Codan 8525B Series, 8528 Series Technical & Service Manual

HF SSB Transceiver

8525B/8528 series

Technical Service Manual

Volume 1

 1989 Codan Pty Ltd.

ACN 007 590 605

Order code 15-02036

Issue 6, November 1995

Head Office

Codan Pty Ltd
ACN 007 590 605
81 Graves Street
Newton
South Australia 5074
International [phone]
Facsimile [phone]

Marketing Offices

Codan (U.K.) Ltd
Gostrey House
Union Road
Farnham, Surrey GU9 7PT
United Kingdom
International [phone]
Facsimile [phone]
Telex 858355

Codan Pty Ltd
Suite 11A, 2 Hardy Street
South Perth
Western Aus tralia 6151
International [phone]
Facsimile [phone]

1 General Information

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

1.2 Specifications .....................................................................................................1-2

1.2.1 General ............................................................................................1-2

1.2.2 Receiver...........................................................................................1-4

1.2.3 Transmitter......................................................................................1-6

1.2.4 Rear Panel Connectors ....................................................................1-7

1.2.5 Front Panel Connector...................................................................1-10

1.3 Options and Accessories ..................................................................................1-11

1.3.1 Options..........................................................................................1-11

Table of Contents

1.3.2 Accessories....................................................................................1-12

1.4 Abbreviations ...................................................................................................1-13

1.5 Circuit Reference Designations........................................................................1-20

1.6 Units.................................................................................................................1-21

1.7 Unit Multipliers................................................................................................1-21

2 Block Diagram

3 Brief Description

3.1 General ...............................................................................................................3-1

3.2 Receive...............................................................................................................3-1

3.3 Transmit .............................................................................................................3-2

3.4 Synthesizers........................................................................................................3-2

3.5 Display, Control and Switching.........................................................................3-3

3.6 External Power Amplifiers.................................................................................3-3

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ii Table of Contents 8525B/8528 Technical Service Manual

4 Technical Description

4.1 Control and Supply Voltages..............................................................................4-1

4.1.1 Supply Voltages ..............................................................................4-1

4.1.2 Power and Control Lines.................................................................4-2

4.1.3 Overvoltage and Reverse Voltage Protection..................................4-2

4.1.4 Transceiver Power On/Off ..............................................................4-2

4.1 5 High Power Transceiver Protection and Power On/Off..................4-3

4.2 Receiver..............................................................................................................4-4

4.2.1 Input Filters.....................................................................................4-4

4.2.2 Mixers, First IF Amplifiers and Filters ...........................................4-4

4.2.3 Noise Blanker..................................................................................4-5

4.2.4 IF Crystal Filter and Amplifier........................................................4-5

4.2.5 Demodulator....................................................................................4-5

4.2.6 Audio Mute .....................................................................................4-6

4.2.7 Volume Control and Audio Amplifier............................................4-6

4.3 Transmitter Exciter.............................................................................................4-7

4.3.1 Microphone Amplifier.....................................................................4-7

4.3.2 Modulator........................................................................................4-7

4.3.3 Crystal Filter....................................................................................4-8

4.3.4 Mixers, IF Filters and IF Amplifier.................................................4-8

4.4 Local Oscillators.................................................................................................4-9

4.4.1 Voltage Controlled Oscillators (VCO)............................................4-9

4.4.2 Phase/Frequency Detectors and Dividers........................................4-9

4.4.3 Lock Signals..................................................................................4-10

4.4.4 Loading of Frequency Data to Synthesizers..................................4-10

4.4.5 1650/1647kHz Local Oscillators...................................................4-11

4.4.6 Clarifier .........................................................................................4-11

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8525B/8528 Technical Service Manual Table of Contents iii

4.5 Microprocessor Controller PCB (04-02451 & 04-03031)................................4-12

4.5.1 Microcontroller Bus ......................................................................4-12

4.5.2 Internal I2C Bus.............................................................................4-13

4.5.3 External Bus..................................................................................4-14

4.5.4 Reset and Watchdog Circuit..........................................................4-14

4.5.5 RS-232 Port (ALE Option Only)...................................................4-14

4.6 Motherboard and Chassis (02-02453)..............................................................4-15

4.6.1 Power Supplies..............................................................................4-15

4.6.2 Antenna Control Facility...............................................................4-15

4.6.3 Extended or Remote Control—Option R......................................4-15

4.6.4 External Microphone Audio Clamp..............................................4-16

4.6.5 Additional Audio Facilities—Option RC......................................4-16

4.6.6 Headphone Output—Option PH ...................................................4-16

4.6.7 CW Facility—Option M................................................................4-16

4.6.8 External Selective Call or RTTY—Option PS..............................4-16

4.6.9 AGC Dump Circuit .......................................................................4-17

4.7 8525B Front Panels and Control Head.............................................................4-17

4.7.1 Display PCB..................................................................................4-17

4.7.2 Switch Matrix................................................................................4-18

4.7.3 Digiswitches..................................................................................4-18

4.7.4 Microphone Amplifier and Interface.............................................4-18

4.8 8528 Front Panel and Control Head.................................................................4-19

4.8.1 Display PCB, LCD........................................................................4-19

4.8.2 Indicator and Display Dimming....................................................4-20

4.8.3 Links..............................................................................................4-20

4.8.4 Switch Matrix................................................................................4-21

4.8.5 Microphone Amplifier and Interface.............................................4-21

4.8.6 8532 Display Head........................................................................4-21

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iv Table of Contents 8525B/8528 Technical Service Manual

4.9 PA and Filters (04-02452)................................................................................4-24

4.9.1 Introduction...................................................................................4-24

4.9.2 PTT Control...................................................................................4-24

4.9.3 Gain Control Stage........................................................................4-24

4.9.4 Pre-Driver Stages ..........................................................................4-24

4.9.5 Driver Stage...................................................................................4-25

4.9.6 Output Stage and Bias Regulator..................................................4-25

4.9.7 Output Filters and Control.............................................................4-25

4.9.8 ALC Control..................................................................................4-26

4.9.9 Transmit Indicator.........................................................................4-27

4.10 PA Exciter Interface.........................................................................................4-27

4.10.1 Introduction...................................................................................4-27

4.10.2 Control Lines.................................................................................4-27

4.10.3 Power On/Off................................................................................4-27

4.10.4 PTT Circuit....................................................................................4-27

4.10.5 Transmit Amplifier........................................................................4-28

4.10.6 Receive Circuit..............................................................................4-28

4.10.7 Antenna Control............................................................................4-28

4.10.8 PA Unit..........................................................................................4-28

4.11 Selective Calling (04-02250)............................................................................4-29

4.11.1 Selective Calling ...........................................................................4-29

4.11.2 Selective Call Detection (04-02250).............................................4-30

4.12 Two-Tone Calling ............................................................................................4-31

4.12.1 Two-Tone Calling .........................................................................4-31

4.12.2 Two-Tone Detection......................................................................4-31

4.13 Option PS—External Signalling Interface.......................................................4-33

4.13.1 External Selective Calling.............................................................4-33

4.13.2 Selective Call Scanning.................................................................4-34

4.13.3 RTTY—ARQ mode......................................................................4-34

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8525B/8528 Technical Service Manual Table of Contents v

4.14 Options.............................................................................................................4-36

4.14.1 F—Fan Assembly..........................................................................4-36

4.14.2 LU—Switched LSB Operation......................................................4-36

4.14.3 L—Lower Sideband Operation.....................................................4-37

4.14.4 M—CW Facility............................................................................4-37

4.14.5 PH—Headphone Output ...............................................................4-37

4.14.6 R—Extended/Remote Control Interface.......................................4-37

4.14.7 AD—Antenna Driver....................................................................4-37

5 Operating Instructions

6 Maintenance

6.1 General ...............................................................................................................6-1

6.1.1 CMOS Devices................................................................................6-1

6.1.2 Circuit Boards .................................................................................6-1

6.1.3 Transmitter Precautions...................................................................6-3

6.1.4 Probe Precautions............................................................................6-3

6.1.5 Surface Mounted Components........................................................6-3

6.2 Fault Diagnosis...................................................................................................6-4

6.2.1 General ............................................................................................6-4

6.2.2 Voltage Measurement......................................................................6-5

6.2.3 Front Panel Controls........................................................................6-6

6.2.4 Logic Levels....................................................................................6-7

6.2.5 No Reception...................................................................................6-7

6.2.6 No Transmission .............................................................................6-8

6.2.7 Unlocked Synthesizer......................................................................6-8

6.2.8 Typical PA Voltages........................................................................6-8

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vi Table of Contents 8525B/8528 Technical Service Manual

6.3 Dismantling and Re-assembly............................................................................6-9

6.3.1 General ............................................................................................6-9

6.3.2 Top and Bottom Covers ..................................................................6-9

6.3.3 Circuit Board Removal....................................................................6-9

6.3.4 PA and Filter Assembly ................................................................6-10

6.3.5 Replacement of PA Transistors.....................................................6-11

6.3.6 Replacement of Escutcheon or Switch Substrate..........................6-12

7 Adjustments

7.1 Introduction........................................................................................................7-1

7.2 Channel Addition (EPROM Replacement)........................................................7-1

7.2.1 Channel Addition, Programming P Channels (8528)......................7-2

7.3 Preset Adjustments.............................................................................................7-5

7.3.1 Test Equipment Required................................................................7-5

7.3.2 Voltage Regulators..........................................................................7-6

7.3.3 Crystal Oven....................................................................................7-6

7.3.4 VCO Adjustments (RF Mixer and Dual Synthesizer PCB)............7-6

7.3.5 45MHz IF Alignment—Receive.....................................................7-7

7.3.6 1650kHz IF Alignment....................................................................7-7

7.3.7 45MHz IF Alignment—Transmit....................................................7-7

7.3.8 Highpass Filters...............................................................................7-9

7.3.9 Noise Limiter.................................................................................7-10

7.3.10 Frequency Adjustment (USB).......................................................7-10

7.3.11 Frequency Adjustment (LSB)........................................................7-10

7.3.12 Mute Adjustments .........................................................................7-11

7.3.13 PA Adjustments.............................................................................7-11

7.3.14 Output Power.................................................................................7-11

7.3.15 Options SD, SDE and SDEM........................................................7-13

7.3.16 Option TD .....................................................................................7-13

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8525B/8528 Technical Service Manual Table of Contents vii

7.3.17 Selective Call Code Settings (8525B)...........................................7-14

7.3.18 Message Length Setting ................................................................7-15

7.3.19 Selective Call Code Settings (8528)..............................................7-15

7.4 Receiver Performance Checks..........................................................................7-16

7.4.1 Sensitivity and Signal + Noise to Noise Ratio..............................7-16

7.4.2 AGC Check...................................................................................7-16

7.4.3 Audio Output.................................................................................7-16

7.4.4 Selectivity (USB Operation)..........................................................7-16

7.4.5 Clarifier Operation ........................................................................7-16

7.4.6 Noise Limiter Operation................................................................7-16

7.5 Transmitter Performance Checks.....................................................................7-17

7.6 Emergency Call Frequencies............................................................................7-18

8 Parts List

8.1 General Information...........................................................................................8-1

8.2 Parts List.............................................................................................................8-3

7.5.1 Frequency......................................................................................7-17

7.5.2 Clarifier .........................................................................................7-17

7.5.3 ALC...............................................................................................7-17

7.5.4 Power Output and Intermodulation Distortion..............................7-17

8.1.1 Ordering Information.......................................................................8-1

8.1.2 Component Substitution..................................................................8-2

9 Diagrams

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viii Table of Contents 8525B/8528 Technical Service Manual

Issue 6 Publication No: 15-02036

8525B/8528 Technical Service Manual 1-1

1 General Information

1.1 Introduction

The Type 8525B Series and 8528 Series transceivers feature synthesized
frequency generation and have sealed membrane controls. The two series
differ only in control and display facilities. All differences are in the front
panel and the controlling EPROM. All transceiver functions are controlled
by a central microprocessor, enabling facilities such as clarifier, emergency
call, etc to be included as standard fit.

The 8525B comprises two versions: one for front panel control and one for
extended control. The extended control transceiver has a blank front panel
and is connected by up to 100 metres of cable to an 8530 control head. The
control head may also be used with the front control version allowing for
both local and extended control. Channel capacity of the 8525B is 99 single
or two-frequency simplex channels in EPROM, and channel display is by a
two-digit LED whose brightness is controlled by ambient illumination.

The 8528 series comprises two basic models: the land-based 8528 and the
marine 8528S. Two versions of each model are available: one for front
control and one for extended control. The extended control transceiver has a
blank front panel and is connected by up to 100 metres of cable to an 8532
or 8531S control head. The control head may also be used with the front
control version allowing for both local and extended control.

Channel capacity of the 8528 is 501 single or two frequency simplex
channels in EPROM and 99 channels in EEPROM. Transmit frequencies
may be front-panel entered by a qualified technician or where permitted, by
the operator. Receive frequencies may be entered by the operator.

The 8528 series has a twenty-digit liquid crystal display showing the
transmit and receive frequencies and channel number, or selective call send
and receive identification. Selective call addresses may be entered by
keypads on the 8528.

For transmission power in excess of 100W, the internal PA in the 8528S is
replaced by a PA driver and an external HF4000 series PA is used. The
external PAs are covered in a separate Technical Service Manual.

Publication No: 15-02036 Issue 6

1-2 General Information 8525B/8528 Technical Service Manual

1.2 Specifications

Specification figures will normally be exceeded by production equipment.
Where relevant, acceptance limits are given in parentheses. All
measurements are made at 13.6V DC, 50Ω source and load, and 25°C

ambient temperature (unless otherwise specified).

1.2.1 General

Specifications
cover

Frequency Range Transmit: 2 to 24MHz

Frequency
Generation

Channel Capacity 8525B: 99 single or two-frequency

8525B series; 8528 series including
8528S and H versions. (H transmission
specifications included in a separate
Technical Service Manual (code 2037),
covering the 4402 and 4404 PA units).

Receive: 0.25 to 30MHz
All frequencies generated by synthesizer.

simplex channels with 10Hz resolution
controlled by plug-in EPROM
8528: single or two-frequency simplex
channels with 10Hz resolution controlled
by plug-in EPROM and 99 EEPROM
controlled channels with 100Hz
resolution.The total number of channels
will depend on the version of 8528 series
to a maximum of 600.
Receive frequencies of the EEPROM—
controlled channels can be entered from
the front panel by the operator.
Transmit frequencies of the EEPROM—
controlled channels can be entered from
the front panel by qualified technical
personnel or (where authorised) by the
operator.

Operating Modes Land: Single sideband (J3E) USB, with

LSB (Option L) or switched sideband
(Option LU) available as options.

Marine: Single sideband (J3E) USB.
LSB may be programmed for specific
frequencies (requires Option L). On
2182kHz J3E or H3E USB selectable.

Frequency
Stability

Long term ageing 1ppm per year.

Issue 6 Publication No: 15-02036

USB: ±2(3)ppm –30°C to +60°C
LSB: ±2(3)ppm ± 20Hz −30°C to +60°C

8525B/8528 Technical Service Manual General Information 1-3

Oven warm-up

1 minute.

time
Controls Sealed membrane switches.
Indicators Refer to illustrations for details.

8525B: All indicators except Tx
automatically dimmed for low ambient
light levels.
8528: All indicators except Tx may be
manually dimmed by front panel controls
if required.

Transmit/Receive
Switching

Approximately 20ms using the Option
PS connection. Transmit/receive
frequencies may be separated by up to
1MHz.

Connectors RF—UHF connector

Extended/Local control (Option R)
required for Control Head operation or
remote control.
Antenna control
Extension Loudspeaker
External Selective Call (Option PS)

External Alarm (for Options SDE and
SDEM) and/or unswitched battery for
remote control (Option PP)
Note: Options PS and PP are mutually
exclusive.

RF Input/Output

50 ohms nominal.

Impedance
Supply Voltage 12V DC nominal, negative earth. Normal

operating range 10.5 to 15V DC.
Maximum operating range 9 to 16V DC.

Overvoltage
Protection

Shutdown at 16V DC (nominal) for
duration of overvoltage.

Supply Current 8525B: Receive, no signal 400–550mA

8528: Receive, no signal, display
illumination on: 400mA; off: 340mA
Transmit: see section 1.2.3

Environmental Ambient

Relative Humidity

Temperature
–30°C to +30°C 95%
+30°C to +60°C from 95% at +30°C

to 30% at +60°C

Derate upper ambient temperature by
1°C per 330m above sea level.

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1-4 General Information 8525B/8528 Technical Service Manual

Cooling Convection or fan (Option F).
Size and weight Transceiver only:

250mm W x 78mm H x 350mm D;

3.41kg
With mounting cradle:

270mm W x 90mm H x 350mm D;

3.92kg
8530 Control Head:

180mm W x 65mm H x 70mm D
With mounting bracket:

190mm W x 78mm H x 70mm D; 700g
8531 Control Head:

248mm W x 78mm H x 70mm D
With mounting bracket: 252mm W x

92mm H x 70mm D; 900g

Finish Case: Silver-grey

1.2.2 Receiver

Type Dual conversion superheterodyne.
IF Frequencies 45MHz, 1650kHz.
Sensitivity 0.25 to 0.5MHz:

Note: Depth measurements (D) include

rear connectors/cables.

Panel surround and heat sink: Matt black

Panel overlay: Lexan—Matt black
Painted surfaces are scratch-resistant

textured polyester powdercoat.

Not specified

0.5 to 2MHz:
2 to 28MHz:

Approximately 8µV PD

0.25(0.35)µV PD [–119(–

116)dBm] for 10dB
SINAD with greater than
50mW audio output.

28 to 30MHz:

0.5µV PD

Input Protection Will withstand 50V rms RF from a 50Ω

source.

Selectivity Greater than 70(65)dB at –1kHz and

+4.2kHz ref SCF USB.
Pass band –6(–8)dB 300 – 2800Hz.
Ripple 1.5(3)dB pp 500 – 2500Hz.

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8525B/8528 Technical Service Manual General Information 1-5

Desensitisation 10dB SINAD reduced to 7dB SINAD.

–1 and +4.4kHz ref SCF (USB)
62(55)dB.
±10kHz 85(80)dB.
±50kHz 100(95)dB.

Image Rejection Better than 90(80)dB
Spurious

Responses

Better than 90(70)dB.
Self generated signals >0.25µV PD;

6.6MHz, 9.998MHz, 13.2MHz,

19.8MHz, 26.4MHz.

Cross Modulation A signal 90(85)dB above a signal

producing 10dB SINAD, modulated 30%
and removed at least 20kHz from the
wanted signal will produce an increase in
receiver noise of less than 3dB.

Blocking As for desensitisation.
Intermodulation To produce a third order intermodulation

product equivalent to a wanted signal
producing 10dB SINAD, two unwanted
signals greater than 30kHz removed
from the wanted signal must have a level
greater than 85(80)dB above the wanted
signal. Third order intercept +10(7)dBm,
not affected by AGC.

AGC Less than 6dB variation in output level

for input variations between 1.5(2.5)µV
and 100mV PD. Fast attack, slow
release.

AF Response –1 typ(–3)dB 300 to 2800Hz.
AF Power and

Distortion

2.5W into 8 ohms: 5% THD
5W into 4 ohms: 5% THD
8W into 2 ohms: 5% THD

Clarifier Land: Nominal ±0.001%

Marine: ±180Hz
Clarifier is automatically reset to
mid-range with channel change.

Inband IMD Better than 25dB IMD with two 50mV

PD RF inputs.

Signal to noise vs
Input Level

5µV PD: 34dB S/N
50µV PD: 53dB S/N
500µV PD: 60dB S/N

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1-6 General Information 8525B/8528 Technical Service Manual

1.2.3 Transmitter

Power Output
Land

100W PEP at 2MHz falling linearly with
frequency to 90W PEP at 24MHz ±0.5dB.

Marine 125W PEP at 2MHz falling linearly with

frequency to 90W PEP at 24MHz ±1dB.
CW or single tone: approximately 60%
of PEP with average PEP control.

Duty Cycle 100% normal speech over full

temperature range.
100% ARQ up to 30°C.
25% continuous data mode (5 minutes
on maximum) at ambient temperature up
to
30°C.
100% all modes up to maximum ambient

of 45°C with Option F.

Supply Current Output power 100/125W

2-tone or CW: 9 to 12A
Average speech: 6A for battery life
calculations.

Protection Safe under all load conditions by limiting

reflected power to 6W and limiting PA
transistor collector voltage swing.
Thermal protection against excessive
heatsink temperature.

AF Response Overall response of microphone and

transmitter rises approximately
6dB/octave 300 – 2800Hz.
Electrical input −6(−8)dB,
300 – 2800Hz.
Ripple 1.5(3)dB pp, 500 – 2500Hz.

Spurious &

Better than 60(48)dB below PEP.

Harmonic
Emissions

Carrier

60(50)dB below PEP.

Suppression
Unwanted

Sideband
Intermodulation

Distortion 2-tone
Test

50(45)dB below PEP (400Hz)
70(65)dB below PEP (1kHz)

100W: 32(26)dB below each tone

38(32)dB below PEP

125W: 30(26)dB below each tone

36(32)dB below PEP

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8525B/8528 Technical Service Manual General Information 1-7

ALC A 10dB increase in input level above

compression threshold produces less
than 0.5dB increase in power output.
Maximum ALC range greater than 30dB.
ALC attack time approximately 1ms.

Residual Noise 65(55)dB below PEP in 3kHz channel.
Transmitter Noise

Floor

Transmitter noise output below cut-off
frequency of harmonic filter in 3kHz
bandwidth typically −67dBm.

Microphone Dynamic type with push-to-talk switch

fitted in case.

1.2.4 Rear Panel Connectors

The following tables show the pin connections, functions, and signal levels
for the rear connectors.

Antenna Select Facility (15-way, D-type Socket)

Pin Number Function Signal Level

1 Channel Number Bit

3

2 Channel Number Bit

4

3N/C

Logic (Open Collector)

Logic (Open Collector)

4 TUNE IN/OUT 5V Logic (Active Low)
5 SCAN (Active

Logic (Open Collector)

Antenna)

6 & 7 N/C

8 PTT out +10V (1kΩ source)
9 Channel Number Bit

Logic (Open Collector)

1

10 Channel Number Bit

Logic (Open Collector)

2

11 TUNED IN 5V Logic (Active Low)
12 & 13 A rail + Battery supply out
14 & 15 0V Ground

N/C = Not Connected
All channel number bits active high. (Pull-up resistors required).

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1-8 General Information 8525B/8528 Technical Service Manual

Option R (15-way, D-type Plug)

Pin Number Function Signal Level

1 Loudspeaker Ref Spec
2 Remote PTT 0 Volts = PTT
3 Receiver Audio

Special *

Output

4 Power On Momentary 0V for

Power ON
5 Data 5 Volt Logic
6 Data Line Enable 5 Volt Logic
7 Clock 5 Volt Logic
8 Transmit Lamp Modulated by Tx

output

9 & 10 0V Ground

11 Transmit Audio I/P Nom. 1.5Vpp (8kΩ

input impedance)

12 Receiver

Demodulator O/P

Nom. 1.5Vpp (1kΩ

output impedance)

(Min. load 5kΩ)

13 Receiver Audio I/P Special*
14 Interrupt 5 Volt Logic
15 A rail + Battery supply out

* Special: adjusted to suit attached equipment

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8525B/8528 Technical Service Manual General Information 1-9

Option PS
Note: Option PS cannot be fitted with internal option SDE or SDEM.

Pin Number Function Signal Level

1 0V Ground
2 Rx FSK Tones

Output

Nom. 1.5Vpp (From
2k ohms)

3 FSK Tx Tones Input 3Vpp (22k ohm Input

Impedance)

4 Quiet Line +10V (ON) or

floating (OFF)
5 Alarm Tones I/P 3Vpp into 100K ohms
6 PTT Input: 0V = PTT
7 Scan +10V output in scan
8 A rail + Battery supply out

Options SDE, SDEM (External Alarm) and PP (Supply for remote
control)
Note: Options SDE or SDEM cannot be fitted with Option PS.

Pin Number Function Signal Level

1 Unswitched Supply Battery +ve (PP only)

2 & 3 External Alarm Contacts rated at 50V

1A DC. Closed when
external alarm
required.

4 Unswitched Supply Ground (PP only)

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1-10 General Information 8525B/8528 Technical Service Manual

1.2.5 Front Panel Connector

Microphone Connector

Pin Number Function

1 PTT ground
2 PTT active low
3 Microphone input
4 Microphone

ground

5 Speaker

connection*
6 Audio output
7 Audio output

*Linked to pin 7 for front panel speaker operation.

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8525B/8528 Technical Service Manual General Information 1-11

1.3 Options and Accessories

1.3.1 Options

The following is a list of the options available with brief descriptions. The
references in brackets indicate more detailed descriptions will be found in
those sections of this manual.

AD Fit antenna driver interface for 8558 automatic tuning

whip antenna.

F Fit fan for continuous data transmission.
LF 1.6 to 2MHz—Receive at 0.25µV sensitivity.
LU Fit for USB and LSB operation on selected channels.

USB or LSB selected on front panel (land only) (4.4.5).

L Fit for LSB only operation on selected channels (4.4.5).
M* CW facility (4.6.7).
PH* Headphone output (4.6.6).
PP Fit unswitched battery power output facility.
PS External selective call interface (4.13).
R* Extended/remote control interface (4.6.3).
SE Programmable 4 digit (internally preset) selective call

encoder (4.11).

SDE Programmable 4 digit (internally preset) selective call

encoder/decoder (4.11).

SDEM Front panel programmable 4 digit (mesh) selective call

encoder with (internally preset) decoder: 8525B only
(4.11).

SD Fit Selective Call Decode: 8528/8528S only.
TD Tone-operated decoder: (4.12).

* Required for front control transceivers only.

Programmable Options (per channel)

E RFDS Emergency call.
O Transmit clarifier (land only).
TE Program two-tone encode (specify frequency).
SE Program Selective Call Encode: 8528/8528S only.
INHIBIT Receive only.

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1-12 General Information 8525B/8528 Technical Service Manual

1.3.2 Accessories

The following is a list of the accessories available.

Code

112 Vehicle installation hardware kit.
114 Marine suppression kit.
117 Vehicle mounting cradle—front entry.

118 Vehicle mounting cradle—top/bottom entry.

Supplied complete with transceiver DC power cable
(6 metres).

121 2-module clamp suitable for locking transceiver with

another unit of compatible mechanical design.
164 Rack mounting frame 483mm (19").
602 Headphone complete with cable and connector.
654 Telephone handset complete with cable and

connector.
641 Desk microphone complete with cable and

connector.
649 Extension loudspeaker.
651PC Program package (8525B/8528 series). For use with

IBM compatible PC.
652 Morse key complete with base, cable and connector.
704 Vehicle interference suppression kit.
726 Channel decoder (1 of 14)—active low. For use with

relay switched antenna systems.
2036 Service manual for type 8528 series.
2037 Service manual for type 4402 and 4404.
Type

8530

Control head complete with 6 metres of interface

cable fitted with connectors and plug-in hand PTT

microphone. Suitable for transceiver 8525B.

(Requires Option RS to be fitted to the transceiver).
Type

8531
Type

8531S
Type

8532

Issue 6 Publication No: 15-02036

As above, suitable for 8528. (Requires Option R to

be fitted to the transceiver).

As above, suitable for 8528S. (Requires Option R to

be fitted to the transceiver).

As above, suitable for 8528. (Requires Option R to

be fitted to the transceiver).

8525B/8528 Technical Service Manual General Information 1-13

1.4 Abbreviations

A/D Analog to digital
A/F Across flats (hexagon)
AC Alternating Current
ACIA Asynchronous Communication Interface Adapter
ADJ Adjust
AF Audio Frequency
AFC Automatic Frequency Control
AGC Automatic Gain Control
ALC Automatic Level Control
ALF Absorption Limited Frequency
AM Amplitude Modulation
ASCII American Standard Code for Information

Interchange
ASSY Assembly
ATU Antenna Tuning Unit
AUX Auxiliary
AV Average
BAL Balance
BALUN Balanced to Unbalanced Transformer
BCD Binary Coded Decimal
BPF Band Pass Filter
BW Bandwidth
C/O Change-over
CAL Calibrate
CCT Circuit
CCW Counterclockwise
CH Channel
CMOS Complementary Metal Oxide Semiconductor
COAX Coaxial
COM Common
CPU Central Processing Unit
CRO Cathode Ray Oscilloscope
CRT Cathode Ray Tube
CSK Countersink

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1-14 General Information 8525B/8528 Technical Service Manual

CW Continuous Wave, Carrier Wave or Clockwise
D/A Digital to Analog
DC Direct Current
DEMUX Demultiplexer
DMA Direct Memory Access
DPDT Double Pole, Double Throw
DPST Double Pole, Single Throw
DRG Drawing
DSB Double Sideband
DTL Diode Transistor Logic
DVM Digital Voltmeter
dB Decibel
dBm Decibel relative to 1mW
EAPROM Electrically Alterable Programmable Read Only

Memory
ECL Emitter Coupled Logic
EDP Electronic Data Processing
EEPROM Electrically Erasable Programmable Read Only Memory
EMF Electromotive Force
EMI Electromagnetic Interference
EPROM Erasable Programmable Read Only Memory
EXT External
F/V Frequency to Voltage
FET Field Effect Transistor
FM Frequency Modulation
FREQ Frequency
FSK Frequency Shift Keying
FTTL Fast Transistor Transistor Logic
GND Ground
GPIB General Purpose Interface Bus
HCMOS High Speed Complementary Metal Oxide

Semiconductor
HEX Hexadecimal or Hexagon
HF High Frequency
HMOS High Speed Metal Oxide Semiconductor
HORIZ Horizontal

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8525B/8528 Technical Service Manual General Information 1-15

HPF High Pass Filter
I/COM Intercom
I/F Interface
I/O Input Output
I/P Input
IF Intermediate Frequency
IMD Intermodulation Distortion
INT Internal
ISB Independent Sideband
JFET Junction Field Effect Transistor
J3E Single Sideband Suppressed Carrier Telephony

Emission
LC Inductance-Capacitance
LCD Liquid Crystal Display
LDR Light Dependent Resistor
LED Light Emitting Diode
LF Low Frequency
LIN Linear Law
LNA Low Noise Amplifier
LO Local Oscillator
LOG Logarithmic Law
LPF Low Pass Filter
LS Loudspeaker or Low Power Schottky
LSB Lower Sideband or Least Significant Bit
LSI Large Scale Integration
LSTTL Low Power Schottky Transistor Transistor Logic
LTU Line Terminating Unit
MAX Maximum
MF Medium Frequency
MIN Minimum
MODEM Modulator-Demodulator
MOL Maximum Operating Level
MOS Metal Oxide Semiconductor
MPU Microprocessor

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1-16 General Information 8525B/8528 Technical Service Manual

MSB Most Significant Bit
MSI Medium Scale Integration
MUF Maximum Useable Frequency
MUX Multiplex, Multiplexer
N/C Normally Closed
N/O Normally Open
NC Not Connected
NMOS N Type Metal Oxide Semiconductor
NOL Normal Operating Level
NOM Nominal
NORM Normal
NPO Zero Temperature Coefficient
NTC Negative Temperature Coefficient
O/C Open Circuit
ODU Outdoor Unit
OMT Orthomode Transducer
O/P Output
OPR Operator
OPT Option
OSC Oscillator
OWF Optimum Working Frequency
PA Power Amplifier
PCB Printed Circuit Board
PCM Pulse Code Modulation
PD Potential Difference
PEP Peak Envelope Power
PH Phase
PIA Peripheral Interface Adapter
PIV Peak Inverse Voltage
PKG Package
PLL Phase Locked Loop
PMOS P Type Metal Oxide Semiconductor
POL Peak Operating Level

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8525B/8528 Technical Service Manual General Information 1-17

POT Potentiometer
PP Peak to Peak
PPM Parts per Million
PROM Programmable Read Only Memory
PSU Power Supply Unit
PTC Positive Temperature Coefficient (Resistor)
PTT Push To Talk
PUT Programmable Unijunction Transistor
PWM Pulse Width Modulation
RAM Random Access Memory
R/C Remote Control
RC Resistance-Capacitance
RCU Remote Control Unit
REF Reference
REG Regulated, Register
RF Radio Frequency
RFI Radio Frequency Interference
RMS Root Mean Square
ROL Reference Operating Level
ROM Read Only Memory
RTL Resistor Transistor Logic
RTTY Radio Teletype
Rx Receive, Receiver
S/C Short Circuit
S/N Signal To Noise
(S+N)/N Signal Plus Noise to Noise Ratio
SCF Suppressed Carrier Frequency
SCR Silicon Controlled Rectifier
SINAD Signal + Noise + Distortion to Noise + Distortion Ratio
SMPS Switching Mode Power Supply
SOT Select On Test
SPDT Single Pole Double Throw
SPST Single Pole Single Throw

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1-18 General Information 8525B/8528 Technical Service Manual

SSB Single Sideband
STTL Schottky Transistor Transistor Logic
SWR Standing Wave Ratio
SYNC Synchronisation
SYNTH Synthesizer
T/R Transmit Receive
TC Temperature Coefficient
TCVR Transceiver
TCW Tinned Copper Wire
TCXO Temperature Compensated Crystal Oscillator
TVRO Television Receive Only
TDM Time Division Multiplex
THD Total Harmonic Distortion
TRIG Trigger
TS Tag Strip
TSM Technical Service Manual
TTL Transistor Transistor Logic
TYP Typical
Tx Transmit, Transmitter
UART Universal Asynchronous Receiver Transmitter
UJT Unijunction Transistor
USART Universal Synchronous/Asynchronous Receiver

Transmitter
USB Upper Sideband
UT Universal Time
UTC Universal Co-ordinated Time
V/F Voltage to Frequency
VA Voltampere
VCO Voltage Controlled Oscillator
VCXO Voltage Controlled Crystal Oscillator
VDR Voltage Dependent Resistor
VERT Vertical
VFO Variable Frequency Oscillator
VHF Very High Frequency

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8525B/8528 Technical Service Manual General Information 1-19

VOX Voice Operated Switch
VSWR Voltage Standing Wave Ratio
VU Volume Unit
WRT With Respect To
WT Weight
XTN Extension
XTND Extend
λ Wavelength
+ve Positive

-ve Negative
∅ Phase, Diameter in mm

Note: EMF is the source voltage behind the output resistance (Rs) and is

independent of the signal generator loading. Many signal generators
are calibrated in PD (across the load) assuming a total load
resistance equal to the generator source resistance; for these, the
EMF is twice the attenuator scale reading.

Figure 1.1 EMF Verses PD

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1-20 General Information 8525B/8528 Technical Service Manual

1.5 Circuit Reference Designations

A Assembly
B Transducer—Microphone, Loudspeaker, etc

C Capacitor
D Diode—small signal and power
E Heating device
F Protection device—Fuse, etc
G Generator—battery, etc
H Signalling/indicating device—Lamp. LED, Buzzer,

etc
IC Integrated Circuit, thick film hybrid
J Jack socket
K Relay, key switch
L Inductor
M Indicating device—meter, etc
PPlug
R Resistor
S Switch
T Transformer, common mode choke
TP Test Point
U Modem, Modulator
V Semiconductor (not including small signal and power

diodes)
X Terminals
Z Quartz Crystal, Crystal Filter, Frequency Network

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8525B/8528 Technical Service Manual General Information 1-21

1.6 Units

A Ampere m metre
C Celsius (degrees) min minute
C Coulomb N Newton
F Farad Pa Pascal
g gram S Siemen
h hour s second
H Henry T Tesla
Hz Hertz V Volt
J Joule W Watt
K Kelvin (degrees) Wb Weber
l litre Ω Ohms

1.7 Unit Multipliers

T tera one million million 10
G giga one thousand million 10
M mega one million 10
k kilo one thousand 10
h hecto one hundred 10
da deca ten 10
d deci one tenth 10
c centi one hundredth 10
m milli one thousandth 10
µ micro one millionth 10
n nano one thousand millionth 10
p pico one million millionth 10

l2

9

6

3

2

-1

-2

-3

-6

-9

-12

n

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1-22 General Information 8525B/8528 Technical Service Manual

Issue 6 Publication No: 15-02036

8525B/8528 Technical Service Manual 3-1

3 Brief Description

3.1 General

This description should be read in conjunction with Block Diagram
03-00636.

The same frequency conversions are used in both transmit and receive
modes, therefore many circuits are common to both modes of operation.
Signal routing is determined by switching and control voltages according to
the mode selected.

3.2 Receive

The RF signal from the antenna passes through the PA low-pass filters, then
via the transmit/receive relay and broadcast filter on the PA PCB, to the
switched high-pass filters on the RF, Mixer, and Dual Synthesizer PCB.
Filters are automatically selected according to the frequency band in use.

The band-pass filtered signal is applied to the first mixer. This double­balanced, diode-ring mixer, driven from VCO1, converts the signal up to a
45MHz IF signal.

The 45MHz signal is amplified and filtered with a 20kHz wide ‘roofing’
filter. The output of this filter is mixed with the output of VCO2 in another
double-balanced, diode-ring mixer. This produces a second IF of 1650kHz.

The second IF signal is passed to the IF amplifier and noise limiter. The
noise limiter is designed to suppress impulse noises such as motor vehicle
ignition noise. The limiter operates by 'gating' the IF signal path for the
duration of the noise pulses.

The crystal filter on the Audio and 1650kHz IF PCB reduces the bandwidth
to 2.5kHz before passing the signal to the 1650kHz tuned AGC controlled
IF amplifier.

The signal is then demodulated to provide an audio signal which is fed via
the mute gate to the digital volume control and audio PA where it is
amplified before being passed to the speaker.

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3-2 Brief Description 8525B/8528 Technical Service Manual

3.3 Transmit

The audio signal is amplified and levelled in the microphone amplifier and
compressor, then fed to the balanced modulator where it is mixed with the
1650kHz (USB) or 1647kHz (LSB) local oscillator to produce a DSB signal.
The resulting signal is filtered by the 2.5kHz crystal filter to produce the
SSB signal (USB or LSB) required for transmission.

In the transmit mode, the IF amplifier is switched to produce a buffer to feed
the mixer where the SSB signal is mixed with the output of VCO2 to
produce the 45MHz IF signal.

After the ‘roofing’ filter, the signal is mixed with VCO1 output to produce
the required channel frequency (2MHz to 24MHz). This is taken, via the
transmit/receive switch, to the PA and Filter PCB where it is amplified and
filtered (band-switched filters) before being passed to the antenna.

Forward and reflected power circuits are used to control the power
amplifier. If a high VSWR is detected, the PA’s output power is reduced to
prevent damage.

3.4 Synthesizers

Two single-loop synthesizers are used, the main synthesizer (VCO1)
generating a 45.25MHz to 75MHz signal i.e. 0.25 to 30MHz, plus 45MHz,
in 2kHz steps, and a ‘vernier’ synthesizer (VCO2) generating 43.352 to

43.350MHz in 10Hz steps.
The synthesizers are controlled by the microprocessor which controls the

transceiver. Serial data is loaded into both synthesizers; the data varying
according to the required channel frequency preprogrammed into memory.

The system uses a single ovened crystal reference oscillator of 6.6MHz. This
reference is also used to provide the 1650kHz (6.6MHz divided by 4) signal
for the audio modulator/demodulator.

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8525B/8528 Technical Service Manual Brief Description 3-3

3.5 Display, Control and Switching

All the transceiver functions are microprocessor controlled with the required
channel frequencies, facilities, etc being preprogrammed into an Erasable
Programmable Read Only Memory (EPROM).

The front-panel and extended (8530, 8532 or 8531S) control head displays,
and switch-pads are controlled via a four-wire serial bus.

Other functions such as filter switching, PTT and tone generation are
controlled either by buffered microprocessor signals or the relay-switched

‘C’ and ‘D’ supply voltages. The microprocessor switches the associated
relay (K1 on the Motherboard), depending on the transceiver mode. The
relay is energised during transmit and de-energised in receive.

3.6 External Power Amplifiers

In suffix H transceivers, the PA is replaced by a PA Exciter Interface
Assembly to drive an external PA assembly. The transceiver is powered
from the external PA.

n

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3-4 Brief Description 8525B/8528 Technical Service Manual

Issue 6 Publication No: 15-02036

8525B/8528 Technical Service Manual 4-1

4 Technical Description

This section of the manual contains a detailed description of the circuitry
used in the land and marine transceivers and associated control heads.
Unless otherwise stated, the description applies equally to the 8525B, 8528
and 8528S transceivers.

All circuit paths of the following PCB assemblies are made through the
Motherboard and Chassis (04-02453), where the connector and pin numbers
of the interconnections are shown:

RF Mixer & Dual Synthesizer (04-02450)
Audio & IF 1650kHz (04-02093)
Microprocessor Controller (04-02451 and 04-03031)
Display PCB (04-02579) (8525B)
Display PCB, LCD (04-02454) (8528)
PA & Filter (04-02452)
PA/Exciter Interface (04-02434)

4.1 Control and Supply Voltages

All switching, except ‘power-on’, is controlled either directly or indirectly
by the microprocessor on the Microprocessor Controller PCB.

4.1.1 Supply Voltages

The supply voltages used are as follows:

• ‘A’ rail—unregulated battery supply.

• ‘B’ rail— +10 Volt regulated supply derived from the ‘A’ rail.

• Volt supplies—individual three-terminal regulators supplied from ‘A’

rail.

• Volt supply—‘pump up’ switching supply on the RF Mixer and Dual

Synthesizer PCB (IC1).

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4-2 Technical Description 8525B/8528 Technical Service Manual

4.1.2 Power and Control Lines

By controlling relay K1 on the Motherboard, the microprocessor on the
Microprocessor Controller PCB generates two switched +10V power and

control lines designated as the ‘C’ and ‘D’ rails. These rails are defined as
follows:

• ‘C’ rail—+10 volts on receive; 0 volts on transmit.

• ‘D’ rail—0 Volts on receive; +10 Volts on transmit

All other control lines come directly from the Microprocessor Controller
PCB and are described in section 4.5.

4.1.3 Overvoltage and Reverse Voltage Protection
(refer Dwg 04-02452)

Overvoltage protection is provided by V3, V2, R4, R5 and V1 on the PA
and Filter PCB. If the battery voltage rises above the 15V reference of V1,
plus V2 Vbe drop and the 0.6V diode drop of D6, transistor V2 turns on and
hence turns off V3 to de-energise the power-on relay K7. This occurs at
nominally 16V. Diode D6 provides reverse voltage protection.

4.1.4 Transceiver Power On/Off

When primary power is first applied to the transceiver, V3 on the PA and
Filter PCB (04-02452) is turned on (by R5 to ground) charging C56 through
D6 and K7 which momentarily pulses on. With primary power applied and
in the power off condition, K7 is de-energised and the dual-coil latching

relay K2 on the Motherboard (04-02453) is in the ‘off’ position.
Switching power on is a hardware operation; switching off is basically a

software operation.
When the power on-off pad on the front panel is pressed, a ground is applied

to the cathode of D4 on the Display PCB. This will forward bias V2 via R12
on the Motherboard. The emitter of V2 is connected to the positive supply
via K7, V3 and D6 on the PA PCB. When V2 on the Motherboard conducts,
latching relay K2 contacts close, operating the power-on relay K7. C56
ensures that there is sufficient current to toggle K2.

Operation of the on/off pad on a control head (when connected), switches on
V2 via the rear panel connector P502 pin 4. This has the same effect as
operation of the front panel on/off pad.

Once power ‘on’ has been established, the diodes connected to the power
on/off switch-pad are reverse biased, and no longer affect the operation of
those lines.

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8525B/8528 Technical Service Manual Technical Description 4-3

Pressing the power on/off pad with relay K2 set to ‘on’ has no effect on V2
on the Motherboard. However, via the front panel I2C bus control, the
microprocessor grounds the Power Off line, J3 pin 5. This energises the ‘off’
coil of K2, toggling the relay back to its ‘off’ state. Contacts of K2 open to
break the energising path for relay K7 on the PA and filter PCB.

4.1.5 High Power Transceiver Protection and Power On/Off

In place of the PA and Filter PCB, transceivers fitted for external PA
operation have a PA/Exciter Interface PCB (08-03691). This does not
contain overvoltage and reverse voltage protection circuits. In these
transceivers the power supply is connected to the PA unit where reverse
voltage protection is provided. Operation of the on/off pad on the transceiver
grounds a line which is taken, via the PA Exciter Interface, to the PA unit.
This energises relays in the unit which in addition to switching power on to
the unit, also switch the supply back to the transceiver. The supply to the
transceiver is taken, via the PA/Exciter Interface, to energise relay K2 and
switch on the transceiver in the normal way.

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4-4 Technical Description 8525B/8528 Technical Service Manual

4.2 Receiver

4.2.1 Input Filters (Refer 04-02452)

The receiver input signal is passed through the PA low-pass filter selected
by the PA band line, the transmit/receive relay K6 and the 2MHz high-pass
broadcast filter to the RF, Mixer & Dual Synthesizer PCB.

Option LF adds a 1nF capacitor C111 in parallel with C47, reducing the
broadcast filters passband to 1.6MHz. D3 and D4 provide protection against
excessive input signals.

4.2.2 Mixers, First IF Amplifiers and Filters (Refer 04-02450)

The receive input signal enters the PCB at pin 1 of J1. Five high-pass filters
are used to complete the bandpass filter characteristics and are switched
according to the frequency band in use by grounding the appropriate select
line (the band 2.0 to 3.1MHz is not filtered on this board). From the selected
filter, the received RF is passed via a 28MHz low-pass filter (C32 to C37
and L15 to L18) to the first mixer.

The first mixer is a double-balanced, diode-ring mixer consisting of
transformers T1 and T2, and diode ring D13 to D16. The 0.25MHz to
30MHz signal is mixed with the output of VCO1 (45.25MHz to 75MHz) to
produce the first IF signal, at 45MHz.

FETs V7 and V8 are switched as a grounded-gate amplifier through diodes
D22 to D24 by the C rail being high and D rail being 0V. The amplified
output is passed, via a crystal roofing filter Z1 (approx. 20kHz bandwidth)
to FET V9.

FET V9 is switched as a source follower by the C and D rails through diodes
D26 to D29. The output is applied to the second mixer, formed by
transformers T5 and T6, and diode ring D31 to D34. The 45MHz signal is
mixed with the output of VCO2 (43.350MHz to 43.352MHz) to produce the
second IF signal at 1650kHz. This signal is passed to both the second IF
amplifier and the noise blanker.

FETs V16 and V17 are switched to form the second IF amplifier as a
grounded gate amplifier by the C and D rails through diodes D35 to D38.

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8525B/8528 Technical Service Manual Technical Description 4-5

4.2.3 Noise Blanker

The 1650kHz IF signal is amplified by the RC coupled stages V15 and V18
and fed to the tuned collector amplifier V20. V21 is an active rectifier.
Noise bursts produce negative going pulses at the emitter of V21 which
drives the monostable flip-flop V25 and V26. The output of V25 and V26
are complementary gating pulses which are fed to FET gates V23 and V24
respectively. With V23 on and V24 off, the IF signal is passed to the crystal
filter. With V23 off and V24 on, the IF signal is blocked.

The average DC component of the collector current of V21 passing through
R65 is amplified by V22 and applied as reverse AGC to V15 and V18. Thus
only impulse type signals produce sufficient rectified output at V21 emitter
to trigger the monostable. C119 sets the monostable pulse width such that
the noise burst has finished before the IF is again enabled.

C rail biases off V15 and V18 in transmit and the noise blanker may be
disabled for test purposes by grounding TP6.

4.2.4 IF Crystal Filter and Amplifier (04-02093)

The 1650kHz IF signal from the noise gate on the RF Mixer and Dual
Synthesis PCB (V23, V24 04-02450) enters the board at P1 and is filtered
by Z1 to pass 1647.2kHz to 1649.7kHz (2.5kHz bandwidth) to the two-stage
AGC amplifier V2 and V3. IF output amplifier V4 drives the demodulator
IC8 and the AGC active rectifier V6. AGC attack time is set by R21/C17
and release time by R24/C17, giving a fast attack/slow release response.
Divider R23/R24 sets the AGC threshold level while the voltage set by the
divider, combined with the gain of IC1a, IC1b and the bias on IC1b positive
input, determines the static AGC voltage. D2 and D3 hold the sources of V2
and V3 positive to enable the AGC voltage applied to gate 2 of these FETs
to go negative with respect to sources for full AGC control.

4.2.5 Demodulator

The IF from V4 is converted to the required audio output by double
balanced mixer IC8. When receiving an RF USB signal, the IF signal is LSB
with an SCF of 1650kHz due to the sideband inversion in the first mixer.
With an RF LSB signal, the IF signal is USB with an SCF of 1647kHz. Thus
the demodulator requires a 1650kHz local oscillator for USB reception and a
1647kHz local oscillator for LSB reception at P2. The oscillators are
described in Section 4.4.5.

The audio output (pin 6) of IC8 passes through a third-order low-pass filter
IC4a and associated components to remove noise above the wanted audio
passband.

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4-6 Technical Description 8525B/8528 Technical Service Manual

4.2.6 Audio Mute

The audio from the output of IC4a is looped back through the Motherboard
to the Audio and IF 1650kHz PCB and applied to the audio mute input
(R63/C37) and the mute gate (V12).

IC4b and IC3a are arranged as a squaring amplifier. The squared signal
charges C38 via D8 during negative excursions and the charge is transferred
to C39 by V13 during positive excursions. The resultant DC voltage on C39
is proportional to the frequency of the audio. IC3b and its associated
components form a low-pass filter with a cut-off frequency of approximately
10Hz. The output from IC3b is a DC voltage varying at the syllabic rate of
the received speech.

IC5a and IC5b form a window comparator whose window width is adjusted
by R73, setting the mute sensitivity. The divider formed by R75 and R76,
together with C42, averages the output of IC3b and provides the reference
voltage for the window comparator. If the output from IC3b rises above or
below this reference by the amount set by R73, then the output of either
IC5a or IC5b will go high.

The comparator outputs are diode ORed into C43 to provide a fast attack,
slow release (3 seconds) mute control signal.

The microcontroller on the Microprocessor Controller PCB can override the
mute control circuit in two ways:

• It can force the unmuted condition by grounding the inverting input of

IC6b (via J3 pin 9). This applies a high to the gate of V12 to switch it
on.

• It can force the muted condition by grounding the gate of V12 via J3

pin 8.

The control circuit is also over-ridden at D7 by the C rail being taken to 0V
in transmit. This forces the mute condition, thus preventing acoustic
feedback during transmission.

4.2.7 Volume Control and Audio Amplifier

The post-mute audio from V12 is passed to the volume control section. The
switches in IC9 control a resistive ladder network used as an attenuator.
Sixteen steps are available with each step, giving approximately 4dB
attenuation for a total of 64dB. The switches in IC9 are controlled by the
microprocessor on the Microprocessor Controller PCB.

IC6a buffers the attenuated audio and passes it to the audio amplifier, IC10.
The audio amplifier is capable of 8 Watts output into a 2 ohm load and gives
approximately 2 Watts into the internal 8 ohm speaker.

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8525B/8528 Technical Service Manual Technical Description 4-7

4.3 Transmitter Exciter

4.3.1 Microphone Amplifier (Refer Dwg 04-02093)

The input to the microphone amplifier is J1, pin 6. Transistor V7 is an audio
clamp which is released only when the microphone PTT button is pressed.
This prevents the microphone picking up background noise and sidetone
from the loudspeaker when transmitting emergency calls etc.

IC2a amplifies the microphone input to approximately 1V pp (gain=100).
The signal is then connected to the AGC detector (IC2b, V10) and the
modulator (IC8).

The output of IC2a to the AGC detector is fed directly to V10, and via
inverter IC2b, to V11. Transistors V10 and V11 form a full wave rectifier to
drive V9, which in turn drives V8 to complete the AGC loop by shunting the
input signal to ground.

The release time-constant of the amplifier is set by R37 and C25. Diode D4
allows the gain to rise exponentially to maximum. The three-terminal
regulator IC7 prevents power supply variations affecting the AGC circuit.

4.3.2 Modulator

The microphone amplifier output is capacitively coupled via C28 to the
temperature compensating resistor network R55 and R56. R55 is an NTC
resistor; as the temperature increases its resistance decreases and so the
audio signal level passed to IC8 increases. This is to compensate for reduced
gain at higher temperatures mainly in the 45MHz IF amplifier.

In the 8528S, the selection of the AM (H3E) mode forward biases V3 on the
Motherboard (04-02453). This switches R17, R27 and C23 (also on the
Motherboard) into circuit between the microphone amplifier output J1 pin 3
and ground. This attenuates the audio signal to the lower level required by
the modulator in this mode. Link Y is inserted in H versions which reduces
the audio drive by approximately 3dB to improve the exciter IMD products.

IC8 is the modulator during transmit and mixes the audio signal with the
1650kHz (1647kHz, LSB) local oscillator to produce a DSB signal output,
centred on the LO frequency, from pin 12.

This signal is then passed via the transmit/receive switch V1 and D1 to the
1650kHz crystal filter.

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4-8 Technical Description 8525B/8528 Technical Service Manual

4.3.3 Crystal Filter

One sideband of the DSB signal from the modulator is selected by the
crystal filter to give an SSB signal. During transmit, the C rail is at 0 Volts,
and forces a low output from IC1b for maximum AGC to V2 and V3. Thus,
none of the transmit signal passes through the IF Amplifier.

4.3.4 Mixers, IF Filters, and IF Amplifier
(Refer Dwg 04-02450)

The SSB signal from the crystal filter is applied to J4 on the RF Mixer and
Dual Synthesizer PCB. As the C rail is at 0 Volts in transmit, the noise
blanker is disabled, switching V23 on and V24 off.

V16 and V17 are switched as a source follower selected by the C and D
rails, via diodes D35 to D37. The output is applied to the double-balanced,
diode-ring mixer formed by T5, T6, and D31 to D34. Here the signal is
mixed with the output of VCO2 (43.352 to 43.350MHz) to produce the
45MHz IF.

The 45MHz IF signal is amplified by V9 switched as a grounded-gate
amplifier by C and D rails, via diodes D26 to D29. The amplified output is
passed to the crystal roofing filter (Z1) to remove the unwanted mixer
products.

V7 and V8 are switched as a source follower by the C and D rails, via diodes
D22 to D25. The output is passed to the double-balanced, diode-ring mixer
formed by T1, T2 and D13 to D16. Here the 45MHz IF signal is mixed with
the output from VCO1 to produce the required channel frequency. This
signal is filtered by the 28MHz low-pass filter then passed by D12 to the
Power Amplifier input.

The receive band-switched filters are switched off while transmitting.

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4.4 Local Oscillators (04-02450)

Two synthesized oscillators are used to drive the first and second mixers.
The first oscillator operates between 45.250MHz and 75MHz in 2kHz steps
to convert the 250kHz to 30MHz received signal to 45MHz for the first IF.
The second oscillator operates between 43.352 and 43.350MHz in 10Hz
steps to convert the 45MHz first IF to the 1650kHz second IF.

Each oscillator consists of a voltage controlled oscillator (VCO), a divider, a
phase/frequency detector and loop amplifier/filter together with a reference
crystal oscillator and divider.

The division ratios required are provided in serial data form from the
microprocessor from data stored in the memory. The reference frequency
crystal (6.6MHz) is held at a constant temperature by a power PTC
thermistor and is divided by 4 to provide the 1650kHz drive to the
modulator/demodulator in USB operation. For LSB operation, a separate
1647kHz crystal oscillator is required.

4.4.1 Voltage Controlled Oscillators (VCO)

As VCO1 and VCO2 are virtually identical, only VCO1 will be described in
detail:

• FET V2 operates as a Hartley oscillator tuned by varicaps D17 to D20.

The output is levelled by schottky diode D21 which generates a
negative bias voltage for V2 gate.

• Unity gain feedback amplifier V3/V4 provides a buffer between the

oscillator and the cascade connected amplifier V5/V6. The amplifier
provides the +7dBm drive to the ring diode mixer and also drives the
divide by 64/65 prescaler IC6.

• VCO2 differs from VCO1 only in having a single varicap because of

the reduced frequency range required.

4.4.2 Phase/Frequency Detectors and Dividers

IC5 generates a DC voltage which controls the varicaps in VCO1 in order to
lock the oscillator to its nominated frequency. A 6.6MHz oscillator in IC5
controlled by the crystal Z2, is divided within the IC to give a 2kHz
reference for a phase/frequency detector also in the IC. VCO1 output is
divided in a prescaler, IC6, and applied to further dividers in IC5, allowing
for the VCO frequency to be changed in 2kHz steps. The divided output is
compared with the 2kHz reference frequency.

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When the phase/frequency difference is considerable, e.g. immediately after
a change of channel, the output from PDB (IC5 pin 2) consists of pulses
with their mark:space ratio proportional to the difference. These are
integrated by R82 and C123 to provide an input to IC2b. The signal is
further filtered by R85 and C127, while the output from IC2b is filtered by
R90, R91, C131, C132, C133 and C134. At the same time PBA pin 1 gives
an analogue output proportional to the phase/frequency difference. This is
also applied to the input of IC2b, via R80. The resulting output drives VCO1
in the direction required to obtain the correct frequency, thus providing a
coarse control of the frequency.

When the phase/frequency difference becomes small the PDB output goes
high impedance and the analogue output from PDA becomes the only input
to IC2b. This provides the fine control of the VCO1 output.

IC7 generates the DC voltage required to control the varicaps in VCO2. A
similar arrangement of prescaler and internal dividers divides the VCO2
output in the same way as VCO1 was divided in IC5. The result is compared
with a reference frequency within IC7. The 6.6MHz crystal oscillator output
from IC5 is applied in IC7 to a divider which can be programmed to provide
reference frequencies between 1.1 and 2kHz. This allows VCO2 frequency
to be changed in 10Hz steps.

Coarse control of VCO2 is provided by the Freq output (pin 6) from IC7.
This consists of low pulses if the VCO frequency is too high, and high
pulses if it is too low. These are integrated in C147 to provide the correcting
voltage to VCO2. Fine control is provided by the Phase output (pin 17).

4.4.3 Lock Signals

Both phase/frequency detectors give an output which is low while the VCO
and reference frequencies are locked, and high when they are unlocked. The
LD output from IC5 pin 3 is applied to the base of V28 and the Charge
output from IC7 pin 4 is applied to V29. Thus the corresponding transistor is
switched on to light the LED indicator (H1 or H2) in its collector circuit if
either loop loses lock. The collector circuits are ORed by D45 and D46 so
that if either collector is low, i.e. one loop has lost lock, an Unlock output is
passed to the Microprocessor Controller PCB.

4.4.4 Loading Frequency Data to Synthesizers

Data containing the required division factors for both synthesizers is applied
from the Microprocessor Controller PCB over the Data line. The serial data
is clocked into IC7 under the control of the signal on the clock line. The first
twenty-eight bits are clocked through IC7 and into IC5, via SR Out (pin 14)
of IC7. When the twenty-eight bits have been clocked they are loaded in IC5
by the Enable 1 pulse from the Microprocessor Controller PCB. Similarly
the next thirty-two bits are loaded into IC7 by the Enable 2 pulse.

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4.4.5 1650/1647kHz Local Oscillators

When receiving (or transmitting) a USB signal, a 1650kHz local oscillator is
required. This is obtained by dividing the buffered oscillator output of IC7
(6.6MHz) by 4 in IC9.

When receiving (or transmitting) an LSB signal, a 1647kHz local oscillator
is required. This is obtained from IC10c and crystal Z3. A high on J3 pin 11
enables IC9 and disables IC10c giving 1650kHz (USB). A low on J3 pin 11
gives 1647kHz.

A tune output is provided from IC10a to the gate of source follower V16 and
V17 when the tune line J3 pin 12 is low. The DC component of the tune
output holds V26 conducting which switches V23 to open circuit,
disconnecting the crystal filter from V16 and V17.

In the 8528S with compatible AM fitted, resistor R76 is removed,
preventing V23 from switching off. This is necessary to permit the carrier to
be re-inserted for AM transmission. Because of excessive loading of SSB
filter Z1 at 1647kHz, the 1650kHz LO is automatically selected in the tune
mode when LSB is selected.

4.4.6 Clarifier

Operation of the clarifier control varies the frequency of VCO2 in 10Hz
steps. This is achieved by varying both the VCO and reference frequency

dividers according to a ‘look-up’ table in the transceiver’s operating system.
Thus the phase comparison frequency is not constant but varies from 1.1 to

2.0kHz.
The range of the clarifier differs between land and marine transceivers:

• Land—The limits of the clarifier are 0.001% (±10ppm) of carrier

frequency. Consequently the number of steps are also a function of the
carrier frequency. At low frequencies dummy steps are inserted to
increase the time the clarifier takes to go from end to end of its range.

• Marine—The limits of the clarifier are ±180Hz at all frequencies, i.e.

±18 steps.

An audible indication is given when the clarifier reaches its upper or lower
limit. The clarifier automatically resets to mid frequency when the channel
is changed.

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4.5 Microprocessor Controller PCB (04-02451 and 04-

03031)

Circuits on the Microprocessor Controller PCB govern the functions of the
transceiver including the operator controls and displays. The microcontroller
on the board accepts inputs from within the transceiver and from the

operator’s controls. Under control of the program in the EPROM, it outputs
the data necessary to achieve the appropriate response to input signals.

The microcontroller is connected with other devices on the board by three
buses: a conventional microcontroller bus (8-bit data, 16 bit address) and

2

two ‘Inter-Integrated Circuit’ (I
Note that during the manufacturing period of the 8528 series transceiver the

microprocessor controller 04-02451 was replaced with that shown against
04-03031.

For reference, both sets of drawings have been included in this manual. The
following description relates to either microprocessor controller.

C) buses.

4.5.1 Microcontroller Bus

The microcontroller (IC11) is an 80C31 which is a member of the Intel 8051
8-bit microcontroller family. It has 128 bytes of internal RAM, no internal
program memory, two counter/timers, two external interrupt sources and an
internal clock oscillator.

The microcontroller operates in a conventional 8-bit data, 16-bit address
configuration. To minimise the pin count, the low-order address bits are
multiplexed with the data on pins 32 to 39 onto a common 8-bit bus. IC13 is
used to latch the low order address bits to facilitate access to the external
non-multiplexed devices. The address latch enable (ALE) signal (IC11
pin 30) indicates to the latch when the address is present on the bus. The
high order address bits are provided directly on pins 21 to 28 of IC11.

The following external devices are connected to the bus:

• IC14—an EPROM containing radio control software and customer

channel information. The microcontroller selects the EPROM via the
program store enable (SE) signal (pin 29).

• IC10—a RAM device supplementing the microcontroller’s internal

RAM. It is selected via the Read or Write signals (IC11 pins 16 and 17)
in conjunction with address line 15 (IC11 pin 28).

• IC12—a triple tone generator used to generate all audio tones required

in the transceiver, e.g. emergency call tones, error beeps, etc. It is
selected via the Read and Write signals in the same way as IC10 but in
conjunction with address line 14 (IC11 pin 27).

Three audio tone outputs are generated within IC12 by dividing the clock
inputs on pins 18, 15 and 9 by factors programmed into its data input. The

clock input signal is derived from the microcontroller’s ALE output. During
those cycles when the microcontroller does not generate the AL E, the Read or

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Write signals are inserted via D3 and D4 to maintain a steady square wave
signal.

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4.5.2 Internal I2C Bus

The internal I2C bus consists of four lines: data, clock, interrupt and
data-line enable. The data and clock lines operate as an I2C bus; the data line
carries data and address bits serially, with their transfer being synchronised
by transitions on the clock line.

Data, clock and interrupt lines connect the microcontroller with 8-bit I/O

IC’s, IC3 and IC4. Each 8 bit I/O IC converts parallel data on its eight ports
to serial data for application to the bus and conversely, converts serial data
from the bus to parallel data on its ports. Each IC has a separate address set
by the connections to its address pins (1, 2 and 3). When a data transfer is
required, the microcontroller first outputs on the data line the address bits of
the required IC and follows this with a Read/Write command. Data is then
read from, or written to, the selected 8-bit IC.

When an input to any of the ports changes, the IC sends an interrupt to the
microcontroller. As all the interrupts are connected to a single input, the
microcontroller must interrogate each IC in turn until the changed data is
read. When this is done, the IC removes the interrupt and the
microcontroller can resume its other operations.

Part of IC3 is used for connection to selective call options. The remainder of
IC3 and all of IC4 carry data to and from various functions within the
transceiver.

The clock and data lines of the internal bus also connect the microcontroller
with IC5 and IC15 which operate as I2C bus receivers. Under control of the
data, each IC accepts serial bits from the bus. A set of bits consists of two
address bits and sixteen data bits. On completion of the transfer of a set, the
IC to which the set is addressed latches the data bits as 16-bit parallel data
on its outputs. These two IC’s provide 32 open drain outputs to various
circuits throughout the transceiver.

The microcontroller is connected via the data and clock lines to the non­volatile memory, IC6. This stores the channel selection, volume control,
clarifier and scan channel settings. When power is switched off, the data is
retained in the memory by battery G1. Drain on the battery is very low,
therefore the battery life is limited mainly by its shelf life i.e. approximately
10 years. A discharged battery will cause the channel, volume, clarifier and
scan channel settings to require resetting each time the transceiver is
switched on, but will allow normal operation of all other facilities.

Also connected to the microcontroller via the bus are two EEPROMs, IC16
and IC17. These provide the non-volatile memory for assemblies 04–02451
to store data entered on the front panel pads. For the 04-03031 version of
assemblies, IC16 only is shown being of a different component type to the
earlier design.

The clock and data lines are also connected to the synthesizer circuits on the
RF, Mixer and Dual Synthesizer PCB via J4. Data for setting the two
synthesizer frequencies is sent over the data lines and on completion of the
transfer, the synthesizer latch line is pulsed.

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The clock and data lines are also taken to connector P1 for application to I/O
ports in the Selective Calling PCB when this is fitted. Provision is made for

all four lines of the bus to be connected via P3 to further I/O IC’s if required.

4.5.3 External Bus

A second four-line bus is provided for connection to 8-bit I/O IC’s and
16-bit open-drain output IC’s on the Display PCB of the front panel and
Extended Control Head. All four lines are buffered in IC1 to allow for the
longer lines to the Extended Control Head and to protect the
microcontroller. Since the buffers can carry data in only one direction, data
read from the control panels is buffered in IC1 and returned to a separate
input of the microcontroller.

To allow control of the transceiver from both the front panel and the
extended control head 8530/1, the 8-bit I/O ports at these locations must
have different addresses. This is arranged by a link on each Display PCB
which is set for extended (E) or direct (D) control, thus allowing boards of
the same design to be used in both locations.

The interrupt line from the Display PCB is ORed with the internal bus
interrupt by diode D5 and is also taken via two buffers in IC1 to a separate
input of the microcontroller. By this means, the microcontroller can detect
whether an interrupt has come from the internal or external bus.

4.5.4 Reset and Watchdog Circuit

The microcontroller is reset by a +5V level on pin 9. This may be applied in
three ways: by applying power to the unit, by the low voltage detection
circuit or by the watchdog circuit.

When power is switched on, the reset input is held high, thus resetting the
microcontroller ready for initial operation.

If the supply voltage (A rail) falls below 8 volts, IC7 output goes low. This
switches on V3 which holds the microcontroller in the reset condition until
the supply voltage is restored.

Schmitt Trigger IC8a, together with R9 and C4, form an oscillator with a
period of approximately 600ms. However during normal operation, the
microcontroller outputs a positive pulse from pin 14 at regular intervals of
less than 600ms. Each pulse switches on V1 which discharges C4 thus
preventing the oscillator from starting. Should the microcontroller operation
fail, or be affected by abnormal inputs, the pulses cease thus allowing the
oscillator to start. Each negative excursion of the oscillator output will
switch on V3, resetting the microcontroller. The reset will therefore be
applied at regular intervals until normal operation is resumed.

4.5.5 RS-232 Port (ALE Option Only)

On 04-03031 there is shown an RS-232 I/O port. V5 is used for the O/P
driver and level translator. V6 is used for input level translation.

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4.6 Motherboard and Chassis (04-02453)

The Motherboard interconnects circuit boards which are mounted directly on
it and provides connections to other assemblies mounted on the chassis. It
also provides connections which link these assemblies to external
connectors. In addition the board carries components associated with these
interconnections and with the power supplies and their switching.

4.6.1 Power Supplies

The Motherboard contains relays with their switching transistors associated
with the power supplies as described in Section 4.1.

The board also contains two voltage regulators, IC1 and IC2. These provide
stabilised DC supplies of 10V and 5V respectively from the A battery
supply. The 10V and 5V supplies are used throughout the transceiver.

4.6.2 Antenna Control Facility

The Antenna control socket J201 on the rear panel of the transceiver
provides an interface for various antenna tuning systems. The socket is
connected to P201 on the Motherboard. Interconnections within the
Motherboard provide access to the 12V supply, 4-bit binary-coded data
indicating the channel selected, and various other outputs for use by
automatic tuning systems. On entering the board the lines are filtered to
prevent RF picked up on the external cable, from interfering with the
operation of the transceiver. The table in Section 1.2.4 shows the pin
connections and the signal level of each function.

4.6.3 Extended or Remote Control—Option R

The Remote Control plug (P502) on the rear panel of the transceiver
provides an interface for extended or remote control equipment. When fitted
the plug is connected to the Remote Control and Filter PCB mounted on the
Motherboard. Transceivers manufactured for extended or remote control
include these components. Transceivers with front panel controls can have
them fitted as Option R.

The extended or remote control lines carry the services shown on the circuit
diagram. Components on the Remote Control and Filter PCB prevent RF
picked up on the external cable, from interfering with the operation of the
transceiver.

The table in Section 1.2.4 shows the pin connections and levels of each
function.

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4.6.4 External Mi cr ophone Audio Clamp

The audio input from the Remote Control connector at pin 11 P502
(Option R) is connected, via C102, R103, R101 and C101 to the Aux
Compressor input of the Audio and IF 1650kHz PCB. FET V101 is
normally biased on via R104 and R105, thus clamping the microphone input
to ground. Operation of the remote PTT switch grounds the junction of
R104 and R105 removing the bias from V101. This allows the microphone
input to be applied to the compressor input.

4.6.5 Additional Audio Facilities—Option RC

Two summing amplifiers, IC401a and IC401b, can be fitted to provide for
special applications. IC401b can accept two inputs from the Audio and IF
1650kHz PCB. Inputs are selected, according to the application, by the
inclusion or omission of R401 and C401 or R402 and C402. R19 is omitted
when the option is fitted and the amplifier output is connected to the Remote
Control connector Option R.

Similarly, by the inclusion or omission of input resistor-capacitor
combinations, IC401a can accept up to three inputs from various sources. Its
output is connected to the pre-volume control input of the IF and Audio
PCB.

4.6.6 Headphone Output—Option PH

A 6.4mm switched jack socket can be fitted to the transceiver front panel
and connected to the loudspeaker circuit to provide for connection of
headphones. The headphone signal is attenuated by a 330Ω resistor in the
line.

4.6.7 CW facility—Option M

A 3.2mm jack socket can be fitted to the transceiver rear panel for
connecting a morse key. When the key is operated, a ground is applied via
the jack socket to the Microprocessor Controller PCB (P2 pin 3). The
microprocessor detects the key function via the I/O port of IC3 and
generates the required PTT and audio tone for transmission.

4.6.8 External Selective Call or RTTY—Option PS

Connector J301 on the rear panel of the transceiver provides an interface for
selective calling or radio teletype equipment. Operation of the external
equipment via this interface is described in Section 4.11.

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4.6.9 AGC Dump Circuit

The synthesizer latch from the Microcontroller PCB pin 1 P10 is used to
turn on V4 thus grounding the AGC line on the Audio and IF 1650kHz PCB
via R23. This discharges the reservoir capacitor in the AGC circuit so that
the receiver has maximum gain immediately after a frequency change. This
is particularly important when in the scan mode.

4.7 8525B Front Panels and Control Head (04-02579)

The front panels of 8525B series transceivers and extended control heads are
similar. Overlays on these panels contain sealed tactile membrane switches.

A 2-digit numeric display and LED’s on a board behind the panel show
through the appropriate part of the overlay when lit. Circuits on a further
board behind the panel interface with the microprocessor in the transceiver
to drive the displays and indicators and decode the operations of the front
panel switches. In the extended control head, circuits on the Microphone
Amplifier PCB compensate for the long lines which connect the head with
the transceiver.

4.7.1 Display PCB

The Display PCB (08-03944) for the transceiver and the Display PCB
(08-03945) for the extended control head differ in size and layout to fit their
individual panels. They are, however, electrically identical.

Common connections of the numeric displays and the LED indicators
(except the Tx indicator) are taken to a positive supply controlled by IC1.
Indicators and segments of the displays are lit by grounding the appropriate
connection via IC2 and IC3.

The conduction of phototransistor V1 increases in proportion to the intensity
of the light falling upon it. Increasing conduction of V1 reduces that of V2
and V3 thus raising the reference voltage to IC1. Consequently, increasing
ambient light raises the output voltage of IC1 thus increasing the display and
indicator brightness. The circuit sensitivity is adjusted by R1.

The Tx indicator LED is lit by grounding its cathode via a transistor on the
PA and Filter PCB.

Two 16-output open drain drivers, IC2 and IC3 ground individual lines in
response to data received on the I2C bus.

A remote 8-bit I/O port, IC4, accepts inputs from switches on the front
panel, and under control of the microprocessor, converts them to a suitable
format and applies them to the I2C bus.

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4.7.2 Switch Matrix

The switches on the panel overlay (substrate) form a 4 x 4 matrix. The row
and column connections are taken to the Display PCB. (A table listing the
switch functions and corresponding connections is given in Section 6.2.3.)
The row connections are polled by the microprocessor, via IC2 by taking
them to 0V one at a time. A switch operation is detected by the
corresponding column connection to IC4 going to 0V. Data indicating the
switch closure is encoded and returned, via the I2C bus, to the
microprocessor.

4.7.3 Digiswitches (8525B)

Transceivers and control heads fitted with selective call mesh (SDEM) have
thumbwheel digiswitches on their front panels. The BCD outputs of these
are applied to the Display PCB. Individual switches are polled so that switch
settings are detected and returned to the microprocessor in a similar way to
the matrix switches.

4.7.4 Microphone Amplifier and Interface (04-02096)

To compensate for the long lines connecting the Control Head to the
transceiver, a Microphone Amplifier and Interface (08-03039), interfaces the
Display PCB with the lines to the transceiver. On entering the board each
line is filtered to prevent interference picked up on the lines from affecting
the operation of the head.

The board contains buffers for the Data, Clock and Enable lines of the data
bus. A 3-terminal regulator, IC1, on the A rail provides the 5V supply for
use by these buffers and the Display PCB.

An amplifier formed by IC3 and its associated components amplifies the
microphone input to a suitable level for application to the lines. V2 on the
input to the amplifier operates as a clamp in the same way as V7 on the
Audio and IF 1650kHz PCB. This circuit is powered by a separate 10V
supply derived from the A rail by zener diode V1.

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4.8 8528 Front Panel and Control Head

The front panels of 8528 series transceivers and extended control heads are
identical. Overlays on the panels contain sealed tactile membrane switches.
A liquid crystal display (LCD) and light-emitting diode (LED) indicators on
a board behind the panel show through the appropriate part of the overlay
when activated. Circuits on the board interface with the microprocessor in
the transceiver to drive the displays and indicators, and to decode the
operations of the front panel switches. In the extended control head, circuits
on a further board compensate for the long lines which connect the head
with the transceiver.

Surface mounted components are used on the Display PCB, LCD. For
removal and replacement of these components it is essential to use tools
specifically designed for the purpose. Repairs to the board should not be
attempted unless the appropriate tools are available.

4.8.1 Display PCB, LCD (04-02454)

The same Display PCB, LCD (08-03745) is used in the transceiver and the
Extended Control Head.

The LCD on the board has twenty digits and two enunciators which allow it
to display the channel number and transmit and receive frequencies of the
currently selected channel. In addition it can display the identification
number of a called or calling selective call station. The LCD is driven by
IC4. The display data is applied to the driver via the I2C bus. The drive
mode is 1:4 multiplexed with 1/3 bias.

The viewing angle is preset by adjustment of R12. This sets the voltage
applied to the non-inverting input of IC1a while the voltage at the junction
of R10 and D1 is applied to the inverting input. The output of IC1a is
proportional to the difference between the two voltages. The temperature co­efficient of D1 applies temperature compensation tending to keep the set
viewing angle constant over a wide range of temperatures.

R12 is normally set during manufacture for viewing at right angles to the
face of the panel. It can be adjusted for viewing angles above or below if
required.

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Six incandescent lamps, H1 to H6, are mounted behind the LCD to provide
back-lighting so that the display can be read in low ambient light. V1 is
mounted in a position which allows ambient light to fall on the transistor.
Conduction of the transistor increases with the light, i.e. the collector
voltage, applied to the comparator IC1b, decreases. IC1b compares this
voltage with the fixed voltage at the junction of R3 and R4. In low ambient
light, V1 collector voltage is higher than the reference voltage, therefore
IC1b output is high. This turns on V2 to light the lamps. If the ambient light
increases to the point where the collector voltage of V1 no longer exceeds
the reference, the output of IC1b goes low, switching off V2 to extinguish
the lamps. The threshold level can be adjusted by R1.

Completion of Link D connects R30 and R31 in parallel with V2. This
provides an alternative path for the lamp current, therefore the LCD back­lighting is only reduced to approximately half brightness in high ambient
light.

Common connections of the LED indicators H8 to H13 are taken to a
positive supply regulated by IC2. The indicators are lit by grounding the
appropriate connection via the 16-output driver IC3.

The Tx indicator LED is lit by grounding its cathode via a transistor on the
PA and Filter PCB.

4.8.2 Indicator and Display Dimming

The current sinking provided in IC3 for the indicator LEDs H8 to H13, can
be varied by operation of the Recall pad on the front panel. Any indicators
lit will be at full brightness when the transceiver is first switched on.
Pressing the Recall pad twice within one second reduces the current sinking
thus reducing the brightness of the indicators. Pressing the pad twice again
restores the original brightness.

The dimming operation is also applied to the LCD back-lighting via IC3 pin

4. Normally high impedance (tristate), this output goes low in the dimmed
condition. This is applied to the base of V2 to override the input from the
comparator IC1b. Thus if the LCD back-lighting H1 to H6 is lit, it will be
extinguished (or reduced to half brightness) when the indicators are
dimmed.

4.8.3 Links

A moveable jumper link closes either Link E for extended control, or Link F
for front panel control. Closing the appropriate link modifies the address of
the I/O port IC5 so that the microprocessor can recognise the type of control
in use.

Link 1 is used in programming the transceiver options (refer Operator’s
Handbook) and link 2 is for possible future use.

Link D is used in the display dimming facility (4.8.1).

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4.8.4 Switch Matrix

Switches on the panel overlay (substrate) form a 4 x 6 matrix. The row and
column connections are taken to the Display PCB. (A table listing the switch
functions and the corresponding connections is given in Section 6.2.3). In
the Display PCB the row connections are polled by the microprocessor, via
IC3, by taking them to 0V one at a time. (Three of the four lines are taken to
0V via open collector drivers V4, V5 and V6). A switch operation is
detected by the corresponding column connection to the 8-bit I/O port IC5
going to 0V. Data indicating the switch closure is encoded and returned, via
the bus, to the microprocessor.

4.8.5 Microphone Amplifier and Interface (04-02096)

The board used in the extended control head to compensate for the long
lines connecting the head to the transceiver is the same as the one used for
the same purpose in the 8525B series. (Refer to Para 4.7.4).

4.8.6 8532 Display Head

The Type 8532 Extended Control Head has a smaller frontal area and depth
(180mm w x 62mm h x 45mm d) than the Type 8531, making it more
suitable for mounting in confined spaces. It has all the features of the 8531
except the loud speaker is external and the control panel is back illuminated
for use at night.

Overlays on the front panel of the control head contain sealed tactile
membrane switches. A liquid crystal display (LCD) and light emitting diode
(LED) indicators on a board behind the panel show through the appropriate
part of the overlay when activated. Circuits on the board interface with the
microprocessor in the transceiver to drive the displays and indicators, and to
decode the operations of the front panel switches.

Surface mounted components are used in the display PCB. For removal and
replacement of these components it is essential to use tools specifically
designed for the purpose. Repairs to the board should not be attempted
unless the appropriate tools are available.

A single PCB, Display PCB (Assembly 08-04666), is used in the Extended
Control Head (Refer to circuit Diagram 04-02875).

The LCD, H14, has twenty digits and two enunciators which allow it to
display the channel number and the transmit and receive frequencies of the
currently selected channel. In addition it can display the identification
number of a called or calling selective call station. The LCD is driven by
IC5. The display data is applied to the driver via an I2C bus. The drive mode
is 1:4 multiplexed with 1/3 bias. The driver is powered by a separate supply
derived from the A rail by diode D7 and zener diode V7.

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The viewing angle is preset by the adjustment of R34. This sets the voltage
applied to the non-inverting input of IC4a, while the voltage at the junction
of R32 and D6 is applied to the inverting input. The output of IC4a is
proportional to the difference between the two voltages. The temperature co­efficient of D6 applies temperature compensation tending to keep the set
viewing angle constant over a wide range of temperatures.

R34 is normally set during manufacture for viewing at right angles to the
face of the panel. It can be adjusted for viewing angles above or below if
required through the hole in the bottom of the case.

Nine incandescent lamps, H1 to H6 and H15 to H17 are mounted on the
board. Six are behind the LCD and the remaining three in various areas of
the board. These provide back lighting so that the LCD and the panel
annotations can be read in low ambient light.

Common connections of the LED indicators H8 to H13 are taken to a
positive supply derived from the A rail and regulated by IC1. The indicators
are lit by grounding the appropriate connection via the 16-output driver IC2.

The Tx indicator LED, H7, is lit by grounding its cathode via a transistor on
the PA and Filter PCB.

The current sinking provided by IC2 for the indicator LEDs H8 to H13 on
the front panel can be varied by the operation of the Recall pad on the front
panel. Any indicators lit will be at full brightness when the transceiver is
first switched on. Pressing the Recall pad twice within one second reduces
the current sinking thus reducing the brightness of the indicators. Pressing
the pad twice again restores the original brightness.

The dimming operation is also applied to the LCD back lighting via IC2
pin 4. Normally high impedance (tristate), this output goes low in the
dimmed condition. This is applied to the base of V1 to override the hold-on
voltage applied via R1. With the transistor switched off the current for H1 to
H6 is drawn through R3 and R4 thus reducing the brightness of the LCD
back lighting. The panel back lighting is not controlled and remains at full
brightness at all times.

A moveable jumper link closes either Link E for normal extended control, or
Link F for special applications front panel control. Closing the appropriate
link modifies the address of the I/O port, IC3, so that the microprocessor can
recognise the type of control in use.

Link 1 is used in programming the transceiver options (Refer to the

Operator’s Handbook). Link 2 is for possible future use.

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4-24 Technical Description 8525B/8528 Technical Service Manual

Switches on the panel overlay (substrate) form a 4 x 6 matrix. The row and
column connections are taken to the Display PCB. (A table listing the switch
functions and the corresponding connections is given in Section 6.2.3.) In
the Display PCB the row connections are polled by the microprocessor, via
IC2, taking them to 0V one at a time. (Three of the four lines are taken to
0V via open collector drivers V2, V3 and V4.) A switch operation is
detected by the corresponding column connection to the 8-bit I/O port, IC3,
going to 0V. Data indicating the switch closure is encoded and returned, via
the bus, to the microprocessor.

To compensate for the long lines connecting the Control Head to the
transceiver, on entering the board each line is filtered to prevent interference
picked up on the lines from affecting the operation of the head. The Clock
line is also buffered by a small amplifier formed by V8 and V9.

The A rail from the transceiver is used by various circuits on the board and
is also applied to a regulator, IC6, to provide the +5V supply for other
circuits within the board.

An amplifier formed by IC4b and its associated components amplifies the
microphone input to a suitable level for application to the lines. V5 on the
input to the amplifier operates as a clamp in the same way as V7 on the
Audio and IF 1650kHz PCB. This circuit is powered by a separate supply
derived from the A rail by zener diode V6.

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4.9 PA and Filters (04-02452)

4.9.1 Introduction

8528S Series transceivers may be fitted with a PA and Filter PCB, forming
an internal PA stage, or with a PA Exciter Interface PCB for use with an
external PA unit. Software appropriate to the board fitted is installed in the
controlling EPROM on the Microprocessor Control PCB. Transceivers fitted
for use with an external PA assembly are identified by a letter H suffixed to
their type numbers on the rear panel. The board fitted for the internal PA
version is described in this section. The PCB fitted for the other version is
described in Section 4.10.

4.9.2 PTT Control

The driver and output stages are permanently connected to the supply
voltage when the Power On relay K7 is energised. Power to the rest of the
PA and the bias for the driver and output stages is switched by V14 when
the PTT input (D rail, P1 pin 2) goes high causing pin 10 of IC1 to sink base
current from V14 through the antenna change-over relay K6 coil. In
transmit, the input stage V7 and V9 becomes active when its emitter circuit
is connected to ground on the C rail through D12 on the RF Mixer and Dual
Synthesizer PCB (04-02450).

4.9.3 Gain Control Stage

The RF input to the PA is terminated in R21 and drives the common base
long tailed pair V7 and V9 through R29 and R30 in parallel for signal
currents.

The input voltage is also buffered by V8 and additional input current fed to
V7 and V9 through L17, C66 and R32 to compensate for PA high frequency
gain roll-off.

The gain of V9 is controlled by the ratio of emitter DC currents in V7 and
V9. The RF input current is split between the emitters of V7 and V9 in
inverse proportion to their input impedance. The gain of V9 is therefore
reduced when the ALC increases the current in V7.

4.9.4 Pre-Driver Stages

The output current of V9 drives the collector-base feedback stage V10
which is direct coupled to the transformer feedback stage V12. High
frequency peaking is provided by C72. D11 and D12 provide temperature
compensation for V10 and V12.

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4.9.5 Driver Stage

The driver stage V16 and V17 is voltage driven from the secondary of T2
and with emitter resistors R52 to R55 provides a current drive to the output
stage. L20, C78 and R56 provide damping for a harmonic resonance in T3.

The bias for V16 and V17 is provided by the current of the pre-driver stages
passing through the diode-connected transistor V13. The bias may be
increased by R46, the current through R46 causing a voltage drop across
R47 in addition to the base-emitter voltage of V13. The current in V16 and
V17 may be measured at the link in the DC feed to T3.

4.9.6 Output Stage and Bias Regulator

The output stage V18 and V19 is current fed from the secondary of T3 with
low frequency current feedback through R58 and R59 from a winding on the
output transformer T4. C93, C94 with C116 and C81 with C82 provide
frequency compensation for T3 and T4 respectively.

The balanced output auto transformer also contains a balun winding so that
one output can be AC coupled to ground and the other output capacitively
coupled to the load.

The bias regulator (V11 and V15) provides a constant voltage to the bases of
V18 and V19. V11 and V15 form a feedback voltage regulator, the output
voltage being the base-emitter voltage of V11. R76 and V23 cause the bias
to increase at low supply voltage (10V approx) to reduce intermodulation
distortion.

V11 is mounted on the PA heatsink and provides temperature compensation
based on heatsink temperature.

When low power is selected (8528S only), V21 is turned on by V20
collector going high. This increases the bias voltage via R71 and therefore
the bias current in V16 and V17. This is required to improve the
intermodulation distortion at low power.

4.9.7 Output Filters and Control

The output of the PA is passed through one of six low-pass filters to remove
harmonics generated in the PA. These filters are relay selected (K1 to K5
and K8) by the band lines generated by the microprocessor controller and
buffered by IC1.

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4.9.8 ALC Control

ALC control is provided from the following sources:

• Forward power

• Reflected power

• Battery voltage

• Low Power control (8528S only)

• Heatsink over-temperature

• Output stage collector swing.

The ALC control inputs are applied to IC2a which provides a fast attack,
slow decay control voltage to the base of V7. In the absence of any ALC
input, the negative input of IC2a is held at 3.9V by voltage divider R16, R17
and R19 and the voltage on C59 and V7 base is held at a similar level. V7 is
cut off and V9 then operates at maximum gain.

The R16, R17 and R19 voltage divider chain is used to control ALC action

for heatsink temperature. When the heatsink temperature exceeds 80°C,
PTC resistor R16 rapidly increases in value, reducing the ALC threshold
level and reducing the power output such that the heatsink does not rise
above 90°C.

The output voltage from the RF bridge T1, L1, R2, R3, C1 and C2 is
rectified by D1 as forward power and by D2 as reflected power. The forward
power output is divided by R9, R10 and R11 and ORed by D8 with the
output of the reflected power detector into the positive input of the ALC
level comparator IC2a. The power output is set by R10.

The output voltage of the forward power detector is averaged by R68, C92
and ORed into the ALC comparator. The value of R68 is chosen so that a
average forward power of 60W will provide ALC control.

When the DC supply drops below 12.5V, V5 starts to conduct, increasing
the voltage across R11. This effectively raises the forward power detector
level, resulting in increased ALC, reducing the output power.

A low on the LO PWR input (P3 pins 1 and 2) will cause V20 to conduct
also raising the current in R11 and reducing the power output.

The peak positive swings of the PA output transistors V18 and V19 are
rectified by D13 and D14, divided by R61 and R62 and buffered by V22
before being ORed into the ALC comparator. Thus excessive swings of the
PA collector voltage reduces the output power to prevent damage to the PA
transistors. The output from V22 is also applied to one input of IC2b where
it is compared with the forward power to the antenna. If the forward power
is very low (or absent) indicating a fault in the antenna circuit, the output
power is reduced to prevent damage to the PA transistors.

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4.9.9 Transmit Indicator

The forward power voltage developed by D1 is divided and applied to V4
base producing a current in V4 proportional to power output to illuminate
the Tx LED indicator.

4.10 PA Exciter Interface (04-02434)

4.10.1 Introduction

8525B and 8528 Series transceivers may be fitted with a PA and Filter PCB
forming an internal PA stage, or with a PA Exciter Interface PCB for use
with an external PA unit. Software appropriate to the board fitted is installed
in the controlling EPROM on the Microprocessor Controller PCB.
Transceivers fitted for use with an external PA unit are identified by a letter
H suffixed to their type numbers on the rear panel. The PA Exciter Interface
is designed for use with Codan Type 4402 or 4404 power amplifiers and is
described in this section. The PCB fitted for the internal PA is described in
Section 4.9.

4.10.2 Control Lines

Control lines from the transceiver are taken through the board for
connection to the external PA unit. The lines are filtered on the board to
prevent RF, picked up on the cable to the PA Unit, from interfering with the
operation of the transceiver.

4.10.3 Power On/Off

The DC supply for the transceiver is connected to the external PA unit.
Operation of the On/Off switch pad on the transceiver front panel grounds
the Power On line to the PA unit. This energises a relay in the PA unit,
supplying 12V to the transceiver. A fuse on the PCB protects the supply
before it is taken to the remainder of the transceiver to form the A rail. The
latching relay on the Motherboard operates in a similar manner as described
in section 4.1.4.

4.10.4 PTT Circuit

The D rail (10V in transmit) switches on V4 in the transmit mode to ground
the PTT line to the external PA unit.

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4.10.5 Transmit Amplifier

The RF input to the board is applied to a grounded base amplifier V1. The
DC path for the emitter of V1 is completed via D12 and the C line (0V) on
the RF Mixer and Dual Synthesizer PCB. R6 in the collector circuit of this
transistor provides a range of approximately 6dB of gain control. The signal
from the collector of V1 is passed to an emitter follower V2, the output of
which drives V3. V3 with T1 forms a fixed gain feedback amplifier. The
secondary of T1 is adjusted for 2.0V pp output by preset R6 to drive the
4402 or 4404 PA units.

4.10.6 Receive Circuit

Received RF from the PA unit is applied to the Rx I/P (P3 pins 9 and 10).
The signal passes to the receiver circuits via a broadcast band high-pass
filter formed by L1 to L3 and C1 to C7. Diodes D1 and D2 limit the output
swing of the signal. In the transmit mode the D rail forward biases D2,
clamping the receiver input to ground.

4.10.7 Antenna Control

Lines to control an external antenna or antenna control unit are also taken
through the board for connection to the Antenna Control connector at the
rear of the transceiver. The Scan, Tune and Tuned In control signals are also
taken to and from the PA unit.

4.10.8 PA Unit

Details of the Codan 4402 and 4404 PA Units are given a separate service
manual Code 2037.

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4.11 Selective Calling (04-02250)

Selective calling facilities may be either internal to the 8525B or 8528, or by
external control using option PS. It should be noted that option PS cannot
be used when internal selective call is fitted.

The following selective call variants are available:

• SE2 4–digit encoder internally preset.

• SDE 4–digit encoder/decoder—separate internal presets for encode and

decode.

• SDEM 4–digit encoder/decoder—internally preset for decode,

externally set for encode.

Note: When an 8530 control head is used with SDEM, the encode

switches only are mounted on the control head while the remainder
of the circuit is in the transceiver. SE2 uses assembly 08-03303;
SDE and SDEM use assembly 08-03300.

Selective Calls are sent as FSK signals using 1700Hz and 1870Hz at 100
Baud.

4.11.1 Selective Calling

The call is initiated by pressing the Call pad on the front panel. On
recognising the request for a call, the microcontroller first checks that the
channel selected is one on which selective calls are programmed. If it is, the
microcontroller generates the required codes including the Called Address
and Self Ident codes set in the transceiver. The settings of each pair of
switches are encoded by the corresponding 8-bit I/O port IC4, IC5, IC6 and
IC7 and sent to the microcontroller as serial data when the appropriate port
is addressed. The Self Ident switches on the PCB are used for all options.
When external digiswitches are fitted (SDEM), the microcontroller
disregards the settings of any internal Called Address switches duplicated on
the panel or control head.

The microcontroller (04-02451) applies the generated code to the tone
generator IC12. The resulting FSK tones from IC12 pin 10 are filtered by
C20 and R21 and applied via J3 pin 9 to the auxiliary compressor input of
the Audio and IF 1650kHz PCB. The FSK tones are also attenuated by R23
and applied as a side tone via J3 pin 10 to the auxiliary LS input of the
Audio and IF 1650kHz PCB. At the same time, the microcontroller
generates a PTT signal so that the FSK tones are transmitted. On completion
of the coded transmission, the PTT signal is removed and the transceiver
returns to the receive mode.

Note: If selective and 2-tone calling are required on the same frequency,

the frequency must be allocated to two channels; one with selective
calling and one with 2-tone calling.

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4.11.2 Selective Call Detection (04-02250)

The Selective Call PCB fitted for Options SDE and SDEM includes circuits
for the detection of selective calling signals. The received audio is applied
via J1 pin 2 to a band-pass filter formed by IC1a, IC1b and their associated
components. The filter has a 3dB bandwidth of 400Hz centred on 1775Hz.
The filter output passes to the phase locked loop IC2.

The voltage controlled oscillator frequency in IC2 is set by C10, R15 and
R16. In the absence of FSK signals, the lock detect output from pin 6 will
fluctuate and the resulting charge on C8 will be less than half the supply
voltage. This is applied via R11 to the comparator input pin 8, resulting in a
low output from pin 7. Some of the output is fed back to the input via R13
to provide hysteresis. The comparator output is taken via D3 to the non­inverting input of IC3b. This results in a low output from IC3b which
effectively cuts off V1. When FSK signals are detected the lock detect
output goes high. The resulting high comparator output reverse biases D3,
enabling IC3b.

When FSK signals are applied, the data appears on the loop phase detector
output pin 11. This is applied to the data filter IC3a and associated
components. This is a low-pass filter at approximately 80Hz which passes
the resulting data to the input of IC3b. The data filter output is also peak
rectified by D1, D2, C17 and C18. The voltages on the two capacitors are
summed at the inverting input of IC3b to give a reference voltage equal to
the mean frequency of the FSK signal. Consequently, IC3b regenerates the
data, ignoring any frequency shift in the RF path.

Demodulated FSK signals are applied via P2 pin 6 to the Microprocessor
Controller PCB (04-02451) on P1 pin 1. From there they are applied to the
microcontroller which decodes the addresses contained in the data. If the
selective call is found to be addressed to the transceiver, the microcontroller
sends a revertive message to the caller. At the same time the controller gives
outputs which activate the necessary audio and visual indications that a
selective call has been received.

If the transceiver PTT is not pressed, on completion of the audio alarm
period, the microprocessor applies a low on J1 pin 6 (04-02451). This is fed
to Jl pin 4 (on 04-02250) which, through V2 and V3, energises relay K1 for
2 minutes, operating an external alarm (if fitted). K1.1 contacts are rated at
50V 1A. Operation of the PTT cancels the alarm.

Refer to section 7.3.15 for setting up instructions.

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4.12 Two-Tone Calling System

The two tone calling system consists of the transmission of two
simultaneous audio tones. The difference frequency is detected to provide a
narrow bandwidth independent of frequency shifts in the RF path.

4.12.1 Two-Tone Calling

The call is initiated by pressing the Call pad on the front panel. On
recognising the request for a call, the microcontroller first checks that the
channel selected has two-tone calling programmed. If enabled, the
microcontroller activates the tone generator IC12 and the required tones are
generated, one from pin 10 and one from pin 13. These pass through the
filters formed by C18, R21, C14, R14, etc. to be mixed at the junction of
R21 and R22. The mixed tones are applied via J3 pin 9 to the auxiliary
compressor input of the Audio and IF 1650kHz PCB. The tones are also
attenuated by R23 and applied as a side tone to the auxiliary LS input of the
Audio and IF 1650kHz PCB. At the same time the microcontroller generates
a PTT signal so that the tones are transmitted.

An RFDS emergency call (as determined by the programming) is
transmitted for 15 seconds after the Emgcy Call pad has been held for at
least 2 seconds. For other calls, the transceiver reverts to the receive mode
as soon as the pad is released.

Note: If selective call and 2-tone calls are required on the same frequency,

the frequency must be allocated to two channels; one with selective
calling and one with 2-tone calling.

4.12.2 Two-Tone Detection

Received audio signals are applied to the Two-tone Decoder PCB (refer to
04-02231) J2 pin 1. IC1b and its associated components form a high-pass
filter, while IC1a and its components form a low-pass filter. Together, these
form a band-pass filter which accepts frequencies between 850 and 1500Hz,
i.e. the band used for 2-tone calling.

The accepted frequencies are applied to an audio compressor circuit formed
by IC2a, V1, V2 and their associated components. Transistor V2 conducts
during positive half cycles of the output of IC2a to charge capacitor C9 so
that the potential on the capacitor is proportional to the amplitude of the
audio signals. This potential controls the gate of V1 thus controlling the
drain-to-source resistance of the FET and consequently the gain of the
circuit. Thus the audio signals at the output of IC2a are maintained at a
constant amplitude.

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The two-tone audio signal is further filtered by a high pass filter formed by
IC2b, IC3a and their associated components. The filter output is applied to a
phase splitter V3, so that equal anti-phase waveforms are produced. Each of
these is applied to one of the two transistors V4 and V5 which operate as a
full-wave rectifier. The output from the emitters of these is a half-wave
sinusoidal waveform containing the sum and difference of the two tones.
This output is applied to the high-Q low-pass filter formed by IC3b and its
associated components removing unwanted frequencies. R25 and R26 set
the peak of the response.

The difference frequency is then applied to the input of tone decoder IC5.
The internal oscillator of the tone decoder is set to 360Hz or 440Hz by the
value of C24, R30 and R31. When the difference frequency matches the
setting of the internal oscillator within a narrow bandwidth (approximately
±15Hz), pin 8 of the tone decoder goes low. Thus the receipt of any two
tones with a difference in frequency corresponding to the frequency to which
the tone-decoder is set (normally 360 or 440Hz) causes this output from the
Two-tone Decoder PCB.

The output is taken via J3 pin 4 to P2 on the Microprocessor Controller
PCB. This activates the tone generator IC12 to generate three short pips to
indicate the receipt of a two-tone call.

Refer to section 7.3.16 for setting up instructions.
Note: The S’call mute facility can be used with the two-tone decoder in

the same way as it is used with selective calling.

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4.13 Option PS—External Signalling Interface

Connector J301 can be fitted to the transceiver rear panel to provide
connection for Codan 8422 or 8511 Selective Call Controllers, radio
teletype (RTTY) or other signalling equipment. When the option is fitted,
J301 is connected to P301 on the Motherboard (refer to 04-02453) and
components associated with the interface are installed on the board. J301 pin
allocations and signal levels are given in Table 1.2.4.

4.13.1 External Selective Calling

Power is applied to the external unit via J301 pins 8 (A rail) and 1(0V).
Audio received on the channel to which the transceiver is tuned is applied
via pin 2 to the external unit. Any FSK signals in this audio are demodulated
and examined for the selective call code preset into the external unit. While
no selective calls are detected, the external unit holds the Quiet Line input to
the transceiver (pin 4) Hi (+10V). In converting this 10V to the levels
required by the transceiver, V303 inverts the Quiet Line input to hold low
the corresponding input to the Microprocessor Controller PCB so that the
audio is not reproduced by the loudspeaker.

On receipt of a selective call code addressed to the transceiver, the external
unit generates tones forming the revertive message to be sent to the calling
station. These are applied to the transceiver via pin 3 while the PTT line
(pin 6) is taken low. The PTT is applied to the Microprocessor Control PCB
and if other conditions are satisfied this switches the transceiver into the
transmit mode so that the tones are transmitted on the channel to which the
transceiver is set. At the same time, the Quiet Line input goes low so that
audio is reproduced as soon as the transceiver returns to the receive mode
and the alarm tones are applied via pin 5 to the auxiliary L/S input so that
the alarm sounds.

When a selective call is initiated at the external unit, the appropriate FSK
signal is generated. The signal is applied to the transceiver at J301 pin 3
while the PTT line is taken low. Consequently if all conditions at the
microprocessor are satisfied the FSK signal is transmitted.

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4.13.2 Selective Call Scanning

With Option PS fitted and Codan 8422 or 8511 units connected to J301, the
scan facility of the transceiver can be used to scan up to eight channels for
selective calls. This mode of operation is initiated by selecting Scan on the
transceiver front panel and Mute on the external equipment. Selection of the
scan mode is indicated to the external equipment by the Scan output line
(pin 7) being taken high via V301 and V302, which convert the levels in the
transceiver to the +10V or 0V levels required by the external equipment.
The external equipment switches the Quiet Line input high in response. This
is inverted by V303 and applied to the Microprocessor Controller PCB. As a
result of this the speaker is muted, the S’call Mute indicator is lit and
scanning is restricted to the first eight channels of the scan program.

When any selective call is detected, the external equipment takes the PTT
line low. This does not cause a PTT function but stops the scan long enough
for the whole selective call message to be decoded. If it is found not to be
addressed to the transceiver, the PTT line is released and scanning is
resumed.

If, however, the selective call is found to be addressed to the transceiver the
alarm tones generated by the external unit are connected to pin 5 of J301.
The alarm tones are applied to the audio circuits so that they are reproduced
by the loudspeaker. They are also applied to the Microprocessor Control
PCB (04-02451) where they are integrated by diode D6 and capacitor C23.
The voltage on C23 falls rapidly to the low logic level and this is detected by
the microprocessor which initiates the transmit mode. The revertive tones
are produced by the external equipment and applied to the transceiver.
During this period the microprocessor ensures that scanning stops and the
Quiet Line input goes high to unmute the receiver. The microprocessor also
lights the Called indicator and the decimal point in the channel display on
the transceiver front panel.

If the call is answered immediately (by the operator pressing the microphone
PTT switch) the alarm tones will cease, the transceiver will leave the scan
mode, i.e. it will not scan again until the Scan pad is pressed and the Called
indicators will be extinguished.

If, however, the call is not answered within a short time the alarm tones will
cease and the audio will be muted again. Scanning will be resumed
approximately 2.5 minutes after this but the Called indicator and the decimal
point will remain lit until the PTT switch is pressed (refer to Fig 4.1.)

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Figure 4.1 Selective Call Scanning

4.13.3 RTTY—ARQ Mode

With Option PS fitted the transceiver can be used for ARQ mode teletype
transmission and reception. The teletype interfacing equipment is connected
to J301 in the same way as the selective calling equipment. Refer to the
table in section 1.2.4 for the pin connections and the levels acceptable for
each function. Note that Option PS cannot be fitted if any of the internally
fitted selective calling function are fitted.

Note: It is essential to use J301 for the PTT connection in this mode. The

switching time of the PTT function at the microphone connecter on
the front panel is unacceptably long for ARQ operation. (The
transmit/receive switching time at J301 is 20ms).

If scanning is used in this mode (depending upon the external equipment
used), it may be necessary to enter the same channel into consecutive
positions in the scan program in order to dwell on each channel long enough
(in the order of 2 seconds) for the detection of RTTY - ARQ mode signals.
This will, of course, restrict the number of channels that can be scanned.
Scanning must be carried out in the Quiet Line mode, i.e. with +10V applied
to the Quiet Line input. The Alarm Tones input must be taken low (0V) by
the external equipment to stop the scanning.

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8525B/8528 Technical Service Manual Technical Description 4-37

4.14 Options

Options may be fitted to the transceivers as described in the sections which
follow.

4.14.1 F—Fan Assembly

Option F is a factory-fitted assembly containing an axial flow fan and its
control circuit. Mounted over the heatsink the fan enhances the heat
dissipation of the transmitter. When the option is fitted, heatsinks are added
to the driver and output transformers on the PA and Filter PCB.

The Fan Controller circuit (assembly 08-03334) switches on the fan if a
transmission lasts longer than 75 seconds, and keeps the fan switched on
even if short breaks (up to 600ms) are made during the transmission.

The circuit is powered from the transceiver A rail (12V) and the positive
side of this is taken through the PCB to the fan. Transistor V2 is in the
ground return line of the fan. The D rail (10V in transmit) is applied at the
PTT input. During reception, the 0V on the PTT input cuts off V1 allowing
C3 to charge via R3. The positive voltage applied to the MR input of IC1
holds the timer in the reset condition. In the reset condition the Q output of
IC1 is low so that V2 is cut off, breaking the return line of the fan supply.

When the PTT is operated the input goes high which switches on V1. This
immediately discharges C3 and the 0V on the MR input allows IC1 to start
timing. After 75 seconds the Q output of IC1 goes high to switch on V2
which completes the supply circuit to the fan.

If transmission stops, the PTT input goes low and V1 is switched off.
However, C3 must charge via R3 before the MR input to IC1 can go high
enough to reset the timer. This takes approximately 600ms and if
transmission is resumed in this period, C3 is immediately discharged again
and the timer is not affected. Consequently short breaks in transmission,
either during the initial timing period or after the fan has been switched on,
have no effect on the cooling.

4.14.2 LU—Switched LSB Operation

Individual channels can be programmed to operate on upper and lower
sidebands; the choice being made and indicated at the USB/LSB pad on the
front panel. For LSB operation, a 1647kHz crystal is fitted in the Z3 location
on the RF Mixer and Dual Synthesizer PCB 08-03740. Adjustment is
carried out as described in section 7.3.11 of this manual. Selection and
operation of the oscillator circuit is described in Section 4.4.5. The required
facilities are programmed into the EPROM.

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4-38 Technical Description 8525B/8528 Technical Service Manual

4.14.3 L—Lower Sideband Operation

Individual channels can be programmed to operate on LSB in lieu of the
normally programmed USB. The LSB crystal must be fitted as for option
LU. The required facilities are programmed into the EPROM.

4.14.4 M—CW Facility

A 3.2mm jack socket can be fitted to the transceiver rear panel and
connected to pin 3 of P2 on the Microprocessor Controller PCB (08-02451).
This provides the connection for an external morse key.

4.14.5 PH—Headphone Output

A 6.4mm switched jack socket can be fitted to the transceiver front panel
and connected to the loudspeaker circuit as shown in 04-02453. This
provides the connection for a headset. Insertion of the jack in the socket
breaks the loudspeaker circuit, thus muting the loudspeaker. The headset
signal is attenuated slightly by a 330Ω resistor in the line, thus making it
unnecessary to adjust the volume when changing to or from headset
operation.

4.14.6 R—Extended/Remote Control Interface

A 15-way connector can be fitted on the transceiver rear panel to provide the
connections for an extended or remote control head to be used in addition to
the controls on the transceiver front panel. The connector is included in the
normal build standard of transceivers manufactured specially for remote or
extended control only. The circuit is described in Section 4.6.3.

4.14.7 AD—Antenna Driver

Option AD enables Type 8525B/8528 Series Transceivers to be used with
Codan Automatic Tuning Whip Antennas Type 8558 while still retaining
the capability of using antennas with preset tuning.

The option consists of a single PCB (Antenna Driver PCB Assembly
(08-04285). The PCB plugs into J202 on the Mother Board of the
transceiver after disconnecting the ribbon cable previously connected to this
plug. The ribbon cable, which carries the antenna control facility to J201 on
the rear panel of the transceiver, is then connected to a plug (P1) on the
Antenna Driver PCB. Two further connections are made: J2 of the Antenna
Driver PCB is connected to J1 of the Microprocessor Board and J3 of the
Antenna Driver PCB is connected to P4 on the PA and Filter Board.

(Refer to Circuit Diagram 04-02724)

The inclusion of the option is indicated to the microprocessor in the
transceiver when the IO device, IC3, on the board responds to being polled,
on the I2C bus to its Clock and Data lines, when the transceiver is switched
on.

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8525B/8528 Technical Service Manual Technical Description 4-39

ADEQUATE RF

When the Tune pad on the transceiver control panel is pressed the
microprocessor causes some RF, at the required transmission frequency, to
be applied to the antenna. At the same time the microprocessor prepares the
IO device, IC3, to respond to the VSWR measurement. Voltages
proportional to the forward and reflected power at the antenna are applied
from the PA and Filter Board to J3 of the Antenna Driver PCB.

The forward power is compared in IC4a with a preset voltage to establish
whether the RF is adequate for the VSWR measurement to be made. If the
voltage proportional to the forward power exceeds the preset voltage the
output of IC4a goes to the low logic level to indicate

. The

result of the comparison is passed, via IC3, to the microprocessor.
The microprocessor has recorded the last used frequency and is given the

new frequency and it uses this information to drive the antenna tuning motor
in the appropriate direction to improve the VSWR. It does this by sending
data to IC3 to switch on one of the transistors, V4, V6, V8 or V10. These
switch on the corresponding Darlington transistors V5, V7, V9 or V11. The
antenna motor is a 4-phase, uni-polar stepper motor and is driven in the
wave mode (i.e. one phase at a time). +12V is applied to one end of all four
windings of the motor and, when switched on, each Darlington transistor
grounds the other end of one winding.

The forward and reflected power are compared in IC4b, IC5a and IC5b. One
input to each of these comparators is from a potential divider on the forward
power voltage, while the other input to each is the reflected power voltage.
The potential divider values are such that the output of IC4b will go low
when the VSWR is less than 3:1, IC5a will go low when it is less than 2:1
and IC5b will go low when it is less than 1.5:1.

The outputs of the comparators are sent to the microprocessor via IC3 and,
as the antenna tuning changes, the microprocessor detects any improvement
in the VSWR. As soon as an improvement is detected, provided the RF has
been found to be adequate, the microprocessor stops the tuning and a
positive voltage is applied to the Load input to the board (J1 pin 7). This
switches on V2 so that the Load output (P1 pin 4) goes low.

The Load output is applied to a circuit in the antenna which controls a 2-coil
latching type relay (K2 on the Auto Antenna PCB). The low level on the
Load input to the antenna causes the load presented by the antenna to be 22
ohms, while a high level on the input causes the load to be 35 ohms. The
microprocessor assesses whether the change of load has improved the
VSWR.

The microprocessor continues the process of changing the antenna tuning
and the load until either the VSWR is found to be less than 1.5:1 or no
further improvement can be made. Provided the final VSWR is less than 3:1
a Pass indication is given. If it is greater than 3:1, or the RF is inadequate, a
Fail indication is given.

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4-40 Technical Description 8525B/8528 Technical Service Manual

On completion of the tune operation a positive voltage is applied to the
Brake input (J1 pin 6). This switches on V1 which bypasses R1 to lower the
output of IC1 to approximately 1.3V. This is applied to the antenna motor
windings. At the same time V5 is switched on so that a small current
(approximately 120mA) is passed through one motor winding. This has a
braking effect so that the antenna tuning does not change with vibration.

Power for the comparator circuit is derived from the A rail by IC2. This
remains operative on completion of the tune operation so that any change in
the VSWR will still be detected. If a transceiver fitted with the option is
used with a non-tunable antenna an excessive VSWR will cause an

‘Untuned’ indication which cannot be corrected. The reflected power
voltage, via R10, is, therefore, taken to the disable line (P1 pin 5). When the
transceiver is used with a non-tunable antenna, pin 3 of the rear panel D
connector (J201) can be linked to pin 14 to disable the comparator so that no
‘Untuned’ indication is given.

When the transceiver Scan mode is selected, the SCAN input (J1 pin 9) is
taken low to switch on transistor V3. The collector current of the transistor
energises relay K1 in the antenna. This switches into circuit a wide-band
amplifier in the antenna which provides good reception over the entire range
of the antenna.

To retain the ability to use a transceiver fitted with the option with a
pretuned antenna, the lines from the Microprocessor Controller Board
carrying the BCD data for presetting the antenna are carried through the
option board to the rear panel D connector (J201).

n

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8525B/8528 Technical Service Manual 5-1

5 Operating Instructions

Refer to the relevant Operator’s Instructions booklets.

n

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5-2 Operating Instructions 8525B/8528 Technical Service Manual

Issue 6 Publication No: 15-02036

8525B/8528 Technical Service Manual 6-1

6 Maintenance

6.1 General

6.1.1 CMOS Devices

A number of Complementary Metal Oxide Semiconductor (CMOS) devices
are used in the transceiver. Although protection is built into most of these,
their extremely high open-circuit impedance makes them susceptible to
damage from static charges. Care must therefore be used when shipping and
handling the devices and in servicing equipment in which they are installed.
The following precautions should be observed:

• Packaging—Replacement CMOS devices are supplied in special

conductive packaging. They should be left in this packaging until
required for use.

• Switch off—Ensure that supplies are switched off before disconnecting

or reconnecting any connections between circuit boards and the
remainder of the transceiver.

• Handling—Handling of circuit boards and particularly touching any

conductive parts should be kept to a minimum.

• Grounding—Anything connected to or touching the circuit board tracks

should be grounded. Observe the following:

- Test equipment connected to a board should be grounded via its
mains lead.

- Static charges which may build up on the person can be discharged
by touching a grounded metal surface with both hands. This should
be done before, and at frequent intervals while, working on circuit
boards.

- Wearing a suitably grounded conductive wrist strap will minimise
the static build up on the person.

6.1.2 Circuit Boards

When servicing printed circuit boards the following should be observed:

• Excessive heat—Excessive heat may lift the track from circuit boards,

causing serious damage. Avoid the use of high powered soldering irons:
a 60W maximum iron, preferably temperature controlled at
approximately 370°C, is sufficient for most tasks. A slightly higher
temperature (425°C) iron may be required for heavier components such
as PA transistors. Apply the iron only long enough to unsolder an
existing joint or to solder a new one.

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6-2 Maintenance 8525B/8528 Technical Service Manual

• Unsoldering—When unsoldering use a solder-sucker or Solderwick to

remove solder. DO NOT USE SHARP METAL TOOLS SUCH AS
SCREWDRIVERS OR TWIST DRILLS AS THESE WILL DAMAGE
THE PRINTED CIRCUIT TRACK.

• Component substitution—Avoid unnecessary component substitution as

this may damage the component, the circuit track or adjacent
components.

• Component replacement—When a component is diagnosed as

defective, or the fault cannot be diagnosed in any other way than by
substitution, observe the following when installing the replacement:

- Axial leads—Components with axial leads, e.g. resistors and
tubular capacitors, can often be replaced without unsoldering the
joints on the boards. The defective component can be removed by
clipping its leads close to the component leaving the leads soldered
to the board. These leads should be straightened so that the leads of
the replacement can be wrapped around them and soldered. After
soldering the excess lead should be clipped off.

- Remove solder—When a component has been unsoldered from the
board ensure the holes are clear of solder before inserting the leads
of the replacement. ON NO ACCOUNT FORCE THE LEADS
THROUGH THE HOLES AS THIS WILL DAMAGE THE
CIRCUIT TRACK PARTICULARLY WHERE PLATED
THROUGH HOLES ARE USED.

- Observe orientation—When replacing diodes, transistors,
electrolytic capacitors or integrated circuits, before removing the
defective component, observe any marking indicating polarity or
orientation. It is essential that these types of components are
installed with the correct connections. If necessary consult the
manufacturer’s data for indications of the polarity of diodes or
capacitors and connectors of transistors.

- Heat sinking—Whenever possible use long-nosed pliers or some
other form of heat sinking on the leads of heat sensitive
components while soldering them to the board.

- Thermal conduction—When replacing transistors which are
mounted on heat sinks ensure good thermal conduction between the
heat sink and the replacement by cleaning the mounting surfaces
and recoating them with a thermal conduction compound such as
Jermyn Thermaflow A30.

• Track repair—Broken or burned sections of printed circuit track can be

repaired by bridging the damaged section with tinned copper wire. The
section where the repair is to be made must be cleaned observing the
precautions outlined above before soldering.

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8525B/8528 Technical Service Manual Maintenance 6-3

• Integrated circuit replacement—In some cases it is possible to desolder

and remove components from the board without damage to the
component or the board. However, integrated circuits with a large
number of connections, mounted on double-sided circuit boards with
plated-through holes are almost impossible to remove intact, and the
operation is likely to damage the circuit boards. To replace these
components their leads must be cut individually until the body of the
component can be removed from the board. Individual leads must then
be unsoldered and removed. Excess solder must be removed before
inserting the replacement component.

6.1.3 Transmitter Precautions

When making measurements of the low level stages of the exciter it is
advisable to remove the drive to the PA stages by disconnecting P2 on the
PA and filter PCB. The supply voltage is applied to the PA at all times when
the transceiver is switched on. Due care should be exercised when
connecting probes.

6.1.4 Probe Precautions

The following should be observed when connecting CRO probes to the
transceiver:

• When connecting probes to the PA assembly, the earth clip lead should

be wound around the body of the probe so that the earth clip just
reaches the probe tip. This reduces stray pick-up.

• The earth clip should be connected to the ground plane immediately

adjacent to the point of measurement to which the probe tip is
connected.

• It is not advisable to connect two probes at the same time, particularly

when one is earthed to the PA ground plane and the other is earthed to
the chassis, as this may cause earth loop problems.

• Probes should be connected after power has been applied to the

transceiver and the test equipment. The earth connection should be
made first and disconnected last.

6.1.5 Surface Mounted Components

Surface mounted components are used on some printed circuit boards. For
removal and replacement of these components it is essential to use tools
specifically designed for the purpose. Repairs to these boards should not be
attempted unless the appropriate tools are available.

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6-4 Maintenance 8525B/8528 Technical Service Manual

6.2 Fault Diagnosis

6.2.1 General

The removal and substitution of components may damage the components
and/or the printed circuit boards. In some cases it is impossible to remove
components without destroying them. It is important therefore to carry out as
much fault diagnosis as possible with components in situ. Specific tests are
described later in this section. The general points which follow should also
be of assistance:

• Spare boards—If spare boards are held in stock, they may be substituted

in order to positively localise the fault to one board.

• Transistor tests (static)—Transistor failures are most often due to open-

circuit base-emitter or base-collector junctions, or a short circuit
between emitter and collector.

These types of faults can often be detected without removing the
transistor, using the ohms range of a multimeter. The two junctions
should both give the appearance of a diode, i.e. high resistance with the
multimeter leads one way round and low resistance when the leads are
reversed. (Polarity depends on whether a PNP or NPN transistor is
being tested.) Resistance between collector and emitter should be high
with the multimeter leads either way round. The circuit diagram should
be examined for parallel paths before a transistor failing these tests is
removed.

• Transistor tests (dynamic)—Some transistor faults can be diagnosed by

measuring voltages within the circuit. One of the most significant
voltage measurements is the base-emitter voltage. The polarity of this
will depend on the type of the transistor (PNP or NPN). A base emitter
voltage between 0.5 and 0.9V should be measured on a forward-biased
base-emitter junction.

With its base emitter junction forward biased the transistor should
conduct. Some indication of satisfactory operation of the transistor can
be obtained by measuring the voltage drop across its collector or emitter
resistor and short circuiting its base to the emitter. The short circuit will
remove the forward bias cutting off the transistor so that the voltage
across the resistor will be considerably reduced.

• Integrated circuits—If there appears to be no output from an integrated

circuit, before replacing the device, it should be ascertained whether the
fault is due to the IC or its load. As a general rule, if changes in input
cause absolutely no changes in the corresponding output the IC should
be suspected. If, however, even a very small change in output can be
detected the load is more likely to be the cause. Depending upon the
circuit, further tests should be made by disconnecting resistors,
capacitors, etc to verify this diagnosis before removing the IC.

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8525B/8528 Technical Service Manual Maintenance 6-5

6.2.2 Voltage Measurement

The circuit diagrams and the relevant circuit notes give voltages at various
points under the various conditions to enable the faulty section of the
transceiver to be located.

The parameters listed below should always be checked first:

1. Supply voltages on the Motherboard and Chassis:

• Main B rail regulator (IC1) 10V ±0.2V

• C rail, B rail voltage in Receive, 0V in Transmit,

• D rail, 0V in Receive, B rail voltage in Transmit,

• Main 5V regulator (IC2) 5V ±0.4V.

2. Supply voltages on the RF Mixer and Dual Synthesizer PCB:

• +5V supply (IC4) 5V ±0.4V

• +26V supply (TP7) 26V ±1V

• +9V supply (V1 emitter) 9V ±0.25V

3. Supply voltage on the Microphone Amplifier and Interface PCB in the
Control Head: 5V regulator (IC1) 5V ±0.4V.

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6-6 Maintenance 8525B/8528 Technical Service Manual

6.2.3 Front Panel Controls

IN THE TEST WHICH FOLLOWS CARE MUST BE TAKEN WHEN
MAKING CONTACT WITH THE CONNECTIONS TO THE SWITCH
SUBSTRATE NOT TO SCRATCH THE SILVER PLATING.

Lack of response to controls may be due to malfunction of one or more of
the sealed membrane switches. These can be tested by disconnecting the
connector from the front panel (or control head) switches at J4 on the
Display PCB and testing between pins of the connector as shown in the table
below. A meter connected between each pair of pins in turn should indicate
open circuit with the corresponding switch not operated and continuity (less
than l00Ω) when the switch is pressed.

Front Panel Switch Test (8528)

Connector Pin 8528S 8528

1 - 2 Mute On/Off Mute On/Off
1 - 4 Scan Scan
1 - 5 AM Mode S’call Mute
1 - 7 Tx Power USB/LSB
1 - 8 Power On/Off Power On/Off.

1 - 10 Tune Tune

3 - 2 2182 Display
3 - 7 Enter Enter
3 - 8 – Emgcy Call

3 - 10 Recall Recall

6 - 2 Volume (up) Volume (up)
6 - 4 Clarifier(up) Clarifier (up)
6 - 5 Channel (up) Channel (up)
6 - 7 Tune Rx (up slow) Tune Rx (up slow)
6 - 8 Call (red) -

6 - 10 Tune Rx (up fast) Tune Rx (up fast)

9 - 2 Volume (down) Volume (down)
9 - 4 Clarifier (down) Clarifier (down)
9 - 5 Channel (down) Channel (down)
9 - 7 Tune Rx (down

slow)

9 - 8 Test Call

9 - 10 Tune Rx (down

fast)

Tune Rx (down slow)

Tune Rx (down fast)

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8525B/8528 Technical Service Manual Maintenance 6-7

Front Panel Switch Tests (8525B)

Connector Pin 8525B

1 - 5 Scan
1 - 6 Audio Mute
1 - 7 S’call Mute
1 - 8 Power On/Off
2 - 5 Tune
2 - 6 Mute off
2 - 7 USB/LSB
2 - 8 Emgcy Call
3 - 5 Clarifier
3 - 6 Channel
3 - 7 Channel
3 - 8 Call
4 - 5 Clarifier
4 - 6 Volume
4 - 7 Volume
4 - 8 -

6.2.4 Logic Levels

If the switches are found to be satisfactory, the lack of response may be
caused by faults in the main microprocessor or the associated data buses. If
this type of fault is encountered, the I2C bus lines should be monitored with
an oscilloscope. This can be done at J1 pins 1 to 4 and P3 pins 1 to 3 on the
Microprocessor Controller PCB. Operation of any of the front panel controls
should cause the exchange of data via the bus lines. This should be seen on
the oscilloscope as rapidly changing levels between +5 and 0V. No data on a
line, or failure of the levels to change cleanly should be investigated further.
Failure may be due to an IC or its load. Refer to 6.2.1. Ensure that the
terminations of all interconnecting ribbon cables are correctly mated and
aligned.

6.2.5 No Reception

If no signal is received, after confirming the supply voltages described in

6.2.2, the signal path should be traced back from the output stages by

applying the appropriate signals at convenient points. Voltages to be
expected at various points are shown in the circuit diagrams.

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6-8 Maintenance 8525B/8528 Technical Service Manual

6.2.6 No Transmission

If no signal is transmitted, after confirming the supply voltages described in

6.2.2, a single tone approximately 1kHz at 20mV rms should be applied to

the microphone input. The voltages to be expected in the transmit mode at
various points with this input applied are shown in the circuit diagram.

6.2.7 Unlocked Synthesizer

Should the synthesizer lose lock (indicated by UL being shown on the
channel display and Unlock indicators H1 or H2 on the RF Mixer and Dual
Synthesizer PCB, being lit) the following procedure should locate the cause
of the failure:

• Verify the correct voltage on Z2 as shown on the circuit diagram

04-02450.

• Verify that the Enable, Data and Clock pulses on J3 pins 3, 4 and 5

respectively are the correct levels (0 or +5V).

• Verify the correct output level from IC6 and IC8 as shown on the circuit

diagram.

• Verify the VCO control voltages at TP8 and TP9 are within the range

shown on the circuit diagram.

• Verify that the VCO frequencies and levels (V6 and V14 emitters) are

as shown on the circuit diagram.

6.2.8 Typical PA Voltages

In order to optimise the amplifier performance the PA amplifier transistors
are matched in pairs identified by a coloured dot. Measurements taken in
this area will depend upon the matched pair of transistors fitted and the
frequency of transmission.

The tables below are a guide to the peak-to-peak voltages to be expected at
specified points in the PA circuit. They are given for full power output when
driven with a two-tone input. For the tests the supply voltage is 13.6V and
the output is terminated into a 50Ω load.

Frequency Battery V18/V19 V16/V17 V12

Current

(MHz)

2 9.7 27 2.4 13.2 0.4 3.2 0.9
4 9.8 27 2.4 13.2 0.5 3.2 0.9
6 9.2 27 1.9 12.0 0.4 2.7 0.9

8 9.0 32 2.5 13.5 0.5 2.7 0.9
16 10.4 24 2.8 14.0 0.7 5.0 1.5
18 8.9 26 2.3 11.5 0.7 6.3 1.3
24 8.9 22 2.4 12.5 0.8 7.5 1.8

Issue 6 Publication No: 15-02036

(Amps)

C

Vpp

B

Vpp

C

Vpp

E

Vpp

C

Vpp

B

Vpp

8525B/8528 Technical Service Manual Maintenance 6-9

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6-10 Maintenance 8525B/8528 Technical Service Manual

6.3 Dismantling and Re-assembly

6.3.1 General

It may be necessary to remove printed circuit boards from the transceiver in
order to carry out certain repairs. The paragraphs which follow give
instructions for the removal and re-installation of boards. While carrying out
these the following general points should be observed:

• Screwdriver—Screws with Pozidrive heads are used in almost all

locations. Ensure that the appropriate screwdriver of the correct size is
used.

• Connectors—The ribbon cable header and multiway connectors used in

some locations can be incorrectly mated with their corresponding
connectors. Care must therefore be taken when re-installing to ensure
that these connectors are correctly mated.

6.3.2 Top and Bottom Covers

To gain access to the boards, the top or bottom cover must be removed from
the transceiver. To remove either cover, the two screws (one on each side)
must be removed and the rear edge of the cover lifted and drawn back so
that the front edge is released from the panel surround.

When re-installing the cover, it should first be placed on the transceiver with
its front edge resting on the two spring clips of the panel surround. The front
edge must then be pressed firmly down while it is slid forward under the
panel surround. When the cover is correctly positioned the rear edge can be
pressed down so that the retaining screws can be re-inserted and tightened.

6.3.3 Circuit Board Removal

Printed circuit boards connected directly to the Motherboard are easily
removed. After disconnecting any flexible leads connected to the board, its
retaining screws can be removed and the multiway sockets on the board can
be withdrawn from the corresponding pins of the Motherboard. This should
be done by progressively raising each socket a small amount so that the
board rises evenly. The procedure is reversed to re-install the boards.

Note: When removing the Audio and IF 1650kHz PCB, the heat sink

secured to the side of the cabinet should be released before the
screws retaining the board. On re-installation it should be secured
last.

To remove the boards mounted behind the front panel, the flexible leads
should be disconnected and the two countersunk screws released from each
side so that the panel can be withdrawn from the cabinet. The two boards
can then be released and separated.

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8525B/8528 Technical Service Manual Maintenance 6-11

6.3.4 PA and Filter Assembly

Note: The PA output transistors can be replaced without removing the

board from the heat sink. Refer to para 6.3.5.

The PA and Filter PCB (08-03021) can be removed as follows:

1. Disconnect:

• Ribbon cable(s) connecting rear panel connector(s) to the

Motherboard.

• Cable loom connecting the Option PS connector (if fitted) to the

Motherboard. (This connection is made by two 4-way connectors.
Note their positions to avoid cross-overs when reconnecting.

• The ribbon cable connecting the PA and Filter PCB to the

Motherboard. (This can be disconnected at both ends and
removed.)

• The 4-way Molex connector connecting two coaxial cables to the

PA and Filter PCB.

2. Release the two screws at each side of the PA and Filter PCB which
secure the board and the heat sink to the chassis side rails.

3. Release the two screws at the rear edge of the Motherboard which
secure it to the heat sink assembly. Note that these two screws are
shorter than the other screws securing the board.

4. Slide the heat sink assembly complete with rear panel and PA and Filter
PCB away from the chassis.

5. Release the four screws securing the PA transistors to the heat sink.
Note that at least one of these screws may be concealed by a capacitor,
one end of which may have to be unsoldered to gain access to the
screw.

6. Release the screw securing the clamp on the two driver transistors. Note
that this screw is longer than the other screws used to secure the board.

7. Release a further two screws which secure the board to the heat sink.

8. Release the earth screw and the three screws that secure the rear panel
to the heatsink.

9. Lift the board clear of the heat sink.

10. If work on the PA and Filter PCB is extensive, the dc power input lead
may be unsoldered so that the rear panel can be taken away from the
board.

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6-12 Maintenance 8525B/8528 Technical Service Manual

11. Before re-installing the board, clean all the components in contact with
the heatsink and the corresponding surfaces of the heat sink. Re-coat the
cleaned areas with Jermyn Thermaflow A30. To re-install the board
reverse the removal procedure taking particular care with the following:

• When remounting the board on the heatsink, ensure that R16 and

V11 are correctly located in their holes in the heat sink and that a
mica insulator is under V15.

• Ensure that the screws removed in disassembly are re-installed in

their correct locations.

• Screws should not be tightened until the reassembly is complete.

To avoid straining connections to the PA transistors the screws
securing these should be tightened last.

6.3.5 Replacement of PA Transistors

Since the PA output transistors are fitted in matched pairs they should be
replaced only in matched pairs. The gain groupings of the SRFH1008
(selected MRF455s) transistors are identified by a coloured dot. Transistors
of the same dot colour only should be fitted.

The PA transistors can be replaced without removing the PA and Filter PCB
from the heatsink. It is necessary to release only the screws securing the
transistors and to unsolder connections as follows:

1. Remove the transistor flange fixing screws.

2. Use a de-soldering tool or ‘Solder-wick’ to remove the bulk of the

solder from each connection. Gently pull the leads away from the PCB
while heating the joint.

3. Clear away any excess solder from the emitter, base and collector pads.

4. Thoroughly clean the transistor mating surface on the heatsink with a
cloth or tissue.

5. Form the leads of the replacement transistor using the discarded
transistor as a guide.

6. Coat the transistor flange with a thin film of thermal compound, e.g.
Jermyn Thermaflow A30.

7. Check the orientation and fit the new transistor. Tighten the flange
fixing screws evenly.

8. Carefully resolder the transistor connections. This should be carried out
quickly using a very hot tipped soldering iron.

9. Re-adjust the bias current. Refer to 7.3.13.

Issue 6 Publication No: 15-02036

8525B/8528 Technical Service Manual Maintenance 6-13

6.3.6 Replacement of Escutcheon or Switch Substrate

In replacing the switch substrate or the escutcheon on either the transceiver
or the control head the two parts can only be removed from the front panel
together and it is impracticable to separate them. It is therefore necessary to
have replacements for both parts (and the window fitted over the numeric
display behind the escutcheon) before the work is undertaken. Proceed as
follows:

1. Release the two screws at each side of the front panel and disconnect
the leads to the panel so that it can be removed completely from the
transceiver or control head. (Be careful to retain the screws for re­assembly. Damage will result if they are replaced with longer screws.)

2. On the transceiver panel only: release the nut retaining the microphone
connector and withdraw the connector and loom through the front
panel.

3. On either transceiver or control head panels fitted with selective calling
switches: disconnect the switches from the Display PCB, then squeeze
the retaining fingers of the switch assembly while pressing the assembly
forward out of the panel.

4. Disconnect the switch substrate connections from the Display PCB.

5. Taking care not to scratch the paint, insert a screwdriver blade or
similar instrument under one of the left hand corners of the escutcheon
and switch substrate and raise the corner. Grip the raised corner and
peel off the escutcheon and substrate towards the right.

6. Withdraw the substrate connections through the slot at the right hand
edge of the panel.

7. Clean away as much sealant as possible from the slot in the right hand
side of the panel. (The slot must be at least clear enough for the
connections of the new substrate to be inserted clearly.)

8. Use a heat gun to soften the connections of the replacement substrate,
then form them to the required shape using the discarded substrate
connections as a pattern.

9. When the connections have cooled the substrate should be offered into
position to ensure that the correct settings have been made. If they are
not correct the connections can be reheated and reset.

10. When the substrate connections have been satisfactorily formed, peel
off the backing sheet from the substrate and position it on the front
panel. When it is correctly positioned, press firmly on its face to ensure
its adhesion to the panel.

11. Connect the switch substrate connections to the Display PCB

12. Test the switch operation as described in paragraph 6.2.3.

13. Refill the slot through which the connections pass with Expandite
Silicone Sealant 88 (Codan Pt No. 71-30000-007). Ensure that the slot
is completely sealed but that no excess sealant remains around the slot,
particularly on the front face.

14. Peel off the backing sheet surrounding the window opening in the
escutcheon and press the replacement window into place.

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6-14 Maintenance 8525B/8528 Technical Service Manual

15. Peel off the backing sheet from the front face of the switch substrate
and place the escutcheon over the substrate. When the escutcheon is
correctly positioned press firmly all over its face to ensure its adhesion
to the substrate.

16. If the panel was originally fitted with selective calling switches use a
scalpel to cut out the required hole in the escutcheon. Refit the switches.

17. Reinstall the front panel on the transceiver or control head. Remake the
connections and test the transceiver.

n

Issue 6 Publication No: 15-02036

8525B/8528 Technical Service Manual 7-1

7 Adjustments

7.1 Introduction

Two types of adjustment are covered under separate headings.

• Channel Addition: change EPROM (no internal adjustments necessary)

• Preset Adjustments: normally factory-set adjustments will require

attention only if components which affect their settings are replaced.

7.2 Channel Addition (EPROM Replacement)

The EPROM is mounted in a socket on the Microprocessor Controller PCB
and may be changed as follows:

1. Place the transceiver upside-down on a bench with the power supply
disconnected.

2. Remove the bottom cover by unscrewing the two retaining screws
(either side of the transceiver) and lift the rear edge of the cover.

3. Two socket-mounted IC’s will be found near the centre of the

transceiver. The EPROM is identified by the number either:

• 90-20283 (8525B)

• 90-20278 (8528)

• 90-20275 (8528S)

4. Carefully remove the EPROM from the socket. (A small screwdriver
may be needed to prise it free.)

5. Plug in the new EPROM.
Note: The notch on the end of the EPROM must be on the left when

viewed from the front of the transceiver, i.e. in the opposite
direction to the other socketed IC.

6. Re-install the bottom cover by placing it on the transceiver so that the
front edge rests on the two spring clips. Press down firmly on the
cover’s front edge and slide it forward until the screw holes are aligned.
Re-install the screws.

Publication No: 15-02036 Issue 6

7-2 Adjustments 8525B/8528 Technical Service Manual

7.2.1 Channel Addition, Programming P Channels (8528)

In 8528 series transceivers, ninety-nine ‘P’ channels can be programmed
from the front panel to suit the user’s requirements. However, to prevent
transmission on frequencies for which the operator is not authorised, option
TxD inhibits the entry of new transmit frequencies by the operator. This can
be overridden by an internal link in order to program new transmit
frequencies.

Enabling

Transmitter programming is enabled by shorting link 1 on the
Microprocessor Interface PCB (refer to Fig. 7.1). The status of the link is
noted by the microprocessor during the initialisation program; therefore it is
only necessary to short the link (e.g. with a pair of tweezers) at switch on.
Once the display panel is lit, the link may be removed but will remain
effective until power is switched off. It should be noted that normal
operation of the transceiver is inhibited in the programming mode. If it is
intended to change or delete existing P channels, refer also to Program
Inhibit Indication below.

Figure 7.1 Microprocessor Interface PCB

Issue 6 Publication No: 15-02036

8525B/8528 Technical Service Manual Adjustments 7-3

Transmit Frequency Programming

Once the link is effective, transmit frequencies can be entered from the front
panel as follows:

Action Display will show Remarks

Select any channel Tx and Rx frequencies

Channel No.

Press Enter Tx____ Next action must be

started within 60

seconds
Press number pads for
transmit frequency
Press number pads for
receive frequency (if
same as Tx, press Enter
only)
Press Enter OPtION

Press Enter P_____
Press number pads for
channel number
Press Enter Tx and Rx frequencies

Tx 12.345.6
Rx____
Rx 12.345.6 250.0 to 30.000.0

___U__

Pxx

P channel number

2000.0 to 24000.0

(23.000 in suffix H)

Refer to Operators

Handbook, section 5

If the display shows

prog inhib, USEd or

Full, refer to the

corresponding

heading below

Program Inhibit Indication

Established P channels can be protected from being accidentally deleted or
over-written by the insertion of Link 2 on the Microprocessor PCB inside
the transceiver (refer to figure 7.1). If an attempt is made to over-write or
delete a channel when the link is installed, the display will show prog inhib
for a few seconds when Enter is pressed. The Tx and Rx parameters can still
be accepted by entering a new channel number and pressing Enter again. To
over-write or delete existing channels, the transceiver must be switched off
and the link unsoldered. The link should be resoldered with the transceiver
switched off when the programming has been completed.

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7-4 Adjustments 8525B/8528 Technical Service Manual

Used Indication

Channel numbers will normally be accepted on pressing the Enter pad.
However, if a channel with the entered number has previously been
programmed and the over-write link is not installed, the display will show
USEd. The channel number can be either:

1. changed by entry of a new number on the number pads, or

2. accepted and the previously programmed frequencies over-written by a
second operation of the Enter pad.

Full Indication

When all ninety-nine P channels have been programmed, on pressing the
Enter pad to accept another channel, the display will show Full. A further
press of the Enter pad will over-write the nominated channel. Alternatively
some redundant channels can be deleted and then re-used. If the over-write
protection link is installed, this must first be removed before channels can be
deleted.

Too Hi, Too Lo Indications

If an attempt is made to program a channel with either a Tx or Rx frequency
outside the range of the transceiver, an error message will be displayed. The
error can be corrected by simply entering a new number on the number pads.

Issue 6 Publication No: 15-02036

8525B/8528 Technical Service Manual Adjustments 7-5

7.3 Preset Adjustments

7.3.1 Test Equipment Required

• A calibrated CRO with 10X probe with 10MΩ and less than 12pF input

impedance. Y amplifier frequency response of at least 25MHz.

• RF dummy load, 50Ω 100W RMS minimum and Power Meter to suit.

• RF signal generator covering the range 250kHz to 30MHz and capable

of providing calibrated signals down to 0.25µV EMF from a 50Ω
source.

• Frequency counter capable of resolving to 1Hz frequencies up to

24MHz.

• Regulated Power Supply which can be set to 13.6V ±0.2V and capable

of supplying 20A peak current.

• Two-tone (i.e. 700Hz and 2300HZ) audio generator capable of

providing 0–l00mV RMS.

• Spectrum analyser suitable for SSB

• Multimeter or meters for measuring voltages (20kΩ/V or better) and

current (l00mA and 1A ranges) and an audio voltmeter.

• Transceiver Test Unit to Codan drawing 04-01868. (The isolating

transformer should be screened to prevent pick-up from nearby mains
transformers.)

• Square-wave generator capable of 5.5V pp at 100Hz.

• Decade resistance box for ease of determining select-on-test (SOT)

resistors. (A resistance box constructed using the E12 series values of
resistors is very useful.)

Before making any adjustments, the supply voltage must be set to 13.6V
±0.2V.

When working on the low level stages, e.g. the receiver and exciter, the PA
may be isolated by removing the dual coax socket to the PA and Filter PCB.
This will prevent unnecessary heating of the heatsink and removes the
possibility of high level RF fields being picked up by test leads which may
cause erroneous measurements or transmitter instability. Note that this also
disconnects the receive path (between the PA and Filter PCB and RF Mixer
and Synthesizer PCB).

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7-6 Adjustments 8525B/8528 Technical Service Manual

7.3.2 Voltage Regulators

None of the voltage regulators used are adjustable, so only their output
voltage can be checked. The location and correct output voltages are as
follows:

1. B rail regulator, 10V ±0.2V; IC1 on Motherboard.

2. +5V regulators, 5V ±0.4V.

• IC2 on Motherboard.

• IC7 on Audio and 1650kHz PCB

• IC4 on RF, Mixer, and Dual Synthesizer PCB.

• IC9 on Microprocessor Controller PCB

• IC1 on Mic Amp and Interface PCB (8530 Control Head only).

3. 26V regulator, 26V ±1V; TP7 on RF Mixer and Dual Synthesizer PCB.

4. 9V regulator, 9V ±0.25 Volts; emitter of V1 on RF Mixer and Dual
Synthesizer PCB.

These voltages should be checked with the transceiver in receive mode.

7.3.3 Crystal Oven

The oven temperature cannot be adjusted but should be checked after
allowing a five minute warm-up period. The temperature should be
approximately 60°C.

7.3.4 VCO Adjustments (RF Mixer and Dual Synthesizer
PCB)

These adjustments must be made with the metal shields in place.

VCO1

1. Select 30MHz frequency.

2. Using a DC Voltmeter or CRO, monitor TP8.

3. Adjust C42 via access hole in shield cover for 22.5V ±0.5V DC on TP8.

4. Select 250kHz; check TP8 is at 2V ±1V DC, and the ‘unlock’ LED for

VCO 1 is not lit.

VCO2

1. Select any frequency.

2. Using a DC Voltmeter or CRO, monitor TP9.

3. Adjust C79 via access hole in shield cover for 2.7V ±0.2V DC

Issue 6 Publication No: 15-02036

8525B/8528 Technical Service Manual Adjustments 7-7

7.3.5 45MHz IF Alignment—Receive

1. Ensure the dual coax socket is connected to the PA and Filter PCB.

2. Connect the signal generator to the antenna socket.

3. Select a channel (USB) and adjust the signal generator output to the
selected channel frequency plus 1kHz.

4. Increase the signal generator output until the signal is heard from the
loudspeaker.

5. Connect the audio voltmeter to the Demodulator output on the Audio
and IF 1650kHz PCB (TP2).

6. Adjust the 45MHz IF trimmer capacitors (outermost holes in shield lid)
to half mesh i.e. screwdriver slots parallel to the long edge of the shield.

7. Adjust L29 and L32 (centre holes) for a maximum signal on the audio
voltmeter.

8. Reduce the signal generator output and repeat step (7) until no further
increase can be obtained on the audio voltmeter (below AGC
threshold).

7.3.6 1650kHz IF Alignment

1. Align the 45MHz IF as above.

2. While still in Rx, peak the audio voltmeter reading by adjusting L1, L2,
and L3 (on the Audio and 1650 IF PCB) and reducing the signal
generator output to remain below AGC threshold.

3. When no further increase in the audio voltmeter reading can be
achieved, check that the signal generator output is below 0.35µV PD for
10dB SINAD.

7.3.7 45MHz IF Alignment—Transmit

CAUTION

THE PROCEDURE WHICH FOLLOWS INVOLVES ALTERNATELY
APPLYING A SIGNAL GENERATOR TO THE ANTENNA SOCKET TO
SET RECEIVER ADJUSTMENTS AND SWITCHING TO TRANSMIT TO
SET TRANSMIT ADJUSTMENTS. THE SIGNAL GENERATOR WILL BE
DAMAGED IF IT REMAINS CONNECTED WITH THE TRANSCEIVER IN
TRANSMIT.

1. Align the 45MHz IF in receive as above.

2. Align the 1650kHz IF as above.

3. DISCONNECT SIGNAL GENERATOR FROM ANTENNA
SOCKET.

4. Disconnect dual coax socket from PA and Filter PCB.

5. Connect audio two-tone source to microphone input and switch
transceiver to transmit.

6. Connect CRO across TP1 on the RF Mixer and Dual Synthesizer PCB.

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7-8 Adjustments 8525B/8528 Technical Service Manual

7. Adjust L29 and C64 (second and fourth holes from left-transceiver
facing toward operator) for maximum amplitude of the displayed
waveform.

8. Set the transceiver to receive and repeat steps (1) to (5) of 7.3.5.

9. Adjust C58 and L32 (extreme left and third hole from left) for
maximum signal on the audio voltmeter.

10. DISCONNECT THE SIGNAL GENERATOR FROM ANTENNA
SOCKET.

11. Switch the transceiver to transmit.

12. Ensure the two-tone signal is applied to the input and observe the
amplitude of the waveform displayed on the CRO:

• If the amplitude is below 200mV pp return to step (7) and repeat

the procedure.

• If the amplitude is greater than 200mV pp, disconnect the dual coax

socket from the PA and Filter PCB and confirm that the amplitude
increases to at least 400mV pp.

13. Reconnect the dual coaxial socket.

Issue 6 Publication No: 15-02036