUSTANEXISTINGTRANSMIT-

TERREMAIN ON THEAIRduring installation ofthenew
equipment? Plan how this is to be done to minimize off-air
time.

• STAGING AREA. An area should be chosen and set aside

to place all boxes and crates that contain the smaller parts
and assemblies. A separate area should be used to stage all
installation material (wire, conduit and accessories, hard­ware, etc.). Each piece of equipment should be inspected
for shipping damage. Inventory all equipment and the con­tents of each box and compare to the packing list.

• UNLOADING. Will the proper lifting and moving equip-

ment be available when the transmitter arrives? Will there
be enough workers to help unload the transmitter?

• EQUIPMENT PLACEMENT. Using astationlayout draw-

ing, determine equipment placement AND IN WHAT OR­DER EQUIPMENT SHOULD BE SET IN PLACE. If
possible, lay out equipment location with lines marked on
the floor.

• HANGING HARDWARE. Ensure that all pipe hangers,

conduit hangers, threaded rod, beam clamps, Unistrut and
Unistrut hardware is on site.

• TOOLS. Ensure thatall necessarytools willbeonsite when

needed. Make sure all tools are in good shape. Check
technical manuals for the transmitter and other equipment
to see if specialized tools are required. Make arrangements
to obtain them if necessary.

• During thevarious stages ofinstallation different personnel

may be employed, (i.e. contractors, hired help, general
labor). Since they may not beinvolvedin theinitial turnon
of the transmitter it is vital that their work be closely
supervised and checked to avoid any damage or failures to
equipment. It is extremely important to prevent debris,
especially metal filings and hardware, from getting dropped
and lodged into the subassemblies of the transmitter.

2.2 Delivery And Storage

The DX-25U is normally delivered mounted on shipping skids.
Smaller components are shipped in cardboard cartons. Any ob­vious damage should be noted at the time of receipt and claims
filed with the carrier.

Equipment capable of handling a 3,000 pound (1,320 Kg) load
will beneeded to unload the transmitter. Extremecare should be
takenduring theunloading operationto prevent injury to person­nel or damage to the equipment.

If the transmitter is to be temporarily stored, all units require
inside storage. Do not stack items except for small cardboard
cartons. The storage area should be dry and clean.

2.3 Returns And Exchanges

Damaged or undamaged equipment should not be returned un­lessa Return Authorizationis issued.Whencommunicating with
Harris Corporation, Broadcast Division, sp ecify the order
number or invoice number. Include complete details regarding
circumstances and reasons for return in the request. Custom or
special order equipment is not returnable. In instances where
return orexchange of equipment is at the requestor conv enience
of the customer, a restocking fee will be charged. Special ship­ping instructions and coding will be provided to insure proper
handling. All returns will be sent freight prepaid and properly
insured by the customer.

2.4 Unpacking

Carefully unpack the transmitter and save all packing material.
Inspect thoroughly for any damage incurred in shipment. Retain
all PACKING CHECK LISTS to help locate and identify any
components or assemblies removed for shipping. Remove any
shipping supports, and straps prior to initial turn on.

Rev. AA: 8/9/2000 888-2297-002 2-1

WARNING: Disconnect primary power prior to servicing.

DX-25U

Table 2-1. Recommended Installation Materials

NOT SUPPLIED

Ground Strap or Rigid Transmission Line

Welding Torch Set
Oxygen and Acetylene Tanks
Welder’s Mask or Goggles
Power Band Saw (can be rented) and Extra Blades
Silver Solder 1/16 inch diameter, 30%-45%, Hard Stay­Silv #45, Aladdin #45,

HARRIS part number 099 0002 238
Pasteflux(EngelhardUltra-Flux1lbjar)HARRISpart
number 099 0002 241
(HARRIS part number 086 0004 040, 16 oz bottle)
Muriatic Acid (quart)
Rubber Hammer
Garden Hose
Baking Soda (two 1-pound boxes)
Three plastic 5-gallon buckets or containers with open tops
Scotch Brite, Scuff Pad/Sand Paper
Steel Wool
Emery Cloth (roll type like plumber uses)

Basic Wiring and Installation

Wire Strippers
Wire Cutters
Lugging Tool
Socket Set 3/8 inch drive with 6 inch extension
Table or Bench
Carpenters Square
Level
Plumb Bob
Chalk Line
Hacksaw and Extra Blades
Wrenches
Prybar
Crowbar
Rope
Saw Horses or Cutting Table
Cable Hoist or Chain-Fall Hoist
Ladders
Files
25-Ft Tape Measure
Claw Hammer

Materials Not Supplied

105kVA wire (See Cabinet Outline for Size)
Copper Strap
Electrical Conduit
Transmission Line
Circuit Breaker or Fused Disconnects
Transmission Line Matching Hardware
Audio Cable
Remote Control Cable

CAUTION

KEEP THE PA CABINET EXHAUST AND ACCESS HOLES COV­ERED DURING INSTALLATION!

2.5 Factory Test Data

During installation and initial turn on procedure, reference will be
madetoFACT ORYTESTDAT A.Thisdataisnormallypackedwith
thetransmitter ormaybeinsertedinthetechnicalmanual.Itincludes
meter readings, measured performance data, frequency determined
parts and adjustments for your transmitter.

2.6 Transmitter Placement

Refer to the DX-25U Cabinet Outline, 839-7855-152, in the
Drawing Package for important DIMENSIONS, WEIGHT,AIR
FLOW, and ELECTRICAL information.

The DX-25U consists of three main components:

• PA Cabinet

• Output Network Cabinet

• Step Start Panel

The PAand Output Network Cabinetsare bolted together on-site
after positioning and leveling.

Depending on the height of the doors at the site, the Cabinets
mayhavetoberemovedfromtheirskids.AlsotheCombiner
Crossover bar and cover on top of the PA Cabinet may have to
be removed and re-installed later.

2.6.1

Removing the Combiner Crossover Bar

NOTE

Only remove the crossover bar if additional height clearance is
needed to move the transmitter during installation.

• Remove the rear access panels from theRF Amp Compart-

ment.

• Locate the combiner crossover bar on top of the Power

Amplifier Cabinet. Refer to sheet 1 of the Cabinet Outline
Drawing, 839-7855-152.

• Remove the 14 screws that hold the Combiner Cover in

place to expose the combiner bar and the Neutralization
Board. Remove the screws holding the N eutralization
Board and unplug J1.

• Remove the two 3/16 inch Allen head bolts holding each

end of the crossover bar to the combiner bar and remove

the bar and Neutralization Board.
Once thePA Cabinet is inposition, it will be necessary to put the
Combiner barback in place. When tightening the hexhead bolts,
torque to 80 inch pounds. After thebar isinstalled, verifythat J1
on the Neutralization Board is connected.

2.6.2

Cabinet Positioning

Remove shipping bolts and carefully move cabinets off of the
skids. The PA Cabinet is the heaviest and should be positioned
first. The center of gravity is near the Power Supply/Driver

2-2 888-2297-002 Rev. AA: 8/9/2000

WARNING: Disconnectprimary power prior to servicing.

Section II - Installation

Compartment end of the cabinet and should be fork lifted from
this end. If roller bars are used, lead with the heavier end.

CAUTION

UNIT MUST NOT BE DROPPED. MAKE A RAMP TO ROLL FROM
SKID TOFLOOR.

NOTE

ALONG THE BOTTOM OF THE CABINET FRAME ARE LEV­ERAGE POINTS FOR ELEVATING THE CABINET TO ALLOW
HEAVIER PRY-BARS TO BE POSITIONED UNDERNEATH
THE FRAME.

After the PA Cabinet is in position, position theOutput Network
Cabinet. Maneuver the Output Network Cabinet carefully when
joining the two cabinets together so as not to damage any of the
exposed fan blades. Make sure that none of the interconnecting
wires are exposed while positioning thecabinets. Shim and level
the cabinets before bolting together.

If the transmitter is to be positioned into a predetermined wall
opening, be sure to allow for the fan blades of the Output
Network Cabinet and any final leveling needed when determin­ing the wall opening dimensions.

Cabinet Bolts

2.6.3

Fastenthecabinets together using the1 inch1/4-20 stainlesssteel
bolts, flatwashers, split washers, and nuts provided in the instal­lation hardware kit. Plastic plugs are provided to cover the
hardware access holes in the cabinets. These will prevent air
leakage for optimum transmitter cooling.

2.6.4

Cabinet Ground Strap

After the two cabinets are bolted together, fasten the 2 inch
copper interconnect strap from the PA Cabinet to 2E1 in the
Output Network Cabinet.

2.6.5

Cabinet Grounding Plate

Unbolt the Cabinet Grounding Plate on top of the PA Cabinet
and rotate 180 degrees. Secure to the PA Cabinet and Output
Network Cabinet with 10-32 screws, flat washers and split
washers.

2.7 Air System And Cooling

Refer to Sheet 2 of the DX-25U Cabinet Outline Drawing,
839-7855-152,for information onairflowCFM,heat dissipation
and duct work dimensions.

Cooling air for the transmitter enters through the rear of the
Output NetworkCabinet and exhausts through the top-front half
of the PA Cabinet. If an exhaust duct is used, static pressure in
the duct must be neutral or slightly negative. Static pressure for
air intakeat the rear of the transmitter must beneutral or slightly
positive. The exhaust grill opening on the top of the PA Cabinet
must not be restricted. The dimension from the front edge of the
transmitter to the exhaust opening is only 2.3 inches (5.84 mm).
This is an important consideration when a wall will be installed
along the front of the transmitter.

2.8 Electrical Installation

Refer to sheet 3 of the DX-25U Cabinet Outline Drawing,
839-7855-152, for electrical information.

Sheet 1 of the DX-25U Overall Schematic, 839-7855-151, de­tails the interconnections between the PA Cabinet and the Step
Start panel.

2.8.1

Step Start Panel

The Step Start panel is intended to be wall mounted and should
be located as close as possible to the PA Cabinet. The conduit
may be attached to any of the four sides or the back of the Step
Start panel, however, there are no knock-out holes provided on
the back ofthe panel. Alltransmitter AC power is routedthrough
the Step Start panel to utilize the MOV surge protectors. The
grounding strap 3E1, should be connected to a good station
ground with 2" copper strap for optimum protection.

2.8.2

AC Power

There are two separate ACpower feeds needed for the DX-25U:
athreephase105 kVAinputforT1,PAPowersupplytransformer
and a single phase 1 kVA supply for T2, Low Voltage supply
transformer. The input voltage for T1 can be from 360 VAC to

PA Supply Voltage 230 VDC Blower Wires #123, #124, & #125

Line Line to Jumper Line Line Frequency
Voltage T1 Tap Voltage 60 Hz 50Hz
502 Vac 485 485 to +4% 502 Vac -4% tap 380 tap
485 Vac 485 485 to 0 485 Vac -4% tap 380 tap
468 Vac 485 485 to -4% 468 Vac -4% tap 380 tap
447 Vac 430 430 to +4% 447 Vac +4% tap 380 tap
430 Vac 430 430 to 0 430 Vac +4% tap 380 tap
413 Vac 430 430 to -4% 413 Vac 485 tap 380 tap
397 Vac 380 380 to +4% 397 Vac 430 tap 0 tap
380 Vac 380 380 to 0 380 Vac 485 tap 0 tap
363 Vac 380 380 to -4% 363 Vac 485 tap 0 tap

Rev. AA: 8/9/2000 888-2297-002 2-3

WARNING: Disconnect primary power prior to servicing.

Table 2-2. Transformer Tapping for T1 and Blowers

DX-25U

Figure 2-1

K2 in step-start panel connections.

505 VAC, 50/60 Hz. The T2 input voltage can range from 198
VACto280 VAC.An optional 1kVAstep-downtransformer may
be purchased if a separate 240 VAC source is not available.

Refer to page three of the DX-25U Cabinet Outline Drawing for
recommended wire and fuse size for the three phase 105 kVA
feed.

NOTE

THIS EQUIPMENT IS DESIGNED FOR CONNECTION TO A
WYE OR CLOSED DELTA THREE-PHASE POWER SOURCE.
EACH AC POWER FEED IS TO HAVE ITS OWN SEPARATE
DISCONNECT.

2.8.3 Interconnection Wiring

Refer to Sheet 1 of the DX-25U Overall Schematic, 839-7855­151, and Table 2-3 for interconnect information between the
cabinets and Step Startpanel. Wires154, 155, 156, 121,519, 96,
and 97 are part of the wiring harness in the PA Cabinet.

The Installation Kit, 992-8102-001, provides an assortment of
wire and lugs to be used between 3TB1 of the Step Start panel
andTB3of the PACabinet. Enough wire isprovided for a conduit
run of approximately 60feet (18.29meters). Seedrawing 3A-10
for component lay out.

The electrical interconnection between the Step Start Panel and
the PA Cabinet should be made by using conduit runs into the
top of the transmitter. Access is available through the bottom of
the transmitterfor use of a wiretrough. If conduit is used, 3 runs
are needed: One forthe 105 kVA 3 phasefeed, one for the1 kVA
single phase feed and one for the step start control cables. The
control cables should be kept separate from any AC input to the
transmitter cabinet. Refer to Sheet 3 of the DX-25U Cabinet
Outlinedrawing,839-7855-152, for drawing details oftheseruns
and the location of the interconnect points.

2.8.4

Transformer Tap Settings

Tapping information for T1 and the blowers will be found on
Table 2-2, wire and fusing information will be found on Sheet 3
of the DX-25U Cabinet Outline drawing, 839-7855-152. Tap­ping information for Low Voltage Supply transformer T2 is silk
screened on the center door and labeled on the transformer.

NOTE

If the line voltage falls between the 4% taps or sags when going
from no load to full load, tap T1 down to the next voltage combi­nation to avoid unnecessary tripping of the Over Voltage Over­load during initial turn on.

2.9 Component Installation

Some components have been removed for shipment, and will
have to be reinstalled. These components include the PA Com­biner Output Bar and cover and Grounding Plate. Under some
shipping conditions, Output Network capacitors may havebeen
removed. For the following information, refer to Sheet 4 of the
Overall Schematic, 839-7855-151, and Sheet 1 of the Cabinet
Outline drawing, 839-7855-152.

2.9.1

PA Combiner Output Bar

Slide the PA Combiner Output Bar through the feed-through
betweenthe PA Cabinet andOutput NetworkCabinet. Fasten the
bar between the Combiner and L1 in the Output Network using
1/4-28 hex screws and lock washers. Make sure the hardware is
securely tightened.

Next, slide the Output Bar C over over the Outp ut Bar. Make
sure no packing materi al is le ft ins i de the cover. Secure it
using 10-32 screws,flatwashers, and splitwashers. Install the
RF Sample Assembly (T6) on top of the cover with 6-32
screws, flat washers, and split washers. Use the silkscreen on
the back of the rear access panel as a guide to ensure t h at the
assembly is installed inthe correctdirection. Wire #96 should
be connected to T6.

After the RF Sample Assembly is installed on the Output Bar
Cover, fasten the two Close-Out Panels over the feed-through
openings. One is mounted on the PA Cabinet side and the other
is mounted on the Output Network Compartment side.

2.10 Transmitter Grounding

The PA and Output Network Cabinets must be grounded in
addition to grounding the Step Start panel. The transmitter must
be grounded to the station ground system with copper strap at
least two inches wide and 0.020 inch thick connected to 2E1 in
the Output Network Cabinet. A two inch strap connects 2E1 in
the Output Network Cabinet with E1/E2 of the PA Cabinet.

Allgroundingpoints in the transmitter haveabrass block to make
grounding connections. Grounding connections external to the
transmitter should be brazed.

2-4 888-2297-002 Rev. AA: 8/9/2000

WARNING: Disconnectprimary power prior to servicing.

Section II - Installation

CAUTION

THE TRANSMITTER MUST BE GROUNDED WITH A LOW IMPED­ANCE PATH. THE OUTER CONDUCTOR OF THE RF TRANSMIS­SION LINE IS NOT ANADEQUATE GROUND.

2.11 RF Output Connection

The RF output connection is a 3-1/8 inch gas barrier EIA flange
connector, (Female). A bullet and O-ring are provided and
packed separately. This combination allows mating to a male or
femaletransmission lineconnector.After theRF output termina­tion is made, make sure the spark gap, E101 in the Output
Network Compartment, is set to 0.125 inches.

2.11.1

It is very useful to be able to switch the transmitter RF output to
a dummy antenna for testing. This testing frequently includes
modulating with tones.Withtone modulation, 100%modulation
of a 30 kW carrier produces an average power of 45 kilowatts.
Sustained asymmetrical modulation of a 25 kW carrier, with

-100% and +140% peaks, will produce 47 kW of average power
that must be absorbed and dissipated by the load. The instanta­neous peak power under these conditions will be approximately
152 kW. When selecting a dummy load, select a power rating

Dummy Antenna Information

Table 2-3. Interconnect Wiring Check List

sufficient for the type of testing to be done. The peak power
should be considered when selecting a water cooled load.

2.12 Customer Interface Connections

The following paragraphs include information on customer in­terface connections for:

• Audio Input

• Frequency and Modulation Monitors

• External Interlocks

• External RF Input

Refer to paragraphs on Remote Control connections for a de­scription of various interface connections.

2.12.1

Use a shielded pair audio cable for connection between process­ing equipment and Audio Input terminal TB3 on the External
Interface. The External Interface is located at the top of the right
side wall in the Center Control Compartment.

Audio input connections are as follows:

Audio Input

• TB3-1: Shield (to transmitter chassis ground).

• TB3-2: Audio (+)

WIRE # FROM TO LOCATION/DESCRIPTION

PA/OUTPUT CABINET INTERCONNECT

154 F17 2TB1-1 OUTPUT NETWORK CABINET
155 F18 2TB1-2 SAME
156 F19 2TB1-3 SAME
121 A27-J7 2J1 SAME
519 A27-J1 A26-J1 OUTPUT SAMPLE BOARD IN OUTPUT NETWORK CABINET
96 A17-J3 T6 PA CABINET, PA COMBINER OUTPUT BAR AND COVER ASSEMBLY
STEP START PANEL INTERCONNECT
13TB1-1TB3-1SINGLE PHASE AC INPUT TO PA CABINET *
23TB1-2TB3-2SINGLE PHASE AC INPUT TO PA CABINET *
33TB1-3TB3-3GROUND

8 CONDUCTOR CABLE

SHIELD 3TB1-4TB3-4 **
BLK 3TB1-5TB3-5 **
BLU 3TB1-6TB3-6 **
BRN 3TB1-7TB3-7 **
GRN 3TB1-8TB3-8 **
ORN 3TB1-9TB3-9 **
RED 3TB1-10TB3-10 **
WHT 3TB1-11TB3-11 **
153TB1-15TB3-15K1 AND K2 COIL VOLTAGE
163TB1-16TB3-16K1 AND K2 COIL VOLTAGE

2 CONDUCTOR CABLE

SHIELD 3TB1-14 TB3-14
BLK 3TB1-17 TB3-17
WHT 3TB1-18 TB3-18

(Note: WHT may be clear or RED.)

* NOTE: #1, #2, are14 AWG. Electric
code may require different wire size being
used.

**NOTE:This is onlyasuggested order tofollow.
Thecolorcoding may varybetweendifferentwire
manufactures. (e.g. BRN substituted with
RED/BLK. YEL substituted with WHT/BLK.
The important thing is to maintain the 1 to 1
interconnect between 3TB1 and TB3.)
NOTE:YELor 8th doesnotneedto be connected.

Rev. AA: 8/9/2000 888-2297-002 2-5

WARNING: Disconnect primary power prior to servicing.

DX-25U

• TB3-3: Audio (-)

• TB3-4: Optionalshield connection,capacitivelycoupled to

transmitter chassis ground.
Audio Input TB3 is shown on sheet 2 of the DX-25U Overall
Schematic, 839-7855-151. The following paragraphs include
additional information which may be useful in planning and
connecting the audio input.

2.12.1.1 Shield Connections

Theaudio inputcable shield shouldbe groundedat only oneend,
either at the processor or at Audio Input terminal TB3-1. Con­nectingthe shield at bothends canresult inground loopproblems
and increased system noise. Audio Input terminal TB3-4 pro­vides an AC coupled path to the transmitter cabinet. In some
installations, lower noise may be obtained by connecting the
audio cable shield to this terminal.

2.12.1.2 Polarity

If the audio processor has output terminals marked “+” and “-,”
connect one wire of the audio pair (typically red) between the
processor’s “+” output and the transmitter’s “+” input terminal,
and connect the other wire (typically black) between the “-”
terminals.

2.12.1.3 Audio Source Impedance

The transmitter uses a Bessel filter at the audio input to obtain
superior overshoot performance. Performance of this filter de­pendson the sourceimpedanceof the processingequipment.The
load impedance of the processing equipment is not necessarily
its source impedance; for example, modern equipment may be
specified for a 600 Ohm load but have a very low source imped­ance, 50 Ohms or less.

2.12.1.4 Selecting Source Impedance

A white Molexconnectoron audioinputcable number 100,from
theExternal Interface, plugs intoJ1,J2, or J3 ontheAnalog Input
Board. Source impedance for each input is silk screened on the
board. For optimum performance and best high frequency re­sponse, use the jack labeled with an impedance closest to the
source impedance of the processing equipment.

This connection is NOTcritical; using the wrong connector may
result in some overshoot or undershoot on square waves and a
slight change in audio frequency response (typically less than 1
dB at 10 Khz). If the processing equipment source impedance
cannotbe determined, try “Rs= 6 00 Ohms” foroldertransformer
equipment, and “Rs less than 50 Ohms” for newer equipment
with direct coupled operational amplifier outputs.

2.12.2

Modulation Monitor Connection

If a modulation monitor is used at the transmitter site, run a 50
Ohm coaxial cable from BNC jack J5 on the Output Monitor to
the modulation monitor input. The Output Monitor is located at
the top of the left side wall of the Center Control Compartment.
If the modulationmonitor has ahigh input impedanceratherthan
an internal 50 Ohm termination, a 50 Ohm, 3 to 5 Watt termina­tion should be placed across the Modulation Monitor input
terminals.

Do not connect the coaxial cable to the monitor until instructed
to do so during the Initial Turn On Procedure.

2.12.3

Frequency Monitor Connection.

If a frequency monitor is used at the transmitter site, a coaxial
cable should be run from BNC jack J5 on the Oscillator to the
frequency monitor input. The Oscillator is located toward the
bottom of theright side wallin the CenterControl Compartment.
The RF sample at J5 is a 5 V peak unmodulated signal, at the
transmitter carrier frequency.

2.12.4

External RF Input

An external RFinput froma frequency synthesizer orAM Stereo
exciter is connected to J2 on the Oscillator. Jumper P3 on the
Oscillator should bemoved to the1-2 position.Jumper P5 onthe
Oscillator should be used to select the correct impedance for the
external RF source. Refer to SECTION A, Oscillator, for addi­tional information.

2.12.5

External Interlock (Failsafe)

The External Interlock should be used for any function which
should turn the transmitter OFF by turning off the high voltage
supply. Examples include Failsafe connections and safety inter­locks on phasor cabinets or other enclosures which could expose
personnel to RF when opened.

More than one External Interlock can be used by connecting the
normally closed interlock switches in series. All External Inter­lock switches should be closed when the interlocked enclosure
or circuit is in the safe condition, and should open when in the
unsafe or fault condition.

External(remote) Controlinterface connections aremadeat TB1
and TB2, at the top of the right side wall in the Center Control
Compartment. Terminal information issilk screened on the back
of the Driver Compartment door.

EXTERNAL INTERLOCKS should b e connected between
TB1-1 and TB1-2.

a. A CLOSED circuit between TB1-1 and TB1-2 allows the

transmitter to turn ON.

b. An OPEN circuit between TB1-1 and TB1-2 turns the

transmitter OFF by interrupting the 24 VAC coil voltage
for the External Interlock relay K3. Thetransmitter cannot
be turned on again as long as there is an OPEN circuit
between the External Interlock terminals. The 24 VAC is
fused at 1 Amp by F24. External interlock contacts and
wiring should be rated for 1 Ampere AC current.

c. If NO External interlocks are used, a jumper must be

connected between External Interlock terminals TB1-1
and TB1-2.

NOTE

Do not use the External Interlock for antenna pattern switching,
which requires only a brief interruption of RF output. The EX­TERNAL PA TURN OFF connection, described under Remote
Control connections later in this section, should be used for that
purpose.

2-6 888-2297-002 Rev. AA: 8/9/2000

WARNING: Disconnectprimary power prior to servicing.

Section II - Installation

2.13 Pre-Turn On Checks (Mechanical)

Although appropriate packaging and shipping precautions are
taken before the equipment leaves the factory, hardware some­times works loose during shipment. The transmitter should be
checked for any debris, loose hardware and loose connections
before applying primary power. Pre-turn on checks and inspec­tion should include:

a. Check for debris and loose hardware, both in the transmit-

ter and in the AC power panel.

b. Check forlooseconnections, in particularatthe following:

1. Filter capacitors

2. High voltage and low voltage supply rectifier diodes

3. Output network clips, insulators, and hardware

4. PA Power supply transformer

5. Low voltage supply transformer

c. Check RF drive cable connectors. Ensure that cable con-

nectors are properly locked into their printed circuit board
sockets.

d. Check ribbon cable connectors. Ensure that cable connec-

tors are properly locked into their printed circuit board
sockets.

e. Ensure that output network connections and coil taps are

tight, especially at high current points. (Over tightening
can strip threads or break bolts, especially where brass
hardware is used).

f. Ensure that no shipping ties, blocks, or tape remain.

b. Underdrive overload test.
c. PA Power Supply check.
d. Fan rotation check.
e. Driver operation check.
f. Raise/Lower Function check.
g. Power Amplifier turn on check.
h. Setting RF monitor levels.
i. Modulation check.
j. Setting Audio Input level.
k. Audio Input phasing check.
l. Battery Backup for controller.
m. Recording normal meter readings.

If all Installation Procedures up to this point have been com­pleted, the transmitter is readyto be poweredup by thefollowing
sequence. Each step assumes the preceding step has been suc­cessfully completed.

WARNING

IFYOUMUST ENTERANYPART OF THETRANSMITTEREXCEPT
THE FRONT NON-INTERLOCKEDCOMPARTMENTS, TURN OFF
THE TRANSMITTER BYDEPRESSING THE “OFF”BUTTON,SET
THE REMOTE/LOCAL SWITCH ONTHE STATUS PANELTO “LO­CAL,” AND REMOVE SINGLE PHASE AND THREE PHASE PRI­MARYPOWERATTHEMAINDISCONNECT.BEFORE REMOVING
PANELS OR OPENING DOORS, VERIFY THAT THE PA POWER
SUPPLY IS DISCHARGED BY CHECKING “SUPPLY VOLTS” ON
THE FRONT PANEL MULTIMETER.

2.14 Pre-Turn On Checks (Electrical)

Before initial turn on, ensure that the following items have been
completed:

a. A ground strap must be properly connected between the

transmitter and the station earth ground.

b. AC input wiring must be properly connected and connec-

tions must be tight.

c. The transmitter RF output must be properly terminated

with a suitable load capable of handling rated output
power. This can be either an antenna system or a dummy

load.
d. External interlocks must be satisfied.
e. Audio input must be properly connected.
f. Monitoring equipment is properly connected.
g. SECTION III, Operators Guide, in this technical manual

should be read and understood.

2.15 Initial T urn On Procedure

The initial turn on procedure provides checks or adjustments in
the following sequ ence:

a. Low Voltage PowerSupply test.

CAUTION

GROUNDING STICKS ARE PROVIDED INSIDE THE TRANSMIT­TER AND SHOULD BE USED TO ASSURE THAT ALL HIGH VOLT­AGE HAS BEEN REMOVED.

CAUTION

WHEN WORKING IN THE FRONT NON-INTERLOCKED COM­PARTMENTS, BE CAREFUL NOT TO GROUND ANY CONNEC­TIONS WHICH ARE STILL ENERGIZED. THIS INCLUDES ALL
LOW VOLTAGE CIRCUITS IF THE LOW VOLTAGE SWITCH CIR­CUIT BREAKERS CB1 AND CB2 HAVE NOT BEEN SET TO THE
“OFF” POSITION.

CAUTION

IFANY ABNORMALITIESARE ENCOUNTERED IN THE FOLLOW­ING STEPS, STOP THE PROCEDURE AND REFER TO THE TROU­BLESHOOTINGSECTION OF THIS TECHNICAL MANUAL.

Find the packetshipped with thetransmitter entitled FactoryTest
Data. This datashould beused as referenceduring theinitial turn
on procedure.

2.15.1

Power Supply Test

This test will be the first application of single phase AC power
to T2, Low Voltage transformer. This will energize the control
circuitry and allow Low Voltage meter readings to be taken.

Rev. AA: 8/9/2000 888-2297-002 2-7

WARNING: Disconnect primary power prior to servicing.

DX-25U

WARNING

TURN OFF SINGLE PHASE AND THREE PHASE PRIMARY
POWER AT THE MAIN DISCONNECT BEFORE REMOVING ANY
REAR PANEL.

a. Removetherear access panelsfromthe RF AmpCompart-

ment and locate the two Fuse Boards (A24/A25).

b. Remove F20 located on High Voltage Transformer T1

terminal deck; F21 and all other fuses on T1 remain.

c. Remove F1 through F8 on the A25 Fuse Board. This will

minimize inrush current in case of a problem in the step
start sequence or control.

NOTE

Before replacing the rear access panels, make sure nothing is
shorting out the supply. With an ohmmeter, measure the resis­tance of the +230 VDC supply between the heavy copper buss
bar on the fuse boards and ground. With the positive lead on the
supply, the resistance should be approximately 500 Ohms +/­10%, which is the resistance of the PA Discharge circuit. Re­member that S9, S10, and S12 short out the supply when any of
the interlocked RF Amp Compartments behind the front doors
are open.

d. Replace the rear access panels and apply SINGLE phase

AC power to the transmitter at the main disconnect. DO
NOTAPPLYTHREEPHASEPOWERATTHISTIME.

e. Turn the LowVoltage Power Supply circuit breakers CB1

and CB2 in the Center Control Compartment to the ON
position.

f. All transmitter front panel ColorStat™ panel indicators

should be either red or green(except for the Remote LED,
which will not be illuminated when in the LOCAL posi­tion). Indicators are bi-color LED’s (except for Local and
Remote indicators, which are single color LED’s).

g. Switch the front panel MULTIMETER to the -8 VDC, +8

VDC, -22 VDC, and +22 VDC positions and check the
readings against the Factory Test Data. These readings
should be within 10% of the Factory Test Data readings.
The readings will be slightly lower when the +230 VDC
PA Power Supply is energized.

NOTE

If the readings are not within 10% of the Factory Test readings,
turn off CB1 and CB2, remove primary AC power at the main
disconnect and review the connections to Low Voltage Trans­former T2.

2.15.2 Underdrive Overload Test

The following test simulates an Underdrive fault condition by
bringing the transmitter up with 3 phase primary AC OFF. The
importance of this test is to assure that the RF drive sensing
circuitry is functional. Proper drive level is critical to prevent
failures in any of the RF amplifiers, RF1 through RF64, in the
Power Amplifier stage. There are other fault conditions that will
be detected by the Underdrive Overload circuit that are not RF
related failures: i.e. a short across the PA Power Supply during
the step start sequence. Refer to Section IV, Overall System
Theory, and Section VI, Troubleshooting, for a more detailed
understanding of Underdrive overload.

a. Single phase AC power ON, three phase AC power OFF.
b. Depress the LOW power button. Observe the following:

1. K1 step start contactor should energize, quickly drop
out, energize, then drop out a second time.

2. The Underdrive LED on the ColorStat™ panel should
change to AMBER on the first drop out, RED on the
second drop out and remain RED until the display is
reset.

3. K2 in the Step Start Panel should not energize.

4. These events should all occur within two seconds.

2.15.3

PA Power Supply Check

This test will be the first application of three phase AC power to
T1 and will energize the +230 VDC, +115 VDC, and +60 VDC
supplies. It will also determine that the step start sequence is
operating properly.

a. Single phase AC power ON, three phase AC power ON.
b. Place the PA OFF switch, S2, located on the Controller,in

the OFF (up) position.

c. Rotate the front panel Multimeter switch to the PA SUP-

PLY +VDC position.

d. Depress the LOW power button and observe one of the

following:

1. Step start sequence completes, PA supply voltage
comesupto+255VDC+/-5%.

or

2. Step start sequence drops out due to an Underdrive
fault. or

3. Step start sequence drops out due to an Overdrive
and/or Overvoltage fault.

e. Ifstepd1. was observed,proceed to theFan Rotationcheck
f. If step d2. was observed, temporarily disable the Under-

drive overload by placing a clip lead on the LED Board
between TP8 and GND (ground), and then repeat the test.

g. If step d3. was observed, retap T1 to lower the supply

voltage and then repeat the test.

h. Depress the OFF button and observe the PA SUPPLY

+VDC on the Multimeter. The voltage should drop to zero
very quickly. If the voltagebleeds offslowly, troubleshoot
thePASupply Discharge Circuit (Crowbar).Refer toSheet
1 of the DX-25U Overall Schematic, 839-7855-151, in the
Drawing Package for details of the PA Supply Discharge
circuit.

i. Repeat the procedure to verify the step start sequence is

functioning properly. The sound of K1 and K2 is propor­tional to their size.

2.15.4

Fan Rotation Check

The fan motors are three phase and correct rotation must be
verified. If necessary, two of three AC input wires must be
interchanged at 2TB1 in the Output Network Cabinet. Use the
following procedure:

2-8 888-2297-002 Rev. AA: 8/9/2000

WARNING: Disconnectprimary power prior to servicing.

Section II - Installation

a. Single phase ON, three phase ON, CB1 and CB2 ON, PA

Off switch S2 OFF.

b. Depress the LOW power button and hold a piece of paper

against one of the air filters at the rear of the Output
Network Cabinet. Correct fan rotation will pull air in and
hold the paper in place.

c. If there is incorrect air flow, the transmitter may turn off

again after20 seconds, andthe AIR INTERLOCK indica-

tor on the ColorStat™ panel will indicate RED.
d. Depress the OFF button.
e. Remove the left upper air filter on the back of the Output

Network Cabinet and observe the rotation of the coasting

fans. (The fans are located at the left side of the Output

Network Compartment as viewed from the back of the

transmitter). All four fans should be rotating CCW (coun-

terclockwise).
f. If the rotation is correct, reinstall the air filter.
g. IF ROTATION IS NOT CORRECT:

WARNING

DISCONNECT SINGLE PHASEAND 3 PHASE PRIMARY POWER
AT THE MAIN DISCONNECT BEFORE REMOVING ANY REAR
PANEL.

1. Remove the left rear access panel from the Output
Network Compartment.

2. Disconnect and interchange ANY TWO of the three
wires 1, 2 or 3 connecting the fans to 2TB1. (Inter­changing any two of three wires to a three phase motor
will reverse its direction of rotation).

3. Reinstall theaccess panelon the OutputNetwork Com­partment.Reapplysinglephase and three phase primary
AC power to the transmitter.

h. Ensure thatall rear panelsare in place.Turnthetransmitter

ON by depressing the LOW power button.

i. When thetransmitter is turnedON, theAIR interlock LED

on the ColorStat™ panel willextinguish. Itwill then come
back on after 20 seconds, as follows:

1. GREEN: Air pressure is okay.

2. AMBER: Air pressure reduced.

3. RED: Air pressure FAULT. The fault will also turn the
transmitter OFF.

An Air Pressure Fault indicates insufficient air. Likely causes
include a back panel not installed or fastened at all points,
incorrect blower tap connections on the primary of T1 or incor­rectly adjusted circuit on the Driver Encoder/TempSense Board.
Correcttapsfor blower wires 123, 124and 125arelisted on Table
2-2. The adjustment procedure for the airflow switch is in SEC­TION V, Maintenance.

2.15.5

Driver Operation Check

If astereo generator,frequency synthesizer, or other external RF
source equipment is used,it mustalready be installed and opera­tional.

a. Remove the clip lead between TP8 and GND on the LED

Board if used in checking out the PA Power Supply.

b. The PA OFF switch S2 on the Controller should be OFF

(up).

c. Apply single phase and three phase primary power at the

main disconnect.

d. Low voltage circuit breakers CB1 and CB2 in the trans-

mitter should be ON.
e. Depress the LOW power button.
f. Rotate the front panel Multimeter switch to the RELA-

TIVE RF DRIVE position and compare the reading with

the Factory Test Data.
g. Using the RF MULTIMETER on the inside of the Driver

Compartment door, compareall readings with the Factory

TestData. Voltagereadings D8Aand D8B maydifferfrom

FactoryTestData, depending onthe AC line voltage.If the

D8A reading is low, check to see if the AUTO circuit on

the Driver Encoder/Temp Sense Board has turned on

Driver Module D7. This will be indicated by the AUTO

LED on the Driver Encoder/Temp Sense Board in the

Driver Compartment. This is a normal function of this

circuit and does not affecttransmitter operation adversely.

If the Underdrive or Overdrive circuits on the transmitter

do not turn the transmitter OFF, the drive level is correct.

NO TUNING OF THE DRIVER STAGE SHOULD BE

NECESSARY.

2.15.6

Raise/Lower Function Check

The normal time interval for running the power level from zero
to 30 kW is approximately 25 seconds. With the +230 VDC PA
Power Supply energized, lower the power level to zero on all 3
positions, LOW, MED, andHIGH, byholding theLOWERpush
button for 10 seconds in each position, while simultaneously
depressing S1 on the Controller. Switch S1 speeds up the
RAISE/LOWER command by more than 5:1. After all three
positions have been LOWERED, turn the transmitter OFF by
pressing the OFF button.

2.15.7

Power Amplifier stage Turn On Check

If a 50 Ohm resistivedummy load is used for initial check ofthe
Power Amplifier stage, only minimum adjustment of the TUN­ING and LOADINGcontrols will be necessary. When the trans­mitter is then connected to the antenna or if the antenna is used
for the initial Power Amplifier stage check, additional adjust­ments to the DETECTORNULL (ANTENNA) circuitry may be
required.

WARNING

DISCONNECT SINGLE PHASE AND THREE PHASE PRIMARY
POWER AT THE MAIN DISCONNECT BEFORE REMOVING ANY
REAR PANEL.

a. Removetherear access panelsfromthe RF AmpCompart-

ment and replace fuse F20 on T1 and all fuses removed

from the A24 and A25 Fuse Boards.

Rev. AA: 8/9/2000 888-2297-002 2-9

WARNING: Disconnect primary power prior to servicing.

DX-25U

b. Replace all rear panels and apply single phase and three

phaseACpower to thetransmitter.TheLowVoltage circuit
breakers CB1 and CB2 should be ON and the PA OFF
switch S2 on the Controller should be OFF (up).

c. DepresstheLOWpowerbutton. The +230 VDCPAPower

Supply should come up as in the RF drive checkout.
Compare all meter readings with Factory Test Data.

d. Turn the PA OFF switch S2 on the Controller to the ON

(down) position.

e. Change the front panel Multimeter switch from the PA

+VDC to the DETECTOR NULL (ANTENNA) position.
The power meter selector switch should be in FWD.

f. Raise the output power by depressing the RAISE button.

Observe the following while raising the power:

1. Power and Current should increase.

2. DETECTOR NULL (ANTENNA) indication may be­gin to rise depending upon the degree of mismatch
between the station load and the factory test load,
especially if the transmitter is loading into the antenna.
This may cause ANT VSWR trips and prevent full
power operation until this circuit is adjusted.

3. If ANT VSWR trips occur, the Antenna VSWR Phase
Angle Detector on the Output Monitor will have to be
adjusted. The Bandpass FilterVSWR Phase Angle De­tector may also require adjusting, but should be done
after the TUNEand LOAD controlsare adjusted forthe
proper PA voltage/current ratio.

Inbrief, adjustC15 andL12 onthe Output Monitorforminimum
reading on the multimeter DETECTOR NULL (ANTENNA)
position. Adjust C16 and any of the coils (L5, L6, L7, or L8)
switched inby S7on theOutput Monitor for a minimum reading
on the DETECTOR NULL (FILTER) position. Refer to SEC­TION V, Maintenance, for the Output Monitor adjustment p ro­cedures.

2.15.7.1 Tuning for Voltage/Current Ratio

a. At 25 kW power output, the PA Supply Current should be

between105Aand123AdependingontheAClinevoltage.
Refer to the Factory Test Data and use the following
information to help tune the transmitter for the correct PA
Supply Voltage/Current ratio.

1. Power Output (meter)

2. PAVoltage

3. PA Current

4. PA Efficiency

5. # of Steps turned on

b. The RAISE control determines the number of steps (PA

Modules) turned on. When a PA Module is ON, the green
LED on the module is illuminated. With a PA Power
Supply voltage of +230 VDC, there should be 23 PA
Modules on for 25 kW output.

c. TheLOADING control adjuststhe PA Supply Current and

power output. With a PA Power Supply voltage of +230

VDC, PA Supply Current should be between 110 and 115
Amps.

d. The TUNE controlis adjustedfor aPEAK in poweroutput.

This control is rather broad, especially at the low end of
the medium wave band.

2.15.8

Setting Modulation Monitor Sample Levels

An adjustable 1 to 10 Volt RMS signal is availableat the Output
Monitor. The RF sample is set for the proper level at the LOW­EST operating power by adjusting 2L7 in the Output Network
Compartment. When theLOW Powersample levelhas been set,
MEDIUMandHIGHpowerlevelsareadjustedbyR7andR8on
the Output Monitor.

a. Set the LOW poweroutput of thetransmitter to thelowest

power that will be required for normal operation.

b. Measure the RF voltage level at the monitor. If the level

must be increased or decreased to meet modulation moni­tor input voltage requirements, the tap on 2L7 must be
adjusted.

WARNING

TURN OFF ALL SINGLE PHASE AND THREE PHASE PRIMARY
POWER AT THE MAIN DISCONNECT BEFORE REMOVING ANY
REAR PANEL.

c. Remove the rear right panel from the Output Network

Compartment.

d. Locate Modulation Monitor sample coil 2L7 connectedto

the RF output flange. To DECREASE the sample voltage,
move the tap closer to the grounded end of the coil. To
INCREASEthesamplevoltage,movethetapawayfrom
the grounded end of the coil. Move the tap 1/4 turn in the
desired direction. Be careful not to short adjacent turns
when positioning the clip.

e. Replace the Output Network Compartment rear panel.

Reapply single phase and three phase AC primary power
at the disconnect switch. Depress the LOW power switch.

f. Measure the RF sample voltage. Repeat the coil adjust-

ment procedure until the desired sample voltage for the
LOW power setting is obtained.

When the LOWpowersample level is satisfactory ,continue with
the following steps:

g. Turn R7, MEDIUM power modulation monitor sample

adjustment, on the Output Monitor fully CCW (counter­clockwise).

h. Turn the transmitter on at MEDIUM power, or, if the

transmitter is already on at another power level, depress
the MEDIUM power switch. Use the RAISE and LOWER
buttons to set the MEDIUM power output.

i. Adjust R7 on Output Monitor until the sample voltage at

the modulation monitor is the SAME as it was inthe LOW
power position. This adjustment can be made while the
transmitter is operating.

2-10 888-2297-002 Rev. AA: 8/9/2000

WARNING: Disconnectprimary power prior to servicing.

Table 2-4. Minimum Recommended Control and

Status Functions for Remote Control

Section II - Installation

Rev. AA: 8/9/2000 888-2297-002 2-11

WARNING: Disconnect primary power prior to servicing.

DX-25U

j. TurnR8, HIGHpower modulation monitor sample adjust-

ment, on Output Monitor fully CCW.

k. Turn the transmitter on at HIGH power,or, if the transmit-

ter is already on at another power level, depress the HIGH
power switch. Use the RAISE and LOWER buttons to set
the HIGH power output.

l. Adjust R8, on Output Monitor, until the sample voltage at

the modulation monitor is the SAME as it was inthe LOW
and MEDIUM position.

m.Switch between LOW, MEDIUM, and HIGH power to

verify that the Modulation Monitor sample is the same for
all three power levels. If necessary, readjust R7 or R8 so
that all sample voltages are the same.

2.15.9

During the modulation check, it is helpful to monitor the RF
envelopeon an oscilloscopeconnected in parallel with theModu­lation Monitor RF sample. Check for proper modulation at
various power levels, as follows:

Modulation Check

a. Connect an audio generator to the audio input.
b. Turn the transmitter on at LOW power. Apply a low level

audio sine wave at 1 kHz. Increase the generator output
until modulation level is approximately 50%.

c. Observethe modulatedRF signalon theoscilloscope. The

modulation envelope should be a smooth sinewave, with
no steps, notches, or other distortion. (If a distorted enve­lopeis observed,checkthe audio generatoroutput with the
oscilloscope before assuming there is a transmitter prob­lem. Sometimes, defective test equipment is the problem
rather than the equipment being tested).

d. Switch to MEDIUM and then to HIGH power. The trans-

mitter will maintain the same modulation level. Again,
observe the modulated RF signal on the oscilloscope. The
modulation envelope should still be a smooth sinewave.

e. Increasethemodulation to95% negativepeak modulation.

Observe the waveform again at all power levels.

f. Check all meter readings against the Factory Test Data

sheets for meterreadings withmodulation. Meter readings
should be close to factory readings.

g. Notethat the frontpanel CURRENT meter reading depends

on power outputAND modulation level.The meterreads the
average current returning to the P A Power supply. Because
PA voltageis fixed, PAcurrentdepends ontransmitteroutput
power,which varies with modulation.

transmitter can accommodate reference levels from -10
dBm to +10 dBm at 600 Ohms.)

b. Switch the transmitter to MEDIUM power. Set the output

level from the audio generator to the station’s reference
value for 100% modulation. (If you are using an output
level meter on the audio signal generator, be sure that the
generator is operating into the proper impedance. Some
audio signal generator meters are accurate only with the
correct load.)

c. UsingR15, Audio GainAdjuston theAnalog Input Board,

adjust for a modulation level of 100%, as read on the
modulation monitor. This completes audio input level ad­justment.

2.15.11

This is not a transmitter check, rather, it is a system check. The
50kW transmitter versionis capableof positive peakmodulation
of +125% or greater at 55 kW carrier power, and even higher
positive peak modulation at 25 kW or less. An audio phasing
check may also be included in the manuals for the audio proc­essing equipment.

In the United States and many other countries, positive peak
modulation up to +125% is permitted. If regulations at your
location permit, and you have audio processing equipment with
asymmetrical output, you may wish to verify that your audio
input is phased correctly.Proceed as follows:

Audio Input Phasing Check (Optional).

a. You will need a program source or audio frequency gener-

ator with asymmetrical output, audio processing equip­ment capable of providingpositivepeaks over +100%,and
a modulation monitor with a negative 100% peak flasher
and an adjustable positive peak flasher. (An audio gener­ator with an asymmetrical audio frequency output is ideal
for this check, but is not available at most stations).

b. Turn the transmitter on, at any power level, and modulate

with asymmetrical audio or with program. Adjust the
program level so that negative peaks just reach -100%.

c. Observe positive peak modulation levels.

• If positive peaks are approximately 100%, you don’thave

a program source with higher positive peaks than negative
peaks, or possibly your processing equipment is not ad­justed properly.

• If positivepeaks areLESS than 100%,try reversingthe two

audio signal leads, either at the audio output supplying the
transmitter or at the transmitter audio input terminals.

• If positive peaks are GREATER than 100%, your audio

input phasing is correct.

2.15.10

TheAudio Inputsensitivityofthe transmitter is adjustedwiththe
AUDIO GAIN ADJ control, R15, on the Analog Input Board.
The Analog Input Board is located below the Output Monitor on
the left wallof the CenterControl Compartment.Use the follow­ing procedure for this adjustment:

2-12 888-2297-002 Rev. AA: 8/9/2000

Setting Audio Input Level

a. Determine the stations’s reference audio level for 100%

modulation. (Typical levels are 0 Dbm or +8 Dbm, but the

WARNING: Disconnectprimary power prior to servicing.

2.15.12

A 1 Farad capacitor supplies backup for AC power failures
shorter than 2 hours.

Three AA alkaline batteries are used for Controller power mode
and power levelmemory backup.These willenable the transmit­ter to return to operation at the correct power level after an AC
power failure longer than two hours at 72 degrees F (25 degrees
C). The batteries should be installed AFTER the Low Voltage

Controller Battery Backup

Section II - Installation

circuits have been energized to prevent battery drain by the 1
Faradcapacitor.Thebatteries canbeinstalled whilethe transmit­ter is ON. (Do NOT use rechargeable batteries, such as NiCad
cells. The cell voltage is not high enough for reliable operation).

2.15.13

We strongly recommend that a permanent record of ALL meter
readings be made, with carrier only (no modulation) and with
modulationat one ormorelevels(-95%should beone level).The
form at the end of this section provides an outline. Data should
be taken usingthe primary ormain antennasystem and adummy
antenna (dummy load) if one is available,becausea dummyload
will provide the most repeatable set of conditions.

Recording Normal Meter Readings

2.16 AM Stereo Installation and Tuning

Hints

The transmitter is stereo ready in terms of interfacing with the
stereo exciter. The most difficult part of making a transmitter
stereo ready is minimizing the IQM, increasing RF bandwidth,
and reducing L-R noise. All these were taken into consideration
inthedesignof the transmitter.This results in atransmitter which
essentially requires NO compromise of efficiency, tuning, or
mono performance to obtain the best stereo performance. When
most controls are set for best stereo performance, the other
performance areas are also optimized.

2.16.1

Connect the RF outputof thestereo exciterthrough aBNC cable
to J2 on the Oscillator. Move Jumper P3 to position 1-2. This
enablestheexternaldriveinput.Move P5 to position1-2formost
TTL exciter RF outputs. This is a 20k Ohm termination. Move
P5to1-3 for a 50 Ohmterminationneededforhigher levelexciter
RF outputs.

2.16.2

The Bandpass tuning is the only control that will affect the
Incidental Phase Modulation (IPM) of the transmitter.Normally,
the Bandpass tuning is adjusted for peak power output on the
power meter, and then turned approximately one turn counter­clockwise off the peak on the inductive side of resonance. This
typically optimizes efficiency, IPM and mono THD and IMD.
The bandpass tuning can be adjusted while the transmitter isON
and modulating.

Interfacing For Stereo

Bandpass Tuning 2C1

NOTE

THE TUNING CONTROL SHOULD NOT BE ADJUSTED
MORE THAN 2000W OFF OF THE POWER PEAK. MOST EF­FICIENT OPERATION OCCURS AT OR NEAR THE POWER
PEAK. TUNING OFF THE PEAK IN THE CAPACITIVE
(CLOCKWISE) DIRECTION CAN REDUCE EFFICIENCY AND
CAUSE EXCESSIVE HEATING OF THE RF AMPLIFIER MOD­ULES.

2.17 Remote Control Connections

The following paragraphs include basic information on remote
control interfacing. RemoteControl connections(including con­nection to extended control panels, if used) can be different for
each installation.

The Customer Interfacefor remote control, metering, and status
monitoring is compatiblewith nearlyallremote controlsystems.

If the remote control system does not have enough channels
available for all control and status functions, Table 2-4 lists the
minimum recommended control and monitor functions.

All connections are made on TB1 and TB2 inthe CenterControl
Compartment. Sheet 3 of the External Interface schematic, 839­7855-090, in the Drawing Package shows the list of remote
control functions with a letter (A through E) placed next to each
terminal number. This letter refers to the Characteristic Key on
the diagram. Each Characteristic Key provides schematics and
information which will be useful when planning interfaces to a
remote control unit or to an extended control panel. For detailed
information, including typical interfaces, refer to SECTION N,
External Interface.

2.17.1

Control Inputs are isolated by optoisolators on the External
Interface. This permits maximum flexibility, allowing use of
voltage sources provided on the External Interface or external
voltage sources. Control inputs can be a relay contact closure,
switch closure, or transistor switch. Control inputs may be iso­lated from ground, or can be switched to ground bya positiveor
negative voltage. Each remote control input uses a “+” and “-”
terminal on TB1 or TB2. Applying a voltage between the termi­nals is the same as depressing the corresponding button or
operating the corresponding switch on the transmitter. Voltage
applied to these terminals should be between 15 and 29 Volts;
corresponding currents are 40 to 70 milliamperes. For conven­ience, +15 VDC and -15 VDC sources (at 175 mA maximum
current) are provided at TB1-11 and TB1-12, respectively.

2.17.2

These remote meter readings monitor voltages, currents, power
levels, and other analog parameters. When a high impedance
remote control system input is connected to a Monitor Output
terminal, the nominal output will be +3.4 Volts (or -3.4 Volts to
monitor -22 V and -8 V supplies) when the transmitteris operat­ing at 25 kW. This allows for some increase in readings while
still remaining within the 4 Volt input limit of remote control
systems suchas theHarris Sentinel. If the remote control system
input is not high impedance, the loading on the monitor output
will reduce the output voltage.

2.17.3

These are open-collector outputs to ground. When the corre­sponding status indicator is illuminated at the transmitter (or
when a ColorStat™ panel LED is RED), the transistor turns ON
and provides a current sink from the status output to ground.
These status outputs can switcha 6or 12 VDClow-currentrelay,

Control Inputs

Analog Monitoring Outputs

Status Monitoring Outputs

Rev. AA: 8/9/2000 888-2297-002 2-13

WARNING: Disconnect primary power prior to servicing.

DX-25U
or can provide a logic LOW output when a pull-upresistor to +5

to +15 VDC is used. Maximum safe voltage at a status output
terminal is +15 VDC and maximum safe current into a status
output is 100 mA.

2.17.3.1 Explanation Of Selected Remote Control Functions

Most remote control functions and monitor outputs arethe same
as the corresponding functions or indications in the transmitter.
Some indications and functions will,however, be explained here
to aid in planning and installing a remote control system.

a. OFF Control: Use for normal transmitter turn-off, for

example, at theend ofthe operating day.The externalOFF
control operates the same as the front panel OFF button
and turns off the PA Power supply.

b. EXTERNAL PA TURN OFF: Use to remove RF output

duringantenna pattern switchingor for otherRF switching
operations. Because the transmitter RF output returns as
soon as the PA Turn Off control input voltage is removed,
PA Turn Off MUST NOT be used for failsafe or safety
interlock purposes. A PA Turn Off input turns all PA
Modules OFF through modulator section action, causing
RF output to go to zero. The PA is held OFF as longas the
External PA Turn Off input is active. As soon as the
External PA Turn-Off signal is removed, the transmitter
immediately comes back upto its pre-set operating power.
The External PA Turn-Off command does NOT turn off
the PA Power supply and will NOT result in any red LED
indication on the ColorStat™ panel. However, segment
nine of DS1 on theModulation Encoder(A37) will illumi­nate RED whenever there is an active PA Off command.
The External PA Turn-Off provides the same function as
the PA OFF switch S2 on the Controller.

NOTE

PA TURN OFF should NOT be used for FAILSAFE purposes, for
interlocks, or for routine transmitter turn off. It is intended to
remove RF output during antenna switching operations.

c. EXTERNAL INTERLOCK is used for Failsafe and for

safety interlocks on any enclosure which would permit
contact with RFoutput, such asphasors orantenna switch­ing equipment.

d. TRANSMITTER ON does not use a separate ON switch.

The transmitter comes ON atthe desiredpowerlevelwhen
a momentary remote control input (at least 100 mS long)
is provided at the LOW, MEDIUM, or HIGH terminals on
TB1. If thetransmitter is already ON, providinganother of
these inputs will change power level without interrupting
RF output.

e. RAISE/LOWER inputs change the RF power. A RAISE

input will cause transmitter power output to INCREASE
for as long as the control input is provided, or until the
transmitter power output reaches the maximum power
limit.A LOWER input willcausetransmitter power output
toDECREASE foras long asthe control inputis provided,
or until the power output reaches zero.

NOTE

When remote control and monitoring is used, delays in data
transmission can delay the remote output power reading while
the RAISE or LOWER controls are being operated. This can
cause the remote output power reading to continue to change for
a short time after the RAISE or LOWER command is stopped.

f. FORWARD and REFLECTED POWER (Remote Meter

Reading) outputs correspond to the transmitter FOR­WARD and REFLECTED power meter indications. Note
that the transmitter power meter scale is not linear. The
voltage outputs at Forward remote output TB1-3 and Re­flected remote output TB1-4 are proportional to the RF
output voltage which is proportional to the square root of
the RF power.

g. BANDPASS FILTERVSWR andANTENNAVSWR(Re-

mote Meter Reading) outputs are uncalibrated, relative
readingsfrom the VSWRphase detectors onOutputMoni­tor.

1. BANDPASS FILTER VSWR corresponds to the DE­TECTOR NULL (FILTER) reading on the transmitter
front panel multimeter. A change in the Bandpass Fil­ter/Output Network of the transmitter will cause the
reading to increase.

2. ANTENNA VSWR corresponds to the DETECTOR
NULL (ANTENNA) reading on the front panel mul­timeter. A mismatch in the antenna system will cause
the Antenna VSWR r eadi ng to increase. Antenna
VSWR is an uncalibrated, relative reading, but is more
sensitive than Reflected Power.

h. RF DRIVE ESTIMATE (Remote Meter Reading) is an

UNCALIBRATED sample of the RF drive to the PA
Modules and corresponds to the RELATIVE RF DRIVE
position on the front panel Multimeter.

i. MONITORSIGNALS:+22VDC, -22VDC,+8VDC,And

-8 VDC (Remote Meter Readings) are samples of unregu-

lated voltages from the lowvoltage powersupply,and will
indicate +3.4 VDC for positive s upplies and -3.4 VDC for
negativesupplies during typical operation.

2-14 888-2297-002 Rev. AA: 8/9/2000

WARNING: Disconnectprimary power prior to servicing.

Table 2-5. Initial Turn-On Meter Readings

Section II - Installation

Rev. AA: 8/9/2000 888-2297-002 2-15

WARNING: Disconnect primary power prior to servicing.



3.1 Introduction

This section contains information on day-to-day operation for
the non-technical operator.

Section III

Operators Guide

Ifany indicatorsarestill redafter operating theRESET,do
not attemptto turn on the transmitterand contactqualified
maintenance personnel.

3.2 Operating Procedures

These procedures describe normal daily operation of the DX­25U AM TRANSMITTER, including:

a. Daily pre-operational checkout
b. Transmitter turn-on procedure.
c. Transmitter turn-off procedure.
d. Transmitter operations.

It is important that the operator be aware of normal transmitter
operation and performance, and note any changes or fault indi­cations. Changes in operation may indicate a need for mainte­nanceorcorrectiveactionbeforemore serious problems develop.

WARNING

ALL SERVICE SHOULD ONLY BE PERFORMED BY QUALIFIED
PERSONNEL.DANGEROUS VOLTAGESORCURRENTS MAYBE
PRESENT INSIDE WHEN DOORS ARE OPEN.

Normal operation and monitoring of the transmitter is accom­plished through front panel controls, meters, and indicator (see
SECTION IIIA: Controls and Indicators).

NOTE

If remote/extended control of the transmitter is used, the station
chief engineer or qualified technical staff member should provide
instructions for operator use.

3.3 Daily Preoperational Checkout

The following paragraphs describe checks to be made before
normal daily turn-on.

a. Check the transmitter maintenance log to make sure that

maintenance performed on the transmitter,or other abnor­malconditions, donot placeany restrictions on transmitter
operation. An example is a requirement to operate at re­duced power.

b. Ensure that the transmitter RF output is properly termi-

nated into the antenna. This may include determining that
antenna pattern switching is correct.

c. Check the ColorStat™ panel on the center front of the

transmitter.If the transmitter is ready for operation, all the
status, interlock, and overload LEDs will be green. If any
LEDs are red, note which indicators are red so that infor­mation may be entered into the station maintenance log,
thenpress theRESETbutton.All indicatorsshould change
to green when the RESET button is pushed and released.

3.4 Transmitter Turn-On Procedure

When the Preoperational Checkout has been completed and no
problems are present, the transmitter is ready to turn on.

a. Set the selector switch below the POWER meter on the

meter panel to FORWARD, if it is not already in that
position.

b. Depress the LOW, the MEDIUM, or the HIGH button,

depending on power level desired. Each of these buttons
will turn the transmitter on at the power level which has
been preset.

c. The button operated should illuminate green, and power

willcome up tothe presetlevel.(Youshould also hear three
‘clicks’ as contactors operate, then the blowers should
start, then power will come up.)

NOTE

During the Step Start sequence, you should see the power meter
go to 1/2 of the power selected, then reach the desired power
level.

d. Should FORWARD POWER not be correct, verify that

you have selected the correct power level (LOW, ME­DIUM, or HIGH). If not, press the proper power level
button, and check forward power again.

e. When the correct power level has been selected, but an

adjustment in power is still needed, press the RAISE
button to INCREASE power, or the LOWER button to
DECREASE power.When you press the button andwatch
theFORWARDPOWERindication,the power will change
slowly. Hold the button in until the power is correct.

NOTE

RAISE and LOWER buttons will only change power level if the
transmitter is operating in the HIGH, MEDIUM, or LOW func­tion. This new power level will become the preset power until it
is changed again. The Raise and Lower buttons will adjust power
from zero to 30 kW on each selected power level.

f. Checktransmitter panel meterreadings for normal values.

If an abnormal meter reading is obtained, qualified main­tenance personnel should be contacted.

3.5 Transmitter Turn-Off Procedure

a. To turn off (de-energize) the transmitter, depress the OFF

button.

b. The HIGH, MEDIUM or LOW lamp will go out, you

should hear the contactor as itde-energizes, and the blow­ers will stop.

WARNING: Disconnect primary power prior to servicing.

888-2297-002 3-1

DX-25U

c. With the POWER switch in the FOR WARD position, the

Powermeter should indicatezero pow er. Supplyvoltage and
supply currentmeters should bothindicate zero readings.

3.6 Transmitter Operations

The following is intended to make the operator aware of some
basic operational characteristics of the transmitter.

Routine Meter Readings

3.6.1

A sample log sheet is provided in Section II, Installation/Initial
TurnOn. Copies can be made of thelog sheetfor stationrecords.
Readings should be taken monthly, at a minimum. However,
more frequent logs can be an important tool in assisting mainte­nance personnel.

3.6.2

Fault Conditions

The following describe s normal transmitter actions for certain
possible fault conditions. It is very important that operators log all
abnormal operation, such as incorrect meter readings, overloads,
fault indications, and transmitter shut-downs. A log or record of
abnormal operating conditionswill be useful to technicalpersonnel
in locating and correcting transmitter or other system problems.

3.6.2.1 T ransmitter Shuts Off

The operator should first check for overload or fault indications
that are RED and log any fault indications that are found. Press
the RESET button on the ColorStat™panel toclear faultindica­tions. ColorStat™ panel indicators should change from RED to
GREEN.

If any LEDs are still RED, contact qualified maintenance per­sonnel.

When the faultindicators are GREEN,follow the normalturn-on
procedure.

If the transmittershuts OFF again,and the sameoverloador fault
indication comeson, try turning the transmitter ON by using the
LOW power button. Under some conditions, a transmitter may
operate satisfactorily at reduced power. If it will not come on at
LOW power, contact qualified maintenance personal.

3.6.2.2 AC Power Failure

If Battery Backup has beeninstalled in the transmitter,no opera­tor action is required for AC power failures less than approxi­mately 96 hours. The transmitter will automatically return to an
on-air condition, at the same power level as before the power
failure.

If Battery Backup has not been installed in the transmitter, no
operator action is required for AC power failures less than
approximately 2 hours. The backup capacitor will hold transmit­ter memory for this time and the transmitter will automatically
return to an on-air condition, at the same power level as before
the power failure.

If AC power failures greater than approximately 2 hours occur
and no Battery Backup has been installed, the transmitter will
remain OFF after AC power returns, and operator action will be

required to turn the transmitter ON and to reset power levels on
the LOW, MEDIUM and HIGH positions.

3.6.2.3 Power Supply Current Fault

During a CurrentFault condition thetransmitter willshut off and
the indicator will change to RED.The transmitterwill attempt to
restart once by itself. If the fault is no longer present, the
transmitter will remain ON. If the fault is still present when the
transmitter attempts to restart, the transmitter will turn OFF and
operator action will be required to restart the transmitter and
possibly correct the problem.

a. If the transmitter turns back on and stays on, check pro-

gram modulation level. If overmodulating on positive
peaks, reduce audio level to reduce modulation.

b. If the transmitter shuts off again and no longer automat-

ically restarts. Log or record the fault, RESET the fault
indicator, and try turning the transmitter on.

c. If the fault occurs again, try operating at LOW power. If

the transmitter will operate at LOW power, operation may
continue on a temporary, emergency basis.

3.6.2.4 Power Reduction

The transmitterwill reduce the power output duringcertain fault
conditions.

If the power output has been reduced, the transmitter has re­sponded to one of the following faults.

3.6.2.5 VSWR Faults

If the BANDPASS VSWR, ANTENNA VSWR, OR VSWR
TEST indicators are RED, the following may be the cause:

a. This may be a normal occurrence during a thunderstorm,

rain storm, blowing sand, or underconditions ofice onthe
antenna and will stop when the weather conditions im­prove.

b. The indicators may flash RED when overmodulation oc-

curs. Reducing modulation to normal levels may correct
the condition.

c. If the indicators remain RED and weather conditions or

overmodulation are not the cause, transmitter and/or an­tenna problems are indicated.

3.6.2.6 Temperature Faults

If the TEMP FAULT LED on the Driver Encoder/Temp Sense
Board, insidethe DriverCompartment door,is RED, the follow­ing may be the cause:

a. Failure of the building cooling system (high ambient tem-

peratures).

b. Possible transmitter mistuning

3.6.2.7 Air Flow Faults

Ifthe AIR FLOWindicator ontheColorStat™ panel isAMBER,
the following may be the cause:

a. Obstruction of air flow at the air intake on the transmitter

rear doors or at the air outlet on the transmitter top.

b. Dirty or clogged air filters.

3-2 888-2297-002

WARNING: Disconnect primary power prior to servicing.

3.6.2.8 RF Amp Fuse

The transmitter will continue to operate safely, although power
output will be slightly reduced. The fault indication can not be

Section III - Operation

RESET. The fault must be corrected to clear the fault indication.
Continue normal operation and contact qualified maintenance
personnel.

WARNING: Disconnect primary power prior to servicing.

888-2297-002 3-3



Section IIIA

Controls and Indicators

Figure3A-1. DX 25 Front View,Controls and Indicators

Table 3A-1. DX 25U Transmitter, Controls and Indicators

REF. CONTROL/INDICATOR FUNCTION

1 LOAD Control Control for matching load to 50 Ohm transmitter impedance.
2 TUNE Control Control for matching load to 50 Ohm transmitter impedance.
3 Switch Board/

Meter Panel
4 ColorStat™ Panel See Table 3A-3 and Figure 3A-3.
5 Driver Control Compartment
6 Center Control Compartment
7 Left Control Compartment
8 Output Network Compartment

Rev AA: 8/9/2000 888-2297-002 3A-1

See Table 3A-2 and Figure 3A-2.

WARNING: Disconnect primary power prior to servicing.

DX-25U

Figure3A-2. Switchboard/Meter Panel

Controls and Indicators

Table 3A-2. Switchboard/Meter Panel

Controls and Indicators

REF. CONTROL/INDICATOR FUNCTION

1 VOLTAGE multimeter Indicates voltages at points selected by the Multimeter Switch (Ref. 11).
2 SUPPLY CURRENT meter Indicates the 230V supply current being supplied to the Power Amplifier.
3 POWER meter Indicates either FORWARD or REFLECTED power at the transmitter output, whichever is

selected by the POWER METER selector switch (Ref. 4).
4 POWER, selector Selects Forward power output or Reflected power, to be read on the POWER meter.
5 OFF, pushbutton Used to turn the transmitter off. (Low voltage supplies remain on as long as AC primary

power is applied).
6 LOWER, pushbutton Used to adjust power level. When the transmitter is in the LOW, MEDIUM, or HIGH

power mode, depress to LOWER power output and hold until desired power is reached. IN-

DICATOR illuminates while power is being lowered.
7 RAISE, pushbutton Used to adjust power level. When the transmitter is in the LOW, MEDIUM, or HIGH

power mode, depress to RAISE power output and hold until desired power is reached. IN-

DICATOR illuminates while power is being raised.
8 HIGH, pushbutton Used to turn the transmitter on at the preset HIGH power level, or to change power to the

preset HIGH power level. INDICATOR: The pushbutton will illuminate when in the HIGH

power mode.
9 MEDIUM, pushbutton Used to turn the transmitter on at the preset MEDIUM power level, or to change power to

the preset MEDIUM power level. INDICATOR: The pushbutton will illuminate when in

the MEDIUM power mode.
10 LOW, pushbutton Used to turn the transmitter on at the preset LOW power level, or to change power to the

preset LOW power level. The pushbutton will illuminate when in the LOW power mode.
11 MULTIMETER switch Selects the desired point to be monitored by the VOLTAGE multimeter.

3A-2 888-2297-002 Rev AA: 8/9/2000

WARNING: Disconnect primary power prior to servicing.

Section IIIA - Controls and Indicators

Figure3A-3. ColorStat™ Panel

Controls & Indicators

Table 3A-3. ColorStat™ Panel

Controls & Indicators

REF. CONTROL/INDICATOR FUNCTION

1 LOCAL/REMOTE, switch Selects LOCAL or REMOTE control of the transmitter. (Remote monitoring is operational

in either the LOCAL or REMOTE switch position.)

2 LOCAL, status indicator Indicates that the remote control inputs to the transmitter are disabled, and only local con-

trol is possible.

3 REMOTE, status indicator Indicates that the transmitter remote control inputs are active. (The transmitter may still be

controlled with the panel pushbuttons as well.)

4 AUDIO INPUT board, -15V

supply status indicator.

5 AUDIO INPUT board, +15 V

supply status indicator.

6 OSCILLATOR, RF output

status indicator

7 BUFFERAMP,RFoutput status

indicator

8 PREDRIVER, RF output status

indicator

Bicolor LED indicator. Indicates status of -15 Volt supply on the audio input board.
GREEN indicates normal operation; RED indicates -15 Volt supply fault.

Bicolor LED indicator. Indicates status of +15 Volt supply on the audio input board.
GREEN indicates normal operation; RED indicates +15 Volt supply fault.

Bicolor LED indicator. Indicates oscillator board RF output status. GREEN indicates nor­mal RF output; RED indicates low or no RF output. (Note 1)

Bicolor LED indicator. Indicates buffer amplifier RF output status. GREEN indicates nor­mal RF output; RED indicates low or no RF output. (Note 1)

Bicolor LED indicator. Indicates predriver RF output status. GREEN indicates normal RF
output; RED indicates low or no RF output. (Note 1)

9 RF AMP, indicator Bicolor LED indicator. Red indicates a failure of an RF Amplifier. Green indicates a nor-

mal, no fault condition. (Note 2)

Rev AA: 8/9/2000 888-2297-002 3A-3

WARNING: Disconnect primary power prior to servicing.

DX-25U

Table 3A-3. Status Panel, Controls and Indicators (Cont.)

REF. CONTROL/INDICATOR FUNCTION

10 OUTPUT MONITOR, BAND-

PASS FILTERVSWR indicator

Bicolor LED indicator. GREEN indicates that there is low reflected power at the input of

the bandpass filter; RED indicates that reflected power at the filter input is above the level

set by the VSWR trip adjust control.
11 OUTPUT MONITOR, +5 V

supply status indicator

12 OUTPUT MONITOR, -5V sup-

ply status indicator

13 OUTPUT MONITOR, ANT.

VSWR, status indicator

Bicolor LED indicator. Indicates status of +5 Volt supply on the Output Monitor board.

GREEN indicates +5 Volts present; RED indicates +5 Volt supply fault.

Bicolor LED indicator. Indicates status of -5 Volt supply on the Output Monitor board.

GREEN indicates -5 Volts present; RED indicates -5 Volt supply fault.

Bicolor LED indicator.Indicates VSWR status. RED indicates that VSWR at the transmit-

ter’s 50 Ohm point is above the threshold set by “VSWR Trip Adjust” control. GREEN in-

dicates low VSWR.
14 VSWR SENSOR, “STATUS”

indicator (VSWR Self-Test re­sult)

Bicolor LED indicator.Indicates result of “VSWR Self-Test.” RED indicates VSWR logic

fault, GREEN indicates VSWR protection logic is functioning normally. (“VSWR self-

test” can be initiated manually with “VSWR SENSOR, Manual Test,” and is performed

automatically whenever AC power is restored.
15 VSWR SENSOR, MANUAL

TEST, pushbutton

Used to test operation of VSWR logic, result of test is displayed on VSWR SENSOR,

STATUS indicator (Ref. 14, above). When the pushbutton is depressed, both the Bandpass

Filter and Antenna VSWR status indicators will momentarily go red, then Sensor Status In-

dicator will indicate green if VSWR logic is functioning properly. DO NOT TEST WHILE

TRANSMITTER IS ON AIR.
16 OVER DRIVE, status indicator RED indicates excessive RF drivelevel to the Power Amplifier. GREEN indicates drive

level is below the Overdrive threshold.
17 UNDER DRIVE, status indica-

tor

18 SUPPLYFAULT, overload indi-

cator

RED indicates low RF drive level to the Power Amplifier.GREEN indicates drive level

above the preset threshold. Red can also indicate a short in the PA power supply.

RED indicates that the Power Supply Protection circuit has detected a PA Power Supply

fault (an imbalance in three phase voltages from transformer T1, caused by loss of one

phase or phase imbalance on incoming primary power. GREEN indicates no fault.
19 OVER CURRENT, overload in-

dicator

20 OVER VOLTAGE, overload in-

dicator

GREEN status indicates normal status; RED indicates that either average or peak supply

current has exceeded preset levels.

Bicolor LED indicator. GREEN indicates normal status; RED indicates main power supply

voltage has exceeded 260 Volts DC.
21 RESET pushbutton Resets the overload indicators; when depressed, overload indicators will change from RED

to GREEN if the cause of the overload has been cleared. Depressing the RESET button

will also reset the Bandpass and Antenna VSWR fault indicators.
22 DOOR INTERLOCKS, status

indicator

23 AIR INTERLOCKS, status in-

Bicolor LED indicator. GREEN indicates all doors are closed; RED indicates that a door is

open, or not fully closed.

Bicolor LED indicator. GREEN indicates proper air flow, RED indicates “air flow fault.”

dicator

24 EXT. INTERLOCKS, status in-

dicator

25 MODULATION ENCODER,

CABLE INTERLOCK, status
indicator

Bicolor LED indicator. Indicates status of external interlocks. GREEN indicates a com-

pleted circuit; RED indicates an open circuit.

Bicolor LED indicator. Indicates status of cables between the MODULATION ENCODER

board and the Combiner/Motherboards. It also indicates when any RF Amplifier module is

not properly inserted into the motherboard. GREEN indicates all cables and modules are

properly installed; RED indicates that a cable or module is not installed or connected.

3A-4 888-2297-002 Rev AA: 8/9/2000

WARNING: Disconnect primary power prior to servicing.

Table 3A-3. Status Panel, Controls and Indicators (Cont.)

REF. CONTROL/INDICATOR FUNCTION

26 DC REGULATOR, B+ supply

status indicator

27 DC REGULATOR, B- supply

status indicator

28 A/D CONVERTER, +15 V sup-

ply status indicator

29 A/D CONVERTER, -15 V sup-

ply status indicator

30 A/D CONVERTER,CONVER-

SION ERROR, status indicator

31 A/D CONVERTER, +5V sup-

ply status indicator

Bicolor LED indicator. Indicates status of +5 Volt supply on the DC Regulator board.
GREEN indicates normal operation; RED indicates +5 Volt supply fault.

Bicolor LED indicator. Indicates status of B- supply on the DC Regulator board. GREEN
indicates normal operation; RED indicates B- Volt supply fault.

Bicolor LED indicator.Indicates status of +15 Volt supply on the A/D Converterboard.
GREEN indicates normal operation; RED indicates +15 Volt supply fault.

Bicolor LED indicator.Indicates status of -15 Volt supply on the A/D Converter board.
GREEN indicates normal operation; RED indicates -15 Volt supply fault.

Bicolor LED indicator. GREEN indicates normal operation of analog to digital (A/D) con­verter. RED indicates conversion error in A/D converter.

Bicolor LED indicator.Indicates status of +5 Volt supply on the A/D Converter board.
GREEN indicates normal operation; RED indicates +5 Volt supply fault.

Note 1

Use S4 on the LED board to display current RF status when PA supplies are off.

Note 2

This indicator is tied to red (blown fuse) indicators on the RF Amplifiers in the
PA section. Refer to Section C.

Section IIIA - Controls and Indicators

Rev AA: 8/9/2000 888-2297-002 3A-5

WARNING: Disconnect primary power prior to servicing.

DX-25U

Table 3A-4. Boards Listed in Alphabetical Order

Analog Input Board A35
A/D Converter Board A34
Buffer Amplifier A16
Combiner Motherboard Binary A8
Combiner Motherboard Main A5 thru A7
Combiner Motherboard Driver A14
Controller A38
DC Regulator A30
RF Drive Splitter A15
Driver Encoder/Temperature Sensor A19
Driver Supply Regulator A22
RF Multimeter A23
External Interface A28
Fuse Board #1 A24
Fuse Board #2 A25
IPA Splitter A18
LED Board A32
Modulation Encoder/Binary A37
Neutralization Board A40
Oscillator A17
Output Monitor A27
Output Sample Board A26
Power Distribution A39
Switch Board/Meter Panel A31
T6/T7 RF Sample Assembly A41

Table 3A-5. Boards Listed in Numerical Order

A5 thru A7 Main Combiner Motherboard
A8 Binary Combiner Motherboard
A14 Driver Combiner Motherboard
A15 RF Drive Splitter
A16 Buffer Amplifier
A17 Oscillator
A18 IPA Splitter
A19 Driver Encoder/Temperature Sensor
A22 Driver Supply Regulator
A23 RF Multimeter
A24 Fuse Board Assembly 001
A25 Fuse Board Assembly 002
A26 Output Sample Board
A27 Output Monitor Board
A28 External Interface
A30 DC Regulator
A31 Switch Board/Meter Panel
A32 LED Board
A34 A/D Converter Board
A35 Analog Input
A37 Modulation Encoder/Binary
A38 Controller
A39 Power Distribution Board
A40 Neutralization Board
A41 T6\T7 RF Sample Assembly

3A-6 888-2297-002 Rev AA: 8/9/2000

WARNING: Disconnect primary power prior to servicing.

Section IIIA - Controls and Indicators

Rev AA: 8/9/2000 888-2297-002 3A-7

WARNING: Disconnect primary power prior to servicing.

Figure 3A-4

Left Control Compartment - Front View

839 7855 117

DX-25U

3A-8 888-2297-002 Rev AA: 8/9/2000

WARNING: Disconnect primary power prior to servicing.

Figure 3A-5

Center Control Compartment -Front View

839 7855 118

Section IIIA - Controls and Indicators

Rev AA: 8/9/2000 888-2297-002 3A-9

WARNING: Disconnect primary power prior to servicing.

Figure 3A-6

Driver Compartment - Front View

839 7855 119

DX-25U

3A-10 888-2297-002 Rev AA: 8/9/2000

WARNING: Disconnect primary power prior to servicing.

Figure 3A-7

Driver Compartment - Rear View

839 7855 126

Section IIIA - Controls and Indicators

Rev AA: 8/9/2000 888-2297-002 3A-11

WARNING: Disconnect primary power prior to servicing.

Figure 3A-8

Left & Center Control Compartments - Rear View

839 7855 121-modified

DX-25U

3A-12 888-2297-002 Rev AA: 8/9/2000

WARNING: Disconnect primary power prior to servicing.

Figure 3A-9

Output NetworkCompartment - Rear View

839 7855 120

Section IIIA - Controls and Indicators

Rev AA: 8/9/2000 888-2297-002 3A-13

WARNING: Disconnect primary power prior to servicing.

Figure3A-10

Step Start Panel, Unit 3

839 7855 139



4.1 Introduction

This section presents the overall principles of operation for the
DX-25U AM TRANSMITTER, including a review of Digital
Modulation, andcircuits not described in SECTIONS A through S.

4.2 Block Diagram Description

Referto Figure4-1,DX-25U BlockDiagram. Most of theblocks
on the diagramrepresent printedcircuit boardsin the transmitter.
The OverallSchematic Diagram, 839-7855-151, in the Drawing
Package, includes many of the same blocks identified as printed
circuit boards.

The Block Diagram description is divided into four sections:

a. RF Section
b. Audio and Modulation Section
c. Controller Section
d. Power Supplies

4.2.1

RF Section

The RF Section includes the Oscillator through the Power Am­plifiers, Bandpass Filter, and Pi Matching Network.

The RF Section generatesan RF signal, then amplifies the signal
to a level high enough to drive the PowerAmplifier stage. In the
Power Amplifier stage, the RF amplifier outputs are combined,
and fed to a Bandpass Filter/Output Network and then to a 50
Ohm RF output point.The PiMatching Networkallows conven­ient matching to loads that are not exactly 50 Ohms.

4.2.1.1 Oscillator

The RF signal path begins at the Oscillator from a crystal
oscillator or from an external source, such as an AM Stereo
Exciter or Frequency Synthesizer.The Oscillator output is fed to
the BufferAmplifier.

4.2.1.2 Buffer Amplifier

The Buffer Amplifier amplifies the Oscillator output and pro­vides a stable input signal to drive the Predriver stage.

4.2.1.3 Predriver

The Predriver stage uses oneof the79 identicaland interchange­able RF amplifiers used in the DX-25U. ThePredriver amplifies
the BufferAmplifier signal to a high enough levelto operate the
14 RF amplifiers used in the Driver stage.

4.2.1.4 Driver Stage

The Driver stageconsists of the combined output of up to 14 RF
amplifier modules. One of the RF amplifier modules is utilized
as a spare and two operate in a reserve capacity to compensate
forACline variations. Oneof these twoDriverModules operates
as a “coarse” adjustment, “ON” or “OFF,” while the other
provides “fine” adjustment for the Power Amplifier stage drive
level. The Driver Supply Regulator provides a variable DC
supply for this module.

Section IV

Overall System Theory

4.2.1.5 Driver Supply Regulator

The Driver Supply Regulator is partof a loop which controls RF
driveleveltothe power amplifier.An “RF Sense” feedback signal
from the RF splitter controls the regulator output voltage to
control the Driver stage output.

4.2.1.6 Driver Encoder/Temp Sense Board

The Driver Encoder/Temp Sense Board provides the turn-on
signals for the 14 Driver Modules. The “RF Sense” feedback
signal from the RF splitter is also fed to this boardfor automatic
control of one module. Other circuits on the board monitor heat
sink temperatureof PA Modules RF1and RF2 and air flow from
the four cooling fans.

4.2.1.7 RF Status Indications: RF Sense D ata Lines

Three RF status indicators on the transmitter ColorStat™ panel
indicateOscillator,BufferAmplifier,orPredriveroutput.AFault
in any section will cause that LED indicator on the ColorStat™
panel to turn RED.

The ColorStat™ panel indicators are normally GREEN. Some
indicators will turn AMBER under certain temporary overload
conditions. If a Fault or abnormal condition exists, the indicator
will turn RED.

Additional indicators are located on the RF amplifiers. Each RF
amplifier is fused, and if an amplifierfault causesa fuse to open,
a red LED will illuminate to indicate the location of the open
fuse.

4.2.1.8 Driver Combiner and RF Splitter

The outputs of the 14 RF amplifiers in the Driver stage are
combinedbytheDriverCombinerand fedto theRF Splitter. The
RF Splitter provides separate RF drive signals to the individual
RF amplifiers in the Power Amplifier.

4.2.1.9 Power Amplifier (64 RF amplifiers)

The PowerAmplifier consists of 64 identical RF amplifier mod­ules. For “Digital Amplitude Modulation,” encoded audio sig­nalsturn ononlyas many RFamplifiersas required atany instant
to generate the carrier and the modulating signal. The 64 RF
amplifiers are referred to as “Steps.”

The physical location of the RF amplifier modules in the Power
Amplifier stage creates a combination of 58 equal RF voltage
“Big Steps” and 6 fractional RF voltage “Binary Steps”.

4.2.1.10 PA Combiner

The 64 RF amplifier modules plug into three Main Combiner
motherboards and one Binary Combiner motherboard to make
up the PA Combiner. The module outputs are series combined
by a torroidal transformer to develop the total RF voltage. The
RF signal is then fed to the Output Network.

4.2.1.11 Output Network

The Output Network transforms the low impedance of the PA
Combiner output to a 50 Ohms output impedance. The imped­ance transformation is accomplished in two sections: the Band­pass Filter stage and the Pi Matching stage.

Rev. AA: 8-9-00 888-2297-002 4-1

WARNING: Disconnect primary power prior to servicing.

DX-25U
The Bandpass Filter stage completes the digital to analog con-

version by filtering out the individual “step” voltages generated
by the RF amplifiers.

The Pi Network provides for impedance matching into antenna
systems that are not perfect 50 Ohm loads.

The Output Network also includes RF sample circuits for the
Output Monitor circuitry to provide power monitoring, and
protection against high VSWR conditions.

4.2.2

Audio and Modulation Section

The Modulation Section of the transmitter accepts an analog
audio input signal and converts it to a digital signal. The digital
signal is then processed, or “encoded”, to control the RF ampli­fierswhichproducethe “Digital Amplitude Modulation”. Circuit
boards in the Modulation section include the Analog Input
Board, Analog to Digital Converter, Modulation Encoder, and
DC Regulator.

4.2.2.1 Audio Input

Audio is fed into the Analog Input Board where it is processed.
This processing includes attenuating the high audio frequencies
for Medium Wave Band channel spacing, and adding a DC
component to determine the carrier power. This (Audio + DC)
signalissent to theAnalog toDigital Converter.Asecond(Audio
+ DC) signal is sent to the DC Regulator.

4.2.3

Controller Section

The Controller section consists of the Controller, LED Board,
External Interface, and the Switch Board/Meter Panel.

4.2.3.1 Controller

The PA Turn-On/Turn-Off command is recognized by the Con­troller from any of the LOW, MED, HIGH, or OFF buttons on
the Switch board or from external inputs. Circuits on the Con­troller energize themain contactorsfor the PAPowerSupply and
provide carrier power control.

4.2.3.2 LED Board

The LED Board contains fault and overload sensing and logic.
It provides 26 LED ColorStat™ panel indications to monitor
transmitter operation. These status indications are also available
as remote statusoutputs from theExternalInterface. Many status
indications are “latched” to provide fault indications until they
are “reset,” even if the transmitter is turned OFF. A battery
backup supply holds status indications in memory if AC power
fails or is turned off. The backup supply also enables the trans­mitter to automatically restart when AC power is restored.

4.2.3.3 External Interface (Remote Control)

The External Interface provides “remote control” inputs, status
outputs, and selected metering outputs. The External Interface
isolates transmitter circuits from remote control inputs to mini­mize thepossibility of damageif improper voltages are acciden­tally applied to the terminal boards.

4.2.2.2 Analog to Digital Converter

The (Audio + DC) signal is sampled at a 400 to 800 kilohertz
rate, depending on the transmitter operating frequency, by the
Analog to Digital Converter circuits. The Analog to Digital
Converter converts the (Audio + DC) signal into a 12 bit digital
signal.

4.2.2.3 Modulation Encoder

The Modulation Encoder converts the 12-bit digital audio infor­mation intocontrol signals which turn the RFamplifier modules
in the Power Amplifier stage ON and OFF, to generate the
transmitter carrier and the instantaneous modulation level.

Otherinputs tothe Modulation Encoder includea PAOFF signal
from the Output Monitor. Supply voltages for the Modulation
Encoder are supplied by the DC Regulator.

The PA OFF signal immediately turns the Power Amplifier
control signals OFF, if a VSWR condition is detected, to protect
the RF amplifier modules. The OFF signal only lasts as long as
the VSWR condition is present, and the transmitter will imme­diately return to normal operation.

4.2.2.4 DC Regulator

The DC Regulator produces the B+ (+5 VDC) and B- voltages
usedby the ModulationEncoder.The (Audio+ DC)samplefrom
the Analog Input Board “modulates” the DC Regulator “B-”
output voltage. The Modulated B- is a bias voltage for the RF
amplifiermodules inthe PowerAmplifierstage which varies the
turn on/turn off times of the modules to optimize distortion and
noise performance.

4.2.4

Power Supplies

Power Supplies are derived from two transformers: T1 provides
the PA Power Supply voltage and T2 provides the Low Voltage
supply.

4.2.4.1 Low Voltage Supplies

TheLowVoltagesupply includes +30 VDC, +22 VDC,-22VDC,
+8 VDC and -8 VDC unregulated supplies. These supplies are
regulated on individual modules for circuit supply voltages.

4.2.4.2 PA Power Supply

The PA Power Supply includes +230 VDC, +115 VDC and +60
VDC supplies for the 58 “Big Step” and 6 “Binary Step” ampli­fiers.

4.3 Low Voltage Power Supply: Circuit

Description

Refer to Sheet 1 of the DX-25U Overall Schematic, 839-7855­151, in the Drawing Package.

A 208 to 270 VAC input is protected by Metal Oxide Varistors
RV1 and RV2 in the Step-Start panel, fed to LowVoltage circuit
breakers CB1 and CB2 and then to transformer T2 in the trans­mitter cabinet.

Low Voltage Supply transformer T2 has two secondary wind­ings.Onewinding provides 24VoltsACtobridgerectifierCR15.
The bridge rectifier positive output is +30 VDC and filtered by
C37. The +30 VDC output is used for contactor status and

4-2 888-2297-002 Rev. AA: 8-9-00

WARNING: Disconnectprimary power prior to servicing.

Section IV - Overall System Theory

Rev. AA: 8-9-00 888-2297-002 4-3

WARNING: Disconnect primary power prior to servicing.

Figure 4-1. DX 25U Block Diagram

DX-25U
supplies the Buffer Amplifier. The winding also supplies 24

Volts AC for the interlock circuit relays.
The other secondary winding is tapped to provide two different

output voltages for bridge rectifiers, CR13 and CR14. The
center tap is grounded, so that each rectifier provides a positive
and negative output voltage. Rectifier CR13 provides +8 VDC
and -8VDC and CR14 provides +22 VDC and-22 VDC. Large
electrolytic filter capacitors are used for all Low Voltage sup­plies. Each capacitor has a bleeder resistor connected across its
terminals to discharge the capacitor when the supply is turned
off. All transmitter circuits, except PowerAmplifier and Driver
Modules, operate from the Low Voltage supply.

4.3.1

Power Distribution Board, A39

The Power Distribution Board distributes the +8 VDC and +22
VDC outputs to individual printed circuit boards, where +5
VDC and +15 VDC regulators and zener diodes provide re­quired voltages for circuits on the boards. The -8 VDC and -22
VDC outputs are also distributed to individual boards where
they are regulated to -5 VDC and-15 VDC.

4.4 PA Power Supply: Circuit Descrip-

tion

The PA Power Supply contactors K1 and K2 in the Step Start
panel are driven by transmitter logic circuits to provide a step­start function on turn-on. Auxiliary contacts on K1 and K2
operatethe PASupply DischargeCircuit (Crowbar)to discharge
the supply when it is turned off. The PA Power Supply trans­formerT1 supplies +230VDC,+115 VDC and+60 VDCforthe
Power Amplifier and Driver stages. Refer to Sheet 1 of the
DX-25U Overall Schematic, 839-7855-151, in the Drawing
Package, for the following description.

4.4.1

Step Start Panel

Eightidentical MOVs (metal oxidevaristors) aremounted inthe
StepStart panelto absorb transientvoltageson theincoming AC
lines. Two are used on the single phase 1 KVA feed, and the
remaining 6 are paired up and placed across the 3-phase lines.

When the PA Power Supply is energized by turn-on/turn-off
logic on the Controller, the single phase AC input voltage
energizes step-start contactor K1 through K101. AC power is
supplied to transformer T1 through three low-resistance high
wattage resistors (R1, R2 and R3). The series resistance limits
surge current as power supply capacitors charge.

When K1 energizes, an auxiliary contact also closes and pro­vides a +30 Volt “K1 has closed” logic signal back to the
turn-on/turn-off logic on the Controller.

After approximately one second, the turn-on/turn-off logic en­ergizes contactor K2 and applies primary power directly to
transformerT1,completingthe step-start sequence. Anauxiliary
contact provides a +30 Volt “K2 has closed” logic signal to the

turn-on/turn-off logic. Approximately one-half second later,
step-start contactor K1 de-energizes.

4.4.2

PA Supply Discharge Circuit (Crowbar)

This circuit is comprised of four powerMOSFETs operating in
parallel to discharge the PA Power Supply anytime K1 and K2
are deenergized.

When the PA Power Supply is energized, transistors Q101,
Q102, Q103, and Q104 function as open switches. In this
condition, K1 is deenergized and K2 is energized. The AUX
contactofK1isclosed and theAUXcontactofK2isopen.When
K2 deenergizes, the AUX contact closes and applies a control
voltagefromthe+230VDC supply tothe gate of each MOSFET.
This voltage switches the MOSFETs ON and discharges the
supply through resistors R101, R102, R103, and R104. Each
MOSFET has a zenerdiode fromgate tosource tolimit thegate
voltage to 10V.

Resistors R105 and R106 provide a secondary discharge path
with a faster time constant than the combination of the individ­ual bleeder resistors mounted across each filter capacitor. The
+115 VDC supply is also discharged through the PA Supply
Discharge Circuit through CR24.

4.4.3

Power Supply Shorting Switches

Switches S9, S10 and S12 short the +230 VDC supply bus to
ground if any Power Amplifier or Driver Compartment inter­locked door is open.

4.4.4

PA Power Supply Transformer T1

Transformer T1 is a three-phase, open frame power transformer
with multiple windings in the primary to allow an input voltage
rangefrom 360VACto505VAC.Threesetsofsecondarywindings
provideACforthe +230 VDCand+115 VDC rectifierassemblies.

4.4.5

Interphase Transformer T3

The +230 VDC, six-phase rectified outputs are combined
through interphase transformer T3. The use of an interphase
transformer reduces the losses in the supply transformer and
rectifiers.

4.4.6

+230 VDC Supply

The +230 VDC 12-phase output of T3 is fed to the distribution
busonFuse Boards A24and A25.A12-phaserectifierassembly
provides a DC output with asmall ripplecomponent at12 times
the power line frequency and reduces the supply filtering re­quired. Aparallel supply reducesthe peak current requirements
of the rectifiers. To further improve overall efficiency, the total
number of rectifiersin each assembly is doubled. Each rectifier
is fused separately to provide built in redundancy.

4.4.7

+115 VDC and +60 VDC Supplies

The third secondary of T1 is wired in a WYE configuration to
provide the +115 VDC supply and the neutral of this winding
supplies +60 VDC. The third secondary of T1 feeds the rectifier
assembly CR16 thru CR21. The +115 VDC output of CR16 ,
CR18, and CR20 is sent to the A24 Fuse Board for the Driver
andBinaryModulesandfilteredbyC19andC7.Theneutral

4-4 888-2297-002 Rev. AA: 8-9-00

WARNING: Disconnectprimary power prior to servicing.

Section IV - Overall System Theory

provides +60VDC for the Predriver and Binary Amplifiers and
is filtered by 10 mH choke L3 and capacitors C10 and C42.

4.4.8

Supply Current Meter, M2

The negative sides of the rectifier assemblies are tied to the PA
Power Supply current meter shunt SH1 in the Driver Compart­ment. The total +230 VDC supply current returns to ground at
this point. The Supply Current meter (M2) on the transmitter
front panel is connected across the shunt.

The voltage across the shunt is proportional to supply current
and also goes to the supply current overloadcircuit on the LED
Boardfrom the SwitchBoard/MeterPanel.Thecurrent overload
circuit also provides a remote supply current metering output.
Refer to SECTION Q, LED Board, for a description of the
supply current overload circuit.

Secondary Winding Fuses

4.4.9

The secondary winding of the PAPower Supply is protected by
fuses mounted on the top of transformer T1: 250 Amp fuses F1
through F12 protect the secondary windings from a shortcircuit
in the +230 VDC rectifier assemblies or output busand 35 Amp
fuses F13,F14, andF15, protect the secondary windings for the
+115 VDC and +60 VDC supplies.

4.4.10

Two25 Amp fuses, F20 and F21 mounted on top of transformer
T1, protect the +60 VDC supply tothe Predriverand theBinary
RF amplifiers.

The +230 VDC supply is fed to the A24 and A25 Fuse Boards
where16individual50Amp fuses feed 5,100uF filter capacitors
mounted near the Combiner Motherboards. Eachcapacitor sup­plies filtering for eight RF amplifier modules.

The +115 VDC supply for the Driverand Binary RF amplifiers
is fed to the A25 Fuse Board. The +115 VDC Binary supply is
fused by 25 Amp fuse F9 and the +115 VDC Driver supply is
fusedby30AmpfuseF10.

On eachFuse Board, aseries diode and parallel resistor provide
a fast discharge path for the capacitor if a fuse opens.

Supply Fuses

Refer to SECTION J, Analog Input Board, for the circuit de­scription.

4.4.12.2 Driver Supply Regulator Sample

Parallel resistors R25-R26 from the +230 VDC supply are
collector load resistors for the Driver Supply Regulator Q2
circuit. Refer to SECTION E, Driver Supply Regulator, for the
circuit description.

4.4.12.3 Supply Fault Sample

An AC sample of power supply “ripple” for the power supply
protection circuit on the LED Board is provided by voltage
dividerR14 and R15. Capacitors C1 and C2 block DC and form
an AC voltagedivider.Transzorb CR11 provides overvoltageor
transient protection. Refer to SECTION Q, LED Board, for a
description of the Power Supply Protection circuit.

4.4.12.4 PA Supply VDC sample

Resistors R16, R17 and R18 form a voltage divided signal for
remote and local PA Supply VDC readings. The signal passes
through the LED Board and then to the Controller. A voltage
followeramplifieron theController providesoutputs tothe front
panelmultimeter and tothe External Interface forremote supply
voltage metering. Refer to SECTION P, Controller, for a de­scription of the PA Supply VDC metering circuit.

4.4.12.5 PA Supply Overvoltage Sample

Resistors R19, R20 and R21 form a voltage divider for the
overvoltage circuit on the LED Board. Refer to SECTION Q,
LED Board, for a circuit description.

4.4.13

The following information highlights circuits that function as a
result of variations in AC line conditions.

4.4.14

The “PAPower Supply” DC overvoltage protection on the LED
Board will shut the transmitter OFF in case of overvoltage
conditions. If high or low supply voltages to the Driver stage
results in excessive RF drive level changes, RF Overdrive or
Underdrive circuits on the LED Board also shut the transmitter
off.

AC Power Protection Circuits

Overvoltage and Undervoltage Protection

4.4.11

The fan motors B1 through B4 are fused separately by 1 Amp
cartridge fuses F11 through F13, F21 through F23, F31 through
F33 andF41 through F43. Fuses F17, F18, andF19 are 15 Amp
cartridge fuses and protect the wiring between the primary of
T1andthefanmotorfuses.

4.4.12

Five +230 VDC sample circuits are located on the two fuse
boards.These fivesamples are routedtothe LED Board,Analog
Input Board, and Driver Supply Regulator. The Analog Input
Board sample is located on the A25 Fuse Board and the other
four samples are located on the A24 Fuse Board.

4.4.12.1 Analog Input Sample

A DC power supply compensation signal for circuitry on the
Analog Input Board comes from R22, R23, R24, R27, and C3.

Fan Motor Fuses

A24 and A25 Fuse Board Sample Circuits

Rev. AA: 8-9-00 888-2297-002 4-5

WARNING: Disconnect primary power prior to servicing.

4.4.15

CircuitryontheLEDBoardprovidesprotectionagainstlossof
phase or line imbalance (Brown-out). If either condition is
detected, a circuit will turn off the PA Power Supply and a
Supply Fault will be indicated on the ColorStat™ panel. Refer
to SECTION Q, LED Board, for a description of the Power
Supply Protection circuit.

Loss of Phase and “Brown-Out” Protection

4.5 Air System and Sensing Circuits

Refer to the DX-25U Cabinet Outline drawing, 839-7855-152,
and Overall schematic, 839-7855-151. Additional information
is also available in SECTION II, Installation/Initial Turn-On,
SECTION V, Maintenance, and SECTION S, Driver En­coder/Temp Sense Board.

DX-25U

4.5.1 Fans

Four fans, B1 through B4, areused in the DX-25U for transmit­ter cooling.Each fan uses a 1/3 HP,dual voltage 3-phase motor,
poweredwhen T1is energized.T1 primary taps also functionas
an autotransformer when the line feed is greateror less than the
+10% tolerance of the motor.

4.5.2

Air Flow and Temperature Sensing

Air flow reduction or loss and over temperature conditions are
sensed by circuitry located on the Driver Encoder/Temperature
Sense Board. The two separate and independent circuits are
detailed in SECTION S. A brief description of their system
operation is as follows:

4.5.2.1 Air Flow Sensing

Circuitry on the Driver Encoder/Temp Sense Board will cause
the AIR INTERLOCK LED onthe ColorStat™ panel to illumi­nate AMBER if one fan has failed, or if the air flow is reduced
to that level. A “Blower Fault” status output is generated by the
External Interface for remote monitoring. Solid state air flow
device U17 on the Driver Encoder/Temp Sense Board sends an
active LOW signal to the LED Board if two fans fail or if air
flow is further reduced. The AIR INTERLOCK LED on the
ColorStat™ panel indicator will illuminate RED and the trans­mitter will turn OFF. An “Air Fault” status output is generated
by the External Interface for remote monitoring.

4.5.2.2 Temperature Sensing

Two temperature probes are attached to the heat sinks of PA
Modules RF1 and RF2. The temperature is sensed by circuitry
on the Driver Encoder/Temp Sense Board which sends a
“LOWER” power command signal to the LED Board if the
temperaturerisesabovea preset threshold. Theamount of power
reduction is determined by the severity of the over dissipation.
As the power is stepped down, the heat sink temperature will
decrease until it is under the threshold of the sensing circuit, at
which time the LOWERcommand willstop. ThePOWERmust
be increasedto the original levelby anoperator induced RAISE
command. A “Temperature Caution” or “Over Temperature”
condition will illuminate LED’s on the Driver Encoder/Temp
Sense Board. A DC temperature sample voltage from the RF1
sensor is available at TB1-10 for remote temperature monitor­ing.

4.6 Interlocks

The interlock circuits protect personnel and external equipment
from dangerous or unsafe conditions. Fora description of inter­lock circuits and logic, refer to SECTION P, Controller. The
interlock circuitry is detailed on the DC Regulator schematic,
839-7855-163, and the DX-25U Overall Schematic, 839-7855­151, in the Drawing Package.

4.6.1

Door Interlock Circuit

The door interlock circuit turns thetransmitter OFFif any ofthe
three interlocked RF Amp Compartment doors are opened. The
DOORinterlockLED on the ColorStat™panelwillilluminateRED.

4.6.2

External Interlock Circuit

The external interlock circuit turns the transmitter OFF if any
external interlock interrupts the normally closed connection
between TB1-1 and TB1-2. The EXTERNAL interlock LED on
the ColorStat™ panel will illuminate RED.

The EXTERNAL interlock circuit includes relay K3, 1 Amp
fuse F24, pull up/pull down resistors on the DC Regulator and
interlocklogic on theController.External Interlockconnections
are detailed in SECTION II, Installation/Initial Turn-On.

4.7 RF Circuits

Refer to Sheet three of the DX-25U Overall Schematic, 839­7855-151, for the following circuit information.

Most of the RF drivecircuits are detailed in their own sections.
TheBlock Diagram Descriptionhasalreadyexplained how each
section contributes tothe drive system inthe DX-25U, therefore
no further information is necessary in this section.

Refer to the following sections for more information:
Oscillator

Buffer
Predriver
Driver
Driver Combiner
Driver Supply Regulator
Driver Encoder/Temp Sense Section S
RF Multimeter

4.7.1

RF Drive Splitter, A15

The combined RF output from the Driver stage feeds the RF
Drive Splitter. The splitter has provision for 256 outputs, two
for each of the 128 PA Modules in a DX-50, however,only 128
are utilyzed in the DX-25U. An additional connector (J17) on
the splitter assembly provides three RF sample signals to other
parts of the transmitter, as follows:

a. To theDriverSupply Regulator and DriverEncoder/Temp

Sense Board: An RF sample for the RF drive Automatic
Gain Control (AGC) loop.

b. To the Analog to Digital Converter: A synchronizing

signal for the analog to digital conversion process.

c. To theLED Board:AnRFdrivesample,for Overdriveand

Underdrive Fault sensor circuits and for “Relative RF
Drive” Metering.

4.7.2

RF Drive Cables

The RF drive splitter outputs, at connectors J1 through J16, are
very low impedance,and the 128 separate RF drive cables tothe
PA Module inputs provide additional isolation so that a fault at
one module input will have little or no effect on other RF drive
signals.

The 16 connectors from the splitter provideconnections fortwo
setsofeight coaxial cables. Eachgroup ofeightcablesfrom each
connector form a cable bundle which goes to aninput connector

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WARNING: Disconnectprimary power prior to servicing.

Section IV - Overall System Theory

on a PA Combiner/Motherboard. All RF drive cables are the
same length, so that all PA Module inputs are in phase.

4.7.3

Power Amplifier Description

InthePowerAmplifierstage,thedigitalinformationconstructed
by the Analog to Digital (A/D) converter is used to switch 64
RF amplifiers ON and OFF . The output combiner sums the
individual units of RF voltage developed by each amplifier.

The Power Amplifier stage may be thought of as a Digital to
Analog (D/A) converter, where the output is a high power,
amplitude modulated, RF signal.

The action of the RF combiner and RF amplifier modules
produce RF voltage “steps” at the combiner output. The power
output of each RF amplifier depends on the total number of
modules switched ON at any time. Switching on twice as many
RF amplifiers willproduce TWICE THEVOLTAGE outputand
FOUR TIMES THE POWER output. If a small number of
modulesareswitched on, eachmodule hasa smallpower output.
If a largenumber of modules are switched on, each module has
a larger power output.

The DX-25U uses a 12-bit digital “word” to control the RF
amplifiers.Ifthe Power Amplifier stage consistedonly ofbinary
“weighted” amplifiers, eachbit would control one amplifierand
it would require 12 RF amplifiers to represent the digital word.
However,the RF voltagedeliveredby the amplifierrepresenting
the most significantbit would haveto be equalto one-halfof the
peak RF voltage created with all 12 modules ON. In an RF
voltage combiner, all RF voltages areadded in series. The same
current flows through all outputs as through the load, and
one-half the peak voltage is also one-half the peak power. The
largest “step”, then, would have to be able to deliver over 75
kilowatts, the next over 37.5 kilowatts, and so on. It is more
practical to use a larger number of smaller power amplifiers.

The DX-25U uses 58 equal RF voltage “Big Step” amplifiers
and six “Binary Step” amplifiers. The binary amplifier outputs
equal 1/2, 1/4, 1/8, 1/16, 1/32 and 1/64 ofa “BigStep” amplifier
output and are controlled by the six least significant bits of the
digital word. The six most significant bits control the total
number of “Big Step” amplifiers ON at any time. Typically, 23
RF amplifiers are ON for a 25kW carrier.

However, AMtransmitterPEAK output powerrequirements are
much greater than the transmitter CARRIER power. Also,
broadcasters may require additional transmitter power to over­come antenna system power losses. For this reason, the carrier
poweroutputof the DX-25Uisrated at upto30kW.The positive
peak modulation capability of an AM broadcast transmitter
depends on the maximum peak power output available from the
transmitter. For example, a +100% modulation peak represents
a peak output power of four times the carrier power, or 100kW
for a 25kW transmitter.This requires that double the number of
amplifiersbe ON forthepeak as comparedwiththe carrier level.
Therefore, at the 100% positive peak modulation of a 25kW
carrier,47 RFamplifiers areON. With a carrier power of30kW,
a 125% positive peak requires a peak output power of 151.9

303.8kW and additional RF amplifiers are turned ON.

SUMMARY: The more“steps” or RF amplifiers thatare turned
on, the more power will be transmitted. The DX-25U uses a
combination of 58 equal RF voltage “Big Step” amplifiers and
six binary weighted RF voltage “Binary Step” amplifiers to
develop the modulated RF envelope. The modulated RF output
is made up of equal VOLT AGE steps, not equal power steps.
Assuming that the supply voltage remains the same, the RF
outputVOLTAGEfromeach module remains the samenomatter
how many other modules are on. Since the combiner secondar­ies are in series, the TOTAL RF VOLTAGE induced on the
combiner rod will increase by the number of STEPS turned on.
Witha constant combiner impedance of approximately8 Ohms,
an increase in RF voltage will increase the RF current in the
combiner and increase the power. The power output from each
amplifier module changes, however, depending on the total
number of amplifier “steps” that are switched on.

4.7.3.1 Switching RF amplifiers On or Off

RF amplifiers are switchedON or OFFby applying orremoving
RF drive to the module with a solid state switching circuit.
Because low voltage,low currentcircuits areused in theswitch­ing process, very little power is consumed.

Combiner Description

4.7.4

Sheet 4 of the DX-25U Overall Schematic, 839-7855-151,
identifies motherboards and module numbers as viewed from
the rear of the transmitter.

The Power Amplifier stage is made up of 64 plug-in RF ampli­fiermodules. These include58 “Big Step”modules RF33-RF90
and six “Binary Step” modules RF91-RF96, which plug into
three Main Combiner/Motherboards, A5-A7, and one Binary
Combiner/Motherboard,A8.

Sixteen ferrite core toroid transformers on each motherboard
combine the RF voltage outputs of the modules through a solid
copper rod which which passes through the center of the trans­formers. The four Combiner/Motherboards are assembled as a
single column.

The PA Combiner RF ground point is at the bottom of the
column. When the A/D Convertersample frequency is one-half
carrierfrequency,a parallel resonantcircuitconsisting of L4and
C4 present a high impedance to the half-carrier frequency.

One Modulation Encoder (A37) takes the digital output from
the Analog to Digital Converter and converts it into control
signals for theRF amplifier modules. Dueto theway the system
would be configured as a DX-50, the RF Amplifier steps con­trolled by theA37 ModulationEncoder areRF33 through RF96
with the last 6 steps being the Binary Steps (RF91 through
RF96).

The PA Modules are also identified by step number on the
interlockedmoduleaccess doors in theCenter ControlCompart­mentandLeft Control Compartment.Eachmodulehastwo LED
“fault” indicators, visible through openings in the access door.
Ifa shorted MOSFETcauses a fuseto open, the “fault”indicator
forthatfuseilluminates.When a module is switched ON, a green
LED indicator illuminates.

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DX-25U
Refer to thefollowing sections foradditional informationon the

PowerAmplifier:

a. SECTION A: RF Amplifier Module.
b. SECTION G: RF Combiners

4.7.5

RF Samples from the Output Combiner

Four RF samples from various points on the RF combiner are
distributed to variouscircuits elsewhere in the transmitter.Refer
to Sheet4 of theDX-25U Overall Schematic, 839-7855-151, to
locate the following sample transformers.

4.7.5.1 T9: Bandpass Filter VSWR Detector

An RF current sample from T9, near the RF ground point inthe
combiner, is fed to the Output Monitor and compared to the RF
voltage sample at the output of the Bandpass Filter section of
the Output Network. During a VSWR condition, a phase shift
in the RF current and voltage samples will be detected by the
phase angle detection circuitry. Refer to SECTION H, Output
Monitor, for additional information on VSWR protection.

4.7.5.2 T6: Oscillator Sync Signal

During a VSWR condition, all power amplifier modules are
quickly turned OFF. Because of the resonant circuits, “ringing
currents” will continue to flowin theoutput network, and in the
RF combiner secondary, for several RF cycles. For maximum
MOSFET reliability during this condition, Q1/Q12 and Q2/Q9
in all PA Modules must switch in phase with these output
network ringing currents. This is accomplished with the Oscil­lator Sync circuitry on the Oscillator.

A current sample from ferrite inductor transformer T6 at the
combiner output is fed to J3 on the Oscillator. A phase shift
network, amplifier stage and an analog switch are used to
synchronize RF driv ephase with outputnetwork ringing current
duringVSWR shut-downs.Refer to SECTION A,Oscillator,for
additional circuit information, and to SECTION V, Mainte­nance, for adjustment procedures.

4.7.5.3 T1: Neutralization Board

T1 on the Neutralization Board is a ferrite toroidal transformer
similar to the ones usedin the RF combiner and is only usedfor
IQM reduction for AM Stereo operation. The transformer is
wired to the Predriveroutput splitter T8.

4.8 Output Network Description

TheOutput Networkofthe DX-25Uis comprisedof a Bandpass
Filter and Pi Matching network and contained in the Output
Network cabinet.

4.8.1

Bandpass Filter

Thebandpass filter/outputnetwork serves asboth an impedance
matching network and filter and consists of L1, C1A, and C1B.
At the very high end of the medium wave band, 1500kHz and
above, C6 is also a part of the bandpass filter. Vacuum variable
capacitor C1A is brought out to the front of the transmitter as
the TUNE control. The TUNE control is adjusted for a peak in

output power. The combiner output impedance is low,approxi­mately 8 Ohms, and is matched to approximately 50 Ohms.

The bandpass filter also “smooths” the small steps in the output
signal that remain after the Digital to Analog conversion by the
PowerAmplifier stage. Any other harmonic and spurious signals
in the RF output are also attenuated by the bandpass filter.

Refer to SECTION V, Maintenance, for information on tuning
and adjustment of the bandpass filter if required by a frequency
change or major component failure.

4.8.2

PI Matching Network

The PI Matching Network consists of C2A, C2B, L3, C3A,
C3B, C4A, C4B and C5. Parallel capacitors C2A and C2B

comprise the first leg of the
tors C3A, C3B, and, at some frequencies, C5 are adjusted and

tuned to 3f
attenuation of the 3rd harmonic to FCC specifications while
passing the carrier frequency. VACuum variable capacitor C4A
is brought out to the front of the transmitter as the LOAD
control. The LOAD control adjusts the amount of Power Am­plifier current.

4.8.3

A spark gap at theoutput ofthe transmitter protects against high
transient voltages causedby lightning orelectrostatic discharge.
This does NOTsubstitute for proper DC grounding chokes, ball
gaps, and other protection at the towers. Set E2 at .090".

. The parallel resonant circuit provides further

c

Spark Gap, E2

πnetwork. Inductor L3 andcapaci-

4.9 Digital Modulation Principles

4.9.1 Digital Terms and Concepts

The discussion of Analog to Digital and Digital to Analog
Conversion includes terms, abbreviations, and concepts which
may not be familiar to some Broadcast Station engineers and
technicians. Most terms will be explainedin the discussion, but
a summary is included for review or reference.

a.

ANALOGrefers to acontinuous rangeof values. Examples

include audio signals from a microphone, a turntable
cartridge, CD, etc.

b.

DIGITAL is related to digits, or discrete quantities. An

analog signal changes continuously, but a digital signal
changes in steps. An analog signal hasan infinite number
of possible values, and a digital signal has a finite, or
limited, number of possible values.

c. BINARY: Has only two possible values. A BINARY

NUMBER is represented using only the digits 0 and 1.
This is useful because a circuit can be two states, either
ON or OFF.

d.

BINARY can alsorefer toa serieswhere each step is either

multiplied or divided by two to get the next step. An
example in the transmitter are the Binary RF amplifier
steps: the 1/2 step; 1/4 step; 1/8 step; 1/16 step; 1/32 step

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WARNING: Disconnectprimary power prior to servicing.

Section IV - Overall System Theory

and 1/64 step. In this series, each step is divided by two
to get the next step. A Binary series could also be 1, 2, 4,

8, 16, 32 etc.
e. BIT :A Binary digit, 0 or 1.
f.

DIGITAL WORD: A digital word is a group of bits repre-

senting a complete piece of digital information. The term

“DIGITAL WORD,” when used here, will always referto

a binary number, which is a series of ones and zeros. The

number of bits in a digital word refers to the total number

of digits (ones and zeros).
g.

MSB: Abbreviation for MOST SIGNIFICANT BIT.Ina

digital word, as in a decimal number,the first digit repre-

sents the largest change, and is the MSB.

LSB: Abbreviation for LEAST SIGNIFICANT BIT.Ina

h.

digital word, as in a decimal number, the last digit repre-

sents the smallest change, and is the LSB.

BIT 1, BIT 2, etc: In a 12-bit digital word, the bits are

i.

numberedfrom 1through 12, whereBit 1 isthe MSB, and

Bit 12 is the LSB.
j.

A/D: Also written “AtoD.” Abbreviation for “Analog to

Digital.”

D/A: Also written “DtoA.” Abbreviation for “Digital to

k.

Analog.”

Some Basic Digital Circuit Concepts used in the following
discussion, and in circuit descriptions, are also included for
review or reference.

In logic circuits, representing a digit by either zero or one is
useful because it can be represented by a switch or a circuit that
is either “OFF” or “ON.” The digits “zero” and “one” may also
be represented by a voltage that is LOW for “zero” and HIGH
for “one.”

In circuit descriptions and on schematic diagrams, the terms
“logic LOW” and “logic HIGH” are used. These terms are also
represented by the letters “L” and “H” on schematic diagrams.

In most logic circuits, normal TTL (transistor-transistor logic)
levels are used. In these circuits, a “logic LOW” is represented
by a voltage between approximately zero and one Volt, and a
“logic HIGH” is represented by a voltage between approxi­mately +3.5 and +5 Volts.

On block diagrams and on schematic diagrams, when a signal
descriptionisfollowedby“-L” or “-H,” the letter indicates the
logic state when the signal is ACTIVE. Examples:

a. “RESET-L” indicates that when the signal is logic LOW,

aRESET will occur,or a RESET commandis beinggiven.
b. “VSWR-H” indicatesthat when the signal is logicHIGH,

a VSWR fault has occurred.

A DIGITAL WORD can represent only a finite number of
quantities, or steps, depending on the number of bits in the
digital word.

a. If n = the number ofbits in the digital word,then: 2

n

=the
number of quantities that may be represented by that
word, including zero. For example, if adigital word has 6

bits, it may represent 2
has 12 bits, it may represent 2

a. “

VAL U E” OF EACH BIT: The least significant bit (LSB)

6

=64 quantities. If a digital word

12

=4096 quantities.

represents one unit. The next least significant bit repre­sents two units. The most significant bit represents one­half of the total quantity that the word can represent. For
example,ina6 bit “digital word”, the number of quantities
that may be represented is 2

6

=64:

1. Bit 1 (MSB) represents 32 units

2. Bit 2 represents 16 units

3. Bit 3 represents 8 units

4. Bit 4 represents 4 units

5. Bit 5 represents 2 units

6. Bit 6 (LSB) represents 1 unit.

4.9.2

Analog to Digital Conversion

An (Audio + DC) signal from the Analog Input Board is con­verted into a series of 12 bit digital words by the Analog to
Digital Converter. The digital signal is then processed by the
Modulation Encoder to provide signals to turn individual RF
amplifiermodulesON andOFF. ThePowerAmplifierstage acts
as the Digital to Analog converter to create a high power,
amplitude modulated, RF output signal.

The DC component of the (Audio + DC) signal controls the
number of amplifiers on for carrier power and is adjusted using
the RAISE and LOWER buttons on the front panel. The audio
signal is then added to the DC component. The (Audio + DC)
signal is then sent to the Analog to Digital Converter.

The Analog to Digital (A/D) conversion process takes place in
three steps:

a. Dividethe time scale into equal intervals by a high speed

sampling circuit.

b. At each time interval, sample and record the amplitude of

the analog signal.

c. For each recorded sample, construct a 12-bit digital word

that represents the analog sample amplitude.

RF Amplifier Control

4.9.3

Refer to SECTION L, Modulation Encoder, for additional in­formation.

The12-bitdigital word is“encoded”on the ModulationEncoder
to control the 58 “BIG STEP” and six “BINARY STEP” RF
amplifiers.The 12-bitdigital wordis dividedinto two groups of
information: The first six bits, B1 through B6, form a six bit
digital word and are used to control the 58 “BIG STEP” RF
amplifiers; the last six bits, B7 through B12, each control a
“BINARY STEP” RF amplifier.

On the Modulation Encoder, bits B1 through B6 are used to
address ROM, Read Only Memory, address locations. In turn,
the data at each address location controls the 58 “BIG STEP”
RF amplifiers.The six most significant bits of the12-bit digital

Rev. AA: 8-9-00 888-2297-002 4-9

WARNING: Disconnect primary power prior to servicing.

DX-25U
word represent the total number of “BIG STEP” RF amplifiers

ON at any time. For example:

a. 010111/XXXXXX = 23 BIG STEPS ON
b. 101111/XXXXXX = 47 BIG STEPS ON
c. 000000/XXXXXX = 0 BIG STEPS ON

The six least significant bits, B7 through B12, each control a
“BINARYSTEP” RF amplifier.If thebit is a “1”, theassociated
amplifierisON.Forexample:

a. XXXXXX/000001 = B12: ON
b. XXXXXX/000011 = B11, B12: ON
c. XXXXXX/010011 = B8, B11, B12: ON

The RAISE and LOWER controlson thetransmitter frontpanel
set the DC component of the (Audio + DC) signal. Under ideal
conditions, for 25 kW carrier, the Analog to Digital Converter
samplesthe DC componentof the (Audio +DC) signaland turns
23 “BIG STEP” RF amplifiers ON and all the “BINARY
STEPS” are OFF:

• 010111/000000

When audio is applied to the transmitter, the audio component
of the (Audio + DC) signal is sampled by the Analog to Digital
Converter and the “BINARY STEP” amplifiers begin turning
ON until the six least significant bits of the 12-bit digital word
are all “LOGIC HIGH”:

• 010111/111111

Asthe audio componentkeeps increasing,the“BINARYSTEP”
RF amplifiers turn OFF and another “BIG STEP” RF amplifier
turns ON:

• 011000/000000

This process repeats until the positive modulation peak is
reached. For 100% modulation of a 25kW carrier, this will
require twice the number of “BIG STEP” RF amplifiers ON as
are required for carrier, or 47. At that instant, the digital word
would be:

• 101111/000000

At this point, the audio component of the (Audio + DC) signal
begins to decrease, and RF amplifier modules begin turning
OFF.

Refer to SECTION J, Analog Input Board, and SECTION K,
Analog to Digital Converter, for additional information.

Amplitude Modulation in the DX-25U

4.9.4

The transmitter Power Amplifier stage uses 64 solid-state RF
amplifier modules. Each RF amplifier can be switched on and
offveryquickly.The RF voltage outputsof the 64 RFamplifiers,
or “steps”, are combined to produce the total transmitter RF
voltageoutput.Theoutput of an AM transmitteris anRF voltage
which varies according to the audio modulating signal input.
Each RF amplifier provides a fixed voltage, and the RF output
can be changed by switching the appropriate number of ampli­fiers ON. A fixed number of amplifiers are switched ON for
carrier. If the audio signal increases, more amplifiers are
switched ON. If the audio signal decreases, amplifiers are

switched OFF. As the audio signal changes from instant to
instant, the number of RF amplifiers that are ON changes.

a. For carrier only,no modulation, only enough RF amplifi-

ers to produce the required RF voltage for carrier power
are switched ON.

b. A positive modulation peak requires a high RF voltageat

thetransmitter outputand a largenumber ofamplifiersare
switched ON.

c. Fora100% negativemodulation peak, whichcorresponds

to zero RF voltage output, all the amplifiers are switched
OFF.

SUMMARY: Amplitude modulation in the DX-25U is accom­plished by turning on only enough RF amplifier modules at any
time to produce carrier and the audio modulating signal at that
moment.

Power Amplifier Stage

4.9.5

The transmitter PowerAmplifier stage contains 64 identical RF
amplifier modules. The amplifier modules are used as 58 equal
RF voltage “BIG STEP” amplifiers, and six fractional RF volt­age “BINARY STEP” amplifiers. The six “Binary Steps” are:

a. B7: = 1/2 “Big Step” RF voltage
b. B8: = 1/4 “Big Step” RF voltage
c. B9: = 1/8 “Big Step” RF voltage
d. B10: = 1/16 “Big Step” RF voltage
e. B11: = 1/32 “Big Step” RF voltage
f. B12: = 1/64 step “Big Step” RF voltage

As the 58 “BIG STEP” amplifiers are turned ON and OFF, the
RFoutput changesin equalVOLTAGE steps,not in equalpower
steps, because of operating characteristics of the output com­biner. For a 100% positive modulation peak, the RF output
voltage of the combiner must double. For a 100% negative
modulation peak, the RF voltageof the combiner must be zero.

For a typical carrier of 25 kW, approximately 23 “BIG STEP”
RF amplifiers are ON. As audio is applied, 47 “BIG STEP” RF
amplifiers turn ON for the 100% positive peak and all modules
turn OFF for the 100% negative peak.

The “BINARY STEP” amplifiers are switched in sequence to
provide smooth transitions between the equal RF voltage steps
created by the “BIG STEP” amplifiers. When all “BINARY
STEP” amplifiers are ON, their total amplitude equals 63/64 of
a “BIG STEP” voltage.

Summary: Digital Modulator

4.9.6

The amplitude modulationprocess in the transmittertakesplace
in three steps.

First, The audio input signal is converted into a digital data
stream, a series of 12-bit digital words, by an Analog to Digital
Converter. This digital data stream is the “digitalaudio” signal.

Second, thedigital data from the Analog to Digital Converteris
encoded by the Modulation Encoder to provide the control
signals required by the Power Amplifier stage.

4-10 888-2297-002 Rev. AA: 8-9-00

WARNING: Disconnectprimary power prior to servicing.

Section IV - Overall System Theory

Third,the control signalsform the ModulationEncoderare used
to switch individual RF amplifiers ON or OFF. The individual
RF amplifiermodule outputs are combined in the RF combiner.
The RF output of the Power Amplifier stage changes in very
small steps, or discrete quantities, and is passed through the
bandpass filter to smooth the step transitions.

4.9.6.1 Digital Modulator Characteristics

The patented Harris Digital Modulator uses new technology
which produces a very high quality, low distortion amplitude
modulated signal for AM broadcasters.

OverallACto RF efficiencyoftheDX-25Uisveryhigh,because
the digital modulator uses very little power and the RF amplifi­ers are highefficiency, solid-state,Class Dswitching amplifiers.

Thetransmitter haslittleor noovershoot ortilt with squarewave
modulation, even at very low audio frequencies and the modu­lation envelope accurately reproduces the audio input signal.

Rev. AA: 8-9-00 888-2297-002 4-11

WARNING: Disconnect primary power prior to servicing.



5.1 Introduction

This section provides general system preventive maintenance
information, board replacement and alignment procedures and a
transmitter frequency change procedure.

5.2 Maintenance

The importance of keeping station performance records cannot
be overemphasized. Separatelogbooks shouldbe maintained for
operation and maintenance. These records can provide data for
predicting potential problem areas and analyzing equipment
malfunctions.

5.2.1

Maintenance Logbook

The maintenance logbook should containa completedescription
of all maintenance activities required to keep the equipment in
operational status.

The following isa list of maintenanceinformation tobe recorded
andanalyzed to providea data base for afailure reporting system:

DISCREPANCY

Describe thenature of the malfunction including all observ­able symptoms and performance characteristics.

TIME/DATE

Time of day and date discrepancy occurred.

CORRECTIVE ACTION

Describe the repair procedure used to correct the mal­function.

DEFECTIVE PART(S)

List all parts and components replaced or repaired and
include the followingdetails:
a. TIME IN USE
b. PART NUMBER
c. SCHEMATIC NUMBER
d. ASSEMBLY NUMBER
e. REFERENCE DESIGNATOR

SYSTEM ELAPSED TIME

Total time on equipment

NAME OF REPAIRMAN

Person who actually made the repair

STATION ENGINEER

IndicatesChief Engineer noted andapprovedthe repairof
the equipment

5.2.2

Preventive Maintenance

Preventive maintenance is a systematic series of operations per­formedperiodically on equipmentand consists ofsixoperations:
inspecting, feeling, tightening,cleaning, adjusting,and painting.

• INSPECT .Inspection is the most important preventive main-

tenance operation because it determines the necessity for the
others.Become thoroughly acquaintedwith normaloperating
conditionsin ordertoreadily recognize andidentify abnormal
conditions. Inspect for the following:

Section V

Maintenance/Alignments

1. Overheating, which is indicated by discoloration, bulg­ing of parts, and peculiar odors.

2. Oxidation.

3. Dirt, corrosion, rust, mildew, and fungus growth.

• FEEL. By checking for overheating, lack of proper venti-

lation or other defects canbe detected and corrected before
serious trouble occurs. Become familiar with operating
temperatures in order to recognize deviations from the
normal range.

• TIGHTEN. Tighten loose screws, bolts, and nuts. Do not

overtighten.

• CLEAN. Clean parts only when inspectionshows thatclean-

ingisrequiredandonlyuseapprovedcleaningsolvent.

• ADJUST. Make adjustments only when inspection shows

that they are necessary to maintain normal operation.

• PAINT. Paintsurfaces with the originaltype ofpaint (using

prime coat if necessary)whenever inspection shows rust or
broken paint film.

Maintenance Of Components

5.2.3

The following paragraphs provide information necessary for the
maintenance of components.

5.2.3.1 Transistors and Integrated Circuits

Preventive maintenance of transistors and integrated circuits is
accomplished by performing the following steps:

CAUTION

USE CARE TO AVOID THE BUILDUP OF STATIC ELECTRICITY
WHEN WORKING AROUNDINTEGRATED CIRCUITS.

a. Inspectthe surrounding area fordirt.Accumulations could

form leakage paths.

b. Use compressed dry air to remove dust from the area.

WARNING

ALWAYS WEAR SAFETY GOGGLES WHEN USING COM­PRESSED AIR.

c. Examineall transistorsfor looseconnections orcorrosion.

Tighten the transistor mounting hardware to no more than
5inch-pounds. Overtightening the transistorhardware will
cause the insulators to short. Torquespecificationfor tran­sistor mounting hardware is 5 inch-pounds.

5.2.3.2 Capacitors

Preventive maintenance of capacitors is accomplished by per­forming the following steps:

a. Examine all capacitor terminals for loose connections or

corrosion.

b. Ensure that component mountings are tight. Do not over-

tighten capacitor mounting straps as excessive pressure
could cause internal shorting of the capacitors.

c. Examine the body of each capacitor for swelling, discol-

oration, or other evidence of breakdown.

Rev. Z1: 06-02-00 Additional info to ECN 46105 888-2297-002 5-1

WARNING: Disconnect primary power prior to servicing.

DX-25U

d. Use standard practices to repair poor solder connections

with a low-wattage soldering iron.
e. Clean cases and bodies of all capacitors.
f. Inspect bleeder resistors when inspecting electrolytic ca-

pacitors.

5.2.3.3 Fixed Resistors

Preventive maintenance of fixed resistors is accomplished by
performing the following steps:

a. When inspecting a chassis, printed-circuit board, or dis-

crete component assembly, examine resistors for dirt or

signsofoverheating.Discolored,cracked,or chipped com-

ponents indicate a possible overload.
b. When replacing a resistor, ensure that the replacement

value agrees with the schematic diagram and parts list.
c. Clean dirty resistors with a small brush.

5.2.3.4 Variable Resistors

Preventivemaintenance of variable resistors is accomplished by
performing the following steps:

a. Inspectthe variable resistors andtightenall loosemountings,

connections, and control knob set-screws (do not disturb

knob alignment). Sliding taps on adjustable resistors should

besnug,butnotexcessivelytight.Overtighteningcan damage

the resistor.
b. Clean dirty resistors with a small brush.
c. Whendirt is difficultto remove,clean withalint-free cloth

moistened with an approved cleaning solvent.

5.2.3.5 Fuses

Preventive maintenance is accomplished by performing the fol­lowing steps:

CAUTION

USE ONLY AN EXACT REPLACEMENT FUSE. FUSES OF THE
SAME SIZE AND/OR RATING FROM A DIFFERENT MANUFAC­TURER MAY NOT FULFILL THE REQUIREMENT FOR EXACT
REPLACEMENT.

a. When a fuse blows, determine the cause before installing

a replacement.
b. Inspect fuse caps and mounts for charring and corrosion.
c. Remove dirt with a small brush.
d. If necessary, tightenfuse clips andconnectionsto the clips.

Fuse clip tension may be increased by pressing the clip

sides closer together.

5.2.3.6 Switches

Preventivemaintenance of switchesisaccomplished by perform­ing the following steps:

a. Inspect switch for defective mechanical action or loose-

ness of mounting and connections.
b. Examine cases for chips or cracks. Do not disassemble

switches.
c. Check contacts for pitting, corrosion, or wear.
d. Operate the switches to determine if they move freely and

are positive in action.

e. Be sure to include an inspection of the power supply

discharge switches S9, S10 and S12 located in the inter­locked RF Amplifier and Driver Compartments.

5.2.3.7 Indicators and Front Panel Switches

Preventive maintenance of indicator lamps and control switches
is accomplished by performing the following steps:

a. To remove an indicator bulb (LOW, MED, HIGH,

RAISEor LOWER) pull outonthe indicator button. The
indicatorlampmay then beremoved.When re-installing
the button, care must be taken to avoid disrupting nor­mal operation.

b. Replacement of a front panel switch requires removal of

the Switch Board behind the meter panel.

5.2.3.8 Printed Circuit Boards

Preventivemaintenance of printed circuitboards isaccomplished
by performing the following steps:

a. Inspect the printed circuit boards for cracks or breaks.
b. Inspect the wiring for open circuits or raised foil.
c. Check components for breakage or discoloration due to

overheating.
d. Clean off dust and dirt with a clean, dry lint-free cloth.
e. Use standard practices to repair poor solder connections

with a 40 Watt soldering iron.

5.2.3.9 Air System

a. The air filters should be routinely washed with soap and

water.Intervals between cleaning will depend on the envi-

ronment.
b. Replace filter when it shows signs of deterioration.

5.2.3.9.1 Fan Motor B1 Through B4 Replacement

It is very probable that the fan blade will be locked or “frozen”
to the shaft of the motor after years of operation. Because of this
it will benecessaryto removethe fanframe and blowerassembly
to service the motor.

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO­CEDURE.

a. Remove the rear panels of the Output Network Compart-

ment.
b. Disconnect the threewires fromthe failedmotor to thefuse

block.
c. Remove the inner fan cage panel which will allow any of

the four fans to be pulled straight back.
d. Remove the 4screwsfasteningthe fan frameto the cabinet

and remove the fan assembly.

Strap the replacement motor in the High Voltage configuration
(460 VAC) shownin Figure5-1. Wirethe motoras shownon the
OverallSchematic,839-7855-151,toensure the motor will rotate
in the same direction as the other three.

5-2 888-2297-002 Rev. Z1: 06-02-00 Additional info to ECN 46105

WARNING: Disconnectprimary power prior to servicing.

Figure 5-1. Blower Motor Strapping

5.3 Module Replacement/Alignment

DX-25U modules can be grouped in three categories when
replacement is required:

• Modules which can be replaced with no adjustments.

• Modules which require preset switch settings or jumper

plug positions

• Modules which require adjustments.

5.4 Modules Which Can Be Replaced

With No Adjustments

The following modulesmay be replaced,orcomponents onthem
can be replaced, without making any adjustments, measure­ments, or preset switch or jumper plug settings:

• Buffer Amplifier(A39)

• Predriver (A10)

• PA Modules (RF33 through RF96)

• External Interface (A28)

• RF Multimeter (A23)

• Fuse Boards (A24/A25)

• Output Sample (A26)

• Drive Splitter (A15)

• Power Distribution (A39)

Buffer Amplifier (A16)

5.4.1

The Buffer Amplifier is broad-band, with no frequency deter­mined components. If the Buffer Amp is replaced and a drive
fault is still indicated on the ColorStat™ panel, check the drive
levelto the PredriverModule using the procedure outlinedunder
Frequency Change Procedure in this section.

5.4.2

Predriver

The Predriver Module is one of 79 identical RF amplifiers used
in the Driver and Power Amplifier stages, and therefore can be
interchanged if required. No adjustments are necessary if the
Predriveris replaced. Turn the transmitter ON and measure both
the Predriver voltage and current on the RF MULTIMETER to

Section V - Maintenance

verify that they are near the measurements indicated on the
Factory Test Data sheet.

NOTE

Adjustment of Predriver tuning control L1 is NOT required when

replacing the module. Changing the Predriver tuning can affect

the setting of the Oscillator sync used for VSWR protection.

If thePredriver is replaced and a Predriver Fault is stillindicated
on the ColorStat™ panel, it is advisable to check the output to
the Driver stage using the procedure outlined under the Fre­quency Change Procedure in this section.

5.4.3

PA Modules

The RF amplifiers used in the Power Amplifier stage are com­pletely interchangeableas required. This can be donequickly by
depressing the OFFbutton, and openingthe interlocked RFAmp
Compartment door inside the Center or Left Control Compart­ment. Remove the defective RF amplifier by pulling it out and
then insert thereplacement RF amplifier.Theinner doorcan then
be closed and the transmitter returned to operation.

If the lowvoltage isleft on when an RF amplifieris removed,the
ColorStat™ panel Cable Interlock LED will illuminate. Depress
thereset buttonto clearthe Interlock indication.Ifthe transmitter
will not turn on and the Cable Interlock LED remains red, check
to make sure the replacement RF amplifier is fully inserted. A
definite resistance should be felt when an RF amplifier is re­moved or inserted in its slot.

Foroptimumperformancethedrivelevelto the PAModules must
becorrect.Itis possible to getan idea ifa replacement PAModule
is operating efficiently by operating the transmitter at full power
with normal modulation for 5 minutes. Shut the transmitter
down,turnofftheLowVoltage at CB1 andCB2andquicklyopen
the interlocked inner door. Compare the temperature of the
heat-sink on the replacement PA Module with other PAModules
in the same area. Ifthe PA Module is hotter than the others, drive
level and phasing should be checked. For Drive Level and
Phasing measurement procedures see “Measuring RF Drive
Level” and “Measuring RF Drive Phasing” paragraphs in the
Troubleshooting section.

5.4.3.1 Handling MOSFET’S

Due to the fragile nature of the gate of a MOSFET, special care
in their handling is required. The gate junction may be destroyed
by static electricity if it is allowed to discharge through the
MOSFET.

NOTE

MOSFET transistors which are in circuit are immune to this

damage.

The MOSFET transistors are shipped in anti-static packaging.
The transistors should remain in this packaging until they are to
be used or tested.

5.4.3.2 Testing MOSFET’s

TheMOSFET’swill have to beremoved fromthe circuitin order
to perform the following test.

Observe the precautions in the paragraph entitled “Handling
MOSFET’S” in this section.

Rev. Z1: 06-02-00 Additional info to ECN 46105 888-2297-002 5-3

WARNING: Disconnect primary power prior to servicing.

DX-25U
The following testapplies toall MOSFET’susedinthetransmit-

ter,but is not necessarily applicableto MOSFET’susedinother
equipment.

The MOSFET’s used in the transmitter may be checked with an
Ohmmeter. However there is a requirement which restricts the
use of some Ohmmeters. If the battery voltage is too low (under
3V) or too high (over 20V) the Ohmmeter cannot be used. A
battery voltage less than 3V will not give an operational check
ofthe transistorand a batteryvoltagegreaterthan 20Vmay result
indamageto the transistorundertest.ASimpson 260, which uses
a 9V battery on the Rx10k scale works quite well.

This test will show how a MOSFET can be switched “on” and
“off” by charging and discharging the gate of the MOSFET.

Connect the positive lead of the Ohmmeter to the drain or case
of the transistor.Connect the negativelead tosource. Alternately
touch a jumper from gate to source and then from gate to drain.
The Ohmmeter should readtowards infinity orat least 2M Ohms
whenthe MOSFET isswitchedoffand less than90k Ohms when
the MOSFET is switched on. (To switch the MOSFET on hard,
near zero Ohms, use +5VDC gateto source signal.) When doing
this test, lay the MOSFET on a flat surface or hold sides of the
case. The resistance of your finger tips and skin will affect the
readings when you touch the leads.

5.4.3.3 Replacing MOSFET’s

When repairing an RF amplifier, it is recommended that all four
MOSFET’s of the failed half of the RF amplifier be replaced.
Even though only one or two of the four MOSFET’s are found
to beshorted, the remaining MOSFET’s may have been stressed
internally and may fail when supply voltage is reapplied. The
repair process would then haveto berepeated which can be very
frustrating. A Blown fuse on one half of the amplifier does not
affect the other half of the amplifier.

MOSFET’s that appear to be undamaged after testing should be
kept as spares for useif newreplacementsare not available.Also
keep in mind that the amplifiers used in the Driver stage and
Power Amplifier stage are identical except that the Driver Mod­ules operate at half voltage. This allows you to rotate a repaired
RF amplifier into a Driver position if so desired.

NOTE

In most cases, the transistor will stick to the heatsink because of
seal created by the transistor pad. This seal will have to be bro­ken before a heatsink can be removed. Pry the transistor out,
away from its heatsink. DO NOT TRY TO PRY THE HEATSINK
AWAY FROM THE PC BOARD WITH TRANSISTORS STUCK
TO THE HEATSINK OR THE PC BOARD MAY BE DAMAGED
AND THE HEATSINK MAY DISTORT. Sometimes the transistor
pad will tear when the seal is broken. Remove stuck pieces and
replace the pad.

a. Remove all the screws from heatsinks and transistors.
b. Remove the heatsinks one at a time starting with the outer

most sink. Break seals on transistor pads as each pair is
exposed.

c. Replace failed transistors. Save and reuse the ferrite bead

on the centerleads of Q3/Q10 and Q4/Q11. Do not solder
leads until heatsinks are in place.

d. Reattach heatsinks in reverseorder as they were removed.

Tightenheatsink and pcboard screws first and thentighten

transistor screws (torque to 3 inch-lbs).
e. Make sure the ferrite beads are on the center leads of

Q3/Q10 and Q4/Q11.
f. Solder transistor leads and trim.
g. Replace blown fuse(s).

5.4.4

RF Drive Splitter Removal

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO­CEDURE.

a. Remove the Driver Compartment rear door.
b. Remove all RF drive cable assemblies from the RF Drive

Splitter.
c. Witha 9/16" socket wrench, removethe boltthat connects

the splitter to the Driver Combiner output rod.
d. Remove thestandoffs for the cable support ring in the four

corners of the splitter and remove the splitter.

5.4.5

RF Drive Splitter Replacement

ReplacementoftheRF DriveSplitteris the reverseoftheremoval
process. Make sure all drive cables are fully inserted and locked
into their sockets.

5.5 Boards Which Require Preset Switch

Settings or Jumper Plug Positions

The following boards have no adjustments, but may havejump­ers or switches that can be preset to match the settings on the
board being replaced.

• Modulation Encoder (A37)

• Controller (A38)

• Driver Combiner/Motherboard (A14)

• Binary Combiner/Motherboard (A8)

• Main Combiner Motherboards (A5-A7)

Modulation Encoder (A37)

5.5.1

When replacing the Modulation Encoder A37, make sure that
binary output jumpers are all in place. Make sure the gold
jumpers for BigStep encoder signals 33 through96 (P-1 through
P-6) arein place. A FlexPatch™ jumper should be in place from
P-15 to P-10. Makesure thatP20 isconnected between J20-1&2
and P21 is connected between J21-1&2. Extra FlexPatch™
jumpers can be stored in P9.

NOTE

To assure the proper connections for J1 through J8, consult
drawing number 839-7855-151.

5-4 888-2297-002 Rev. Z1: 06-02-00 Additional info to ECN 46105

WARNING: Disconnectprimary power prior to servicing.

Section V - Maintenance

5.5.2 Controller A38

Once a new Controller is installed the low voltagecircuit break­ers CB1 and CB2 can be turned on. Check to see that regulator
faultindicator DS1 isnotilluminated. A dc voltmetercan beused
to ensure thatthe regulatorsare operational. Check the following
test points for the indicated voltage.

TP2 ....... +5VDC

TP3 ....... +15VDC

TP7 ....... -15VDC

NOTE

Do not install battery backup BT1 through BT3 until the Control­ler has been installed and power has been applied for at least 1
minute. This will allow time for C44, backup supply capacitor, to
fully charge.

Once the regulator voltages havebeen measured, ensure that the
PA turn off switch S2 is in the ON position (down). The trans­mittercan nowbe turned ONby depressingthe LOW,MEDIUM
OR HIGH button. The power output on all three power levels
will be zero. R eset the transmitter output to the desired powerby
pressing the RAISE button. Referto the Operation Sectionof the
manual for further information.

5.5.3

Binary Combiner/Motherboard, Main Com­biner/Motherboards (A5-A8).

The Binary Combiner/Motherboard contains jumpers J30
through J33 to select the proper amplitude of Binary Steps B-7
through B-10. Set these jumpers to thesame configuration as the
board to be replaced or referto theFactory TestData sheet. Also,
ensure that JP1-JP8 and J30-J33 are configured properly for the
board to be replaced. Check and set the taps on efficiency coils
L1 through L16 on every motherboard to be replaced.

5.5.3.1 Combiner Motherboard Removal

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO­CEDURE.

Allcombinermotherboards are of similarconstruction andthere­fore will require the same basic procedure.

a. Remove all 16 RF amplifiers from the front.
b. Disconnect all wiring and cables from the motherboard

being replaced.

c. Removeonly as muchof the combiner cover as necessary.

2. On the Driver/Combiner motherboard, the secondary
rod will have to be removed through the top of the
transmitter.If top removal is not possible becauseof an
overhead clearance problem, remove the RF Drive
Splitter and lower the rod down to where it will rest on
T1. Continue with therest ofthe motherboardremoval.
Tilt the motherboard back and continue to slide the rod
down and forward across the top of T1.

e. Remove the motherboard fastening hardware beginning

with the two 4-40 screws in the front center card guide
support. The 4-40 screws to be removed can be identified
by the six inch (15cm)aluminum rods attachedto the card
guide support bar.

f. Carefully remove the motherboard from the rear of the

transmitter.

5.5.3.2 Combiner Motherboard Installation

Replacement of the MainCombiner/Motherboards is essentially
the reverse of the removal procedure.

a. During installation of a motherboard, it may not appear to

fit in as easily as it came out. This is due to the blue card
guides not fitting back in their slots at the same time.

1. Install the board, using only a few of the screws to
mount the board to the supports.

2. From the front of the RF Amp Compartment, place the
card guides into their respective slots.

b. Once the motherboard has been fully mounted, insert the

allen screws that bolt the Combiner rods together but do
not fully tighten.

c. Loosen the two set screws in the fiberglass supportson the

motherboard that hold the rod in place. Now tighten the
Allen screws on the copper rod to 150 inch/lbs.

d. Re-tighten the setscrews on the motherboard. Replacethe

Combiner cover, RF amplifiers, and the interconnection
plugs.

NOTE

Replace all combiner cover screws. The majority of combiner RF
ground current flows through the combiner covers.

5.5.4 Driver Combiner/Motherboard A14

The Driver Combiner/Motherboard has no adjustments. There
are taps on efficiencycoils L2-L15 that needto beplaced intheir
proper frequency determined location. Set these coil taps to the
same configuration as the board to be replaced or refer to the
Factory Test Data sheet.

CAUTION

LOCATE AND REMOVE ANY HARDWARE THAT IS DROPPED. IF
LOST HARDWARE IS REPLACED, MAKE SURE NONE OF IT HAS
LODGED ON ANY RF AMPLIFIER MODULE.

d. Depending on which board isbeing removeddo one of the

following two steps.

1. On the Main and Binary motherboards, remove thetwo
end screws from the combiner secondary rod.

Rev. Z1: 06-02-00 Additional info to ECN 46105 888-2297-002 5-5

WARNING: Disconnect primary power prior to servicing.

5.6 Printed Circuit Boards Which Re-

quireAdjustments

The remaining boards in the transmitter haveadjustments which
must be checked and possibly preset before applying high volt­age. Some controls may need further adjustment after applying
high voltage. The following paragraphs describe these boards
and adjustments.

DX-25U
These boards are:

• Analog to Digital Converter (A34)

• Analog Input Board (A35)

• Oscillator (A17)

• Driver Supply Regulator (A22)

• DC Regulator (A30)

• Output Monitor (A27)

• LED Board (A32)

• Driver Encoder/Temp Sense Board (A19)

• Switch Board/Meter Panel (A31)

5.6.1

Analog to Digital Converter (A34)

The Analog to Digital Converter contains two adjustments, two
DIP switches, andtwo sets ofjumpers. The firststepin replacing
the Analog to Digital Converter is to make sure that switches S1
and S2 are set the same as the board to be replaced. S1 sets the
A/D sample phasing and is critical to the proper operation of the
transmitter. Set jumper P10 and P11A/B to the same settings as
on the board to be replaced.

5.6.1.1 Delay Adjustment: R78

The delay adjustment is normally set during factory testing of
the board but can be checked and adjusted if needed using the
following procedure:

a. Once the new Analog to Digital Converter is installed,

apply low voltage to the transmitter and verify that all
LED’s on the ColorStat™ panel are green.

b. Locate the PA TURN-Off switch S2 on the Controller and

move it to the OFF (up) position.

c. DepresstheLOW power button on thefront panel andnote

that the transmitter completes the step-start sequence and
that the +230 Vdc supply energizes.

d. The PA OFF indicator LED of DS1 on the Modulation

Encoders (A37) should be illuminatedand thereshould be
no rf output.

e. Connect a scope with a minimum bandwidth of 30

MHz to TP3 on the Analog to Digital Converter and
ground the probe on TP19, 20, or 21. This is the
conversion pulse for the A/D converter IC. The width
of the positive portion of th e pulse should be approxi­mately 40ns.

f. Adjust the pulse to the correct width with R78. The Con-

versionErrorLED DS1 onthe AnalogtoDigitalConverter
should be green.

Table 5-1. Analog Input Board A35 Preset Controls

5.6.1.2 Offset Adjustment: R7

The Offset adjustment i s set during factory testing of the board,
but can be adjusted if needed. The most significant affect of the
offset adjustment is on modulation tracking. In other words,
equal modulation percentage at all power levels. To check the
setting of the Offset adjustment:

a. Operate the transmitter at 25 kW and modulate with a

100Hz tone at 95%.

b. Operate the transmitter at 5.0 kW and measure thepercent of

modulation. If it is within 1%of the level noted at 25 kW, no
further adjustment of the Offset control is necessary .

c. If the modulation is not within 1% of the level noted at 25

kW, adjust R7 to bring the modulation level within 1%.

d. Operate the transmitter at 25 kW and note the percent of

modulation. Adjust R7 if necessary.

This adjustment will affect the power output on all power set­tings, but will have the most affect at low power. Normally,
satisfactory modulation tracking should be obtained within two
turns of where the control was previously set.

5.6.2

Analog Input Board (A35)

The Analog Input Board has five adjustments that are preset
during factory tests. It is normally advisable to recheck these
settings using the procedures given here, however it may be
necessary to install the board quickly to return the transmitter
to the air. In this case, the adjustments can be set to the same
resistance valuesas o n the board tobe replaced and acomplete
set-up procedure can be p erfor med l ater. This procedure as­sumes the controls on the board to be replaced have not been
changed from their factory settings. The controls to be preset,
and the most convenient measurement points given are s hown
in Table 5-1.

5.6.2.1 Dither Frequency Adjust: R41

Dither Frequency adjustment is factory set and should not need
any further adjustment. To check the dither frequency:

a. Connect a frequency counter to TP10.
b. Adjust R41 fora nominal frequency of 72 kHz. This is not

critical and can vary anywhere from 70 kHz to 74 kHz.

5.6.2.2 Maximum Power Adjust: R27

a. Turnon the transmitter atLOW power with nomodulation.

If the Maximum Poweradjust is set correctly the transmit­ter should come up at the previously set low power level.

b. If it does not, adjust R27 for the correct low power output.

CONTROL FUNCTION MEASUREMENT POINTS RECORDED VALUE
R15 AUDIO GAIN ADJ. U6-2 to R16 (Left side)
R27 MAX. POWER ADJ. TP3 to ground
R41 DITHER FREQ ADJ. U3-2 to junction of R38, R39, CR11
R43 DITHER LEVEL ADJ. TP9 to ground
R84 OFFSETADJ. R83 (right side) to ground
R85 GAINADJ. U5-5to ground

5-6 888-2297-002 Rev. Z1: 06-02-00 Additional info to ECN 46105

WARNING: Disconnectprimary power prior to servicing.

Section V - Maintenance

c. Depress the HIGH power button. If HIGH power was

previouslyset for 25kW,the transmitter output shouldnow
be approximately 25 kW.

d. Depress the RAISE button until the power output reaches

30 kW or stops raising.

1. If themaximum poweroutput isless than30 kW, adjust
R27 for 30 kW output.

2. If the power output exceeds 30 kW, adjust R27 so that
the transmitter power will not exceed 30 kW.

5.6.2.3 Offset Adjustment: R84 (Modulated B-)

a. Connect a scope to the modulated B- output at A30 TP30

of the DC Regulator.

b. Operate the transmitterat 5.0 kWandmodulate 100% with

a 100 Hz sine wave.

c. Set up the scope at 1.0 Volt per division to measure a dc

coupled audio waveform.You should view adistorted sine
wave of approximately 2.0 Vp-p on a -3.0 VDC negative
offset. See Figure 5-7.

d. Adjust R84 so that the positive peak of this waveformjust

begins to clip, then back off the control slightly. Note that
the waveform will reach the clip point as it moves more
positive. This positive peak of audio corresponds to the
modulation envelope negative peak.

5.6.2.4 Audio Gain Adjust: R15

The Audio Gain Adjust is normally factory preset for 100%
modulation with an audio input level of +10.0 dBm. It can be
adjustedfor 100%modulationwith audioinput levels from -10.0
dBm to +10.0 dBm. To adjust:

a. Operate the transmitter at the desired output power and

slowly increase the output of the audio generator to the

desired level.
b. Adjust R15 for 100% modulation.
c. TheRF outputmay vary whenadjusting thiscontrol. Once

R15is adjusted, thepoweroutputmay need to bereset with

the RAISE and LOWER buttons.

5.6.2.5 Dither Level Adjust: R43

The Dither control is preset and there should be no need for
readjustment. If it is desired to check the setting of the control,
use the following procedure.

a. Operate the transmitter at approximately 1.0 kW output

and modulate with a 100 Hz tone at 95%.

Table 5-2. Oscillator A17 Preset Jumpers

b. Use a scope to display one cycle of demodulated audio

from the modulation monitor.

c. Expand the vertical sensitivity of the scope to display only

a portion of the waveform.

d. Adjust R43 maximum counterclockwise. At this point it

should be possible to see some of the individual Digital
Modulation voltage steps. (It may be possible to see the
steps better at a lower modulation level,but it may also be
necessary to externally sync the scope with the audio
generator).

e. While observing the individual steps, adjust R43 clock-

wise until the individual steps can no longer be distin­guished. This should occur within two turns of R43. Ad­ditional clockwise adjustment of the control may appear
to furthersmooth out the steps but will result in additional
noise on the waveform.

f. Only increase the Dither Level enough to just smooth out

small step transitions. Other, slightly larger, steps or
glitches will be seen at low power and modulation levels.
This is normal. Do not use R43 to try and smooth these
out. Never use more than three clockwise turns of R43.

5.6.3

Oscillator (A17)

Preset the replacement board before installation by placing all
jumpers in the same positions as in the board to be replaced.
These are identified in Table 5-2.

a. Set S1, a four section DIP switch, to the same setting as

the board to be replaced.

b. Adjust the tuning slug of L4 with a non-inductive tuning

toolfor approximately thesame amountof penetrationinto
the coil.

c. Removethe heater assemblies from crystals Y1 and Y2.An

angle bracket bolts to the PC board and holds the crystal
heaters in place.

d. Carefully remove the crystalsand reinstallthem onthenew

board. Install the heater assemblies on each crystal.

5.6.3.1 Carrier Frequency Adjust: C1/C3

a. Select crystal oscillator and heater Y1 by movingjumpers

J1andJ6toposition1-2.
b. Turn the low voltage on for 15 to 20 minutes.
c. Connect a frequency counter to BNC connector J5 and

adjust C1 with a non-inductive tuning tool for the correct

carrier frequency.

JUMPER FUNCTION
P1 CRYSTAL SELECT
P2 CRYSTAL FREQUENCY DIVIDER SELECT
P3 INTERNAL/EXTERNAL OSCILLATOR SELECT
P4 SINGLE/COMBINED TRANSMITTER SELECT
P5 EXTERNAL INPUT TERMINATION SELECT
P6 CRYSTAL HEATER SELECT

Rev. Z1: 06-02-00 Additional info to ECN 46105 888-2297-002 5-7

WARNING: Disconnect primary power prior to servicing.

DX-25U

d. To set the output frequency of crystal Y2, turn off the low

voltage and move jumpers J1 and J6 to positions 1-3.
Repeat the above procedure by adjusting C3.

5.6.3.2 Oscillator Sync Adjustment: S1/L4

TheOscillator Syncadjustment is criticalto theproper operation
of the VSWR circuitry. If the circuit is not adjusted properly,
damage to the RFamplifiers could result during a VSWR condi­tion.

a. Using a dual trace scope connect channel one to TP5 and

channel two to TP4 on the Oscillator. Sync the scope to
channel one.

b. Operate the transmitter at full power and note a 5.0 Vp-p

square wave at the carrier frequency on channel one.
Channel two will also have a 5.0 Vp-p square wave dis­played.

c. Adjust the scope to display one or two cycles of RF. If the

positive going edges of the two waveforms are lined up,
no further adjustments are required. Refer to Figures 5-9
and 5-10, Oscillator Sync waveforms.

d. If the two waveforms are not in phase and adjusting L4

does not line up the positive going edges, then different
combinations of capacitance can be switched in by S1.

e. When switching in different values of capacitance use the

least amount necessary to achieve phase alignment of the
two signals.If too muchcapacitance is used there may not
be enough signal input at TP4.

f. Operate the transmitter at 5.0 kW and make sure there is

still a signal present at TP4. Thetwo signals may not beas
well aligned as at full power.

5.6.4

Driver Supply Regulator (A22)

The Driver Supply Regulator sets the proper voltage to Driver
8A and 8B.Twocontrols and one switchmust be set properly for
correct transmitteroperation. If theRegulator is notoperational,
note DRIVER8A voltage on the Factory Test Data sheet and the
normal transmitter log reading.

5.6.4.1 Removing The Driver Supply Regulator Assembly

a. Turn off the primary AC power at the main disconnect.
b. Remove the clear plastic safety cover over the Driver

Supply Regulator.

c. Disconnectall cables fromthe assembly.Removethe bolts

holding the assembly to the transmitter wall.

5.6.4.1.1 Removing Printed Circuit Board From The Heat Sink

The printed circuit board is mounted on the heat sink, using six
screws and spacers. The seven MOSFET’s are soldered to the
printed circuit board, and are mounted on the heat sink using
screws, compression washers, and insulator pads.

Table5-3. DriverSupplyMeasurements

To remove the printed circuit board, the seal between the transis­tors and the insulator pads will have to be broken. Use a long
knife or ice pick to slide underneath the pc board to pry the
MOSFET off the pad. The pad may tear or peel when the seal is
broken. Always replace damaged pads.

5.6.4.2 PresetAdjustments

To prevent drive overloads, it is recommended that the two
adjustments be presetby measuring the resistanceof the controls
on the board to be replaced. The most convenient measurement
locations are shown in Table 5-3.

NOTE

Before proceeding with any adjustments, determine that the AC
line voltage is at normal voltage levels. If the voltage is either
higher or lower than normal, recheck your adjustments when the
AC line has returned to normal.

5.6.4.2.1 Open Loop Adjust: R2

a. SetS1 on theDriverSupply Regulatorto theOPEN LOOP

position.

b. On the Controller,switch PA TURN-OFFswitch S2 tothe

OFF (up) position.

c. Turn on Low Voltage at CB1 and CB2 and depress the

LOW power button. The +230 VDC supply should be
energized but there should be no RF output.

d. Note DRIVER D8A voltage. If the voltage is close to the

normal voltage or to the recorded voltage on the Factory
Test Data sheet, then no further adjustment of the Open
Loop control is necessary.

e. If adjustment is necessary, use an insulated tuning tool to

adjust R2 for normal operating voltage on D8A.

5.6.4.2.2 Closed Loop Adjust: R12

a. Switch S1 to the C LOSED LOOP pos ition.
b. Adjust R12 (Closed Loop Adjust) so that DRIVER D8A

voltage is the same as the Open Loop voltage.

c. Return the PA TURN-OFF switch to the PA-ON position

and readjust R12 for the correct reading at normal power
output.

5.6.5

DC Regulator (A30)

The DC Regulator has two adjustments which should be preset
before the transmitter PA Powersupply is turned on. Refer to the
Factory Test data sheet for the LCD Multimeter readings on the
DC Regulator.

a. Place P1 in the TEST position.
b. Turn on the Low Voltage supply with CB1 and CB2 and

monitor TP8 with an external meter. Adjust for +2 VDC
with R1.

CONTROL FUNCTION MEASUREMENT P OINTS MEASURED RESISTANCE
A22R2 OPEN LOOP ADJ U2-2 to ground
A22R12 CLOSED LOOP R14(Left side) to R13(Left)

5-8 888-2297-002 Rev. Z1: 06-02-00 Additional info to ECN 46105

WARNING: Disconnectprimary power prior to servicing.

Section V - Maintenance

c. Adjust the voltage of “REG B B- OUT” with R93 and

“REGAB-OUT” with R51 for approximately 2.5 VDC
with the Low Voltage ON.

d. Turn the transmitter ON and modulate 100% with a 1 kHz

tone at 25 kW.

e. Adjust the “A”and “B” voltages as needed to match those

recorded on the Test Data Sheet. These voltages will nor­mally be between -4.5 and -5.3 Volts.

NOTE

Place P1 in the NORMAL position after adjustments are com­pleted. This is important to prevent overheating of the regulator
transistors whan the transmitter is OFF and there is no air flow
through the compartment.

5.6.6 Output Monitor (A27)

The Output Monitor performs three main functions:

• Forward and reflected power metering

• VSWR overload sensing

• Modulation monitor sample level adjustment

All of these functions must be calibrated for proper transmitter
operation. Set all jumpers and switches listed in Table 5-4 to the
same position as on the board to be replaced.

Since all of these circuits require adjustment while the transmit­ter output network is set to 50 + j0 Ohms, it is preferred that the
transmitter be operated into a 50 Ohm load. This procedure can
be performed into the antenna, but operating the transmitter into
a load will make measurements easier due to the lackof interfer­ence, compared to that existing on the antenna system.

5.6.6.1 DETECTOR NULL (Antenna) Adjustment

a. Set the PA TURN-OFF switch S2 on the Controller to the

OFF (up) position.

b. Depress the LOW power button. The PA Supply voltage

should be present but no power should beindicated on the
Forward Power meter.

c. Depress and hold the LOWER button for approximately

30 seconds.

d. Set the PA TURN-OFF switch S2 on the Controller to the

ON (down) position and hold the RAISE button until the
transmitter output power is approximately 2.5kW.

e. Using a Dual trace scope, connect a 10x probe on channel

1 to TP6 and a 10x probe on channel 2 to TP5. A signal
should be visible at both TP6 and TP5.

f. While depressing momentary button switch S5, set the

Normal/Calibrate switch S8to the Calibrateposition. Note
that the signal at TP5 has dropped in amplitude.

g. Adjust capacitor C29 for minimum signal at TP5. This

signal will contain mostly harmonics of the carrier fre­quency. It may be necessary to add additional capacitance
with S9-1 andS9-3 atthe low endof the band or additional
inductance with S9-2 and S9-4 at the high end of the band
to achieve a minimum signal.

h. Set theNormal/Calibrate switch S8to theNormal position

and release momentary pushbutton switch S5. Make sure

that the vertical sensitivity of both channels of the scope

are the same.
i. Connect both scope probes to TP6 to ensure that both

traces are the same amplitude. Return the other probe to

TP5.
j. Set the time base on the scope to display 2 to 3 cyclesof RF.
k. Adjust C15 to make the signal at TP6 the same amplitude

as TP5. Note that the two signals are probably not in phase

with each other. See Figure 5-4.
l. Using a non-inductive tuning tool, adjust L12 to phase

align the two signals. It may be necessary to readjust C15

to make the two signals equal in amplitude. Note that it

may not be possible to get both signals equal in amplitude

using C15 until some adjustment of L12 is made.
m.If, by adjusting L12, it is not possible to align the two

signals in phase, select a different value of capacitance

across L12 byswitching in oneor moresectionsof S6then

readjusting L12 for an in phase signal.
n. Note that as the amplitude and phase of the two signals are

matched, the meter reading in the DETECTOR NULL

(Antenna) position will null. Fine adjustments of these

controls will be made at full power once the Bandpass

Filter controls are set.

5.6.6.2 DETECTOR NULL (Bandpass Filter) Adjustment

a. UsingaDualtracescope connect a 10x probefrom channel

1 to TP10 on the Output Monitor. Connect a 10x probe

from channel 2 to TP1. A signal should be visible at both

TP1 and TP10.
b. While depressing the momentary pushbutton switch S5,

set the Normal/Calibrate switch S8 to the Calibrate posi-

tion.Notethat the signalatTP10 has droppedinamplitude.
c. Adjust capacitor C21 for minimum signal at TP10. Also

note that the minimum residual signal will contain mostly

harmonics of the carrier frequency.
d. If a minimumcannot be achieved due to the capacitorC21

running out of range, use S1 to select a different value of

capacitance (C3 or C5), or a different value of inductance

(L2 or L3) to null out the signal at TP1. Note that some

frequencies may not require any added reactance. Nor-

mally capacitance is added at the low endof the frequency

band and inductance is added at the high end of the band.

Table 5-4. Output Monitor A27 Jumper/Switches

JUMPER/SWITCH FUNCTION
P3 Directional Coupler select
P1 Directional Coupler select
S8 Normal/Calibrate
S6 Antenna VSWR Phasing
S7 Bandpass VSWR Phasing
S2 Bandpass VSWR Amplitude
S9 Antenna Null Detector Resonance
S1 Bandpass Filter Null DetectorResonance

Rev. Z1: 06-02-00 Additional info to ECN 46105 888-2297-002 5-9

WARNING: Disconnect primary power prior to servicing.

DX-25U

e. SettheNormal/Calibrate switch S8 totheNormalposition,

and release momentary pushbutton switch S5. Make sure
that the vertical sensitivityof both channels of thescope is

the same.
f. Set the time base on the scope to display 2 to 3 cycles ofRF.
g. Adjust C16 to make the signal at TP1 the same amplitude

as TP10, and also note that it may not be possible to get

both signals equal in amplitude using C16 until some

adjustment of L5 through L8 (selected by S7) is made.

Capacitance can be added with S2 if the signal cannot by

nulledwithC16.
h. Note that the two signals are probably not in phase with

each other.See Figure 5-5.
i. Using a non-inductive tuning tool, adjust L5 through L8,

depending on which one is selected by the DIP switch S7,

to phase align the two signals. It may be necessary to

readjust C16 to make the two signals equal in amplitude.
j. If, by adjustingthe selectedvariableinductorL5-L8, is not

possible to align the two signals in phase, select another

value of variable inductance with S7. Note that as the

amplitude and phase of the two signals are matched the

DETECTOR NULL (Filter) position on the Multimeter

will also null.

5.6.6.3 Fine Tuning

a. Withthe transmitter operating at2.5kW,both the DETEC-

TOR NULL (Antenna) and the DETECTOR NULL (Fil-

ter)positions onthefront panel multimetershould indicate

near zero.
b. Toprevent possible modulationmonitor damage, turnboth

the MEDIUM and HIGH power modulation monitor ad-

justment controls R7 and R8 full CCW.
c. Bring the transmitter to 25 kW and note the DETECTOR

NULL(Antenna) position onthe multimeter.Ifthereading

is now above zero, null this reading using both C15 and

L12.
d. Note the DETECTOR NULL (Filter) indication on the

multimeter. If it is above zero, null it using C16 and L5

through L8, depending on what was selected by S7. The

final adjustments will be made into the antenna at full

operating power.
e. Modulate the transmitter with a 10 kHz tone, or one which

causes the greatest upward deflection on the DETECTOR

NULL (Antenna) meter reading, and recheck nulls.
f. Use a digital voltmeter or a dc coupled oscilloscope and

adjust for minimumvoltage at TP8andTP9 withreference

to ground.

5.6.6.4 Trip Threshold Adjustment

The overload settings for the Antenna and Bandpass circuit are
listed in theFactory Test Data sheet. Afterthe replacement board
has been installed, set the overload settings as follows:

5.6.6.4.1 Antenna VSWR overload

a. Turn on the Low Voltage at CB1 and CB2.
b. Connect a voltmeter to TP4.

c. AdjustR24 until thevoltagematches the FactoryTestData

sheet.

If the Factory Test Data sheet is unavailable or if it is necessary
to verify the original overload setting, use the following proce­dure:

a. Verify that the DETECTOR NULL (Antenna) reading

on the front panel multimeter is nulled (zero) at full

power.
b. Press LOW power, and adjust the RF output for 3.5 kW.
c. Depress the OFF button.

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO­CEDURE.

d. Remove the rear panels from the Output Network Com-

partment.
e. Reverse the Antenna VSWR current sample by placing

P1 from 1-2 and P2 from 2-3 on the Output Sample

Board.
f. Replace the rear panels on the Output Network Compart-

ment.
g. Restore primary AC voltage at the main breaker.
h. Depress the LOW power button.
i. Switch the front panel multimeter to the DETECTOR

NULL (Antenna) position. Note that the meter reads up-

scale.
j. Adjust R24 until the transmitter indicates an ANTENNA

VSWR condition on the ColorStat™ panel.
k. Turn the transmitter OFF.

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO­CEDURE.

l. Remove the rear panels from the Output Network Com-

partment.
m.Place P1 and P2on theOutput SampleBoard inthe Normal

position.
n. Replace the rear panels on the Output Network Compart-

ment.

5.6.6.4.2 Bandpass VSWR Overload

a. Turn on the Low Voltage at CB1 and CB2.
b. Connect a voltmeter to TP10.
c. Adjust R23 for 1.0 VDC for a DX-50; 0.8 VDC for a

DX-25U.

5-10 888-2297-002 Rev. Z1: 06-02-00 Additional info to ECN 46105

WARNING: Disconnectprimary power prior to servicing.

Section V - Maintenance

5.6.6.5 Forward/Reflected Power Adjustments C6 and C40

a. With the transmitter operating at 25 kW and no modula-

tion, read theReflectedpowerindication on thefront panel
meter.

b. Adjust C40, Reflected Balance control, to null the meter

indication. Note that C30 is added by P2 at the low end of

the band to allow the meter indication to null.
c. Depress the OFF button.
d. Locate P1 and P3 on the Output Monitor. Move both

jumper plugs from position 1-2 to position 1-3.
e. Turn the transmitter back on at full power. Note that the

Reflected meter positionnow indicates forwardpowerand

the Forward meter positionnow indicates reflected power.
f. Operate the Forward/Reflected meter switch to the For-

ward power position. Adjust C6 to null this indication.
g. Depress the Off button and move jumpers P1 and P2 to

position 1-2.

5.6.6.6 Modulation Monitor Sample Adjustments

Refer to the Initial Turn-On procedure in SECTION II, Installa­tion/Initial Turn-On, for the procedure to set the Modulation
monitor sample adjustments.

5.6.7

LED Board (A32)

The LED Board contains five overloads which can be preset to
the correct reference voltage or resistance before the board is
replaced. The Factory TestData sheets list the voltage setting for
each overload except the Power Supply overload.

If the FactoryTestData information is not available, measure the
test pointvoltages on the original boardbefore removingit from
the transmitter. Use Table 5-5 to record the voltages for future
reference.

5.6.7.1 LED Board Replacement

To replace the LED Board:

a. Turn off the Low Voltage supply at CB1 and CB2.
b. Remove all cables from the LED Board.
c. Remove the pushbutton caps from the VSWR Self Test

switchS2andtheResetswitchS3ontheColorStat™

panel.
d. Remove the screws holding the LED Board to the Center

Control Compartment door and remove the board.
e. Using an Ohmmeter,measure the resistance from theright

side of R92 to ground. Record this measurement in Table

5-6 for future reference. Adjust R86 on the replacement

LED Board for the same resistance reading.
f. Install the replacement LED Board.
g. Turn on the Low Voltage supply at CB1 and CB2.
h. Using a digital multimeter, set the following test point

voltages to match those recorded on the Factory Test Data

sheets:

1. TP6 (R42): Average Current Overload

2. TP5 (R41): OverdriveOverload

3. TP7 (R68): Peak Current Overload

4. TP8 (R67): Underdrive Overload

If it is not possible to preset the replacement board voltages or if
the correct overloadoperation needs to be verified,the following
procedure for setting each overload should be used.

5.6.7.2 Overload Adjustment Procedures

The following proceduresare used to set individualoverloadson
the LED Board.

5.6.7.2.1 Drive Overloads

These overloads protect the RF amplifiers from drive levels
below 20.0 Vp-p or above27.0 Vp-p. The nominal drive levelis
21 to 25.0 Vp-p, measured at the MOSFET gate on the RF
amplifiers. The first step in setting the drive overloads is to
remove the supply voltage to all the RF amplifiers so that no
damage will occur while the drive level is varied. Next the drive
level will be varied to the overload limits and the overloads will
be set.

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO­CEDURE.

a. Remove all AC primary power from the transmitter at the

AC main breaker.

b. Remove the RF Amp Compartment and Driver Compart-

ment rear access panels.

c. Removethe following fuses:

1. A25: F1-F8

2. A24: F1-F9

3. F20: (on top of T1)
d. Reinstall the access panels.
e. LocateBig StepRF amplifierRF33 behindthe interlocked

RF Amp Compartment access door.

f. Connect10x scope probe with an extended tip, Harris part

610-1131-000, through the door to the anode of CR3 in
front of the heatsink. Ground the probe to door.

g. Set up the scope to measure an RF waveform of approxi-

mately 23.0 Vp-p.

h. Restore AC primary power at the main breaker.

NOTE

When measuring RF amplifier drive amplitudes or phasing, the
RF amplifier to be measured must be turned “ON” to give a
correct drive measurement. The drive waveform of an “OFF” RF
amplifier will be below 0.0 VDC and the peaks may be clipped.

Table 5-5. LED Board Preset Voltages

TEST POINT CIRCUIT FUNCTION MEASURED DC

VOLTAGE
TP5 OVERDRIVE
TP8 UNDERDRIVE
TP7 PEAK CURRENT
TP6 AVERAGE CURRENT

Rev. Z1: 06-02-00 Additional info to ECN 46105 888-2297-002 5-11

WARNING: Disconnect primary power prior to servicing.

DX-25U

i. Toturn onan RF amplifier,depress the LOWpowerbutton

and note that the PA Power supply voltage comes up as
indicated on the front panel multimeter but no RF power
or PA current is indicated.

j. Depress the RAISE button to illuminate the green LED

indicator on RF33.

k. Measure the peak-to-peak drive level on the scope moni-

toring the drive.

l. The waveform shouldmeasure from 21.0to 25.0 Vp-pand

it should be centered on the 0.0 VDC line of the scope.

m. If the waveform falls totally below the 0.0 VDC line of the

scope, the Step 1 RF amplifier is turned “OFF”.See
Figures 5-3 and 5-4 for drive waveforms.

n. Record the reading on the RF Multimeter position for

DRIVERD8Aand D8B. The DriverSupply Regulatorwill
be set back to this voltage once the overloadsare adjusted.

5.6.7.2.2 Underdrive Overload: R67

To adjust Underdriveoverload R67, the drive mustbe reduced to

18.0Vp-p.
a. To reduce the drive, depress the OFF button and allow the

PA Voltage to discharge.

b. Open the interlocked door inside the DriverCompartment

and remove Driver Module D1.

c. Remove both supply voltage fuses and re-insert the RF

amplifier into position.
d. Close the interlocked door and depress the LOW button.
e. Driver D8A voltage will be higher than normal and there

may be a voltage reading on the D8B position.
f. Measure the drive level at RF33 on the scope. If the drive

level is above 18.0 Vp-p, repeat the above procedure for

Driver Module D2.
g. Continue removing fuses from Driver M odules D3

through D5 until the level decreases to 18.0 Vp-p.
h. When a level of 18.0 Vp-p is achieved, adjust R67 on the

LED Board untilthe transmitterturns OFF anddisplays an

Underdrive Fault on the ColorStat™ panel.
i. Replace all fuses removed from the Driver Modules to

restore the Driver stage to the correct output.
j. Press the LOW power button. The transmitter PA Power

supply should energize, there should be no power out, and

the drive level to the PA Modules should be the same as

measured originally.

5.6.7.2.3 Overdrive Overload: R41

Toadjustthe OverdriveOverload, the RF drivemust be increased
to 26.0Vp-p.

a. To adjust the Overdrive overload, place switch S1 on the

DriverSupply Regulator in the OPEN LOOP position and

Table 5-6. LED Board Preset Resistances

note the voltage of D8A and D8B on the RF MULTIME­TER.

b. Adjust R2 onthe DriverSupply Regulator clockwise (CW)

until the drive level reaches 26.0 Vp-p. If the voltage will
not reach 26.0Vp-p, place S1 on theDriverEncoder/Temp
Sense Board tothe ON position. This willturn on thespare
Driver Module D6.

c. Adjust the OverdriveoverloadR41 ontheLED Boarduntil

the transmitter turns OFF and an Overdrive overload is
indicated on the ColorStat™ panel.

d. Turn the OPEN LOOPadjustment R2two turnsCCW and

return S1 on the Driver Encoder/Temp Sense Board to the
OFF position if moved.

e. Depress the LOW power button. The PA Power supply

should energize andthere should be no RF output fromthe
transmitter.

f. Readjust R2 to the same voltage as earlier recorded on the

multimeter DRIVER D8A and place S1 in the CLOSED
LOOP position. The RF drive level should be the same as

first noted.
g. Remove the 10x scope probe from the RF amplifier.
h. Turn the transmitter OFF and note that PA Voltage de-

creases to zero on the multimeter.
i. Remove primary AC power from the transmitter at the

main breaker.

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO­CEDURE.

j. Remove the Driver Compartment and RF Amp Compart-

ment rear access panels and replace all the P A Power

Supply fuses.
k. Replace all panels and restore primary AC power to the

transmitter at the main breaker.

5.6.7.3 Peak Current Overload: R68

a. Turn the PEAK current overload R68 fully CCW.
b. Operate the transmitter at 25 kW and modulate at 100%

with a 20 Hz sine wave. Increase the audio level 1.4 dB.

Adjust the PEAK current overload R68 for an overcurrent

trip.

5.6.7.4 Average Current Overload: R42

a. Operatethe transmitterat 25kW andmodulate with20 Hz

at 100%.
b. Increase modulation 0.5 dB.

CONTROL FUNCTION MEASUREMENT POINTS MEASURED RESISTANCE
R86 POWER SUPPLY FAULT R92(Right side) to ground

5-12 888-2297-002 Rev. Z1: 06-02-00 Additional info to ECN 46105

WARNING: Disconnectprimary power prior to servicing.

Section V - Maintenance

c. Adjust the AVERAGE CURRENT overload R42 until the

OVERCURRENT LED lights AMBER.
d. Modulate the transmitter with 100 Hz at 100%.
e. Increasethe audio in1 dB steps.The PA current shouldnot

increase further than near full scale on the PA current

meter.
f. Further increase the audio level until the transmitter shuts

off and recycles back on. If the audio level is not reduced,

the transmitter may shut OFF and stay OFF at this time.
g. The Overcurrent LEDwillilluminate RED andcan be reset

from the ColorStat™ panel.
h. Turn the transmitter back ON and verify that the transmit-

ter takes at least +10 dB of audio overdrivebefore shutting

OFF.

5.6.7.4.1 Average Current Overload Test

a. Modulate the transmitter 100% with 400 Hz sine wave.
b. Verify it will continue to operate with at least +10.0 dB

over 100% audio overdrive.
c. The transmitter should trip OFF from a Current Overload

with between 10.0 and 15.0 dB of overdrive.
d. The supply current should not exceed 250 amperes during

the test.

5.6.7.4.2 Program Modulation Test

If OVERCURRENT overloads occur during program modula­tion conditions, the PEAK CURRENT OVERLOAD may be
backed off 1-2 turns, but the voltage at TP7 must NOT exceed

11.5 VDC.

5.6.7.5 Power Supply Protection Overload R86

a. Operate the transmitter at maximum output power. Modu-

late the transmitter with 120 Hz at 100% modulation.
b. Increase the audio modulation 0.5dB (6%). Note: Use

100Hzmodulation if operatingat50Hz ACline frequency.
c. Adjust R86 clockwise until the transmitter shuts OFF with

a Power Supply Protection Overload. Note hat the Supply

Fault LED on the ColorStat™ panel is RED and that the

transmitter will not recycle for this fault.
d. Adjust R86 1/4 turn counter-clockwise.
e. Depress the reset button to reset the fault indicator to

GREEN.

NOTE

It is not required to depress the RESET button on the Color­Stat™ to restart the transmitter. The RESET button only clears
the fault indication.

f. Depressthe HIGH powerbutton to operate thetransmitter

at maximum output power.
g. Modulate the transmitter at 100% with 120/100 Hz. The

transmitter should not trip OFF witha PowerSupply fault.

5.6.8

Driver Encoder/Temp Sense Board (A19)

If the powersupply circuits onthe original boardare operational,
measure the following Test Point voltages on the original board
and adjust the new board to the same voltages:

• TP1 (R17): Driver Threshold Reset

• TP2 (R19): Driver Threshold ON

• TP3 (R49): Temp Cal

• TP4 (R50): Step 1 Temp

• TP5 (R51): Step 2 Temp

• TP6 (R60): Temp Thresh

• TP11 (R98): Air Flow

After the replacement board voltages have been calibrated, en­sure that JP1, JP2 and JP3, J5, S1 and S2 are in the correct
position to correspond to the original board. The transmitter is
now ready for operation.

If it is necessary to calibrate the replacement board, use the
following procedure:

5.6.8.1 Over Temperature Circuits

Set the Over Tempera ture circuits on the Driver Encoder/Temp
Sense Board as follows:

NOTE

The transmitter should be OFF and at room temperature when
the Over Temperature circuits are calibrated.

a. Adjust R49 to set TP3 for 2.73 VDC.
b. Determine the ambient temperature in degrees centigrade

and multiply this value by 0.1 (25 degrees C X 0.1 = 2.5).

c. Adjust R50 to set this value at TP4, and R51 to set this

value at TP5.

d. Set the voltage at TP-6 to 7.0 VDC with R60.

5.6.8.2 Auto Driver circuits

The Auto Driver circuit should turn the Auto Driver Module D7
ON when the D8B voltage on the RF MULTIMETER reaches
maximum, approximately +115 VDC. The Auto Driver circuit
should turn the Auto Driver Module D7 OFF when the D8A
voltage on the RF MULTIMETER reaches zero.

Set the Auto Driver circuit thresholds on the Driver En­coder/Temp Sense Board as follows:

a. Turn the transmitter ON at LOW power.
b. Make sure S2 on theDriver Encoder/TempSense Board is

in the AUTO position

c. LocatetheDriverSupply Regulatorin the DriverCompart-

ment. Move S1 to the OPEN LOOP position.

d. Record the D8A and D8B voltage on the RF MULTIME-

TER.

e. Adjust R2 on the Driver Supply Regulator until the D8B

voltage reaches maximum, +115 VDC.

f. AdjustR17on the DriverEncoder/TempSense Boarduntil

the AUTO DRIVER LED DS1 illuminates.

g. Adjust R2 on the Driver Supply Regulator until the D8A

voltage reaches zero.

h. Adjust R19onthe DriverEncoder/TempSense Boarduntil

the AUTO DRIVER LED DS1 turns OFF.

i. Adjust R2 on the Drive Supply Regulator so the D8A and

D8B voltagesare the same as recorded at the beginning of
the procedure.

j. Return S1 on the DriverSupply Regulator tothe CLOSED

LOOP position.

Rev. Z1: 06-02-00 Additional info to ECN 46105 888-2297-002 5-13

WARNING: Disconnect primary power prior to servicing.

DX-25U

5.6.8.3 Air Flow Monitor Circuits

Set the Air Flow Monitor circuits on the Driver Encoder/Temp
Sense Board as follows:

NOTE

For all adjustments and verifications, allow the air sensor at
least one minute to stabilize. Fans are easily shut off by removing
two of the three fuses mounted near each fan in the Output Net­work Compartment.

a. Operate the transmitter for a minimum of 15 minutes at

full power at100% modulation with a 1 kHz tone. All fans
should be operational.

b. With the DRIVER COMPARTMENT DOOR OPEN, ad-

justthe AIR FLOWCAL adjustment R98fora +2.25 VDC
reading at TP11.

c. Close the Driver Compartment door and verify that this

reading increases to +2.5 VDC (+/- 0.05 VDC).

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO­CEDURE.

d. Removethe left rear accesspanel fromthe Output Network

Compartment to expose the four cooling fans.
e. Remove any two of three fuses from one of the fans.
f. Replace the left rear panel on the Output Network Com-

partment.
g. Reapply primary AC voltage and turn the transmitter ON

at HIGH power.
h. Modulate at 100% with a 1 kHz tone.
i. Observe that the transmitter will remain ON with ALL

DOORS CLOSED.
j. The AIR INTERLOCK LED on the ColorStat™ should

turn AMBER after a few minutes of operation.
k. The voltage at TP11 on the Driver Encoder/Temp Sense

Board should now be between +2.8 and +3.1 VDC.

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO­CEDURE.

l. Removetheleftrear access panelfromtheOutput Network

Compartment to expose the four cooling fans.
m.Remove any two of three fuses from a second fan.
n. Replace the left rear panel on the Output Network Com-

partment.
o. Reapply primary AC voltage and turn the transmitter ON

at HIGH power. Modulate at 100% with a 1 kHz tone.
p. The transmitter should shut OFF within 15 minutes and

the AIR LED on the ColorStat™ panel should turn RED.

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO­CEDURE.

q. Removethe left rear accesspanel fromthe Output Network

Compartment to expose the four cooling fans. Replace the
fuses and therear panel. Restoreprimary AC voltage to the
transmitter.

5.6.9

Switch Board/Meter Panel (A31)

There are three adjustments on the Switch Board/Meter Panel.

5.6.9.1 Forward Power Calibrate R14

This adjustment is calibrated at the factory by measuring trans­mitter power output in a calorimetric dummy load. This calibra­tion adjustment should not be changed unless some means of
accurately measuring transmitter poweris available. If no exter­nal RF Power measuring device is available and the Switch
Board/Meter Panel or PowerMeter is replaced, a close approxi­mationofoutput powercan be determinedby usingthe efficiency
factor of the transmitter, number of PA Modules ON and PA
Current as recorded on the Test Data sheet. Forward Power
Calibration adjustment R14 should then be adjusted for the
correct power reading on the front panel meter.

5.6.9.2 Reflected Power Calibrate R13

Once the ForwardPowerreading has beencalibrated, operatethe
front panel to the REFLECTED position and moveP1 and P2 on
the Output Monitor board to the 1-3 position. The Power Meter
will now read Forward Power on the Reflected Power position.
Adjust R13 for the correct power reading on the Power Meter.
Return P1 and P2 on the Output Monitor board to the 1-2
position.

5.6.9.3 P A Volt Meter Calibrate R8

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO­CEDURE.

a. Open the Driver Compartment door and locate the Driver

Supply Regulator.Remove the clear protective cover.

b. Attach the positive lead of a volt meter capable of reading

+250 VDC to where wire #6 connects to the PA Power
Supply discharge switch S1. Attach the negative lead to
transmitter ground.

c. Replace the Driver Supply Regulator clear protective

cover.
d. Apply primary AC voltage to the transmitter.
e. Operate the transmitter at normal operational power.
f. Adjust R8 on the Switch Board/Meter Panel so the front

panel multimeter PA Supply +VDC reading corresponds

to the reading on the external meter.

5-14 888-2297-002 Rev. Z1: 06-02-00 Additional info to ECN 46105

WARNING: Disconnectprimary power prior to servicing.

Section V - Maintenance

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO­CEDURE.

g. Remove the Driver Supply Regulator clear protective

cover and remove the external meter connections.

h. Replace the Driver Supply Regulator clear protective

cover.

5.7 Frequency Change Procedure

The following is a step by step procedure for changing the
frequency of the transmitter. If a complete frequency change is
desired this procedure can be followedin order. If only a specific
tuning procedure is desired (ex. Driver tuning) then only that
section needsto be looked at. In some cases, time is a considera­tion in changing frequency of the transmitter. To allow a faster
frequency change, procedures which are performed to achieve
optimum transmitter performance, yet are not critical to the
reliable operation of the transmitter ,are coveredafter the section
on “Basic Frequency Change.”

5.7.1

Test Equipment Required for Frequency Change

The following is a list of the test equipment required to perform
afrequencychange. A frequencychange shouldnotbeattempted
unless the proper equipment is used.

• Audio Generator and Distortion Analyzer

• Oscilloscope

• Frequency Counter

• Modulation Monitor

• Digital Multimeter (preferred)

• Vector Impedance meter or Impedance Bridge

• Frequency programmable RF Generator (must operate up

to 3 times carrier frequency)

• RF Load, 125 kW average dissipation

• Function Generator (optional)

NOTE

Output Network setup can be performed with a Vector Impedance
Meter or Impedance Bridge and RF Generator. A Vector Imped­ance Meter is usually faster but can be sensitive to interference.
An Impedance Bridge can be more effective in an RF environ­ment.

5.7.2 Frequency Determined Components

Refer to the FD Chart, 839-7855-137, in the Drawing Package.
Install all the proper parts listed for the new desired frequency.
Oscillator crystal frequencies and part numbers are on drawing
817-1280-025.

Note that the FD chart for the output network components is
organizedin bandsA throughL. The mountinghardware, straps,
plates, etc. are all listed on the parts list. When installing these
components,makesureall connections aretight. Use specialcare
when handling vacuum capacitors.

Table 5-7. Frequency Determined Jumpers and Switches

FREQUENCY

BOARD

DETERMINED

PRESETS
ANALOGINPUT A35 R85, R84, R43
OSCILLATOR A17 P2, S1, J3
A TO D CONVERTERA34 P10, S1
OUTPUT MONITORA27 S1, S2, S6, S7, S9, P2
DRIVER COMBINER A14 J14, J15, J16-J32
BINARY COMBINER A8 J30-J33, L1-L16
MAIN COMBINERS A5 thru A7 L1-L16
SWITCH/METER PANEL A31 R1, R2
LED BOARDA32 R41, R67
DC REGULATOR A30 R51, R93

5.7.3

Frequency Determined Jumpers and Switches

Refer to the Tuning Chart, 839-7855-140, in the Drawing Pack­age. Many of the frequency determined components on the
transmitter are permanentlyinstalled and arechanged by moving
the desired jumper, coil tap or switch position. The Frequency
Tuning chart lists all jumpers, taps, and switches that need to be
set per frequency. Some of these settings are considered presets
and may need to change during a specific procedure. A list of
boards with frequency determined jumpers, taps, preset adjust­ments, and switches is shown in Table 5-7.

Output Network Dry Tune

5.7.4

Presetall output networktapsaccording to the FrequencyTuning
chartwith all FDcapacitorsin place. If ahighpowerdummy load
is not available,a smallterminating 50Ohm resistor canbe used.

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO­CEDURE.

Using a VectorImpedance meter orImpedance Bridge,adjust the
following output network sections to their appropriate X

C

or X
values at the desired carrier frequency. The sections should be
isolated (disconnected) from the rest of the other components
and only the necessary connecting straps and hardware should
be used to make the measurements. See table below.

2C2

13∠-90, 0 -j13 (2C2A and
2C2B capacitors only)

2C4

75∠-90, 0 -j75 (2C4A and
2C4B capacitors only)

2L3

42∠90, 0 + j42 (2L3 coil
only)

At frequencies where C5 is installed in series with 2L3:

• Adjust 2L3forA∠+90 or 0 + jAwhere A =100,000/(1.257

x Fo) + 42.

L

Rev. Z1: 06-02-00 Additional info to ECN 46105 888-2297-002 5-15

WARNING: Disconnect primary power prior to servicing.

DX-25U

• Exa mpl e: for a ca rrier frequency of 1000 k H z; A =

100,000/(1.257 x 1000) + 42 = 121.5 = 121.5 ∠90 or 0 +
j121.5.

5.7.4.1 Third Harmonic Trap 2L3, 2C3

After setting the XLfor 2L3, reconnect 2C3A (and 2C3B if
necessary), but still isolate the trap circuit from 2C2 and 2C4
sections. Set the Vector Impedance meter to 3f

and adjust 2C4A

c

for resonance (maximum impedance).

5.7.4.2 LOAD and TUNE

Reconnect all sections in the output network except the Com­biner Output connection. Connect the Vector Impedance meter
at 2C6 with 2C6 disconnected from the combiner output pipe or
at 2L1 if 2C6 is not used at your frequency. Adjust the TUNE

and LOAD controls for 8∠0or8+j0.

5.7.4.3 Half Frequency Trap 1L4, 1C4 (820 kHz - 1705 kHz)

Calculate the value to tap 1L4 as follows:

a. 1L4(Ohms) = 1000/(3.14 x Fo x C4)
b. Example: for Fo = 1000 kHz, C4 = 0.04 Mfd. 1L4(Ohms)

= 1000/(3.14 x 1000 x 0.04) = 7.96 Ohms.

Using a vector impedance meter, or RF generator and bridge,
adjust 1L4 tap for the correct resistance and install the 1L4/C4
combination in the transmitter.

5.7.5

RF Circuits Checkout

Before primary AC voltage is restored to the transmitter, make
sure all FD components have been properly installed and all FD
jumpers, coil taps, and switches have been properly set. Check
all connections in the output network for proper tightness, and
make sure that all panels removedfor the frequency changehave
been installed.

5.7.5.1 Oscillator A17

NOTE

Remember that the RF is held off by the VSWR-H input at J7-5
on the Oscillator until the Power Amplifier stage is turned on.
Use S1 on the LED Board to turn the output of the Oscillator ON
during troubleshooting and setup.

a. Connect a frequency counter to J5 (frequency monitor

sample).

b. Apply primary AC voltage to the transmitter and turn on

the low voltage at CB1 and CB2. Allow the Oscillator to

warm up for approximately 10 minutes.
c. Adjust trimmer C1 for the exact carrier frequency.
d. If asecond crystalisinstalled in Y2,setthe jumpers P1and

P6 topositions 1-3. After a short warmup, adjustC3 to the

carrier frequency.
e. IfanexternalRF source isused, i.e.stereo generator,verify

that it is setup for the correct frequency.

NOTE

Recheck the frequency after 30 minutes. This must be repeated
for both crystals.

5.7.5.2 Buffer Output (A16)

a. Turnoffthe low voltage to thetransmitterat CB1 andCB2.
b. Locate the Predriver in the Driver Compartment.

c. Attach a 10x scope probe, Harris part #610-1131-000,

through the ventilation slots in the interlocked Driver

Compartment door to the anode of CR3.
d. Reapply the low voltage at CB1 and CB2.
e. Press S1 on the LED Board. Verify that the drive level to

the Predriver is approximately 8 to 12.0 Vp-p. See Figure

5-2.
f. The amplitude may be adjusted with R2, Buffer Voltage

adjust.
g. Move the probe and check the anode of CR4 on the

Predriver for the same level.

5.7.5.3 Predriver Tuning

WARNING

ENSUREALLPRIMARYACPOWERISREMOVEDFROMTRANS­MITTER AND THAT A GROUNDING STICK HAS BEEN USED TO
DISCHARGE ANY RESIDUALPOTENTIAL WHERE POWERHAS
BEEN APPLIED BEFORE PERFORMING THE FOLLOWING
STEPS.

a. Remove the rear access panels from the Driver Compart-

ment and the left rear RF Amp Compartment.
b. Remove the following PA Supply fuses:

1. A25: F1-F8

2. A24: F1-F10

3. F20: (on top of T1)

c. Make sure the proper value of C1 is installed and T8 is

tapped per the initial setting from the Tuning Chart.
d. Replace the rear access panels on the DriverCompartment

and the RF Amp Compartment
e. Reapply primary AC power to the transmitter.
f. Temporarily defeat the Underdrive Fault by connecting a

jumper between TP8 and ground on the LED Board.
g. Set S1 and S2 on the Driver Encoder/Temp Sense Board

to the OFF position.
h. Set JP1, JP2, and JP3 on the Driver Encoder/Temp Sense

Board to the ON, 1-2, position.
i. Insert the 10x probe, Harris part #610-1131-000, through

theventilationslotsintheinterlockedDriverCompartment

andattachittoCR3orCR4ofRFDriverD1.
j. Depress the LOW power button. The PA Power supply

contactors will energize and apply the +60 VDC Predriver

supply voltage.
k. Adjust Predriver Tuning L1 for a peak in amplitude while

observing the waveform on the Driver Module D1. (L1

tuning may be fairly broad at some frequencies.) The

amplitude should be between 18 to 21 Vp-p.
l. If RF levelis too low, change the tap setting on T8. Check

all Driver Modules, D2 through D14, for consistent RF

level.

5-16 888-2297-002 Rev. Z1: 06-02-00 Additional info to ECN 46105

WARNING: Disconnectprimary power prior to servicing.

Section V - Maintenance

WARNING

ENSUREALLPRIMARYACPOWERISREMOVEDFROMTRANS­MITTER AND THAT A GROUNDING STICK HAS BEEN USED TO
DISCHARGE ANY RESIDUALPOTENTIAL WHERE POWERHAS
BEEN APPLIED BEFORE PERFORMING THE FOLLOWING
STEPS.

m. Remove the rear access panel from the Driver Compartment

and replace Driver supply fuse A24F10. Replace the panel.

5.7.6

Initial Driver Tuning and Setup

Refer to SECTIONVI, Troubleshooting, for additionalinforma­tion on measuring RF drive. Make sure all jumpers and switch
settings on the following boards are in the correct position:

Driver Supply Regulator A22

S1 to Open Loop.

Driver Encoder/Temp Sense Board (A19)

S1 to OFF
S2 to OFF
JP1, JP2, and JP3 ON.

FD Chart and Tuning check list:

1C1
1C3
1C4
1L4
1L5
Efficiency Coil Tap Settings
T10 - (Note: Attach both leads together on chassis
ground for initial tuning.)

NOTE

When measuring RF amplifier drive amplitudes or phasing, the
RF amplifier to be measured must be turned “ON” to give a
correct drive measurement. The drive waveform of an “OFF” RF
amplifier will be below 0.0 VDC and the peaks may be clipped.

a. Drivertuning must bedone withall Power Amplifierstage

controlsignals ON. To turnallmodules “ON”, temporarily
remove P1 on the Analog Input Board.

b. Connect the 10x scope probe, Harris part #610-1131-000,

toCR3 or CR4 onPAModule RF33 throughthe ventilation
slots in the interlocked RF Amp Compartment.

c. DepresstheLOWpowerbuttonand note thatthe PA Power

supply voltage comes up as indicated on the front panel
multimeter but no RF power or PA current is indicated.

d. With the scope dc coupled, note that an RF sine wave is

now displayed on the scope. The waveform should nor­mally measure between 22.0 and 25.0 Vp-p and it should
be centered on the 0.0 VDC line of the scope. The drive
level may be lower than 20.0 Vp-p at this time. The
GREEN LED on the PA Module should be ON.

e. If the waveformfalls totally below the0.0 VDC line of the

scope, the PA Module is turned OFF. See Figures 5-3 and
5-4 for drive waveforms.

f. To turn a PA Module ON, first make sure that the PA

TURN-OFF switch S2 on the Controller is set to ON
(down) position.

g. Press the RAISE button until the desired PA Module turns

ON as indicated by the correct drive waveform. Note that
green LED indicators will light if drive level is high
enough. TheRF MULTIMETER should also indicate cur­rent on the DRIVER IDC position.

h. The Driver/Combiner is now adjusted forresonance. Reso-

nance is indicated by a peak inthe driveamplitude andthe
DRIVER IDC reading on the RF MULTIMETER.

1. Resonance is achieved by adjusting the length of the
strap, 1L5, that connects the Driver Combiner center
conductor (copper rod) to C3. This strap is located in
the topof the Driver Compartment and can be accessed
through a panel on top of the transmitter.

2. The length of the strap must be increased if changing to
a lower frequencyand decreasedif changingto a higher
frequency.

3. A temporary strap can be made from 2" wide copper.
Punchholesevery1" toalloweasy changingof the length.

4. The strap shouldbeadjusted in 1-2"increments inorder
to see the affects and obtain resonance. When the final
length is found, cut a strap to length for the final setup.

i. Resonance is indicated by a peak in the drive amplitude and

in the DRIVER IDC reading on the RF MUL TIMET ER.

j. Once a resonance peak is achieved, the drive amplitude is

adjusted by:

1. BUCK/BOOST transformer T10

2. The number of Driver Modules ON: JP1, JP2 and JP3
on the Driver Encoder/Temp Sense Board

3. Driver Regulator Adjustment (D8A/D8B)

k. Monitor both the drive waveform on the scope and the

DRIVER D8A voltage on the multimeter.

l. Adjust the OPEN LOOP adjustment R2 on the Driver

Supply Regulator clockwise.

m.As R2 is adjusted, the drive will increase along with the

voltage on the multimeter.

n. Adjust R2 until the drive level reaches 24.0 Vp-p on the

scope or the DRIVER D8A voltage reaches 100 Volts.

1. If 24 Vp-p driveisindicatedonthescopeandtheDRIVER
D8A voltage is between 40 and 95 VDC continue on to
paragraph,“Closed LOOP Adjustment.”

2. If drivelevel is greaterthan 24 Vp-pand DRIVERD8A
voltage is less than 30 VDC, turn one Driver Module
OFF at a time by placing JP1, JP2 or JP3 on the Driver
Encoder/Temp Sense Board in the OFF position.

3. If all three jumperplugs arein theOFF positionand the
drivelevel is still too high, T10 may be used to reduce,
or“BUCK”,theamplitude of thedrive.Thisis achieved
by attaching one lead of thetransformer winding tothe
driver/combiner bar and the other lead to the combiner
bar cover.The lengthof theDriver Tuning strap 1L5 may
need to be changed to achieve resonanceif T10 is used.

o. If the drive level is less than 22 Vp-p and DRIVER D8A

voltage is greater than 90 VDC, BUCK/BOOST Trans-

Rev. Z1: 06-02-00 Additional info to ECN 46105 888-2297-002 5-17

WARNING: Disconnect primary power prior to servicing.

DX-25U

former T10 may needto beused to increase, or “BOOST”,
the amplitude of the drive. This is achieved by attaching
reversingthe leadconnections on the transformer winding
tothe combiner barand the combinerbar cover.The length
of the Driver Tuning strap 1L5 may need to be changed to
achieve resonance if T10 is used.

p. The optimum combination is:

1. Driver Current of 22 Amps or less

2. All Driver Modules active (D1-D5 and D9-D14)

3. The Drive Regulator operating with DRIVER D8A
voltage between +40 and +95 VDC

4. BUCK/BOOST transformer T10 out ofcircuit orin the
“BOOST” mode

q. When the optimum drive level is obtained, refer to the

Driver Encoder/Temp Sense section for the AUTO Driver
circuit adjustment procedure.

5.7.6.1 Closed LOOP Adjustment

a. When the RF drive level has been set at 24.0 Vp-p, note

the DRIVER D8A voltage.

b. Set the LOOP select switch S1 on the Driver Supply

Regulator to the Closed position.

c. AdjustClosed Loop adjustment R12,on the DriverSupply

Regulator, for the same reading on the DRIVER D8A
reading. The RF drive level should remain between 22.0
and 24.0 Vp-p. The LOOPselect switch is normally leftin
the Closed position for normal operation.

NOTE

MAKE SURE TO REINSTALL P1 ON THE ANALOG INPUT
BOARD IN THE NORMAL POSITION BEFORE PROCEEDING
WITH INITIAL TUNING AT LOW POWER.

d. Remove jumper between TP8 and ground on the LED

Board. This activates the Underdrive Overload circuitry.

5.7.6.2 Underdrive/Overdrive overloads

Ifit is desiredto setthe Underdriveand Overdriveoverloadsrefer
to the “Underdrive and Overdrive Overload Setting” paragraphs
in the LED Board adjustment procedure.

5.7.6.3 RF Drive Phase

Even though not necessary for a frequency change, the RF drive
phasing and Drain Phasing can be checked. For DriveLevel and
Phasing measurement procedures see “Measuring RF Drive
Phasing,” paragraph and “Measuring RF Amplifier Drain Phas­ing,” paragraphs in SECTION VI, Troubleshooting.

5.7.7

Initial Tuning At Low Power

WARNING

ENSUREALLPRIMARYACPOWERISREMOVEDFROMTRANS­MITTER AND THAT A GROUNDING STICK HAS BEEN USED TO
DISCHARGE ANY RESIDUALPOTENTIAL WHERE POWERHAS
BEEN APPLIED BEFORE PERFORMING THE FOLLOWING
STEPS.

a. Remove the rear access panels from the Driver Compart-

ment and the left rear RF Amp Compartment and replace
the following PA Power Supply fuses:

1. A25: F1-F8

2. A24: F1-F9

3. F20: (on top of T1)

b. Apply primary AC power to the transmitter. Turn on the

low voltage at CB1 and CB2.

c. Verify that all LED’s on the ColorStat™ panel are illumi-

nated Green.

d. Verify that the transmitter is properly terminated into a

Dummy load.

e. Turn the PA TURN-OFF Switch on the Controller to the

OFF (up) position.

CAUTION

REMOVE JUMPER FROM THE LED BOARD A32 FOR PROPER
DRIVE SENSING BEFORE PROCEEDING.

f. Depress the LOW power button. The PA Power supply

should energize, butno RF output or PA current should be
indicated.

g. Connect a meterto measure between 0.0 and +3.0 VDC to

TP7 on the Analog input Board.

h. Press and hold the Fast Power Set switch S1 on the Con-

troller.While holding S1 down, press the LOWER button
on the front panel. The voltage at TP7 should quickly drop
to zero. The power output of the transmitter is now set to

zero.
i. Set the PA TURN-OFF switch tothe ON (down) position.
j. Press the RAISE button. Power output should begin to

increase along with the PA current indication.Continue to

raise power until the power meter indicates 5.0 kW.
k. Change the Multimeter selection switch from PA +VDC

to FILTER NULL. Power meter selector switch should be

in FWD.
l. Adjust theTUNING control for maximum outputas noted

on the front panel Power meter.
m.FILTER NULL and ANTENNANULL maybegin to rise.

Adjust the Antenna and bandpass filter null on the Output

Monitor if necessary. Refer to the adjustment procedure

for the Output Monitor in this section.

5.7.7.1 Modulated B-Check

a. Modulate the transmitter at 5.0 kW with a 100 Hz sine

wave at 100% modulation.
b. Connect a scope probe to TP6 or TP30 on the DC Regula-

tor.Displayed will be the Modulated B-waveform similar

to the one in Figure 5-7.
c. Setthe scopefor 1 voltper division, dccoupled and the0.0

VDC line on the top graticule. The positive peak of the

waveform should be from -2.0 to -2.7 VDC. The negative

peak should occur between -3.0 and -4.5 VDC.
d. If the waveform is not within these tolerances then the

Modulated B-Adjustments should be set using the proce-

dure in the Analog Input Board and DC Regulator para-

graphs in this section.

5-18 888-2297-002 Rev. Z1: 06-02-00 Additional info to ECN 46105

WARNING: Disconnectprimary power prior to servicing.

Section V - Maintenance

5.7.7.2 A/D Phasing Check

Make sure that the switches and jumpers are preset according to
the Frequency Tuning chart.

a. Operate the transmitter at 5 kW modulated at 100% with

a 10 kHz tone.

b. Feed the demodulated output of the Modulation monitor

to the Distortion Analyzer.

c. Measure the 10 kHz THD. It should typically be 1.5% or

less.

1. If the 10 kHz THD is 2% or higher, check 1 kHz THD.
If this, too, is 2% or higher, see the troubleshooting
sectionon “higher than normal audio distortion.”

2. If the distortion is 2% or less, the10 kHz distortion can
be reduced by selecting different combinations of ca­pacitance and inductance with S1 on the Analog to
Digital Converter.

NOTE

If during the final adjustment of the A/D phasing at full power,
the THD at 10 kHz is much higher than the 1 kHz THD, the
Modulated B- adjustment may not be correct and should be re­checked. See the adjustment procedures under the Analog Input
Board and DC Regulator replacement sections in this section.

5.7.8 Tuning At High Power

a. Press MEDIUM then RAISE to slowly raise power to 15

kW.This should occurwith approximately14 PAModules

turned ON.
b. Press HIGH then RAISE to slowly raise power to 25 kW.
c. At 25 kW power output, the PA Supply Current should be

between105 and 122ampswith 23 PA Modules ON.Refer

to the Factory TestData anduse the following information

to help tune the transmitter:

Power Output (meter)
PA Voltage
PA Current
P A Efficiency
Antenna Null
Filter Null
# of Steps turned on

d. The TUNE control is adjusted for a peak in power output.

This control may be ratherbroad, especiallyat thelow end

of the medium wave band.
e. The LOADING control adjusts the PA Current for a given

number of PA Modules ON.
f. Use the number of steps turned on from the Factory Test

Data to help determine if the LOADING needs adjusting.

1. If the desired # of steps is 23 and more than 25 are on,
press the LOWER button until 23 steps are ON and
increase the PA Current with the loading control.

2. If less than 23 PA Modules are ON, press RAISE until
23 PA Modules are turned ON and decrease the PA
Current with the loading control.

g. Perform A/D Phasing check at 25 kW.Refer to Figure5-6.
h. Check the Oscillator Sync Adjustment. Referto the Oscil-

lator adjustment procedures.

5.7.9

Completion of Basic Frequency Change of
Transmitter

Thefollowingproceduresallowthe transmitter to bechecked and
adjusted for optimum performance. They are not critical for
reliability and basic performance of the transmitter.

5.7.9.1 Binary RF Amplifier Phase Alignment

Binary RF amplifier phase alignment is performed to achieve
optimum stereo performance. This alignment is performed by
measuring the phase difference betweenthe BigStep andBinary
RF amplifiers at the output of each RF amplifier and adjusting
the tap on theEfficiencycoil for minimum phase difference.The
taps on the Binary RF amplifier efficiency coils have already
been preset per the frequency tuning chart.

Because the Binary RF amplifiers B7-B12 are designed to pro­duce output RF voltages less than that of the “Big Step” RF
amplifiers, they do not always operate at the same output phase
of RF as the Big Steps. This phase difference can be anywhere
from 0 to 30 degrees. Because theRF amplifiersoperate atlower
voltage potentials, this situation does not affectreliability. How­ever,it can have an affecton theamount ofIPM(Incidental Phase
Modulation) products.

a. Connect the 10x probe, Harris part #610-1131-000, to the

drainTP1testpoint of Q3 onPAModule RF33 by inserting
the probe through the ventilation slots on the interlocked
RF Amp Compartment door.

NOTE

THE 10X SCOPE PROBE, HARRIS PART #610-1131-000 MUST
BE USED. ALSO ENSURE THAT THE SCOPE PROBE IS
PROPERLY GROUNDED.

b. Set thescopeon AC coupled, 50Volts per divisionwith the

trace centered on the screen.

c. Connect the external sync of the scope to J5 on the Oscil-

lator and make sure the scope sync is set to External.
d. Operate the transmitter at 25 kW with no modulation.
e. Adjust the Horizontal vernier on the scope so that one full

RF cycle occupies 9divisions onthe screen.Each division

now equals 40 degrees of phase shift.
f. Using the Horizontal positioning and triggering level on

the scope, place the transition time of the scope on the

center vertical line of the screen.
g. Increase the vertical sensitivity of the scope to expand the

waveform.
h. Switch the scope to the X10 position and readjust the

horizontal position so that the RF transition again crosses

the center line of the scope. This will be the reference for

thephase measurements. IfanotherRF amplifier transition

occurs at the first large division on the right, this RF

amplifier is operating at 4 degrees lagging from the refer-

ence.
i. Check thedrain of Q3 on RF33 to RF35 toverify that they

are all within +/-5 degrees of each other .
j. Check the drainof Q3 ontheBinary RFamplifierB7. Note

that Q3 is now the right hand MOSFET when viewingthe

module from the front.

Rev. Z1: 06-02-00 Additional info to ECN 46105 888-2297-002 5-19

WARNING: Disconnect primary power prior to servicing.

DX-25U

k. Operate the transmitter again at 25 kW. If no waveform

appears, then depress the RAISE or LOWER button until
this step turns on.

l. As the power is changed, the Binary amps are turning on

and off at different rates. The vertical sensitivity of the
scope can be increased since B7 through B10 operate at
1/2 the supply voltage.

m.If the Phasing of B7 is within +/-5 degrees, move to B9.

1. If the phase difference is greater,then the tap on L1 can
be changed to put the B7 RF amplifier in phase.

2. Typically, more of the inductor will be shorted out for
the Binary amps than for the Big Steps.

3. Do not reduce Efficiency coil turns to less than 1/2 of
the given Frequency determined value.

n. Continue to check the phasing on the remaining binary

amps. Note that the amount of active turns on the Effi­ciencycoils willtend to beless onthe smallerbinary steps.

5.7.9.2 Binary Amplitude Alignment

The output of the Binary RF amplifiers can vary from frequency
to frequency. To optimize the linearity of the modulation signal,
the output amplitude of the Binary RF amplifiers can be varied
in 5% increments.Triangle or rampmodulation mustbe usedfor
this procedure.

a. To check the Binary alignment, operate the transmitter at

5 kW output with 10%, 100 Hz triangle modulation.

b. Connect the external sync input of the scope to the output

of the generator.

c. Onthescope, display thedemodulated outputofthe modu-

lation monitor.

d. Expand the vertical and horizontal display to view the

positive going portion of the ramp.

e. Connect the other channel of the scope to the Modulation

Encoder gold jumper for Big Step RF amplifier RF37.

f. Raise or lower the modulation until a transition from 0.0

to 5.0 Volts can be seen on the Step 37 display. From this
displayitis possible tosee atwhich pointinthemodulation
ramp Step 37 is being turned on.

g. Move the scope probe to the Modulation Encoder signal

for Step 38.

h. Raise and lower the modulation until Step 38 can be seen

toturn on. A smalltransitionin the demodulated waveform
may be noticed wherea big step turns on. See Figure 5-11.

i. By making small changes in both the power level and

modulation level, it should be possible to display the
demodulated ramp between two big steps. This is the area
to look at for binary alignment.

j. If the binary alignment is proper,the transitions between big

steps will be smooth. If, for instance, the 1/2 Step, B7,
amplitude is too low, you will see a transitionapproximately
halfway between Step 37 and Step 38. This is the half step

transition. You may also see that there is now a transition
at Step 37 and Step 38 also. See Figures 5-11 and 5-12.

WARNING

ENSUREALLPRIMARYACPOWERISREMOVEDFROMTRANS­MITTER AND THAT A GROUNDING STICK HAS BEEN USED TO
DISCHARGE ANY RESIDUALPOTENTIAL WHERE POWERHAS
BEEN APPLIED BEFORE PERFORMING THE FOLLOWING
STEPS.

k. Determining which binary step amplitude is not lining up

with the others can take some trial and error.

1. For example,to change thebinary amplitude for the 1/2
Step, open the interlocked inner door.

2. Remove the 1/2 Step RF amplifier B7 and Big Step RF
amplifier RF33.

3. Look through the slots of the removed amps and move
J30 on the motherboard to the next desired position.

4. Reinsert the modules and check the ramp linearity.

5. Repeat for all Binary Steps B7 through B11 using J31,
J32and J33, ifnecessary.Binary RF amplifierB12 does
not have amplitude adjustments.

NOTE

The ramp may not appear to be perfect, even at what appears to
be optimum binary alignment. This is because the displayed ramp
is at LOW power at around 10% modulation.

CAUTION

ENSURE THAT THE BINARY JUMPER PLUGS ARE PROPERLY
INSERTED AND SEATED. A LOOSE OR MISSING JUMPER WILL
CAUSEBINARY TOROID DAMAGE.

5.7.10 Other Adjustments

These adjustments should normally not change during a fre­quency change, but it is advisable to check their setting for
optimum transmitter performance.

5.7.10.1 Audio Gain Adjust: A35R15

This sets the proper audio modulation level into the transmitter.
Refer to the paragraphs on theAnalog Input Board replacement.

5.7.10.2 Offset Adjust: A34R75

This control adjusts the Modulation Tracking of the transmitter
or,in other words,how equally itwill modulateat differentpower
levels.Refer tothe paragraphsontheAnalog to DigitalConverter
replacement procedure.

5.7.10.3 Dither Adjust: A35R43

This control reduces the small modulation steps caused by the
Digital Modulation process. Refer to the paragraphs on the
Analog Input Board replacement.

5.7.10.4 Modulated B- Adjustments

This signal optimizes the switching time of the PA Modules and
affects high frequency distortion and response. Refer to the
paragraphs on Analog Input Board and DC Regulator replace­ment for B- adjustments.

5-20 888-2297-002 Rev. Z1: 06-02-00 Additional info to ECN 46105

WARNING: Disconnectprimary power prior to servicing.

Section V - Maintenance

Figure 5-2

Predriver Input drive waveform,measured at the anode of CR3

or CR4 on Predriver module (2Vp-p per division)

Figure 5-3

RF DriveWaveform at RF AmplifierGate of Q3 (anode of

CR3). RF Amplifier turned OFF. (5Vp-p per division)

Figure 5-4

RF Drive Waveform at RF Amplifier, Gate of Q3 (anode of

CR3). RF AMplifier turned ON. (5Vp-p per division)

Figure 5-5

Antenna VSWR Detector voltage and current samples as meas-

ured at Output Monitor A27TP1 and TP2. Also typical of Band-

pass filter samples.

Rev. Z1: 06-02-00 Additional info to ECN 46105 888-2297-002 5-21

WARNING: Disconnect primary power prior to servicing.

DX-25U

Figure 5-6

Demodulated Audio of 100 Hz sinewave at 30% modulation at

25 kW. Scope view of positive peak expanded to show glitches

due to improper A to D phasing adjustment.

Figure 5-7

Modulated B- waveform at DC Regulator A30TP30. 5 kW op-

eration with 100 Hz, 100% modulation. 1V per division, 0.0

Vdc at top line.

Figure 5-8

Modulated B- waveform at DC Regulator A30TP30. 25kW op-

eration with 100 Hz, 100% modulaltion.

Figure 5-9

Oscillator sync samples at Oscillator A17TP4 and TP5. Trans-

mitter operating at 25 kW with no modulation. The two sam-

ples are not in phase.

5-22 888-2297-002 Rev. Z1: 06-02-00 Additional info to ECN 46105

WARNING: Disconnectprimary power prior to servicing.

Section V - Maintenance

Figure 5-10

Oscillator sync samples at OscillatorA17TP4 and TP5. Trans-

mitter operating at 25 kW with no modulation. The two sam-

ples are in phase.

Figure5-12

Demodulated audio.Transmitter operaeting at 5kW with 100

Hz, approximately 10% triangle modulation.

Top Trace - Demodulated audio.

Bottom Trace - Modulation Encoder waveform of Step 6. Insuf-

ficient 1/2 Binary Step output shown.

Figure 5-11

Demodulated audio.Transmitter operating at 5 kW with 100

Hz, approximately10% triangle modulation

Top Trace - Demodulated Audio

Bottom Trace - Modulation Encoder waveform of Step 6. Good

binary alignment shown.

Rev. Z1: 06-02-00 Additional info to ECN 46105 888-2297-002 5-23

WARNING: Disconnect primary power prior to servicing.



Troubleshooting

6.1 Introduction

This section of the technical manual contains troubleshooting
procedures for the DX-25U.

Problems that could cause an OFF AIR situation and how they
relate to the front panel ColorStat™ indicators, including over­loads and interlocks will be discussed first.

This section is a troubleshooting guide for the transmitter as a
system. Foradditional information related to anindividual mod­ule or board, refer to the section of the manual for that particular
board.

This section contains techniques and guidelines to assist the
engineer in isolating the problem more quickly. The engineer
using this section of the manual must have the proper test
equipment available and has a good working knowledge of the
transmitter and the operation of the individual modules and
boards.

Table 6-0. Troubleshooting the Transmitter

Section VI
Troubleshooting

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

Symptom: Transmitter will not turn ON - No ColorStat™ panelindicatorsareilluminated.................. 6-6

PossibleCauses.......................................................................... 6-6

LossofACPower.................................................................... 6-6

Loss of +5V Supply on LED Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6

LooseConnectors..................................................................... 6-6

Symptom: Transmitter will not turn on - all ColorStat™ panelindicatorsareilluminatedGreen. ............ 6-6

PossibleCauses.......................................................................... 6-6

+5BCircuitNotUpToOperatingVoltage................................................. 6-6

ContactorTurnOnLogicOnTheController............................................... 6-6

ContactorDriveCircuitry............................................................... 6-6

+30VDCFeedbackAuxiliarySignal..................................................... 6-6

240VACCoilVoltage................................................................. 6-6

OpenContactorCoilOnK1orK2....................................................... 6-6

Symptom: Transmitter will not turn on - one or more ColorStat™ panel indicators are illuminated RED. . . . . . 6-7

PossibleCauses.......................................................................... 6-7

Symptom: Transmitter will turn ON but immediately turns OFF - one or more ColorStat™ panel indicators illumi-

nate RED. The transmitter may try to turn on twice and a fault indicator illuminates AMBER then RED.. . . . . 6-7

PossibleCauses.......................................................................... 6-7

Symptom: Transmitter turns On (LOW, MEDIUM or HIGH buttons illuminate) but there is no power output and

no PA current is indicated. Supply voltage is indicated on the multimeter.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7

PossibleCauses.......................................................................... 6-7

PATurn-OffCommandGivenToTransmitter.............................................. 6-7

Type4orType5Fault ................................................................ 6-7

PowerOutputIsSetToZero............................................................ 6-7

Symptom:Transmitterisrunning,butpowerislowerthannormal..................................... 6-7

PossibleCauses.......................................................................... 6-7

PowerReductionCircuitryActivated..................................................... 6-7

Symptom: Unable to raise power past a certain point. ColorStat™ panel ANT and/or FILTER LED indicate RED. . 6-8

Possiblecause ........................................................................... 6-8

Symptom: Unable to raise power past a certain point. No ColorStat™ panel indicators Illuminated RED. . . . . . 6-8

Possiblecause ........................................................................... 6-8

Analog Input Board maximum power adjustment R27 misadjusted or defective. . . . . . . . . . . . . . . . . . . 6-8

Analog Input Board, half power step up circuit may have failed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8

PA Turn on/Turn Off control signals on Modulation Encoder A37 may be incorrect.. . . . . . . . . . . . . . . 6-8

OpenfusesonFuseBoardsA24orA25................................................... 6-8

Symptom: Transmitter turns ON (Low, Medium, or High Indicators Illuminate) but will not modulate. . . . . . . . 6-8

Section VI

Rev. R: 11-11-96 888-2297-002 6-1

WARNING: Disconnect primary power prior to servicing.

DX-25U

PossibleCauses.......................................................................... 6-8

Modulation not reaching transmitter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8

Analog Input Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8

ColorStat™ panelOvercurrentFaultIndication.................................................... 6-8

Random Faults With Program Audio. Possible Causes: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8

Faults With Tone Modulation. Possible Causes:. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9

OverloadsonTurnOn.PossibleCauses:...................................................... 6-9

ColorStat™ panelOvervoltageFault............................................................. 6-9

PossibleCauses.......................................................................... 6-9

SupplyVoltageTooHigh.............................................................. 6-9

ColorStat™ panelSupplyFault................................................................ 6-10

PossibleCauses......................................................................... 6-10

InputAC3PhaseLineImbalance ...................................................... 6-10

Open+115VDCSupplyRectifierFuse.................................................. 6-10

Failed PA Power Supply Transformer T1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10

LowFrequency,HighLevelModulation................................................. 6-10

OverloadSettings.................................................................... 6-10

ColorStat™ panelUnderdriveFault ............................................................ 6-10

PossibleCauses......................................................................... 6-10

HighVoltageSupplyShort............................................................ 6-10

Failed Driver Supply (+115 VDC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10

FailedPredriverSupply(+60VDC)..................................................... 6-10

NoDriveToTheDriverStage......................................................... 6-11

Driver Module Failure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11

ExcessiveRFAmplifierFailures ....................................................... 6-11

Driver Supply Regulator Failure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11

Driver Supply Regulator Loop Select. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11

DefectiveDriverEncoderSignals....................................................... 6-11

SevereDriverMistuning.............................................................. 6-11

ColorStat™ panelOverdriveFault ............................................................. 6-11

PossibleCauses......................................................................... 6-11

HighACLineVoltage ............................................................... 6-11

DefectiveDriverEncoderSignals....................................................... 6-11

RFAmplifierON/OFFCircuitry ....................................................... 6-11

Driver Supply Regulator Failure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11

ColorStat™ panelDoorInterlock.............................................................. 6-12

PossibleCauses......................................................................... 6-12

DefectiveInterlockSwitch ............................................................ 6-12

deleted03-01-96 .................................................................... 6-12

BadConnectionatDCRegulator....................................................... 6-12

ColorStat™ panelExternalInterlock............................................................ 6-12

PossibleCauses......................................................................... 6-12

ExternalInterlockTerminalsOpen...................................................... 6-12

ExternalInterlockFuseF24 ........................................................... 6-12

ExternalInterlockRelayK3........................................................... 6-12

DC Regulator Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12

ColorStat™ panelAirInterlock................................................................ 6-12

Possiblecauses ......................................................................... 6-12

Fans Not Operating Properly, Failed/Running Backward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12

DirtyFilters........................................................................ 6-12

TopAirExhaustRestricted............................................................ 6-12

Rear Access Panel Open . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12

AirInterlockSensingCircuitry......................................................... 6-12

AirInterlockDetectorU17............................................................ 6-12

ColorStat™ panelOscillatorFault.............................................................. 6-13

ColorStat™ panelBufferFault................................................................ 6-13

6-2 888-2297-002 Rev. R: 11-11-96

WARNING: Disconnectprimary power prior to servicing.

Section VI - Troubleshooting

ColorStat™ panelPredriverFault .............................................................. 6-13

ColorStat™ panelRFAmpFault............................................................... 6-13

ColorStat™ panel Analog Input Board: +15V and -15V Supply Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13

ColorStat™ panel Analog to Digital Converter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13

+15V, -15V, and +5V Supply Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13

Conversion Error Fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13

ColorStat™ panel Modulation Encoder: Cable Interlock Fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13

PossibleCauses......................................................................... 6-14

RF Amplifier Module Not In Place. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14

Modulation Encoder Cable Not In Place. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14

ColorStat™ panelDCRegulatorB+andB-SupplyFaults.......................................... 6-14

ColorStat™ panelOutputMonitorFaults........................................................ 6-14

+5Vand-5VSupplyFaults ............................................................... 6-14

VSWRFaults........................................................................... 6-14

GeneralDiscussionOfVSWRProtectionoftheTransmitter ................................. 6-14

First Stage VSWR Protection 6-15
Second Stage VSWR Protection 6-15

AntennaVSWRFault................................................................. 6-15

Possible Causes of VSWR Overloads 6-15
Possible Causes of Arcing 6-15
Possible Causes of Transient Signals 6-15
Load Impedance Changes 6-15

Bandpass Filter VSWR Fault. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16

Bandpass Filter VSWR Caused By Problems In The Output Network 6-16

Symptom:LossOfPositivePeakCapability...................................................... 6-16

PossibleCauses......................................................................... 6-16

PowerSupplyVoltageLow............................................................ 6-16

AudioProcessorEquipmentDefectiveorIncorrectlySet .................................... 6-16

IncorrectTransmitterTuning........................................................... 6-16

Transmitter Operated In FlexPatch™ Mode............................................... 6-16

FailedRFAmplifier.................................................................. 6-16

Symptom:HigherThanNormalAudioDistortion ................................................. 6-16

PossibleCauses......................................................................... 6-16

FailedRFAmplifier(s)................................................................ 6-16

TransmitterMistuning ................................................................ 6-16

OperatingIntoaBandwidthRestrictedAntenna ........................................... 6-16

Low RF Drive Level To The PA Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16

AdditionalTipsForTroubleshootingAudioTHD.......................................... 6-16

Symptom:ConsistentLossofRFAmplifiers..................................................... 6-17

RepeatedLossofSameRFAmplifierInAnyPosition.......................................... 6-17

PossibleCauses ..................................................................... 6-17

Defective Transistor Pad 6-17
Defective Turn On/Turn Off Circuitry 6-17
Improper RF Drive 6-17
Poor Solder Connection on PC Board 6-17

ConsistentLossOfAnRFAmplifierInOneParticularPosition.................................. 6-17

PossibleCauses ..................................................................... 6-17

Improper RF Drive 6-17
Improper Drain Phasing 6-17
Defective Output Toroid 6-17
Improper Control Signal 6-17

Consistent Loss Of Modules In Random Positions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-17

PossibleCauses ..................................................................... 6-17

Analog to Digital Phasing Improperly Set 6-17
Modulated B- Improperly Set 6-17
Improper VSWR Circuit Operation 6-17

Rev. R: 11-11-96 888-2297-002 6-3

WARNING: Disconnect primary power prior to servicing.

DX-25U

Improper Overload Settings 6-17

Improper Air Flow 6-17

Transmitter Mistuning 6-17

OtherTroubleshootingTechniques............................................................. 6-18

Handling MOSFET’s .................................................................... 6-18

Testing MOSFET’s...................................................................... 6-18

FindingAMissingStep.................................................................. 6-18

Using FlexPatch™ forBypassingaFailedPAModule......................................... 6-18

Using FlexPatch™ for Isolating Modulation Encoder/RF Amplifier Problems. . . . . . . . . . . . . . . . . . . . . . . 6-19

MeasuringDriveLevel................................................................... 6-19

MeasuringDrivePhasing................................................................. 6-19

ExcessiveDrivePhaseDifference ...................................................... 6-20

RFAmplifierDrainPhasing............................................................... 6-20

ExcessiveDrivePhaseDifference ...................................................... 6-20

MeasuringSteps50-123:TESTSWITCH................................................ 6-21

6-4 888-2297-002 Rev. R: 11-11-96

WARNING: Disconnectprimary power prior to servicing.

Section VI - Troubleshooting

Table 6-1. DX 25U Fault Types

TYPE 1 TURNS PA POWER SUPPLY OFF (Manual Restart Required)

EXT External Interlock
AIR Air Flow Fault
DOOR Door Interlock
SUPPLY FAULT PA Power Supply Protection
OVERVOLTAGE PA Power Supply Overvoltage Protection
CABLE INTERLOCK Modulation and Driver Encoder Cables
B+ DC Regulator B+ Regulated Fault
B- DC Regulator B- Regulator Fault
+5V Output Monitor +5V on board Regulator Fault
+15V Output Monitor +15V on board Regulator Fault
REPEATING TYPE 2 FAULT See NOTE below

TYPE 2 TURNS PA POWER SUPPLY OFF (Recycles Back ON)

UNDER DRIVE RF Drive Level to PA LOW
OVER DRIVE RF Drive Level to PA HIGH

OVER CURRENT * PA Power Supply +230 Vdc Overload
* Transmitter will attempt to limit current first, then shut off high voltage if limiting is not sufficient.
NOTE: If Type 2 fault condition is still present when the transmitter cycles back on,or repeats within 2.4 seconds after the

transmitter cycles back on, it becomes a Type 1 fault and will require a manual restart.
TYPE 3 LOWER POWER OUTPUT

BANDPASS VSWR Output Monitor detects sucessive VSWR’s

ANT.VSWR Output Monitor detects sucessive VSWR’s

TEMPERATURE Driver Encoder/Temp Sensor detects an over temperature condition
TYPE 4 PA TURN OFF ONLY (PA Kill only,High Voltage Remains ON)

BANDPASS VSWR Output Monitor detects one shot VSWR

ANT.VSWR Output Monitor detects one shot VSWR

+15V A/D Converter on board +15V Regulator Fault

-15V A/D Converter on board -15V Regulator Fault

+5V A/D Converter on board +5V Regulator Fault

+15V Analog Input on board +15V Regulator Fault

-15V Analog Input on board -15V Regulator Fault

TYPE 5 CLEAR MODULATOR DIGITAL AUDIO DATA

CONVERSION ERROR A/D Converter
TYPE 6 FAULT DISPLAY ONLY

RF AMP PA RF Amplifier Fuse Blown

Rev. R: 11-11-96 888-2297-002 6-5

WARNING: Disconnect primary power prior to servicing.

DX-25U

6.2 Symptom: Transmitter will not turn

ON - No ColorStat™ panel indica­tors are illuminated.

6.2.1 Possible Causes

6.2.1.1 Loss of AC Power

If no indicators on the ColorStat™ panel are illuminated, check
for correct unregulated Low Voltage supply readings on the
front panel multimeter. Make sure that the Low voltage power
supplybreakers CB1 and CB2aresettotheON position. Finally
checkthe ACprimary powerto thetransmitter to ensurethat the
fuses or circuit breakers have not opened.

6.2.1.2 Loss of +5V Supply on LED Board

IftheColorStat™panel LED’s are notilluminated, butthe unregu-
lated Low Voltage readings are correct, this indicates a problem
with the Controller or LED Board. The +5 VDC supply for the
LED Board is developed on the Controller.Check for +5 VDC at
the Controller on TP2. DS1 on the Controller will illuminate any
time any of the three regulatorson the controller fails.

6.2.1.3 Loose Connectors

Check for loose or improperly installed connectors on both the
LED Board and Controller. Also check the connectors on the
Power Distribution Board A39.

NOTE

Do not remove any plugs with the power on.

WARNING

TO MEASURE THE VOLTAGES IN THE FOLLOWING STEP RE­MOVE PRIMARY VOLTAGE AT THE MAIN BREAKER OR FUSE
PANEL. THEN REMOVE THE STEP START PANEL COVER AND
ATTACH YOUR MEASURING DEVICE TO THE TERMINALS
LISTED.

6.3.1.3 Contactor Drive Circuitry

Measure the +15 VDC on 3TB1-8 in the Step Start panel. If
voltage is not present or does not drop when the LOW button is
depressed, the wire or cable continuity is broken at some point
or solid state relay K101 is defective. If K1 energizes but K2
does not, measure for +15 VDC at 3TB1-10. It should be
present, and drop to close to zero shortly after K1 energizes. If
it does not, check for wire continuity or defective K102.

6.3.1.4 +30 VDC Feedback Auxiliary Signal

The +30 VDC AUX signal sent back to the Controller to indicate
K1and K2haveenergizedmay be missing.Measurefor +30 VDC
at 3TB1-5. If not present, measure at TB3-5 in the transmitter.

a. Voltage present at TB3-5: Continuity problem between

transmitter and step-start panel.

b. Voltage not present at TB3-5: Troubleshoot +30 VDC

supply.

If+30 VDC ispresentat3TB1-5 in thestep-start panel,measure
the return voltage at 3TB1-6. When K1 energizes, +30 VDC
should be present 3TB1-6.

6.3 Symptom: Transmitter will not turn

on - all ColorStat™ panel indicators
are illuminated Green.

6.3.1 Possible Causes

6.3.1.1 +5B Circuit Not Up To Operating Voltage

If the +5Bsupply whichuses the1 farad capacitorbackup isnot
operational then the transmitter will not turn ON. If the trans­mitter does not have a good set of backup batteries BT1-BT3
installed and the transmitter has been off for more than two
hours, the backup capacitor C44 requires approximately 1 m­inute to recharge. This keeps the transmitter from turning ON.
Measure TP6 on the Controller and verify that the +5B voltage
is present. If it is not, troubleshoot the +5B supply.

6.3.1.2 Contactor Turn On Logic On The Controller

Measure the voltage at Q10-1 on the Controller. It should be
approximately +15 VDC. While monitoring this voltage, de­press the LOW power button. The voltage should drop to near
0 VDC for approximately 1 second. If it does not, troubleshoot
the Controller contactor drive logic circuits. If the voltage does
drop down or isnotpresent, the problemisin the contactordrive
circuitry.

NOTE

If the contactor circuit for K1 (step start contactor) is inopera­tive, no sound from the Step Start panel will be heard when the
LOW, MEDIUM, or HIGH button is depressed. If the contactor
circuit for K2 (run contactor) is inoperative, two clicks will be
heard when the transmitter LOW, MEDIUM, or HIGH button is
depressed. This is the step start contactor energizing and de-en­ergizing.

6.3.1.5 240 VAC Coil Voltage

WARNING

TOMEASURETHEVOLTAGESINTHE FOLLOWINGSTEPFIRST
REMOVE ALL VOLTAGES APPLIED TO STEP START PANEL
AND TRANSMITTER. YOU CAN THEN REMOVE THE STEP
START PANEL COVER AND ATTACH YOUR MEASURING DE­VICE TO THE TERMINALS LISTED. ROUTE LEADS OUT OF
STEP START PANEL AND THEN REATTACH STEP START
PANEL COVER. NOW APPLY POWER AND RUN TEST. AFTER
TEST IS COMPLETED, REMOVE ALL POWER, REMOVE
COVER,REMOVE LEADS, AND REATTACH COVER.

Measure the voltage between 3TB1-15 and 16. The 240 VAC
from K1 and K2 coils should be present when CB1 is switched
on. Measure for 240 VAC on K1 and K2 coils when the LOW,
MEDIUM or HIGH buttons are depressed. If not present, pos­sible defective K101 or K102.

6.3.1.6 Open Contactor Coil On K1 or K2

Measure the resistance of each contactor coil. The nominal
resistance should be approximately 200-250 Ohms for K1 and
10-20 Ohms for K2.

6-6 888-2297-002 Rev. R: 11-11-96

WARNING: Disconnectprimary power prior to servicing.

Section VI - Troubleshooting

6.4 Symptom: Transmitter will not turn

on - one or more ColorStat™ panel
indicators are illuminated RED.

6.4.1 Possible Causes

See the “Troubleshooting ColorStat™ Panel Indicator Faults”
paragraph in this section.

6.5 Symptom: Transmitter will turn ON

but immediately turns OFF - one or
more ColorStat™ panel indicators il­luminate RED. The transmitter may
trytoturnontwiceandafaultindi­cator illuminates AMBER then RED.

6.5.1 Possible Causes

In the case of the Overcurrent, Overdrive and Underdrive over­loads, the transmitter will try to restart one time before indicat­ing a fault. This indicates that the fault still exists and must be
repaired before the transmitter will become operational. See the
“Troubleshooting ColorStat™ panel Indicator Faults” para­graph in this section.

To check, remove the wire connected to the Customer Remote
control terminal strip TB1-22.

6.6.1.2 Type 4 or Type 5 Fault

Regulator faults that occur on the Analog to Digital Converter
and the Analog Input Board will generate a Type 4 fault and
cause a PA Turn-Off command. A Type 5 fault is generated by
the Analog to Digital Converter conversion error fault circuit
and will also produce a PA Turn-Off command. If any of these
fault indicators are illuminated on the ColorStat™ panel, refer
to the “Troubleshooting ColorStat™ panel Indicator Faults”
paragraph in this section.

6.6.1.3 Power Output Is Set To Zero

No transmitter output power and the PA OFF LED segment
DS1-9 on the Modulation Encoder is not illuminated, indicates
thatthe output powerhas beenloweredto zero. Pressthe RAISE
button to see if power begins to rise. If it does, hold the RAISE
button until the desiredoutput poweris reached.Reset theother
power levels to the desired output power. A power reset nor­mally occurs only when the battery backup power supplyon the
Controller

discharges and the transmitter AC power has been off for over
one hour. Replace the batteries and check the +5V “B” supply
on the Controller if this is a common occurrence.

If the power is zero and cannot be increased by the RAISE
control, investigate the power control circuitry on the Analog
Input Board and the Controller.See the specificsection foreach
of these boards for circuit information.

6.6 Symptom: Transmitter turns On

(LOW, MEDIUM or HIGH buttons
illuminate) but there is no power out­put and no PA current is indicated.
Supply voltage is indicated on the
multimeter.

6.6.1 Possible Causes

6.6.1.1 P A Turn-Off Command Given To Transmitter

The PA Turn-Off command will allow the PA Power Supply to
energize,butwill notallow anyofthe RF amplifiersto beturned
ON to produce power output. To check for a PA Turn-Off
command, open the Center Control Compartment door and
observe section DS1-9on the Modulation Encoder. If the trans­mitter PA Power Supply is energized and DS1-9 is illuminated
RED, a PA Turn-Offcommand is being given tothe transmitter.
Check the following items for a PA Turn-Off command:

a. PA Turn-OffSwitch S2 onController settoOFF Position.

Check to make sure this is set to the ON (down) position.

b. External PA Turn-Off Circuit Activated

The External Interface allows the use of an External PA Turn­OFF command for customer applications such as Day/Night
switching on a Phasor. If this feature is connected, make sure
that the device associated with the PA Turn-Off is not at fault.

6.7 Symptom: Transmitter is running,

but power is lower than normal.

6.7.1 Possible Causes

6.7.1.1 Power Reduction Circuitry Activated

a. ANTENNALED on the ColorStat™ panel is RED. This

indicates a VSWR problem in the load, phasor,combiner
or antenna system. Press the RESET button on the Col­orStat™ panel. If the LED turns GREEN, press RAISE
and set power back to normal. This indicates that the
VSWR problem is not active anymore. If the LED will
not reset GREEN, the problem is still active and must be
investigated. If further testing does not reveal a problem
in the RF load that the transmitter is connected to, inves­tigate components in the phase angle detector on the
Output Monitor. refer to SECTION H, Output Monitor,
for additional information.

b. BANDPASS LED on the ColorStat™ panel is RED,

power cannot be raised. This indicates a VSWR problem
in the matching network between the poweramplifierand
output network stage in the transmitter. Likely causes are
defectivevacuumcapacitors.Ifcapacitors in thebandpass
circuit are not defective, investigate components in the
phase angle detector(s) on the Output Monitor. Refer to
SECTIONH, Output Monitor,foradditional information.

Rev. R: 11-11-96 888-2297-002 6-7

WARNING: Disconnect primary power prior to servicing.

DX-25U

c. No ColorStat™ panel indicators are illuminated RED.

Check the Over Temp LED DS2 on the Driver En­coder/Temp Sense Board. If it is RED, the circuitry has
initiated a Temperature Induced Lower command to the
Controller. Possible causes include transmitter tuning or
RF combiner problems resulting in excessive heat sink
temperature on PAModules RF33 and RF34 or defective
temperature monitoring circuitry. Refer to SECTION S,
Driver Encoder/Temp Sense Board and the transmitter
tuning instructions in the initial turn-on paragraphs in
SECTION II, INSTALLATION.

6.8 Symptom: Unable to raise power

past a certain point. ColorStat™
panel ANT and/or FILTER LED in­dicate RED.

6.8.1 Possible cause

If the ANT and/or FILTER ColorStat™ panel indicators are
Illuminated RED. The VSWR sensor(s) trip and power reduc­tion circuitry is activated. There are two possible causes:

a. Thephaseangle detector(s) ontheOutputMonitor are not

nulled yet on a newinstallation or recent antenna system
work. Refer to SECTION II, Installation/Initial Turn On,
for the phase angle detector null procedure.

b. If detectors are nulled and transmitter has been operating

normally for sometime,troubleshoot the causeof VSWR
problems.

appropriate Encoder output. Refer to SECTION L, Modulation
Encoders, for additional information.

6.9.1.4 Open fuses on Fuse Boards A24 or A25.

If a fuse has opened, there willbe no voltage toa group of eight
amplifiers.

6.10 Symptom: Transmitter turns ON

(Low, Medium, or High Indicators
Illuminate) but will not modulate.

6.10.1 Possible Causes

Because audio is added to a DC voltage relative to the power
output level, any problem that would affect the (Audio + DC)
signal would also affect the power level.The DC isadded tothe
audioearly in theanalog input circuitry. Ifthetransmitter power
output control functions normally but there is no modulation,
the problem is either before the transmitter or isoccurring inthe
first few stages of the Analog Input Board.

6.10.1.1 Modulation not reaching transmitter

Verify that audio is reaching the audio input terminals on the
External Interface.

6.10.1.2 Analog Input Board

Only the circuitry associated with Analog Input Board compo­nentsU6,U9,and U7 wouldaffectmodulation butnotthe power
control. Refer to SECTION J, Analog Input Board, for addi­tional information.

6.9 Symptom: Unable to raise power

past a certain point. No ColorStat™
panel indicators Illuminated RED.

6.9.1 Possible cause

6.9.1.1 Analog Input Board maximum power adjustment

R27 misadjusted or defective.

Check associated circuitry and monitor at test points TP3, TP1,
and TP4. Refer to SECTION J, Analog Input Board, for more
detailed information.

6.9.1.2 Analog Input Board, half power step up circuit may

have failed.

Check Q7 and Q8 circuitry.Refer to SECTION J, Analog Input
Board, for more detailed information.

6.9.1.3 PA Turn on/Turn Off control signals on Modulation

Encoder A37 may be incorrect.

If this type of failure is of such magnitude to limit full power
output, severe audio distortion should also be noted. Check for
thecorrect numberof stepsto be turnedon by viewingthe green
LED’son eachof the PA Modules.If LED’sare not illuminated,
i.e. steps 1-48, check to see if the control signal is at the

6-8 888-2297-002 Rev. R: 11-11-96

WARNING: Disconnectprimary power prior to servicing.

6.11 ColorStat™ panel Overcurrent

Fault Indication

TheOvercurrentFaultmonitorsthesupplycurrent of the PAPower
Supply and will generate a TYPE 1 Fault any time the PA current
reaches a preset overload level. Both average and peak current are
monitored and combined for the Overcurrent Fault.

6.11.1

Random Faults With Program Audio. Possi­ble Causes:

a. OVERMODULATION: Random faults with modulation

often indicates that the peak overload current is being
exceededby overmodulation ofthetransmitter.Checkthe
modulation level with an oscilloscope ifthe calibrationof
the modulation monitor is in question.

b. LOW FREQUENCY SIGNALS: If the modulation level

is correct, there may be sub-audible signals feeding into
thetransmitter.Takenote of whentheoverloadsoccurand
tryto relatethe overloads to aparticular source. Turntable
rumble, especially during start-up, can be of such level to
cause Overcurrent overloads. A switchable high-pass fil­ter in the program line, some audio processors have
switchable low frequencycut-off filters, will filter outthe

Section VI - Troubleshooting

sub-audible signals withoutdegrading the ONAIR sound
of the transmitter.

c. DC OFFSET: Some audio processors have a DC offset

which can shift with modulation, at either a sub-audible
or audio rate. This shifting offset voltage will generate
carrier shift, and if the “offset” shifts in a positive direc­tion at the same time as positive modulation peak occurs,
an Overcurrent overload could occur .

WARNING

ENSURE ALL PRIMARY AC POWER IS REMOVED FROM THE
TRANSMITTER AND THATTHE GROUNDINGSTICK HAS BEEN
USED TO DISCHARGE ANY RESIDUAL VOLTAGE WHERE
POWERHASBEENAPPLIEDBEFOREPERFORMINGTHE FOL­LOWING STEPS.

d. OPEN +230 VDCRECTIFIERFUSE:An open 250Amp

fuse for one of the +230 VDC supply rectifiers can cause
excessivepower supply ripple which can cause the trans­mitter to trip off with an Overcurrent overload when
modulated with low frequencies. Remove the rear panel
to thePA Power Supply andcheck allrectifier fuses. If an
open fuse is detected, check the associated rectifier for a
short.

e. OVERLOADADJUSTMENT:Tocheck the setting of the

Overcurrent overloads, refer to procedures in SECTION
V, Maintenance.

CAUTION

DO NOT MAKE ANY ADJUSTMENTS TO THE OVERCURRENT
OVERLOAD SETTINGS UNTIL ALL OF THE PRECEDING
CHECKS LISTED ABOVE HAVEBEEN PERFORMED. TRANSMIT­TER DAMAGE COULD OCCUR IF THE OVERCURRENT OVER­LOADSARE IMPROPERLY SET.

inSECTIONP, Controller,andSECTION J, Analog Input
Board, for additional information.

WARNING

ENSURE ALL PRIMARY AC POWER IS REM OVED FROM THE
TRANSMITTERAND THATTHE G ROUNDING STICK HASBEEN
USED TO DISCHARGE ANY RESIDUAL VOLTAGE WHERE
POWERHASBEENAPPLIEDBEFOREPERFORMING THE FOL­LOWING STEPS.

c. PA POWER SUPPLY SHORT: If the transmitter will not

turn ON with the PA OFF switch in the OFF position, the
problem could be in the PA PowerSupply. Isolate the PA
Power Supply from the RF amplifiers by removing F1
through F9 on Fuse Board A24, F1 through F8 on A25,
and F20 located on T1. Measure the resistance to ground
of the PA Power Supply +230 VDC bus bar on the Fuse
Boards. The resistance should be approximately 500
Ohms. If it is not, troubleshoot the PA Power Supply
Discharge Circuit (crowbar). Refer to the DX-25U Over­all Schematic, 839-7855-151,in theDrawingPackage for
circuit details.

6.12 ColorStat™ panel Overvoltage Fault

The Overvoltage Fault indicates that the PA Power Supply
voltage is excessive.

6.12.1

6.12.1.1 Supply Voltage Too High

Possible Causes

6.11.2 Faults With Tone Modulation. Poss ibl e Causes:

If the transmitter is being tested with tone modulation it is
possible to generate Overcurrent overloads with high level low
frequency modulation. Second,many audio test generators will
also have a DC offset voltage in their output when they are
switched from one frequency range to another; this offset can
cause an overload. Third, if the transmitter is turned ON with a
high level, low frequencytoneat theaudioinput, overloadsmay
occur due to the surge current produced as the transmitter is
ramping up to power with full modulation.

6.11.3

Overloads on Turn On. Possible Causes:

a. OVERMODULATION: If the transmitter indicates an

Overcurrent overload on turn on, the most likely cause is
low frequency, high level modulation. Lower the modu­lation level before turning on the transmitter.

b. POWER CONTROL CIRCUIT: A problem in the power

The PA Power Supply voltage will be at its highest with the PA
Power Supply ON and the power output at zero. The supply
voltage will be at its highest. The transmitter should not incur
an overload in this condition.

If the transmitter has been operating with the proper supply
voltage for some time, check to see if thepower line voltage has
increasedfor thenormal operating voltage.Measurethe AC line
voltage and tap both the high and low voltage transformers for
the correct voltage. If the transmitter will remain ON long
enough to measure the PA Supply +VDC on the front panel
multimeter,compare thisreading tothat recordedon thefactory
Test Data Sheet. If the reading is significantly higher, the PA
PowerSupply transformer primary tapping must be changed. If
the transmitter will not stay on long enough for a measurement,
tap the transformer down to obtain the next lowest supply
voltage.

NOTE

control circuit that would release the PA OFF command
before thePAPower Supply capacitors had fully charged
could cause an Overcurrent overloadon turn on. Turn the
PA OFFswitchto the OFF(up)position on theController.
If the transmitter turns ON, refer to the turn-on sequence

The highest tap setting for T1 is 485/+4% (502VAC). If line
voltage is higher than 505VAC it will be necessary for the utility
company to lower the line voltage.

NOTE

Rev. R: 11-11-96 888-2297-002 6-9

WARNING: Disconnect primary power prior to servicing.

DX-25U

WARNING

ENSUREALLPOWERIS REMOVEDFROMTRANSMITTERAND
THAT THE GROUNDING STICK HAS BEEN USED TO DIS­CHARGE ANY RESIDUAL VOLTAGE WHERE POWER HAS
BEEN APPLIED BEFORE T HE TAPS ON THE HIGH VOLTAGE
AND/OR LOW VOLTAGE TRANSFORMERS ARE CHANGED.

For example if the supply voltage reading is too high, remove
all AC power to the transmitter and retap the PA PowerSupply
transformer T1 to the next highest primary number. If the
transformer is presently tapped to 430/0, change the tapping to
430/+4% to reduce the supply voltage.

6.13 ColorStat™ panel Supply Fault

ThePAPowerSupplyFault circuit will notallowthetransmitter
to operate if an imbalance exists in the 3-phase AC input. This
will cause excessive heating of the PA Power Supply trans­former. If the overload occurs only intermittently, the power
supply balance is marginal and is probably faulting on low
frequency modulation peaks. If the fault occurs consistently,
even without modulation, the problem is more serious and
should be attended to immediately.

the threephase line voltages are wellbalanced. If the balance is
marginal, then Supply fault overloads could occur.

6.13.1.5 Overload Settings

Refer to SECTION V, Maintenance, for the Power Supply
Protection overload adjustment procedure.

CAUTION

DO NOT MAKE ANY ADJUSTMENTS TO THE POWER SUPPLY
PROTECTION OVERLOAD SETTINGS UNTIL ALL OF THE PRE­CEDING CHECKS LISTED ABOVE HAVE BEEN PERFORMED.
TRANSMITTER DAMAGE COULD OCCUR IF THE OVERLOADS
ARE IMPROPERLY SET.

6.14 ColorStat™ panel Underdrive Fault

NOTE

The transmitter may indicate an UNDERDRIVE overload if a
direct short exists in the PA Power Supply. This could be caused
by shorted MOSFET transistors in the PA Supply Discharge cir­cuit (crowbar) or a defective shorting switch. The overload indi­cated could be an UNDERDRIVE fault if the Driver Supplies do
not reach their proper voltage. An Underdrive condition could
be sensed before an OVERCURRENT overload is detected.

6.13.1

6.13.1.1 Input AC 3 Phase Line Imbalance

Possible Causes

Measure the 3 phase line voltages. They should be within 5%
of each other. If the line voltages are not in balance, the utility
company should be contacted to rectify the problem. Note that
line imbalance not only affectsthe transformerheating, but will
also degrade the transmitter AM signal-to-noise performance.

WARNING

ENSURE ALL POWER IS REMOVED FROM THE TRANSMITTER
AND THAT THE GROUNDING STICK HAS BEEN USED TO DIS­CHARGE ANY RESIDUAL VOLTAGE WHERE POWER HAS
BEEN APPLIED BEFORE PERFORMING THE FOLLOWING
STEPS.

6.13.1.2 Open +115 VDC Supply Rectifier Fuse

Remove the rear panel of the Driver Compartment and check
F13, F14, and F15. If an open fuse is detected, check the
associated +115 VDC supply rectifier.

6.13.1.3 Failed PA Power Supply Transformer T1

If the rectifiers check good and the AC line voltage balance is
within specifications, the PA Power Supply transformer may
have failed windings in one of the sections. It is sometimes
possible tocompare resistance readings for each primary wind­ingtolocateafailedwinding

6.13.1.4 Low Frequency, High Level Modulation

The sensing circuit for the Supply overload fault detects the
amount of full wave power supply ripple on the +115 VDC
supply.This 100Hz/120Hzcomponentwill be the greatest when
the transmitter ismodulated atthis audio frequencyrange. High
modulation levels with frequencies from 90-140Hz could be
sensed as a fault. This condition should normally not occur if

WARNING

ENSURE ALL POWER IS REMOVED FROM THE TRANSMITTER
AND THAT THE GROUNDING STICK HAS BEEN USED TO DIS­CHARGE ANY RESIDUAL VOLTAGE WHERE POWER HAS
BEEN APPLIED BEFORE PERFORMING THE FOLLOWING
STEPS.

6.14.1 Possible Causes

6.14.1.1 High Voltage Supply Short

Remove AC power to the transmitter and remove the Driver
Compartment andRF Amp Compartmentrear access panels.
Isolate the PA Power Supply from the RF amplifiers by
removing F1 through F9 on Fu se Boa rd A24, F1 through F8
on A25, and F20 located on T1. Measure the resistance to
ground of the PA Power Supply +230 VDC bus bar on the
Fuse Boards. The resistance should be approximately 500
Ohms. If it is not, troubleshoot the PA Power Supply Dis­charge Circui t (crowbar). Refer to the DX-25U OverallSche­matic, 839-7855-151, in the Drawing Package for circuit
details.

6.14.1.2 Failed Driver Supply (+115 VDC)

Note the PA Supply +VDC readingon the front panel multime­ter while pressing the LOW button. If the voltage deflects
upward, but the DRIVER +VDC indication on the RF MUL­TIMETER does not, there is a problem in the driver supply
voltage circuit. Check F10 on the Fuse Board A24. Also check
F13, F14, F15, and C7, C8, C9. Refer to the DX-25U Overall
Schematic in the Drawing Package for circuit details.

6.14.1.3 Failed Predriver Supply (+60 VDC)

Observe the voltage rise in the PREDRIVER +VDC position
on the RF MULTIMETER. It should reach approximately 50
Volts within 1 second. If it does not deflect upscale, checkF21,

6-10 888-2297-002 Rev. R: 11-11-96

WARNING: Disconnectprimary power prior to servicing.

Section VI - Troubleshooting

C10, L3. Refer to the DX-25U Overall Schematic in the Draw­ing Package for circuit details.

6.14.1.4 No Drive To The Driver Stage

The Driver Modules require a minimum of 20Vp-p of drive to
each module. This drive is generated on the Oscillator, and
amplified by the Buffer and Predriver. If any of these stages is
inoperative an Underdrive fault will occur. With only the Low
Voltage on, the Oscillator,Buffer,andPredriverIndicatorLED’s
should all be GREEN. Depress S1 on the LED Board toturn the
Oscillator ON and activate the RF Sensecircuits. The Predrive r
LED should now change to RED until the PA Power Supply is
energized. At that time, the Predriver LED will change back to
GREEN when all drive levels are normal in all stages. If the
OscillatororBufferLED’sareRED,troubleshootthatparticular
stage. To measure the RF drive to the Driver Modules, refer to
the “Measuring RF Drive” procedure in SECTION V, Mainte­nance.

6.14.1.5 Driver Module Failure

The transmitter has a gain controlled Driver stage with redun­dant Driver Modules to maintain drivelevel if a module fails. If
additional modules fail, it may not be possible for the control
circuits to keep the drive level within limits. Check for a Driver
Module failure by viewing the LED’s on each of the Driver
Modules. Depress the LOWpower buttonand noteif any of the
LED’s illuminate RED before the transmitter shuts OFF. If any
LED’s illuminate RED, remove all power from the transmitter
and replace or repair the modules. If a spare module is not
available, exchange the defective module with a PA Module,
starting with RF90.

6.14.1.6 Excessive RF Amplifier Failures

Thefailureofalargenumber of PAModules could load thedrive
level down enough to cause an Underdrive Overload. To check
for this type of failure, observe the PA Module LED’s through
the interlocked innerdoors. Depress theLOWpowerbutton and
note any LED’s that illuminate during the step-start sequence.
If more than five LED’s are RED on any combination of PA
Modules, repair the modules before proceeding with further
attempts to troubleshoot an Underdrive problem.

6.14.1.7 Driver Supply Regulator Failure

If the Driver voltage is present, (DRIVER +VDC deflecting to­wards +110 VDC during the step-start sequence), the Drive r
Supply regulator can be checked. Operate the RF Multimeter
switch to the DRIVER D8A position. Depress the LOW power
button. The meter indication should deflect upward before the
transmittershuts backdown.If the readingdoesnotdeflectupward,
the Driver Supply regulatorcould bedefective.Refer toSECTION
E for more information on DriverSupply Regulator .

6.14.1.8 Driver Supply Regulator Loop Select

IftheDriverSupply Regulator is theproblem,it may be possible
togetthe transmitter operationalby switchingthe regulatorloop
selectswitch S1 to the OPENLOOP position.Thisswitch is set
throughthe coverovertheDriverSupplyRegulatorinthe Driver
Compartment. If the transmitter will operates,the problem is in
the Closed Loop regulator section of the Driver Supply Regu­lator.The transmitter will operate normally with the loop set in

the OPEN position, but it will not have the Automatic gain
control feature enabled.

6.14.1.9 Defective Driver Encoder Signals

Check the PA Module control signals from the Driver En­coder/Temp Sensor Board, A19. Most of the GREENLED’son
the Driver Modules should illuminate when the PA Power
Supply is energized. Under normal conditions D6 and D7 are
held off in reserve capacity depending where S1 and S2 are
positioned. Another modulein theD5, D9, orD10 position may
be factory selected to be off. Refer to SECTION S, Driver
Encoder/Temp Sense Board, for additional information.

6.14.1.10 Severe Driver Mistuning

The control circuits will normally compensate for slight tuning
problems. If the mistuning is severe the regulator will not be
able to keep the drive level in range. To check the Driver stage
tuning, refer to “Driver Adjustments” in SECTION V, Mainte­nance. It is possible for one of the Driver tuning capacitors to
have failed and change the tuning. Failed capacitors can some­times be identified by checking for abnormal temperature after
operating for a short time.

6.15 ColorStat™ panel Overdrive Fault

6.15.1 Possible Causes

6.15.1.1 High AC Line Voltage

The RF drive regulation should compensate for most line vari­ations. However, if the AC line increases are greater than the
regulation capabilities of the Driver,an OVERDRIVEoverload
can occur.

6.15.1.2 Defective Driver Encoder Signals

Some of the Driver Amplifiers are selected to be off under
normalconditions. If circuitryfailson the DriverEncoder,some
or all of these modules could turn on which could create an
OVERDRIVE fault. Refer to SECTION S, Driver En­coder/Temp Sense Board, for additional information.

6.15.1.3 RF Amplifier ON/OFF Circuitry

The on/off circuitry on the amplifiers could fail in such a way
as to permanently activate the RF amplifier. The most probable
cause would be a shorted Q5 or Q6. See SECTION C, RF
Amplifier, for more information.

6.15.1.4 Driver Supply Regulator Failure

If the transmitter incurs an Overdrive fault as soon as the PA
Power Supply is energized, it is still possible to see if the
regulatorhas aproblem before thetransmitter turns off.Operate
the RF MULTIMETER tothe DRIVER D8A position. Monitor
the meter reading while depressing the LOW power button and
note the meter deflection during the step-start sequence. It will
probably deflect near the +115 VDCmark. Nowoperate theRF
MULTIMETER switch to the DRIVER D8B position. Again
depress the LOW power button and note the meter deflection.
If the meter deflects upward toward the +115 VDC mark, the
DriverSupply regulatoris turning onboth sections due to some

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DX-25U
malfunction. Refer to SECTION E, Driver Supply Regulator,

for additional information.

6.16 ColorStat™ panel Door Interlock

Refer to SECTION M, DC Regulator, for a simplified diagram
of the interlock circuitry. If a Door Interlock fault is indicated
check to make sure all doors are securely closed especially
where the plunger contacts the interlock switch itself.

6.16.1

Possible Causes

WARNING

ENSUREALLPOWERIS REMOVEDFROMTRANSMITTERAND
THAT GROUNDING STICK HAS BEEN USED TO DISCHARGE
ANY RESIDUAL POTENTIAL WHERE POWER HAS BEEN AP­PLIED B EFORE PERFORMING THE FOLLOWING STEPS.

6.16.1.1 Defective Interlock Switch

If the doors appear tobe closing properly,remove all powerand
ohmmeter each half of each switch for continuity when the
switch plunger is depressed.

6.16.1.2 deleted 03-01-96

deleted

6.16.1.3 Bad Connection at DC Regulator

Check the pullup resistors and interconnect wiring between the
DC Regulator and the Controller.

6.17 ColorStat™ panel External Interlock

Refer to SECTION M, DC Regulator,for a simplifiedinterlock
diagram.

6.17.1

6.17.1.1 External Interlock Terminals Open

Iftheexternalinterlock feature was not utilized, ajumper should
beconnectedbetween TB1-1 and TB1-2.Ensurethatthejumper
wire is properly connected. If a device requiring an external
interlock is connected to these terminals, (Phasor door inter­lock, dummy load, etc.) make sure that this device is providing
closed contacts in the normal operating condition.

6.17.1.2 External Interlock Fu se F24

Theexternal interlock terminalsTB1-1 and TB1-2areprotected
by fuse F6 in the Center Control Compartment. Check the fuse
and replace if open. Make sure that there are no shorts on the
External interlock line.

6.17.1.3 External Interlock Relay K3

If F6 is good and the external interlock connections are
closed, verify that K3 is energized when the low voltage is
on and the external interlock is closed. If it is not, suspect an
open coil of K3.

6.17.1.4 DC Regulator Components

Check the components and interconnect wiring on the DC
Regulator.

Possible Causes

6.18 ColorStat™ panel Air Interlock

Airinterlock problems willbeeither intermittent, ascould occur
if an air filter becomes blocked, or constant, as during a two or
more fan failures.

6.18.1

6.18.1.1 Fans Not Operating Properly, Failed/Running

Possible causes

Backward

Thetransmitter will turnON for approximately20 seconds with
NO air flow. Press the LOW power button and open the Center
Control Compartment door. A steady stream of air should be
felt coming from the holes in the interlocked inner door.If there
is little or no air flow, verify that the fans are operating in the
proper direction byplacing apiece of paperon any ofthe filters.
If the fan rotation is correct, the paper will stick to the filter.

If the fan rotation is incorrect, refer to the fan rotation check in
SECTION II, Installation/Initial Turn-On.

6.18.1.2 Dirty Filters

Temporarily remove the air filters from the rear panel of the
Output Network Compartment. If the transmitter now operates
with the filters removed, clean or replace the filters.

6.18.1.3 To p Air Exhaust Restricted

If air is exhausted through air duct installed on the top of the
transmitter, a restriction can generate an Air Interlock. If an
exhaust restriction is suspected, operatethe transmitterwith the
Driver Compartment door open. Because the exhaust is no
longer going through the top, the transmitter should not incur
an Air interlock.

6.18.1.4 Rear Access Panel Open

The air system will not operate correctly with any rear access
panel open or partially open, due to the loss of air pressure in
the Driver Compartment.

6.18.1.5 Air Interlock Sensing Circuitry

The Air interlock sensing circuitry consists of U12, U13, U10
on theLED Board. Measure theDC voltageat J5-1 on the LED
Board. Depress the LOWpower on button. If the voltage at this
point goes to approximately +5 VDC and does not go LOW
before the transmitter turnsOFF,then the air interlock switch is
operating and the LED Board circuitry is suspect. Refer to
SECTION Q, LED Board, for additional information.

6.18.1.6 Air Interlock Detector U17

If the logic LOW does not appear at J5-1 on the LED Board,
and all the above checks have been made, then the air interlock
detector U17 on the Driver Encoder/Temp Sense Board or
connecting circuitry is suspect. Refer to SECTION S, Driver
Encoder/Temp Sense Board for additional information.

6.19 ColorStat™ panel Oscillator Fault

NOTE

Use S1 on the LED Board while troubleshooting.

If the transmitter is incurring Underdrive faults, and the Oscil­lator ColorStat™ panel LEDis illuminated RED, the Oscillator

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Section VI - Troubleshooting

output is not sufficient. Refer to SECTION A, Oscillator, for
additional information.

6.20 ColorStat™ panel Buffer Fault

If the transmitter is incurring Underdrive faults, and the Color­Stat™ panel Buffer LED is illuminated RED, the Buffer Am­plifier output is not sufficient. Refer to SECTION B, Buffer
Amplifier, for additional information.

6.21 ColorStat™ panel Predriver Fault

If the transmitter is incurring Underdrive faults, and the Pre­driver ColorStat™ panel LED is illuminated RED during PA
Supply turn on, the Predriver output is not sufficient. Refer to
SECTIONC,RFAmplifier,andSECTIOND,DriverCom­biner/Motherboard, for additional information.

6.22 ColorStat™ panel RF Amp Fault

The function of the RF AMP fault indicator is to alert the
operator that an RF amplifier has a blown fuse. This normally
indicates shorted MOSFET transistors on the PAModule. This
circuit only illuminates the RED LEDon the ColorStat™ panel
and triggers a remote output if connected. The transmitter will
continue to operate normally, with slightly reduced power
and/or slightly increased distortion, depending on whichampli­fier has failed.

In brief,this circuit mimics the RF amplifier blown fuse indica­tors on each PA Module. The LED will illuminate RED at a
steady state or flash to themodulation orprogram leveldepend­ing on which step has a blown fuse.

NOTE

Whenever all fuses on A24 and A25 are removed, the RF Amp
Fault will illuminate whenever the PA Power Supply is ener­gized or P1 on the DC Regulator is in the TEST position. This is
normal.

NOTE

It is sometimes possible for the regulator circuit to lock into a
fault condition if the output is accidentally shorted. If this oc­curs, turn off the Low Voltage at CB1 and CB2 for approxi­mately 1 minute to allow the power supply to discharge. Reap­ply Low Voltage and note if the regulator is now reset.

6.24 ColorStat™ panel Analog to Digital

Converter

6.24.1 +15V, -15V, and +5V Supply Faults

If any of the Analog to Digital Converter LED’s (+15V, -15V,
or +5V) are RED on the ColorStat™ panel, this indicates that
the Supply has failed. The transmitter will generate a PA Turn­Off command so no power output will be produced. With the
Low Voltage on, measure thevoltage onboth sides of F1 on the
Analog to Digital Converter and verify that it is approximately
+22 VDC. Measure the voltage on both sides of F2 and verity
that it is approximately -22 VDC. Measure the voltage on both
sides of F4 andverify thatit isapproximately +8 VDC. If a fuse
has failed, replace the fuse and try again. If the fuse fails again,
troubleshoottheregulatorcircuit.Refer to SECTION K, Analog
to Digital Converter, and SECTION M, DC Regulator, for
additional information.

NOTE

It is sometimes possible for the regulator circuit to lock into a
fault condition if the output is accidentally shorted. If this oc­curs, turn off the Low Voltage at CB1 and CB2 for approxi­mately 1 minute to allow the power supply to discharge. Reap­ply Low Voltage and note if the regulator is now reset.

6.24.2 Conversion Error Fault

The Analog to Digital Converter requires a sample of the RF
driveto develop thesample frequency forthe A/DconvertorIC.
If this sampleis missing or thereis a faultin the analogto digital
conversion process, the Conversion Error LED on the Color­Stat™ panel will illuminate REDand circuitryon theAnalog to
Digital Converterwill generate a PA Turn-OFF command. The
PA Power Supply will remain energized but no RF output will
be produced. Refer to SECTION K, Analog to Digital Con­verter, for additional information.

6.23 ColorStat™ panel Analog Input

Board: +15V and -15V Supply Faults

Ifeither the +15Vor-15V Fault LED’son the ColorStat™panel
are RED,this indicates that the supply has failed. Thetransmit­ter will generate a PA Turn-Offcommand and no power will be
produced. Withthe low voltageon, measure the voltage at both
ends of F2 on the Analog Input Board and verify that it is
approximately +22 VDC. Measure the voltage on both sides of
F3 and verify that it is -22 VDC. If one fuse has failed, replace
the fuse and try again. If the fuse fails again, troubleshoot the
regulatorcircuit.Refer to SECTIONJ, Analog InputBoard, and
SECTION M, DC Regulator, for additional information.

Rev. R: 11-11-96 888-2297-002 6-13

WARNING: Disconnect primary power prior to servicing.

6.25 ColorStat™ panel Modulation

Encoder: Cable Interlock Fault

A Cable interlock will prevent a transmitter turn on command
from being generated. The step-start sequence will not begin
and the PA Power Supply will not energize. This prevents
possibledamage to combinertoroids andRF amplifiersif cables
are removed or amplifiers are not installed in place on the
Combiner/Motherboards. The cableInterlock feature is accom­plished through the individual Modulation Encoder and DC
Regulatorcables.Ifthetransmitterwillnot turn on andthe Cable
Interlock LED isRED, check thefollowing.Refer to SECTION

DX-25U
L, Modulation Encoder, for additional information and circuit

description to help facilitate troubleshooting an interlock prob­lem.

NOTE

Place P1 on the DC Regulator in the TEST position and use the
DS1 Bar Graph LED’s on each Modulation Encoder to help
locate the interlock fault. Refer to the Cable Interlock chart on
sheet 4 of the Modulation Encoder schematic, 839-7855-134, in
the Drawing Package. After troubleshooting has been com­pleted, place P1 in the NORMAL position.

6.25.1 Possible Causes

6.25.1.1 RF Amplifier Module Not In Place

Refer to the DS1 Bar Graph LED display on each Modulation
Encoder to isolate a fault to a group of eight PA Modules.
Remove all AC primary power and open the inner front door
exposing the RF amplifiers. Make sure all PA Modules are
seated properly. Some resistance is normal when inserting and
removing Amps. If all modules are seated, swap one module at
a time with a known good module to isolate the problem to a
particular module. It is possible for a shorted diode on the
module to cause a false Cable Interlock indication.

6.25.1.2 Modulation Encoder Cable Not In Place

Locate the Modulation Encoder ribbon cables on the right side
of the Modulation Encoder. Make sure all are seated properly
with the black “ears” fitting over the connector.Remove all AC
primary power and open the inner front door exposing the RF
amplifiers. The Modulation Encoder ribbon cables connect to
the Combiner/Motherboards with the same type connectors on
the side of the motherboards facing the front of the transmitter.
The appropriate RF amplifier must be removed to allow access
to the ribbon connectors. Check each of these connectors for
proper seating.

6.26 ColorStat™ panel DC Regulator

B+ and B- Supply Faults

If either the DC Regulator B+ or B- Fault LED’sare RED, this
indicates that the supply has failed or a cable is not properly
seated on the board. Refer to SECTION M, DC Regulator, for
additional information.

NOTE

It is sometimes possible for the regulator circuit to lock into a
fault condition if the output of the regulator is accidentally
shorted. If this occurs, turn off the Low Voltage supply at CB1
and CB2 for approximately 1 minute to allow the power supply
to discharge. Reapply Low Voltage and note if the regulator is
now reset.

6.27 ColorStat™ panel Output Monitor

Faults

6.27.1 +5V and -5V Supply Faults

If either of the Output Monitor +5V or -5V Fault LED’sonthe
ColorStat™ panel are RED, this indicates that the supply has
failed. The transmitter will turn OFF and will not be able to be
turned ON until the fault is cleared. With the Low Voltage on,
measure the voltage on both sides of F1 on the DC Regulator
andverifythat it isapproximately +8VDC. Measurethevoltage
on both sides of F2 and verify that it is approximately -8 VDC.
If a fuse is open, replace the fuse and try again. If the fuse fails
again, troubleshoot the regulator circuit. Refer to SECTION H,
Output Monitor, and SECTION M, DC Regulator, for addi­tional information.

NOTE

It is sometimes possible for the regulator circuit to lock into a
fault condition if the output of the regulator is accidentally
shorted. If this occurs, turn off the Low Voltage supply at CB1
and CB2 for approximately 1 minute to allow the power supply
to discharge. Turn the Low Voltage back on and note if the
regulator is now reset.

6.27.2 VSWR Faults

6.27.2.1 General Discussion Of VSWR Protection of the

Transmitter

A discussion of VSWR protection is included here to aid the
station technical and engineering staff in determining when
VSWRoverloads may indicatea problem thatshouldbe located
and corrected. The VSWR protection built into the transmitter
is both for the protection of the transmitter and the protection
of external equipment which might be installed between the
transmitter and the antenna system. Operating at high power
with a VSWR condition can result in high voltages or currents
that can result inarcing, overheatingof components,or compo­nent failure. The VSWR overloads and limits set in the trans­mitter protection circuitry should not be bypassed or increased
beyond the recommended limits set at the factory.

CAUTION

VSWR OVERLOAD LIMIT SETTINGS THAT EXCEED RECOM­MENDED VALUES MAY RESULT IN COMPONENT DAMAGE OR
FAILURE .

The transmitter uses two nearly identical circuits to generate a
VSWR faultfrom two different locations. The Antenna VSWR
monitors the output load of the transmitter.The Bandpass Filter
VSWR circuit detects a VSWR that occurs anywhere in the
transmitter output network and combiner. Should any output
network part fail, the transmitter will be protected.

The Bandpass Filter VSWR circuit will also sense any antenna
load changes, but its sensitivity is set lower. An antenna VSWR
will be detected by the Antenna VSWRcircuit first and then by
the Bandpass Filter VSWR circuit.

The first step in VSWR protection is to try to clear the fault.
Most VSWR faults can be cleared by reducing the transmitter
power output to zero for a brief period of time. This zero power

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WARNING: Disconnectprimary power prior to servicing.

Section VI - Troubleshooting

output is accomplished by turning all PAModules OFFthrough
modulator action. This occurs in less than 20 milliseconds and
may not be noticed by listeners, or will be noticed as a slight
“click” or “pop.” If a VSWR fault cannot be cleared by turning
the PA Modules OFF for a short period several times, the
transmitter will reduce power.

6.27.2.1.1 First Stage VSWR Protection

a. SYMPTOM: VSWR LED flashes RED, carrier level

pauses at half-powerand then returns to normal power; a
slight “Pop or Click” isheardontheair.

b. ACTION: This is the first step in the VSWR protection.

The VSWR detectors act in less than a millisecond to
detectaVSWRfaultandturnoffthetransmitterRFoutput
for approximately 20 milliseconds or less. The VSWR
status indicator flashes RED for approximately one-half
second, then returns to GREEN. If the VSWR condition
is no longer present, no additional action will occur. The
VSWR LED will not stay RED since this is not a serious
type of VSWR condition. These types of VSWR actions
can occur indefinitely, as long as they do not occur for a
consistent period of longer than one second.

6.27.2.1.2 Second Stage VSWR Protection

a. SYMPTOM:The VSWR LEDlatchesRED, theLOWER

button illuminates and the PA Current and Power meters
read low. Within 10 to 30 seconds, the LOWERindicator
extinguishes and the poweroutput and PACurrent stayat
a lower than normal power level.

b. ACTION: The VSWR sensor has detected a serious

VSWR problem.The firststage ofVSWR protection was
attempted, butthe fault existed for more than 1 second of
continuous recycling.A LOWER command was givento
the transmitter to fold back the power to a level at which
the transmitter could still operate. The power level will
remain there until given a RAISE command. If the fault
still exists the transmitter will again LOWER the power
and disregard the RAISE command. In this case, the
source of the VSWR fault must be repaired before full
power operation can continue.

6.27.2.2 Antenna VSWR Fault

6.27.2.2.1 Possible Causes of VSWR Overloads

VSWR overloads during stormy weather may occur normally,
and may be no cause for concern. Proper installation of static
drain and static discharge equipment in the antenna system can
minimize, but not eliminate, this problem.

Causes of VSWR overloads may be listed in three categories.
They will first belisted, then will be discussed in more detail in
the following paragraphs.

a. ARCING in the impedance matching network, phasor,

switching equipment, transmission line, tuning equip­ment, or at the tower ball gaps. Once an arc occurs,
transmitter output power would sustain the arc. When the
transmitter power output is removed, the arc will go out
(unless there is some other voltage source to keep it
going).

b. TRANSIENTS, or other signal pickup, fed back into the

transmitter output from the antenna system.

c. COMPONENT FAILURES causing a change in load

impedance at the transmitter output connector.

6.27.2.2.2 Possible Causes of Arcing

Common causes of arcing include:

a. Defective vacuum capacitors. VSWR overloads will prob-

ably occurat a certain power levelor under modulation.

b. Static discharge or discharge due to lightning, across ball

gaps, guy wire insulators, or possibly acrosscomponents
already operating close to their voltage ratings. Static
charge buildup can occur on towers that do not have
provision made for static discharge, such as static drain
chokes. Charge buildup can also occur on insulated guy
wire segments. Static charge buildup can occur before,
during or after rain, snow, or even blowing dust or sand.

c. Dirt build up or moisture (including condensation) on

insulating surfaces, causing the voltagebreakdownrating
to be reduced. VSWR overloads will probably occur on
modulation peaks.

d. Condensation inside a transmission line may cause re-

duced breakdown voltage of the line. This can occur if
pressurized gas filled lines lose pressure or if the dehy­drator in the line pressurization unit fails. VSWR over­loads will probably occur on modulation peaks.

e. In new systems, insufficient voltage rating of compo-

nents, such as capacitors or insulators, or spark gaps that
are set too close.

6.27.2.2.3 Possible Causes of Transient Signals

TRANSIENT signal pickup may occur during thunderstorms,
even from distant lightning strokes in some cases. Lightning
strikes may induce currents in towers, causing currents on the
transmission lines that can reach the phase detectors and give a
VSWR overload indication.

Other station signals can also induce voltages and currents in
antenna systems that are large enough to be detected by the
phasedetectorand cause VSWRoverloads.The solutioninsuch
cases may be a trapor filter in the antenna impedance matching
network or phasor.

6.27.2.2.4 Load Impedance Changes

The Reflected Power reading and DETECTOR NULL (An­tenna) reading on the front panel multimeter are the best indi­cations of the antenna operating impedance, once the system is
initially tuned into the antenna. An impedance change in the
transmitter loadwill change the DETECTOR NULL (Antenna)
indication and, to a lesser extent, the reflected power .The load
impedance should be checked with proper impedance measur­ing equipment and corrected if possible.

“Dummy loads” should also be treated with caution. Dummy
load resistance or impe d anc e may change with time, and
dummy load resistance or impedance may also change as the
load heats up when poweris applied. If reflected powerchanges
after power has been applied to the load, this is probably the
cause.

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WARNING: Disconnect primary power prior to servicing.

DX-25U

6.27.2.3 Bandpass Filter VSWR Fault

6.27.2.3.1 Bandpass Filter VSWR Caused By Problems In The

Output Network

If a problem occursin the output network ofthe transmitter due
to a component failure, the Bandpass Filter VSWR circuit will
protect the transmitter until the component can be replaced.

a. Do NOT attempt to further raise Power.
b. Do NOT change Tuning or Loading controls

NOTE

During normal operation if a short term VSWR occurs, such as
that caused by lighting or static discharge on the antenna sys­tem, both the Antenna and Bandpass filter indicators may flash.
The antenna VSWR circuit is set to trip slightly before the Band­pass filter circuit. If only the Bandpass Filter indicator flashes,
this indicates a problem in the output network.

6.28 Symptom: Loss Of Positive Peak

Capability

6.28.1 Possible Causes

6.28.1.1 Power Supply Voltage Low

If the supply voltage forthe RF amplifiers islowerthan normal,
the positive peak capability will be reduced. Nominal PA Sup­ply +VDC should be between +225 and +235 VDC at 60 kW
output power. If the voltage is not close to the factory test data
sheet, the PA Power Supply transformer T1 must be retapped.
Refer to SECTION II,Installation, for information on selecting
the proper transformer taps.

6.28.1.2 Audio Processor Equipment Defective or Incorrectly Set

Many problems with positive peaks are due to defective or
incorrectly setup processing equipment. Check the manual for
that particular piece of equipment for setup and service infor­mation.

6.28.1.3 Incorrect Transmitter Tuning

If the transmitter Loading and Tuning controls have not been
set properly ora changein the antennaimpedance has occurred,
positive peaks can be affected. Refer to the initial turn on
procedurein SECTIONII, Installation,forTuningandLoading
adjustment procedure.

6.28.1.4 TransmitterOperatedInFlexPatch™ Mode

IfseveralPAModules failed and othermodules weresubstituted
using the FlexPatch™ feature, then the transmitter will have a
slightly reduced positive peak capability. The defective PA
Modules must be replaced and the transmitter returned to its
normal operating configuration before full positive peak capa­bility will return.

6.29 Symptom: Higher Than Normal

Audio Distortion

6.29.1 Possible Causes

6.29.1.1 Failed RF Amplifier(s)

If audible distortion is heard on the air and the problem is not
in theprogram content oraudio processing equipment, the next
step is to determine if an RF amplifier has failed. If any RF
amplifier module between position RF33 to RF56 fails, the
carrier power will decrease. The failure of a higher step will not
be noticed as a drop in power. The audio distortion will also be
slightly higher but may not be noticeable with only one RF
amplifier failed. If an amplifier has failed, a substitute may be
used by the FlexPatch™ method. Refer to the FlexPatch™
procedure in this section.

6.29.1.2 Transmitter Mistuning

The transmitter will tune into a wide range of loads and still
produce very good audio performance. Refer to the initial turn
on procedure in SECTION II, Installation, for Tuning and
Loading procedures

6.29.1.3 Operating Into a Bandwidth Restricted Antenna

If the Distortion is poor, especially at the higher audio frequen­cies, then the antenna impedance at the sideband frequencies
may be incorrect. Operate the transmitter into a known good
dummy load. Ifthe highfrequency distortion improves,suspect
the antenna system.

6.29.1.4 Low RF Drive Level To The PA Modules

Low RF drive levels can cause higher than normal distortion.
Typically, the RF drive to the PA Modules should be between
20 and 25 Vp-p. Refer to this section for information on meas­uring RF drive levels.

6.29.1.5 Additional Tips For Troubleshooting Audio THD

If the distortion problem cannot be found using the above
means, an excellent way to determine if the distortion is in the
Analog Input or the analogto digitalconversionprocess OR the
digital to analog conversion process in the Power Amplifier or
Output Network stage, is to measure the distortion out of the
Digital to Analog convertor circuit on the Analog to Digital
Converter. This sample is an actual reconstructed audio sample
from the Analog toDigital Converter. If any distortion is occur­ring in the Analog Input Board, or in the Analog to Digital
conversion process, it will show up here. Connect a scope or a
distortion analyzer to J2 on the Analog to Digital Converter. If
the distortion is present here, troubleshoot the audio source,
Analog Input Board, or the Analog to Digital Converter. If the
distortion is not present, the distortion is occurring in the D to
A process and could be in the Modulation Encoder, RF ampli­fiers, or output Network.

6.28.1.5 Failed RF Amplifier

If an RF amplifier fails, the positive peak capability of the
transmitter will decrease. The power output will also decrease
if one of the first 23 PA Modules fail. Check the RF amplifier
fault LED’s to see if any are illuminated.

6-16 888-2297-002 Rev. R: 11-11-96

WARNING: Disconnectprimary power prior to servicing.

Section VI - Troubleshooting

6.30 Symptom: Consistent Loss of RF

Amplifiers

Any type of consistent or repeated failure indicates a problem
on the RF amplifier or in the location in the transmitter.

The most common method of troubleshooting an RF amplifier
aftera failure istoput the repairedamplifierin a knownworking
step, and to put the working amplifier where the failure first
occurred. This will tell whether the amplifier fault was caused
by the position it was in or by the amplifier itself.

6.30.1

6.30.1.1 Possible Causes

6.30.1.1.1 Defective Transistor Pad

A torn or damaged MOSFET insulator pad can short the tran­sistor. Check for debris lodged in the pad or sharp burr or high
spot on heat sink.

6.30.1.1.2 Defective Turn On/Turn Off Circuitry

Check all low level circuitry on the RF amplifier for defective
components. If onlyone side ofamplifier fails, thenconcentrate
on that circuitry. If nothing significant is found, check the drive
phasing with the amplifier fuses removed. Refer to “Measuring
Drive Phasing,” in this section.

6.30.1.1.3 Improper RF Drive

Refer to the “Measuring RF Drive Level” procedure in this
section. If RF drive is not correct and the transistors are not
defective, suspect RF drive transformers T1 and/or T2 and gate
protection transorbs.

6.30.1.1.4 Poor Solder Connection on PC Board

Inspect all solder connections, especially the drive transformer
loadsofT1and/orT2.

6.30.2

6.30.2.1 Possible Causes

6.30.2.1.1 Improper RF Drive

The RF drive to the PA Modules must be between 20 and
25Vp-p for proper operation. The phase of the drive must also
be within five degrees of the other modules. Refer to the
paragraphs in this section on “Measuring RF Drive Level,” and
“Measuring RF Drain Phasing” for drive amplitude and phase
measurement procedures. Causes of improper drive amplitude
and phasing are defective RF amplifier transistors, defectiveRF
drive cable, or poor motherboard connections.

6.30.2.1.2 Improper Drain Phasing

Just as the RF drive must be within five degrees of the other
modules, thephase of the drain switching waveformsof the RF
amplifierMOSFET’smust also be in phasewithin five degrees.
Even if the RF drive is correct, other problems can cause the
drain phasing to be out of tolerance. To measure the drain
phasing, refer to the paragraphs in this section on “Measuring
RF Amp Drain Phasing.” Causes of improper drain phasing are
a poor connection or wrongtapping ofthe efficiency coil forthe

Repeated Loss of Same RF Amplifier In Any
Position

Consistent Loss Of An RF Amplifier In One
Particular Position

RF amplifier, incorrect MOSFET transistors, or a defective RF
amplifieroutput toroid.An RF amplifier thatfails fromphasing
problemswill operate forashort time beforefailure. Duringthis
condition, the module will operate hotter than the other mod­ules. This is a good indication of a module operating out of
phase.

6.30.2.1.3 Defective Output Toroid

The output toroid for each RF amplifier must couple the RF
output to the combiner. If the toroid is defective, the amplifier
will not operate efficiently and fail. Check the toroid for cracks
or signs of arcing. Some inspection can be done through the
holes in the combiner cover, but a more through inspection
requires removal of the combiner cover. Refer to the Main
Combiner/Motherboard replacement paragraphs in SECTION
V, Maintenance.

6.30.2.1.4 Improper Control Signal

The Turn-on/Turn-off control signals from the Modulation En­coder should be at consistent levels for all steps. Check the
Encoder outputs to compare these signals.

6.30.3

6.30.3.1 Possible Causes

6.30.3.1.1 Analog to Digital Phasing Improperly Set

Consistent Loss Of Modules In Random Positions

An improperly set analog to digital phasing circuit will cause
random failures of RF amplifiers especially at the higher steps.
See paragraphs on “Analog to Digital Phasing Check” in SEC-
TION V, Maintenance.

6.30.3.1.2 Modulated B- Improperly Set

An improperly set Modulated B- can cause random RF ampli­fier failures. See paragraph on “Overall Modulated B- Adjust­ment” in SECTION V, Maintenance.

6.30.3.1.3 Improper VSWR Circuit Operation

If the VSWR protection circuit, including the Oscillator Sync
circuit, innot set properly, random failures RF amplifiers could
occur during VSWR conditions. To test the VSWR circuitry,
depress the ColorStat™ panel VSWR SENSOR Manual Test
Button. At that time both the Bandpass Filter and Antenna
VSWR LED’s should momentarilyilluminate RED,then return
GREEN. Refer to the paragraphs on “Output Monitor” and
“Oscillator Sync” adjustments in SECTION V, Maintenance.

6.30.3.1.4 Improper Overload Settings

If an overload is improperly set or not working, the RF amplifiers
could fail during an overload condition. Refer to the “Overload
Adjustment Procedures ”in SECTION V, Maintenance.

6.30.3.1.5 Improper Air Flow

Insufficient air flow should be detected by the Air interlock
circuitry and should shut the transmitter OFF. If the circuit is
defective or defeated, overheating modules could cause prema­ture failures. Refer to SECTION V, Maintenance, for aircircuit
adjustment procedures.

6.30.3.1.6 Transmitter Mistuning

Transmittermistuning could causethe Power Amplifierstage to
be inefficient and cause modules to run hotter. Refer to the

Rev. R: 11-11-96 888-2297-002 6-17

WARNING: Disconnect primary power prior to servicing.

DX-25U
transmittertuning procedure inSECTION II,Installation/Initial

turn on, for additional information.

6.31 Other Troubleshooting Techniques

6.31.1 Handling MOSFET’s

Due to the fragile nature of the gate of a MOSFET, special care
intheirhandling is required. Thegate junctionmay bedestroyed
ifstaticelectricityis allowed to discharge through the MOSFET.
For example, a static charge could build up on a person as they
walk across a carpet and discharge across the MOSFET if it is
not protected by antistatic packaging.

NOTE

MOSFET transistors which are in circuit are immune to this
damage.

The MOSFET transistors are shipped in antistatic packaging.
The transistors should remain in this packaginguntil theyare to
be used or tested. Proper precautions should be observed to
ground any potential static charge before handling the MOS­FETS.

6.31.2

The MOSFET’s will have to be removed from the circuit in
order to perform the following test.

Observe the precautions in the paragraph entitled “Handling
MOSFET’S” in this section.

The MOSFET’s used in the DX-25U transmitter may be
checked with an ohmmeter. However, there is a requirement
whichrestricts the useof someohmmeters.If the battery voltage
is too low (under 3V) or too high (over 20V) the ohmmeter
cannot be used. A battery voltage less than 3V will not give an
operational check of the transistor and a battery voltage greater
than 20V may result in damage to the transistor under test. A
Simpson 260, which uses a 9V battery on the Rx10K scale
works quite well.

This testwill show how a MOSFET can be switched “ON” and
“OFF” by charging and discharging the gate of the MOSFET.

Connect the positive lead of the ohmmeter to the drain or case
ofthetransistor.Connect thenegativeleadtosource. Alternately
touch a jumper from gate to source and then from gate to drain.
The ohmmeter should read towards infinity or at least 2 me­gOhmswhen the MOSFETisswitched “OFF” and lessthan 90k
Ohms when the MOSFET is switched “ON.” When doing this
test, laythe MOSFETon a flat surface orhold sides of the case.
The resistance of your finger tips and skin will effect the
readings when you touch the leads.

6.31.3

It ispossible to havea failed PA Module without the Fault LED
being illuminated. It should be possible to see this problem on
the detectedaudio waveform. If a “Big Step” PA Module is not
operatingproperly,an errorinthe demodulated audio waveform
will be present as seen in Figure 6-1. This kind of error is

Testing MOSFET’s

Finding A Missing Step

apparent only with triangle (linear ramp) tone modulation. If a
PA Module failure is suspected, operate the transmitter at 30
kW at full modulation. Display the demodulated audio output
of a modulation monitor on a dual trace scope. If a big step
amplifier is not working, an error will be noticeable on the
display. To determine which amplifier is at fault, connect the
second channelof the scope to a probe andlook at the output of
the Modulation Encoder for each big step until the transition
from 0to 5VDC occurs at the same point in time as the error on
thewaveform.SeeFigure6-1.

Becausethere are 58“Big Steps”, it helpsto knowwhereto start
tolook onthemodulation encoder.A good ruleis thatthehigher
the positive peak level where the error occurs, the higher the
step number. A 100% modulated signal at30 kW is NOT using
allthebigsteps — steps RF48through RF58are only ON during
positive peaks. To check these steps, a non-symmetrical audio
waveform should be used in order to modulate the transmitter
with a steady state tone at 125% positive peak without causing
overmodulation, and carrier shift, on the negative peaks.

Figure 6-2 shows a non-symmetrical ramp modulating to125%
positive peak and only 50% negative peak. An error is also
shown near the top of the positive peak indicating a big step
failure at approximately “Big Step” RF55. The Modulation
Encoder waveform is also shown for that step. Once the step
causing the error is located, the PA Module can be changed. If
substituting the PA Module does not remove the error, then the
problem may exist on the Modulation Encoder.

6.31.4

Using FlexPatch™ for Bypassing a Failed
PA Module

FlexPatch™ is a Harris feature to allow the engineer to patch a
failed RF amplifier from an active step position, such as step
RF6, to a step position that is only used for positive peak
modulation, suchas step RF90. Module RF90will nowoperate
as module RF6. This patching is done at a “TTL” level basis
and can be done while the transmitter is on the air.

Because approximately 47 RF amplifiers are used to create 50
kW carrier, a failed module in the RF6 position will result in a
power output drop of approximately 2% and a slight THD
increase. The transmitter will operate with no other problems in
this condition. However,FlexPatch™ willallow thetransmitter
to resume operation at full power and optimum modulation
clarity with only a slight loss of positivepeak capability.

Refer to Figure 6-8 for the following procedure. To use the
FlexPatch™ feature to bypass a failed PA Module:

a. On the Modulation Encoder controlling the failed RF

amplifier,locate and removethe gold jumper forthefailed
RF amplifier to be patched.

b. Locate and remove the gold jumper for step RF90 on

Modulation Encoder A37. Step RF90 is selected because
it would only be ON during the highest positive peaks.

c. Locate and remove one of the FlexPatch™ jumpers and

connect one end to the OUTPUT signal side from where
the jumper plug was removed for the failed amplifier.

6-18 888-2297-002 Rev. R: 11-11-96

WARNING: Disconnectprimary power prior to servicing.