BK Precision 1794 Instruction Manual

B

K

PRECISION

Instruction Manual



Model 1794 High Current Power Supply

Limited Two Year Warranty

B & K Precision Corp. warrants to the original purchaser that its product and
the component parts therof, will be free from defects in workmanship and
materials for a period of two years from the data of purchase.

B & K Precision Corp. will, without charge, repair or replace, at its' option,
defective product or component parts. Returned product must be
accompanied by proof of the purchase date in the form a sales receipt.

To obtain warranty coverage in the U.S.A., this product must be registered
by completing and mailing the enclosed warranty card to B & K Precision
Corp. 22820 Savi Ranch Parkway Yorba Linda, CA-92887, within fifteen
(15) days from proof of purchase.

Exclusions: This warranty does not apply in the event of misuse or abuse of
the product or as a result of unanthorized alternations or repairs. It is void if
the serial number is alternated, fefaced or removed.

B & K Precision Corp. shall not be liable for any consequential damages,
including without limitation damages resulting from loss of use. Some states
do not allow limitation of incidental or consequential damages, so the
above limitation or execlusion may not apply to you.

This warranty gives you specific right and you may have other rights, which
vary from state-to-state.

Model Number: ______________ Date Purchased: __________

TABLE OF CONTENTS

SECTION PARTICULARS PAGE NO.

1 GENERAL INFORMATION 1
2 SPECIFICATIONS 2
3 INSTALLATION 3
4 OPERATING INSTRUCTIONS 5
5 SERVICE INSTRUCTIONS 10
6 PART LIST 16
7 SCHEMATICS 21
8 CIRCUIT DIAGRAMS 28

SECTION - 1

GENERAL INFORMATION

1.1 DESCRIPTION :

The 1794 High Current Regulated D.C. Power Supply is completely solid and suitable
for bench operation or standard 19" rack operation. It is a well regulated constant
voltage / constant current supply which delivers 0-32V at 0-20Amps and can be
adjusted continuously throughout the output range. When the supply is used as a
constant voltage source the front panel VOLTAGE controls can be used to limit the
output voltage. When the unit is used as a constant current source, the front panel
CURRENT controls can be used to limit the output current. The unit will automatically
cross over from constant voltage to current mode and vice-versa if the output
current or voltage exceeds these limits.
Output voltage and current are continuously monitored on two front panel meters.
The load terminals and remote sense terminals are provided on the front panel.
Either the positive or negative output terminal may be grounded or the power supply
can be operated floating at upto a maximum of ±300VDC above ground.

1.2 OUTPUT ON/OFF SWITCH :

The output voltage and Current can be preset without connecting the power supply
to the load. With the OUTPUT ON/OFF switch in the OFF position, the maximum
voltage and current limits can be set. Pressing the OUTPUT ON switch connects
the output of the power supply to the load.

1.3 LIMIT SETTING :

The voltage and current limits can be set when the output is not connected to the
load. With the OUTPUT ON/OFF switch in the OFF position and the LIMIT switch
pressed, the voltage and current limits can be set with the front panel VOLTAGE
and CURRENT potentiometers.

1.4 REMOTE PROGRAMMING ( With External 5K Pot ) :

The output voltage of the unit can be programmed between 0 to 32V by means
of an external remote programming voltage resistor. An external resistance
programming selector link is provided on the rear panel for this purpose.

1.5 REMOTE SENSING :

The degradation in regulation which occurs at the load due to voltage drop in the
load leads can be eliminated by using the power supply in the remote sensing
mode operation. The remote sensing mode is selected by a push switch on the
front panel.

1

SECTION - 2

SPECIFICATIONS

OUTPUT VOLTAGE : 0-32V DC continuously variable with coarse

and fine voltage controls.

LOAD CURRENT : 0-30 Amp max., continuously variable with

coarse and fine controls.

CONSTANT VOLTAGE MODE
REGULATION
LINE : Less than ± 0.01% ± 2mV for ±10% change in

line voltage.

LOAD : Less than ± 0.01% ± 2mV for load change from

zero to full load.

RIPPLE & NOISE : Less than 1mV rms max.(20 Hz - 20 MHz)

CONSTANT CURRENT MODE
REGULATION
LINE : Less than ± 0.05% ±10mA for ±10% change in

line voltage.

LOAD : Less than ±0.05% ±10mA for change in output

voltage from 0 volts to maximum output voltage.

RIPPLE & NOISE : Less than 6mA rms.
OVERLOAD PROTECTION : Automatic overload and short circuit protection.

TRANSIENT RESPONSE : 100µsecs to within 10mV of set output voltage

for load change from 10% to 90%.

STABILITY :

Total drift within 8 hours,after : ± 0.2% ± 10mV in constant voltage mode.
warm up under constant line, : ± 0.5% ± 10mA in constant current mode.
load & temp.
PANEL METERS : Digital panel meters (marked V for voltmeter and

A for ammeter) are provided with an accuracy of
± 3 counts.

MODE INDICATION : Respective LED lights up when the unit is

working in CV or CC mode.

SET LIMIT/ ON-OFF : Push Switches for Limit ( V & A ) and output
CONTROLS ON/OFF setting.
OUTPUT CONTROLS : Single turn coarse and fine voltage and current

controls are provided on the front panel

EXTERNAL PROGRAMMING : Output voltage can be programmed by a variable

resistance of 5K ( Pot )

OPERATING TEMPERATURE : 0-40°C.
INPUT VOLTAGE : 120/230V AC, ± 10% 47 - 63Hz single phase.
DIMENSIONS : 19"(W ) x 15.75"(D) x 5.24"(H).
WEIGHT : 50 lbs. Net.

2

SECTION - 3

INSTALLATION

3.1 INITIAL INSPECTION :

As soon as the power supply 1794 unit is unpacked, inspect for any damage that
may have occurred during transit. Save all packing material until inspection is
completed. If any damage is found, notify the carriers immediately. Our authorised
representatives should also be notified.

3.2 PHYSICAL CHECK :

This check should confirm that there are no broken knobs or connectors, that the
cabinet and panel surfaces are free of dents and scratches and the meters are not
scratched and cracked.

3.3 ELECTRICAL CHECK :

The power supply unit 1794 should be checked against electrical specifications.
An in-cabinet performance check will verify proper operation.

3.4 INSTALLATION DATA :

The power supply unit 1794 is shipped ready for bench operation. It is necessary
only to connect the unit to a rated source of power (120V AC) and it is ready for
operation.

3.5 LOCATION :

The power supply 1794 unit is fan cooled. Sufficient space should be kept around
the unit while in operation, so that unit do not remain in confined space or close to
another heating source.The ambient temperature of the area around the unit should
be less than 40°C.

3.6 RACK MOUNTING :

The unit is in rack size and can be rack mounted in a conventional rack using
standard mounting screws and the rack attachments optionally supplied.

3.7 INPUT POWER REQUIREMENTS :

The power supply 1794 unit may be operated continuously from input
voltage of 120V or 230V AC 47 - 63Hz power source. It is factory wired for 120V AC
operation.

3

3.8 INPUT TAP SELECTION FOR 230V OPERATION :

In case of 230V Mains operation please ensure that shorting links on the Tap Selector
board is changed to 230V, marked on the PCB for both Mains Transformer &
Auxilary Transformer. as per the following diagram.

Please make sure the input varistor is replaced by 20D 361K or equivalent for
230V Mains Operation at the Input of EMI Filter located at bottom side of the unit.

3.9 REPACKAGING FOR SHIPMENT :

To ensure safe shipment of the power supply 1794 unit, it is recommended that the
package designed for the unit be used. The original packaging material is reusable.
Be sure to attach a tag to the unit specifying the owner, and the fault observed with
a brief description.

TAP SELECTOR BOARD

4

SECTION - 4

OPERATING INSTRUCTIONS

4.1 TURN ON SETTING PROCEDURE :

The following procedure describes the use of controls and indicators for
Constant Voltage and Constant Current Mode of Operation.

CONSTANT VOLTAGE (CV) MODE :

a. Set ‘POWER ON’ Switch & keep the OUTPUT ON/OFF switch to OFF
position ( ).
b. Press LIMIT switch and adjust the VOLTAGE controls till the desired voltage is
indicated on Voltmeter ( ).
c. Press OUTPUT switch to ON position and observe that CV LED lights ( )

CONSTANT CURRENT VOLTAGE (CC) MODE :

a. Turn off the suppy. Short circuit the output terminals of the power supply
& turn on the supply.
b. Keep the OUTPUT ON/OFF switch in the OFF position ( ).
c. Press LIMIT switch and adjust the CURRENT controls till the desired current is
indicated on Ammeter ( ).
d. Press OUTPUT switch ON position and observe that CC LED lights ( ).
e. Remove the short circuit.

4.2 SET LIMIT CONTROL (VOLTAGE AND CURRENT) :

To preset the output Voltage and Current, proceed as follows :
a. Set the OUTPUT switch to OFF position ( ) ( Release Position ).
b. Press the LIMIT switch and adjust the desired output voltage with the
VOLTAGE control potentiometers ( ).
c. Release the LIMIT switch, set the OUTPUT switch to ON position and check
the output voltage on the voltmeter ( ).
d. Set the OUTPUT switch to OFF position ( ).
e. Press the LIMIT switch and adjust the desired output current limit with the
CURRENT control potentiometers ( ).
f. Release the LIMIT switch, set the OUTPUT switch to ON position and check
the output current on the ammeter ( ).
g. Set the OUTPUT switch to OFF position ( ).
h. Connect the load to the Output terminals and set the OUTPUT switch to ON
position ( ).

4.3 LOAD CONNECTIONS :

The load should be connected to the power supply output terminals using separate
pairs of connecting wires. This will minimize mutual coupling effects between loads
and will retain full advantage of the low output impedance of the power supply.
Each pair of connecting wires should be as short as possible and twisted or shielded
to reduce noise pick up. (If a shielded pair is used, connect one end of the shield to
ground and leave the other end unconnected).

5

4.4 Positive or negative voltage can be obtained from this supply by grounding either
one of the output terminals or one end of the load. Always use two leads to connect
load to the supply, regardless of where the setup is grounded. This will eliminate
any possibility of the output current return paths through the power source ground
which would damage the line cord plug. This supply can also be operated upto
±300V DC above ground, if neither output terminal is grounded.

4.5 REMOTE SENSING :
Warning : Do not Operate the Unit in 'REMOTE SENSE' mode without

ensuring proper 'REMOTE/ 4 TERMINAL' Connections. Serious
Damage to Unit or Equipment under test could result.

Remote sensing is used to maintain good regulation at the load and reduce the
degradation of regulation which could occur due to the voltage drop in the leads
betweeen the power supply and the load. Remote sensing is accomplished by
connnecting the load to remote sense terminals on the front panel. The leads from
the sensing( +S and -S) terminals to the load will carry much less current than the
load leads and it is not required that these leads be as heavy as the load leads.
However, they must be twisted or shielded to minimize noise pickup.

For reasonable lengths of load leads, remote sensing greatly improves the
performance of the supply. However, if the load is located at a considerable
distance from the supply, added precautions must be observed to obtain
satisfactory operation. Notice that the voltage in the load leads subtracts directly
from the available output voltage and also reduces the amplitude of the feedback
error signals that are developed within the unit. Because of these factors, it is
recommended that the drop in each load lead does not exceed 0.5Volts.

NOTE : Due to the voltage drop in load leads, it may be necessary to

readjust the current limit in the remote sensing mode.

Observance of the precautions in para. 4.8 and 4.9 will result in a low dc output
impedance at the load. However, another factor that must be considered is the
inductance of long leads.This causes a high ac impedance and could affect the
stability of the feed back loop seriously enough to cause oscillations. If this is the
case, it is recommended that the following actions be taken:

a) Adjust R6 to remove oscillations, or to achieve best possible transient

response for given long load lead configuration.

b) If performing adjustment in step (a) above does not remove oscillation,

disconn-ect output capacitor C3 ( L1791-FP-CNT-0603 PCB ) and connect
a capacitor having similar characteristics (approximately the same capaci
tance, the same voltage rating or greater, and having good high frequency
characteristics) directly across load using short leads.
Readjust R6 as in step (a) above after making this change.

6

4.6 OUTPUT CAPACITANCE :

Internal capacitor C3 ( L1791-FP-CNT-0603 PCB ) connected across the output
terminals of the power supply, helps to supply high current pulses of short duration
during constant voltage operation. Any capacitance added externally will improve
the pulse current capability, but will decrease the safety provided by the constant
current circuit. A high current pulse may damage load components before the
average output current is large enough to cause the constant current circuit to
operate.

The effects of the output capacitor during constant current operation are as follows:
a.The output impedance of the power supply decreases with increasing frequency
b. The recovery time of the output voltage is longer for load resistance changes.
c. A large surge current causing a high power dissipation in the load occurs
when the load resistance is reduced rapidly.

4.7 REVERSE VOLTAGE LOADING :

A diode CR1 is connected across the output terminals. Under normal operation,
the diode is reverse biased (anode connected to the negative terminal ). If a reverse
voltage is applied to the output terminals ( positive voltage applied to the negative
terminal), the diode will conduct, shunting current across the output terminals to the
forward voltage drop of the diode. This diode protects the series transistor and the
output electrolytic capacitors.

4.8 REVERSE CURRENT LOADING :

Active loads connected to the power supply may actually deliver a reverse current
to the power supply during a portion of its operating cycle.
An external source cannot be allowed to pump current into the supply without loss
of regulation and possible damage to the output capacitor. To avoid these effects,
it is necessary to preload the supply with a dummy load resistor so that the power
supply delivers current through the entire operation cycle of the load device.

4.9 EXTERNAL PROGRAMMING :

Remote external programming can be achieved changing the external program
link on rear panel and connecting a variable resistor of 5K across the terminals as
shown on the rear panel.

7

OPERATORS SAFETY SUMMARY

The general safety information in this part of the summary is for both operating &
servicing personnel. Specific warnings and cautions will be found throughout the
manual where they apply, but may not appear in this summary.

TERMS IN THIS MANUAL :
CAUTION : Statements identify conditions or practices that could result in

damage to the equipment or other property.

WARNING : Statements identify conditions or practices that could result in

personal injury or loss of life.

TERMS AS MARKED ON EQUIPMENT :
CAUTION : Indicates a personal injury hazards not immediately accessible

as one reads the markings,or a hazard to property, including the
equipment itself.

DANGER : Indicates a personal injury hazard immediately accessible as one

reads the marking.

POWER SOURCE : This product is intended to operate from a power source that
does not apply more than 130V rms between the suply conductors or between
either supply conductor and ground. If the Internal Tap is selected for 230V AC
operation, then the input voltage should not be more than 250V rms. A protec­tive ground connections by way of the grounding conductor in the power cord is
essential for safe operation.

GROUNDING THE PRODUCT : This product is grounded through the
grounding conductor of the power cord. To avoid electrical shock, plug the
power cord into a properly wired receptacle before connecting to the product
input or output terminals. A protective ground connection by way of the
grounding conductor in the power cord is essential for safe operation.

DANGER ARISING FROM LOSS OF GROUND : Upon loss of the protective
ground connection, all accessible conductive parts (including knobs and controls
that may appear to be insulating) can render an electric shock.

USE THE PROPER POWER CORD :

Use only the power cord and connector specified for your product.
Use only a power cord that is in good condition.
Refer cord and connector changes to qualified service personnel.

8

USE THE PROPER FUSE : To avoid fire hazard, use only fuse of the correct type,
voltage rating and current rating as specified in the parts list for your product.
For 115V operation 20A, For 230V operation 10A Slow blow.

Refer fuse replacement to qualified service personnel.
DO NOT OPERATE IN EXPLOSIVE ATMOSPHERES : To avoid explosion, do

not operate this instrument in an explosive atmosphere unless it has been
specifically certified for such operation.

DO NOT OPERATE UNIT WITHOUT COVERS : To avoid personal injury, do
not operate this product without covers or panels installed.

SERVICING SAFETY SUMMARY

FOR QUALIFIED SERVICE PERSONNEL ONLY

Refer also to the Operators Safety Summary

DO NOT SERVICE ALONE :

Do not perform internal service or adjustment of this product unless another person
capable of rendering first aid and resuscitation if present.

USE CARE WHEN SERVICING WITH POWER ON :

Dangerous voltages exist at several points in this product. To avoid personal injury,
do not touch exposed connections or components while power is on.

Disconnect power before removing protective panels, soldering, or replacing
components.

POWER SOURCE :

This product is intended to operate from a power source that does not apply
more than 130V rms between the supply conductors or between either supply
conductor and ground. A protective ground connection by way of the grounding
connector in the power cord is essential for safe operation.

9

SECTION - 5

SERVICE INSTRUCTION

5.1 GENERAL :

The instrument has been tested throughly and then released for dispatch.Normally,
the unit works satisfactorily under all condition. However due to ageing / misuse or
malfunctioning, the unit may become defective. In case, the user wants to carryout
Servicing, the following instructions will be helpful in rectifying the defects.

The Servicing shall be done only by Qualified personnel.

5.2 TROUBLE SHOOTING TECHNIQUES :

5.2.1 INTRODUCTION: The trouble shooting procedure is arranged in an order that checks
the simple trouble possibilities before proceeding to extensive troubleshooting.

5.2.2 CONTROL SETTINGS : Incorrect control settings can seem like a trouble that
does not actually exist. If there is any question about the correct function or
operation of any control, see the Operating Instructions section.

5.2.3 SYSTEM AND ASSOCIATED EQUIPMENT: Before trobleshooting the 1790 (349),
check for proper input fuse as per part list is intac. Check the line voltage at the
power source before connecting the mains cord is rated for the correct input voltage.
Verify that the front panel controls are kept at minimum position. The output terminals
must be free from any load connecting leads.

5.2.4 VISUAL CHECK : Many troubles can be located by visual clues such
as unsoldered connections, broken wires, damaged circuit board, damaged
components, etc.

5.2.5 INTERNAL ADJUSTMENT :Check the Adjustment of the suspected circuit if the
trouble is obviously in a certain circuit. The trouble may only be a result of
misadjustment and may be corrected by readjustment.

5.2.6 CIRCUIT ISOLATION: Note the trouble symptoms. These often identify the circuit
in which the trouble is located. When trouble symptoms appear in more than one
circuit,check the effected circuits by making waveform and voltage measurements.

5.2.7 COMPONENT CHECKING : If a component cannot be disconnected from its
circuits, then the effects of the associated circuitry must be considered when
evaluating the measurement. Except for soldered-in transistors and integrated
circuits, most components can be lifted at one end from the circuit board.

5.2.8 TRANSISTORS AND IC's : Turn the power switch off before removing or
replacing any semiconductor. A good check of transistor operation is actual
performance under operating conditions. A transistor can most effectively be

10

checked by substituting a new component for it ( or one which has been checked
previously). However, be sure that circuit conditions are not such that a
replacement transistor might also be damaged. If substitute transistors are not
available, use a dynamic tester. Static-type testers are not recommended, since
they do not check operation under simulated operating conditions. An approved
anti-static suction-type desoldering tool must be used to remove soldered-in
transistors.

Integrated circuits can be checked with a voltmeter, test oscilloscope, or by
direct substitution. A good understanding of the theory of operation is essential to
troubleshooting circuits using IC's. Operating waveforms, logic levels, and other
operating information for the IC's are given in the theory of operation. Use care when
checking voltages and waveforms around the IC's so that adjacent leads are not shorted
together. A convenient means of clipping a test probe to the 14 pin in-line IC is with an
integrated circuit test clip. This device also doubles as an extraction tool.

5.2.9 DIODES : Do not use an ohmmeter that has a high internal current. High currents
may damage a diode. A diode may be checked for an open or shorted condition by
measuring the resistantce. With an ohmmeter scale having an internal source of
between 800mV and 1.5V, the resistance should be very high in one direction and
very low when the leads are reversed.

5.2.10 RESISTORS : Check the resistors with an ohmmeter. Resistor tolerances are
given in the Electrical Parts List. Resistors do not normally need to be replaced
unless the measured value varies widely from the specified value.

5.2.11 CAPACITORS : A leaky or shorted capacitor can be detected by checking
resistance with an ohmmeter on the voltage rating of the capacitor. The resistance
reading should be high after initial charge of the capacitor. An open capacitor can best
be detected with a capacito meter, or by checking whether it passes ac signals.

5.3 REPAIRS & MAINTENANCE :

5.3.1 Remove the mains cord from the source before removing top cover of the unit.

5.3.2 To open the cover, remove screws from Top & Bottom Covers. Disconnect
Grounding of both covers.

5.3.3 If there is no output, check the input supply Voltage and the Fuse continuity, etc.
Check both Fans are working

5.3.4 If the output is high and unregulated, check for open Voltage adjust potentiometer
or shorted mosfet, etc.

5.3.5 Similarly, if the current is unregulated, check for value change in the Shunt
Resistor. It may get open.

11

5.3.6 If Fuse blows, connect the unit through a variable AC Source with a current meter

REFER TO OPERATORS SAFETY SUMMARY AND SERVICE SAFETY SUMMARY PRIOR TO PERFORMING ANY SERVICE.

and monitor the current at no load. If the current is very high (more than 2 ampere
or so ) load, check for Bridge Rectifier short or shorted Diode, or Secondary
Winding short, etc

5.3.7 If output voltage or current DPM's are not reading, check for loose connections
especially in crimping, soldering, of Connectors.

WARNING

THE FOLLOWING SERVICING INSTRUCTIONS ARE FOR USE BY

QUALIFIED PERSONNEL ONLY.TO AVOID PERSONAL INJURY, DO NOT
PERFORM ANY SERVICING OTHER THAN THAT CONTAINED IN
OPERATING INSTRUCTIONS UNLESS YOU ARE QUALIFIED TO DO SO.

-

12

5.4.3 VOLTAGES AT VARIOUS PINS OF IC LM324 IS AS FOLLOWS ( IC-101 ) :-

IC PIN NO. VOLTAGES & CHECK CONDITIONS

PIN1 -- 0V ( WHEN V POT KEPT AT MIN. POSITION IN CV MODE )
PIN1 -- +5V ( WHEN V POT KEPT AT MAX. POSITION IN CV MODE )
PIN2 & 3-- +4.9V AT V POT MAX. POSITION.
PIN4 -- +12V ALL CONDITION
PIN7 -- 0V AT CC POT MIN. POSITION.
PIN7 -- 0.5V AT CC POT MAX. POSITION.
PIN8 -- +1V TO 3.5V IN CV MODE.
PIN8 -- +10V TO +11V IN CC MODE.
PIN9 -- 0V.
PIN10 -- -6mV.
PIN14 -- +10 TO 11V IN CV MODE.

5.4.4 A. CHECK VOLTAGES AT D3 CATHODE +2V (CV) +10V (CC)

B. CHECK VOLTAGES AT D4 CATHODE +10V (CV) + 2V (CC)
C. CHECK VOLTAGES AT Z101 CATHODE -4.7V
D. CHECK VOLTAGES AT Z102 CATHODE +10V

5.4.5 CHECK ALL REGULATORS OUTPUT WITH RESPECT TO OUTPUT

+VE COMMON.

REF. PCB NO. PART NO. PIN NO. VOLTAGES

U1 IC7812 3 +12V
U2 IC79L05 3 -5.0V
U3 IC7805 3 +5.0V

CHECK WITH RESPECT TO OUTPUT -VE AS COMMON.

U4 IC7812 3 +12.0V

All the above readings are approximate values. The given voltages & waveforms
are refering to a normal working unit. If find any Voltage or Waveform absent at
given points, then trace back to the associated circuits. This complets basic trouble
shooting test points.

5.5 PARTS ORDERING & REPLACING :

5.5.1 OBTAINING REPLACEMENT PARTS : Most electrical parts can be obtained
through your local distributor or representative. However you should be able to
obtain many of the standard components from a local commercial source in

13

your area. Before you purchase check the electrical parts list for the proper value
rating, tolerance and description.

5.5.2 ORDERING PROCEDURE :When ordering replacement parts from B+K Inc.
please include the following minimum information :

1. Power Supply Type ( 1794 ).

2. Power Supply Serial Number ( For example, 03080001 ).

3. A description of the part ( if electrical include the circuit number ).

5.5.3 CIRCUIT BOARDS : If a circuit board is damaged beyond repair, the entire
assembly including all soldered-on components can be replaced. To remove or
replace a board, proceed as follows :-

1. Disconnect all Sockets & leads connected to the board ( both soldered

lead connections and solderless pin connections ).

2. Remove all mounting screws holding the board in the instrument.Remove

any knobs, etc. that would prevent the board from being lifted out of the
instrument.

3. Lift the circuit board out of the unit. Do not force or bend the board.

4. To replace the board, reverse the order of removal. Use care when

replacing pin connectors. If forced into place incorrectly positioned, the
pin connectors may be damaged.

5.5.4 TRANSISTORS AND IC's : Transistors and IC's should not be replaced unless
they are actually defective. If removed from their sockets during routing
maintenance, return them to their original sockets. Unnecessary replacement or
switching of semiconductor devices may affect the calibration of the instruments.
When a transistor is replaced, check the operation of the part of the instrument that
may be affected.

Replacement semiconductors should be of the original type ora direct replacement.
When removing soldered-in transistors, use an approved anti-static suction-type
desoldering tool to remove the solder from the holes in the circuit board.

An extracting tool should be used to remove the 14 pin integrated circuits to pre­vent damage to the pins. If an extracting the pins. Pull slowly and evenly on both
ends of the IC. Avoid having one end of the IC disengage from the socket before
the other end.

14

CAUTION

Static discharge can damage any semiconductor

component in this instrument.

This instrument contains electrical components that are susceptible to damage
from static discharge. Static voltages of 1KV to 30KV are common in unprotected
environments.
Observe the following precautions to avoid damage :

1. Minimize handling of static-sensitive components.

2. Transport and store static-sensitive components or assemblies in their original
containers, on a metal rail, or on conductive foam. Label any packages that
contains static-sensitive assemblies or components.

3. Discharge the static voltages from your body by wearing a wrist strap while han­dling these components. Servicing static-sensitive assemblies or components should
be performed only at a static-free workstation by qualified service personnel.

4. Nothing capable of generating or holding a static charge should be allowed on the
work station surface.

5. Keep the component leads shorted together whenever possible.

6. Pick up components by the body, never by the leads.

7. Do not slide the components over any surface.

8. Avoid handling components in areas that have a floor or work-surface covering
capable of generating a static charge.

9. Use a soldering iron that is connected to earth ground.

10. Use only special anti-static suction-type or wick-type desoldering tools.

15

FAULT FINDING PROCEDURE

( A ) FUSE BLOWS

FUSE BLOWS

Is Line Voltage OK ? Adjust Proper Line Voltage.

Y

Is Fuse Rating OK ? Insert proper rating Fuse.

Y

Is Mains Transformer Tapping OK ? Adjsut the Tap according

Y

Are Power Diodes Connected

on Heat Sink Short ? Replace the Damaged Diodes.

N

Is Remote Sense Switch Pressed Release the Remote Switch

in Normal Condition ? in Normal Load Condition.

N

Is Device Drop setting proper on Load ?

N

Turn ON the Unit at Rated input Voltage. Load the Unit at
max rated Voltage and Current. Adjust the Voltage between

Output Positive and Capacitor Positive between

4.5V to 5V by using Please VR1 on PREG PCB

N

N

N

Y

Y

to required voltage.

1. Check Line Voltage for 230V AC or 115V AC, whichever applicable.

2. Check whether Fuse Present is 20A for 115V AC I/P and 10A for 230VAC I/P.

3. Check the Transformer Tapping, whether it is configured accordingly to I/P

Line Voltage.

4. The Power Diodes connected in Bridge configuration on Heat sink might be

short.

5. If Remote Sense Switch is pressed on Load, in Normal Operation, there is

chance unit may take High Current and Fuse might Blow.

6. If Device Drop measured across O/P Positive and Capacitor Positive is not

between 4.5V to 5V at rated I/P Voltage and rated O/P Voltage and Load
condition, there is a chance of Fuse getting blow in Load Condition.

Note : 4.5V to 5V Device Drop at Full Load.

16

( B ) UNREGULATED O/P VOLTAGE

UNREGULATED O/P VOLTAGE

Is IC LM324 OK ? Replace the IC.

Y

Is Voltage Pot Open OR Replace the Voltage Pot and
Pot Wire is Reaching PCB ? Ensure proper continuity

Y

Are Mosfets OK ? Replace the Damaged

Y

Is Gate Wire of Mosfet Ensure proper continuity of

Reaching PCB ? Gate Wire and PCB.

Y

Are Diodes D3 and D4 on Replace the Damaged
Front Panel PCB OK ? Diode.

Y

Is INT & COM Terminal on Back Panel Short ?

N

Short INT COM Terminal when
External Programming

is not in use.

N

N

with the PCB

N

Mosfet.

N

N

1. Check IC LM324 on Front Panel PCB (L1791-FP-CNT0603).
It might be damaged.

2. Check Voltage Potentiometer (5K Pot), whether it is open or whether the
wires associated with Voltage Potentiometer is reaching the Front Panel PCB.

3. Check the Mosfets and Zener Diodes across Mosfet.
It might be Short.

4. Check Diodes D3 and D4 on Front Panel PCB.
It might be Leaky or Short.

5. Ensure proper continuity of Gate Wire on Front Panel PCB and Mosfet module.

6. Ensure INT and COM Terminal at Back Panel are short when External
Programming is not in use.

17

( C ) NO OUTPUT VOLTAGE.

NO O/P VOLTAGE

Is IC LM324 OK ? Replace It.

Y

Is Voltage Pot Short ? Replace It.

N

Is Voltage Selection Preset Open ? Replace It.

N

Are Zener Diodes Z1 & Z2, Preset

VR1 on Preg. PCB OK ? Replace It.

Y

Are all the Auxillary Winding Ensure their proper

Reaching PCB ? Connectivity with PCB.

Y

Is there proper contact of Load Ensure proper Connectivity
Terminal with Terminal Nuts of Load Terminals and
Inside Unit. Terminal Nut inside.

N

Y

Y

N

N

Y

1. Check IC LM324 on Front Panel PCB L1791-FP-CNT-0603.
It might be Damaged.

2. Check the voltage Potentiometer. It might be Short.

3. Check Voltage Selection Preset R22 on Front Panel PCB. It might be Open.

4. Check Preset VR1 on Pre-regulator PCB (PS-PREG-1791-0503).
It might be Open.

5. Check whether Auxillary Transformer Windings are reaching PCB.

6. Check whether there is proper contact between Load Terminals and Terminal
Nuts inside the Unit.

18

( D ) NO DPM INDICATION.

NO DPM INDICATION

Is Display Completely Blank ? Is Continuity between

N

Does Display shows only Ensure proper Continuity.

dot Indication ?

Y

Is Continuity between IC7905 & Ensure proper Continuity.

DPM's OK ?

Y

Is Continuity between ANGD &

DPM's OK ? Ensure proper Continuity.

Y

Is DPM IC OK ?

N

Replace It.

Y

IC7805 & DPM OK ?

N

N

N

1. If there is no Indication at all ensure that there is proper Continuity
between +5V Wire of DPM's and IC7805. It might be possible that +5V Wire
Crimping might be loose or IC7805 might be Damaged.

2. It DPM shows only a Dot Indication. Check whether -5V from IC 7905 is
reaching DPM's. It might be possible that -5V Wire Crimping might be loose
or IC 7905 might be Damaged.

3. If DPM shows Dot Indication, check whether AGND ( Grey Wire ) is reaching
DPM's.

4. If DPM shows Dot Indication, check DPM IC7107. It might be Damaged.

19

( E ) DPM NOT READ ( 000 ).

DPM NOT READ ( V & I )
( 000 )

Is DPM IC 707 OK ? Replace It.

Y

Does Hi and Low Wire Ensure proper Continuity.

Reach DPM's ?

Y

IC TL431 OK ? Replace It.

Y

Is minimum Resistance

of Pot High ?

Y

Replace Pot.

N

N

N

1. Check whether Voltage Hi and Voltage Low Wire is reaching DPM.

2. Check DPM IC 7107. It might be Damaged.

3. Check minimum Output Voltage of Potemtiometer.
It should be less than 150mV.

4. See that Power Supply is earthed properly.

5. Check IC TL 431 ( Z1 and Z2 ) on Front Panel PCB.
It might be Damaged.

6. If Current DPM does not read then, check whether Current Hi and Low Wire
is reaching Current DPM.

20

( F ) UNIT NOT TAKE CURRENT.

UNIT NOT TAKE
CURRENT

Is Current Set Preset R21 OK ? Replace It.

Y

Is Current Pot Short ? Replace It.

N

Is Wiring Related to Ensure proper Continuity.
Current Pot OK ?

Y

Are Load Terminals connected
properly with Nut inside Unit ?

N

Ensure proper Terminal Contact.

N

Y

N

1. Check Current set Preset R21. It might be open.

2. No Contact of Load Terminals with the Terminal Nut inside the Unit.

3. Current Pot might be Shorted.

6. Check Wiring related to Current Pot.

21

( G ) UNIT DIRECTLY GOES TO CC.

UNIT DIRECTLY GOES
TO CC

Is Shunt Resistor open OR Replace Shunt Resistor

Dry Soldered ? if necessary.

N

Is IC LM324 OK ? Replace It.

Y

Is Current select Preset Replace It.
R21 Open ?

N

Is Remote Sense Terminal
Pressed ?

Y

Ensure Remote Sense Switch is
Unpressed in Normal Condition.

Y

N

Y

1. Check the Shunt Resistor ( 0.025E / 25E ).
It might be Open or Dry Soldered.

2. Check IC LM 324. It might be Damaged.

3. Current select Preset R21 might be Open.

6. Remote Sense Terminal might be Pressed on No Load.

22

( H ) NO CURRENT CONTROL.

NO CURRENT CONTROL

Is Current Pot Open ? Replace it.

N

Are Wires related to Current Pot OK ?

Y

Ensure proper Connectivity.

Y

1. Check the Current Potentiometer. It might be Open.

2. Check all Wires related to Current Potentiometer.

( I ) POOR LINE REGULATION.

POOR LINE REGULATION

Improper Device Drop Setting. Keep the Input Supply at rated I/P.

N

Leaky Summing Capacitor C4. +Ve Adjust that Vtg. between 4.5V

Y

Replace It.

Y

Keep the O/P Supply at rated Vtg.
and rated Load. Measure Vtg.
across O/P +Ve and Capacitor

to 5V with Preset VR1 ( 5K ) on
PREG PCB.

1. Check whether Device Drop is between 4.5V to 5V. It might not be proper.

2. Leaky Summing Capacitor C4 on PREG PCB.

23

( J ) POOR LOAD REGULATION.

POOR LOAD REGULATION

Load Terminal Nut inside Unit

might be Loose.

N

Is there any Oscillation Reduce Resistor R6 ( 2.2K ) on

in CV Mode ? F/Panel PCB until Oscillation dies

N

Is Capacitor C4 and C2 Leaky ?

Y

Replace It.

Y

OUT. If Oscillation still Prevail
reduce R9 to 2K on F/Panel PCB.

1. Load Terminal might be Loose. Ensure Terminals are tightened properly with
Terminal Nuts inside the Unit.

2. There might be Oscillation at Output in CV Mode. Adjust Resistor R6 & R9
until Oscillations die out.

3. Check Capcitor C4 and C2 on PREG PCB. It might be Leaky.

( K ) CV, CC, LED NOT GLOW.

1. Check the LED's. it might be Leaky, Open or Short.

24

( L ) EXTERNAL PROGRAMMING NOT OK.

EXTERNAL PROGRAMMING

NOT OK

Is Pot connected across INT Ensure 5K Pot connected

& COM OK ? between INT and COM

Y

Is there proper Continuity between
PROG Terminal and O/P +Ve ?

N

Ensure proper Connectivity.

N

Terminal.

1. Check whether 5K Pot is connected between INT and COM Terminal.

2. Check whether there is proper Continuity between PROG Terminal and
Output Positive.

(M ) REMOTE SENSE NOT OK.

1. Check whether there are Oscillations at Output, when Remote Sense Switch
is pressed. If Oscillations are present reduce Resistor R6 and R9 on Front
Panel PCB, until Oscillations die out.

25

SECTION - 6

PART LIST & SCHEMATICS

1. L1791-FP-CNT-0603 PCB ASSY

Reference Part Reference Part
Designator Description Designation Description

RESISTORS

R1 1K R22 500E(3206F)
R2 Not Used R23 33K
R3 100E R24 10K
R4 3.3K R25 10K
R5 1K R26 100E(3206F)
R6 2.2K R27 Not Used
R7 4K7 R28 100E(3206F)
R8 6K8 R29 1K
R9 6K8 R30 560E
R10 15K R31 820E
R11 1K R32 100E(3206F)
R12 1K R33 Not Used
R13 1K, 10W R34 100K
R14 1K R35 Short
R15 1K R36 1.5K
R16 3.3E R37 Short
R17 100E R38 330K
R18 2K2 R39 100E
R19 2K R40 1K8
R20 3K6 R41 12K
R21 500E(3206F)

ZENERS

Z1 TL 431 Z2 TL 431

DIODES

D1 1N4148 D4 1N4148
D2 1N4148 D5 1N4007
D3 1N4148 D6 1N4007

CAPACITORS

C1 1KPF/100V, DISC C5 10µF/35V, ELE
C2 1µF/100V, ELE C6 10µF/35V, ELE
C3 220µF/50V, ELE C7 Not Used
C4 1KPF/100V, DISC C8 0.1µF/230V

IC

U1 LM324

SWITCHES

SW1-3 2P2W SW PUSH BUTTON

TERMINALS

T1 60A / 1000V O/P +Ve RED TERMINAL ( 4mm )
T2 60A / 1000V O/P -Ve LIGHT BLUE TERMINAL ( 4mm )
T3 10A / 1000V Sns +Ve RED / BANANA TERMINAL (25mm L)
T4 10A / 1000V Sns -Ve BLACK / BANANA TERMINAL(25mm L)
T5 EARTH BRASS BINDING NICKEL PLATED ( 4mm )

26

1. L1791-FP-CNT-0603 PCB ASSY

Reference Part Reference Part
Designator Description Designation Description

CONNECTORS

J1 J2.54 - 3MSL, 3 PIN SIL 2.54mm ST LOCK MALE
J1 J2.54 - 3FSL, 3 PIN SIL 2.54mm ST LOCK FEMALE

CLAMPS

SW1 SINGLE PUSH SWITCH MTG. CLAMP
SW2 DOUBLE PUSH SWITCH MTG. CLAMP
SW3 DOUBLE PUSH SWITCH MTG. CLAMP

SWITCHES

SW1 GREY COLOUR CAP FOR PUSH SWITCH
SW2 GREY COLOUR CAP FOR PUSH SWITCH
SW3 GREY COLOUR CAP FOR PUSH SWITCH

2. PS-PREG-1791-0503 PCB ASSLY

Reference Part Reference Part
Designator Description Designation Description

RESISTORS

R1 3K3, 2W, MOR R15 100K, 0.25W, 5%, MFR
R2 270E, 2W, MOR R16 39K, 0.25W, 5%, MFR
R3 324E, 0.25W, 5% MFR R17 10K, 0.25W, 5%, MFR
R4 82K, 0.25W, 5% MFR R18 10K, 0.25W, 5%, MFR
R5 10K, 0.25W, 5% MFR R19 3K9, 0.25W, 5%, MFR
R6 180K, 0.25W, 5% MFR R20 390E, 0.25W, 5%, MFR
R7 8K2, 0.25W, 5% MFR R21 1K5, 0.25W, 5%, MFR
R8 330K, 0.25W, 5% MFR R22 4.7E, 0.25W, 5%, MFR
R9 180K, 0.25W, 5% MFR R23 3K9, 0.25W, 5%, MFR
R10 4K7, 0.25W, 5% MFR R24 15E, 0.25W, 5%, MFR
R11 820E, 0.25W, 5% MFR R25 10E, 0.25W, 5%, MFR
R12 6K8, 0.25W, 5% MFR R26 10E, 0.25W, 5%, MFR
R13 12K, 0.25W, 5% MFR R27 270E, 2W, MOR
R14 10K, 0.25W, 5% MFR

CAPACITOR

C1 33µF, 50V ELE C5 0.1µF, 63V
C2 4.7µF, 50V ELE C6 470µF, 50V, ELE
C3 2.2µF, 50V ELE C7 0.1µF, 230V AC
C4 1µF, 63V ELE

DIODES

D1-22 1N4007 ( 200V / 1A )

CONNECTORS

J1 J2.54-6MSL, 6 PIN SIL 2.54mm LOCK MALE
J1 J2.54-6MSL, 6 PIN SIL 2.54mm LOCK FEMALE
J2 J2.54-4MSL, 4 PIN SIL 2.54mm LOCK MALE
J2 J2.54-4MSL, 4 PIN SIL 2.54mm LOCK FEMALE
J3 J2.54-5MSL, 5 PIN SIL 2.54mm LOCK MALE
J3 J2.54-5MSL, 5 PIN SIL 2.54mm LOCK FEMALE

27

2. PS-PREG-1791-0503 PCB ASSLY

Reference Part Reference Part
Designator Description Designation Description

TRANSISTORS

Q1 BC 109 Q4 BC 557
Q2 MPSA12 Q5 BC 547
Q3 BC 557

ZENERS

Z1 4.7V, ½W Z2 10V, ½W

PRESET

VR1 5K ( 3206F ) HOR

TRANSFORMER

T1 EE25 PULSE TX.

3. Z-DPM/01 REV - 01 X 2

Reference Part Reference Part
Designator Description Designation Description

RESISTORS

R1 39K, 0.25W, 5%, MFR R6 2K4, 0.25W, 5%, MFR
R2 470K, 0.25W, 5%, MFR R7 330K, 0.25W, 5%, MFR
R3 1M, 0.25W, 5%, MFR R8 330K, 0.25W, 5%, MFR
R4 SEL ( INPUT ) R9 6K8K, 0.25W, 5%, MFR
R5 10K, 0.25W, 5%, MFR

PRESETS

PR1 3K, LIN, VER (REF ADJ)

CAPACITOR

C1 220µF/50V, EL C6 0.1µF, 100V, MP
C2 0.1µF/100V, MP C7 10µF, 50V, ELE
C3 0.01µF/100V, MP C8 0.1µF, 50V, CD
C4 0.1µF/100V, MP C9 10µF, 50V, ELE
C5 0.1µF/100V, MP C10 0.1µF, 50V, CD

IC's

IC1 7107 DECODER DRIVER
VR1 TL - 431

FND's V, DPM DS1 KLS 563G GREEN DS3 KLS 563G ( GREEN )

FND's A, DPM DS1 KLS 563 I RED DS3 KLS 563I ( RED )

LEDS

LED1 3mm GREEN FOR CV ( V DPM )
LED2 3mm RED FOR CC ( A DPM )

MISCILLANEOUS

J1 2.54 PITCH, 3 PIN M J3 2.54 PITCH, 4 PIN M
J2 Not Used

DS2 KLS 563G GREEN

DS2 KLS 563 I RED

28

4. PS-AUX-DC-L1791-0503 PCB ASSLY

Reference Part Reference Part
Designator Description Designation Description

CAPACITORS

C1 470µF/35V, ELE C8 100nF, 50V, DISC
C2 47µF/35V, ELE C9 470µF, 35V, ELE
C3 10µF/35V, ELE C10 470µF, 35V, ELE
C4 10µF/35V, ELE C11 220µF, 35V, ELE
C5 100nF/50V, DISC C12 100nF, 50V, DISC
C6 Not Used C13 100nF, 50V, DISC
C7 Not Used C14 10µF, 35V, ELE

ICs

U1 LM 78T12 U3 LM 7805
U2 LM 7905 U4 LM 7812

BRIDGE

BR1 W06M, 600V, 3A

DIODES

D1 1N4007 D4 1N4007
D2 1N4007 D5 Not Used
D3 1N4007 D6 Not Used

CONNECTORS

J1 J2.54ST/10, 10PIN SIL 2.54mm ST LOCK MALE
J1 J2.54ST/10, 10PIN SIL 2.54mm ST LOCK FEMALE
J2 J2.54ST/12, 12PIN SIL 2.54mm ST LOCK MALE
J2 J2.54ST/12, 12PIN SIL 2.54mm ST LOCK FEMALE

5. PS-PWR-L1791-0703A PCB ASSLY

Reference Part Reference Part
Designator Description Designation Description

RESISTORS

R1 0.1E, 3W, W/W R10 100K, 0.25W 5% MFR
R2 0.1E, 3W, W/W R11 100K, 0.25W 5% MFR
R3 0.1E, 3W, W/W R12 100K, 0.25W 5% MFR
R4 0.1E, 3W, W/W R13 100E, 0.25W 5% MFR
R5 0.1E, 3W, W/W R14 100E, 0.25W 5% MFR
R6 0.1E, 3W, W/W R15 100E, 0.25W 5% MFR
R7 100K, 0.25W 5% MFR 100E, 0.25W 5% MFR
R8 100K, 0.25W 5% MFR 100E, 0.25W 5% MFR
R9 100K, 0.25W 5% MFR 100E, 0.25W 5% MFR

ZENERS

R1-12 12V, ½W

TRANSISTORS

Q1-6 IRFP 150, MOSFET

MISCELLANIOUS

M7-8 M3 / M4 CLICK NUT
DQ1-3 ST MALE FASTNER 2 PIN
SQ1-3 MOUNTING

29

6. FRONT PANEL ASSLY

Reference Part Reference Part
Designator Description Designation Description

POT

VR1 5K, WWPOT VR3 500E, WWPOT
VR2 1K, WWPOT VR4 50E, WWPOT

SWITCH

SW1 16A/250V ON/OFF SWITCH

7. MAIN CHASSIS ASSLY

Reference Part
Designator Description

RESISTOR

R1 4.7K/2W, MOR
R2 0.025E / 25W SHUNT

DIODES

CR1 U30D40C (30AMP/400V DUAL DIODE)

CAPACITORS

C1 6 X 15,000µF, ELE
C2 1µF / 250V AC CAP X 2

RECTIFIER HEATSINK ASSLY

DIODES

CR1 40HFR20 CR3 40HF20
CR2 40HFR20 CR4 40HF20

SCR1 2906, 25A
SCR1 2906, 25A

FILTER 15A/20A, RFI FILTER
VARISTOR 130V OR 230V TYPE 20D 361
FAN1 12V / 24V INNOVATIVE ( HI COOL )

FAN2 12V / 24V INNOVATIVE ( HI COOL )

8. BACK TROUGH ASSLY

Reference Part
Designator Description

FUSE1 20AMP, TYPE F ( 115V ) 8 Amp TYPE T ( 230V ) ( 6.5mm X 32mm )
MAIN BOARD 3 PIN AMERICAN 12 AWG 3 CORE CABLE
REMOTE TERMINAL 4 WAY 10A TERMINAL BLOCK

30

FRONT PANEL LAYOUT

31

1. Power ON / OFF Switch. 6. Current Fine Control. 11. Output ON / OFF Switch.

2. Voltage Coarse Control. 7. CV Mode INdication. 12. Output Terminals.

3. Voltage Fine Control. 8. Current DPM. 13. Remote Sense Switch.

4. Voltage DPM. 9. CC Mode Indication. 14. Remote Sense Terminal.

5. Current Coarse Control. 10. Output Preset Switch. 15. Earth Terminal

BACK PANEL LAYOUT

32

1. INPUT FUSE HOLDER 3. INT / EXT PROGRAM TERMINAL 5. COOLING FAN.

2. MAINS CORD. 4. PROTECTIVE GROUND 6. VENTILATION.

TOP VIEW

33

BOTTOM VIEW OF MODEL : 1790 / 1791

34

FRONT PANEL CONTROL PCB OF 1790 / 1791

35

POWER MODULE PCB OF 1790 / 1791

36

PRE REGULATOR / AUX. P/S PCBs 1790 / 1791

37

Service Information

Warranty Service : Please return the product in the original packaging with

proof of purchase to the below address. Clearly state in writing the perfor­mance problem and return any leads, connectors and accessories that you
are using with the device.

Non-Warranty Service : Please return the product in the priginal packag­ing to the below address. Clearly state in writing the performance problem
and return any leads, connectors and accessories that you are using with the
device. Customers not on open account must include payment in the form of
a money order or credit card. For the most current repair charges contact
the factory before shipping the product.

Return all merchandise to B & K Precision Corp. with pre-paid shipping. The
flat-rate repair charge includes return shipping to locations in North America.
For overnight shipments and non-North America shipping fees contact B & K
Precision Corp.

B & K PRECISION CORP.

22820 Savi Ranch Parkway
Yorba Linda, CA-92887

Phone: [phone]
Fax : [phone]

Include with the instrument your complete return shipping address, contact
name, phone number and description of problem.

B

K



PRECISION

SN: 480-808-9-001

Printed in India
(M-00198)

B & K PRECISION CORP.

22820 Savi Ranch Parkway
Yorba Linda, CA 92887
www.bkprecision.com