Agilent 6626A Service Manual

SERVICE MANUAL

MULTIPLE OUTPUT LINEAR SYSTEM

DC POWER SUPPLY

AGILENT MODELS 6625A, 6626A,

6628A, and 6629A

Agilent Part No 06626-90003

Agilent Model 6625A, Serial 3738A-01389 through 01408
US37380101 and up

Agilent Model 6626A, Serial 3737A-02259 through 02328
US37370101 and up

Agilent Model 6628A, Serial 3738A-00683 through 00727
US37380101 and up

Agilent Model 6629A, Serial 3738A-00968 through 00997
US37380101 and up

* For instruments with higher Serial Numbers, a change page may be included.

5

Microfiche Part No. 06626-90004 Printed in Malaysia: September, 2001

CERTIFICATION

Agilent Technologies certifies that this product met its published specifications at time of shipment from the factory. Agilent
Technologies further certifies that its calibration measurements are traceable to the United States National Bureau of
Standards, to the extent allowed by the Bureau’s calibration facility, and to the calibration facilities of other International
Standards Organization members.

WARRANTY

This Agilent Technologies hardware product is warranted against defects in material and workmanship for a period of three
years from date of delivery. Agilent software and firmware products, which are designated by Agilent for use with a
hardware product and when properly installed on that hardware product, are warranted not to fail to execute their
programming instructions due to defects in material and workmanship for a period of 90 days from date of delivery. During
the warranty period Agilent Technologies will, at its option, either repair or replace products which prove to be defective.
Agilent does not warrant that the operation of the software, firmware, or hardware shall be uninterrupted or error free.

For warranty service, with the exception of warranty optio ns, this product must be returned to a service facility designated
by Agilent. Customer shall prepay shipping charges by (and shall pay all duty and taxes) for products returned to Agilent
for warranty service. Except for products returned to Customer from another country, Agilent shall pay for return of
products to Customer.

Warranty services outside the country of initial purchase are included in Agilent’s prod uct pr ice, only if Customer pays
Agilent international prices (defined as destination local currency price, or U.S. or Geneva Export price).

If Agilent is unable, within a reasonable time to repair or replace any product to condition as warranted, the Customer shall
be entitled to a refund of the purchase price upon retur n of t he pr oduct to Agilent.

LIMITATION OF WARRANTY

The foregoing warranty shall not apply to defects resulting from improper or inadequate maintenance by the Customer,
Customer-supplied software or interfacing, unauthorized modification or misuse, operation outside of the environmental
specifications for the product, or improper site preparation and maintenance. NO OTHER WARRANTY IS EXPRESSED
OR IMPLIED. Agilent SPECIFICALLY DISCLAIMS THE IMPLIED WARRANTIES OF MERCH ANTABILITY AND
FITNESS FOR A PARTICULAR PURPOSE.

EXCLUSIVE REMEDIES

THE REMEDIES PROVIDED HEREIN ARE THE CUSTOMER’S SOLE AND EXCLUSIVE REMEDIES. Agilent
SHALL NOT BE LIABLE FOR ANY DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
DAMAGES, WHETHER BASED ON CONTRACT, TORT, OR ANY OTHER LEGAL THEORY.

ASSISTANCE

The above statements apply only to the standard product warranty. Warranty options, extended support contracts, product
maintenance agreements and customer assistance agreements are also available. Contact your nearest Agilent
Technologies Sales and Service office for further information on Agilent’s full line of Support Programs.

SAFETY SUMMARY

The following general safety precautions must be observed during all phases of operation, service, and repair of this
instrument. Failure to comply with these precautions or with specific warnings elsewhere in this manual violates safety
standards of design, manufacture, and intended use of the instrument. Agilent Technologies assumes no liability for the
customer’s failure to comply with these requirements.

BEFORE APPLYING POWER.

Verify that the product is set to match the available line voltage and the correct fuse is installed.

GROUND THE INSTRUMENT.

This product is a Safety Class 1 inst rument (provided with a protective earth terminal). To minimize shock hazard, the instrument chassis
and cabinet must be connected to an electrical ground. The instrument must be connected to the ac power supply mains through a three­conductor power cable, with the third wire firmly connected to an electrical ground (safety ground) at the power outlet. For instruments
designed to be hard-wired to th e ac power lines (supply mains), conn ect the protective earth terminal to a protective conductor before any
other connection is made. Any interruption of the protective (grounding) conductor or disconnection of the protective earth terminal will
cause a potential shock h azard that could result in personal inju ry. If the instrument is to be energized via an external autotransformer for
voltage reduction, be certain that the autotransfo r mer common terminal is connected to the neutral (earthed pole) of the ac power lines
(supply mains).

FUSES.

Only fuses with the required rated current, voltage, and specified type (normal blow, time delay, etc.) should be used. Do not use repaired
fuses or short circuited fuseholders. To do so could cause a shock o r fire h azard.

DO NOT OPERATE IN AN EXPLOSIVE ATMOSPHERE.

Do not operate the instrument in the presence of flammable gases or fumes.

KEEP AWAY FROM LIVE CIRCUITS.

Operating personnel must not remove instrument covers. Component replacement and internal adjustments must be made by qualified
service personnel. Do not replace components with power cable connected. Under certain conditions, dangerous voltages may exist even
with the power cable removed. To avoid injuries, always disconnect power, discharge circuits and remove external voltage sources before
touching components.

DO NOT SERVICE OR ADJUST ALONE.

Do not attempt internal service or adjustment unless another person, capable of rendering first aid and resuscitation, is present.

DO NOT EXCEED INPUT RATINGS.

This instrument may be equipped with a line filter to reduce electromagnetic interference and must be connected to a properly grounded
receptacle to minimize electric shock hazard. Operation at line voltages or frequ encies in excess of those stated on the data plate may
cause leakage currents in excess of 5.0 mA peak.

SAFETY SYMBOLS.

Instruction manual symbol: the product will be marked with this symbol when it is necessary for the user to refer to the
instruction manual (refer to Table of Contents) .

Indicates hazardous vo ltages.

Indicate earth (ground) terminal.

The WARNING sign denotes a hazard. It calls attention to a procedure, practice, or the like, which, if not correctly
performed or adhered to, could result in personal inj ury. Do not proceed beyond a WARNING sign until the
indicated conditions are fully understood and met.

The CAUTION sign denotes a hazard. It calls attention to an operating procedure, or th e like, which, if not correctly
performed or adhered to, coul d result in damage to or destruction of part or all of the product. Do not proceed
beyond a CAUTION sign until the indicated conditions are fully understood and met.

DO NOT SUBSTITUTE PARTS OR MODIFY INSTRUMENT.

Because of the danger of introducing additional hazards, do not install substitute parts or perform any unau thorized modification to the
instrument. Return the instrument to an Agilent Technologies Sales and Service Office for service and repair to ensure that safety features
are maintained.

Instruments which appear damaged or defective should be made inoperative and secured against unintended operation until they can be
repaired by qualified service personnel

SAFETY SUMMARY (continued)

GENERAL

Any LEDs used in this product are Class 1 LEDs as per IEC 825-1.

ENVIRONMENTAL CONDITIONS

This instrument is intended for indoor use in an installation category II, pollution degree 2 environment. It is designed to
operate at a maximum relative humidity of 95% and at altitudes of up to 2000 meters. Refer to the specifications tables for the
ac mains voltage requirements and ambient operating temperature range.

SAFETY SYMBOL DEFINITIONS

Symbol Description Symbol Description

Direct current Terminal for Line conductor on permanently

installed equipment

Alternating current Caution, risk of electric shock

Both direct and alternating current Caution, hot surface

Three-phase alternating current Caution (refer to accompanying documents)

Earth (ground) terminal In position of a bi-stable push control

Protective earth (ground) terminal Out position of a bi-stable push control

Frame or chassis terminal On (supply)

Terminal for Neutral conductor on
permanently installed equipment

Terminal is at earth potential
(Used for measurement and control
circuits designed to be operated with
one terminal at earth potential.)

Herstellerbescheinigung

Diese Information steht im Zusammenhang mit den Anforderungen der
Maschinenläminformationsverordnung vom 18 Januar 1991.

* Schalldruckpegel Lp <70 dB(A) * Am Arbeitsplatz * Normaler Betrieb
* Nach EN 27779 (Typprüfung).

Manufacturer’s Declaration

This statement is provided to comply with the requirements of the German Sound Emission Directive,
from 18 January 1991.

* Sound Pressure Lp <70 dB(A) * At Operator Position * Normal Operation
* According to EN 27779 (Type Test).

Off (supply)

Standby (supply)
Units with this symbol are not completely
disconnected from ac mains when t his switch is
off. To completely disconnect the unit from ac
mains, either disconnect the power cord or have
a qualified electrician install an external switch.

Edition 2 September, 2001
© Copyright 2001 Agilent Technologies, Inc.

CONTENTS

Section I
INTRODUCTION

1-1 SCOPE ……………………………………………………1-1
1-2 SAFETY CONSIDERATIONS…………………………..1-1
1-3 INSTRUMENT AND MANUAL
IDENTIFICATION……………………………………….1-1
1-4 FIRMWARE REVISIONS ……………………………….1-1

Section II

PRINCIPLES OF OPERATION

2-1 INTRODUCTION ………………………………………..2-1
2-2 OVERALL BLOCK DIAGRAM DESCRIPTION

(Figure 2-1) ………………………………………………..2-1
2-3 AC Input Circuits ………………………………………..2-1
2-4 GPIB Board .……………………………………………... 2-1
2-5 Front Panel ……………………………………………….2-1
2-6 Output Boards …………………………………………2-1
2-7 GPIB BOARD (Figure 2-3) ……………………………... 2-3
2-8 GPIB Interface …………………………………………... 2-3
2-9 System MicroComputer…………………………………2-4
2-16 Output Boards Interface ………………………………...2-4
2-20 Front Panel Interface …………………………………...2-5
2-23 Bias Supply and Start Up ……………………………….2-5
2-24 OUTPUT BOARD ……………………………………… 2-7
2-25 Secondary Interface Circuits
(Figure 2-4) ……………………………………………… 2-7
2-37 Power mesh and Control Circuits
(Figure 2-5) …………………………………………… 2-10

Section III
VERIFICATION

3-1 INTRODUCTION ………………………………………..3-1
3-2 TEST EQUIPMENT REQUIRED ………………………3-1
3-3 OPERATION VERIFICATION TESTS ………………. 3-1
3-4 PERFORMANCE TESTS ………………………………..3-1
3-5 Introduction………………………………………………3-1
3-6 Measurement Techniques ………………………………3-1
3-10 Constant Voltage (CV) Tests ……………………………3-4
3-19 Constant Current (CC) Tests…………………………..3-10
3-27 EXTENDTED TESTS …………………………………...3-13
3-28 Output Drift Tests ……………………………………...3-13

Section IV

TROUBLESHOOTING

4-1 INTRODUCTION ………………………………………..4-1
4-2 ELECTROSTATIC PROTECTION ……………………..4-1
4-3 REMOVAL AND REPLACEMENT …………………...4-2
4-4 Top Cover Removal …………………………………….4-2
4-5 Gaining Access to Assemblies in the Supply …………4-2
4-6 GPIB Board Removal ..………………………………….4-2
4-8 DUSTCOVERS …………………………………………..4-4
4-9 Replacing the Power Module U338 ……………………4-4
4-10 Front Panel Removal …………………………………….4-4
4-11 Chassis Mounted Components ………………………...4-5
4-12 TEST EQUIPMENT REQUIRED ……………………….4-5
4-13 FUSE REPLACEMENT………………………………….4-5
4-14 INITIAL TROUBLESHOOTING AND BOARD

ISOLATION PROCEDURES ..…………………………4-8
4-15 Power-On Self Test ………………………………………4-8
4-16 Connector P201 Jumper Positions ……………………...4-9
4-17 ERROR Codes and Messages…………………………...4-9
4-18 GPIB BOARD AND FRONT PANEL

TROUBLESHOOTING PROCEDURES ……………...4-13
4-19 Test Setup ……………………………………………….4-13
4-20 Post Repair Calibration………………………………...4-13
4-21 Setting the Model Number

(MODEL Command) …………………………………...4-13
4-22 Signature Analysis Testing …………………………4-14
4-23 Test Setup for S.A. ……………………………………...4-14
4-24 Firmware Revisions (ROM? Command)……………..4-14
4-25 OUTPUT BOARD TROUBLESHOOTING

PROCEDURES ………………………………………….4-30
4-26 Test Setup ……………………………………………….4-30
4-27 Post Repair Calibration ………………………………..4-30
4-28 Self Exercise Routine on the Output Board ………….4-30
4-29 Troubleshooting Analog Multiplexer U323 and

Readback Using VMUX? Command …………………4-49
4-30 Understanding and Troubleshooting the Signal

Processor (U327)………………………………………...4-50
4-33 Power Module Signals …………………………………4-54
4-34 Miscellaneous Trouble Symptoms and Remedies ….4-54

Section V

REPLACEABLE PARTS

5-1 INTRODUCTION ………………………………………..5-1
5-2 HOW TO ORDER PARTS ………………………………5-1

i

CONTENTS (Cont. )

Section VI
CIRCUIT DIAGRAMS

6-1 INTRODUCTION ………………………………………..6-1
6-2 FUNCTIONAL SCHEMATIC DIAGRAMS …………..6-1
6-3 COMPONENT LOCATION ILLUSTRATIONS ……...6-1

Appendix A
LOGIC SYMBOLOGY

ii

LIST OF FIGURES

Figure Page

2-1 Agilent 6625A, 6826A, 6628A and 6629A Multiple Output Power Supplies, Block Diagram ……………………………...2-2
2-2 Output Operating Ranges for Agilent Models 6625A, 6626A, 6628A and 6629A …………………………………………2-3
2-2 HP-IB Board, Block Diagram ……………………………………………………………………………………………………..2-6
2-4 Output Board, Secondary Interface Circuits, Block Diagram ………………………………………………………………….2-9
2-5 Output Board, Power Mesh and Control Circuits, Block …………………………………………………………………….2-12
2-6 Voltage and Current Control Circuits, Simplified Schematic ………………………………………………………………..2-13
2-7 Typical Output Range Characteristics ………………………………………………………………………………………….2-14
2-8 Typical Downprogramming Characteristics Below 2.0 V …………………………………………………………………….2-16
2-9 Overvoltage Protection Circuits, Block Diagram ……………………………………………………………………………...2-17
3-1 Operating Ranges Available in Models 6625A, 6626A, 6820A and 6629A …………………………………………………..3-3
3-2 Current Monitoring Resistor Setup ………………………………………………………………………………………………3-4
3-3 Basic Test Setup …………………………………………………………………………………………………………………….3-5
3-4 Transient Recovery Time Test Setup …………………………………………………………………………………………….3-6
3-5 Transient Response Waveform. …………………………………………………………………………………………………..3-7
3-6 Negative Current Limit (- CC) Readback Accuracy ……………………………………………………………………………3-9
3-7 Down Programming Speed Test Setup ………………………………………………………………………………………3-12
3-8 CV Down Programming Speed Test Waveform ………………………………………………………………………………3-13
3-9 CV Up Programming Speed Test Setup ………………………………………………………………………………………..3-13
3-10 CV Up Programming Test Waveform ………………………………………………………………………………………….3-14
3-11 Fixed OV Protection Test Setup …………………………………………………………………………………………………3-14
3-I2 OV External Trip Test Connection ……………………………………………………………………………………………3-14
4-1 Agilent 6625A, 6626A, 6628A and 6629A Multiple Output Supplies, Assembly Locations ……………………………….4-3
4-2 HP-IB, Board, Fuse and Test Point Locations …………………………………………………………………………………...4-6
4-3 Output Board 1 and 2 Fuse and Test Point Locations ………………………………………………………………………….4-7
4-4 Output Board 3 and 4 Fuse and Test Point Locations ………………………………………………………………………….4-9
4-5 Initial Troubleshooting and Board Isolation …………………………………………………………………………………..4-13
4-6 HP-IB Board and Front Panel Troubleshooting ……………………………………………………………………………….4-15
4-7 Signature Analysis Test Setup ………………………………………………………………………………………………….4-17
4-8 Output Board Troubleshooting …………………………………………………………………………………………………4-29
4-9 Output Board Waveform During Self Exercise Routine ……………………………………………………………………4-35
4-10 DAC/Amplifier Circuit Troubleshooting ……………………………………………………………………………………...4-36
4-11 Overvoltage Troubleshooting …………………………………………………………………………………………………4-37
4-12 Output Held Low Troubleshooting ……………………………………………………………………………………………..4-39
4-13 Output Held High Troubleshooting …………………………………………………………………………………………….4-42
4-14 OV Circuit Will Not Trip Troubleshooting ……………………………………………………………………………………..4-43
4-15 Signal Processor U327, Overvoltage Circuit, Simplified Schematic …………………………………………………………4-46
4-16 Signal processor U327, Power-On/Start-Up Circuit, Simplified Schematic ………………………………………………4-46
4-17 Signal processor U327, Status Monitor Circuit, Simplified Schematic ………………………………………………………4-52
4-18 Status Problems Troubleshooting ……………………………………………………………………………………………….4-53

6-1 Power Distribution Schematic ….…………………………………………………………………………………………………6-3
6-2 GPIB Board, Component Location ……………………………………………………………………………………………….6-5
6-2 GPIB Board, Schematic Diagram ...……………………………………………………………………………………………….6-6
6-3 Output 1 & 2 Board, Component Location ..…………………………………………………………………………………..…6-7

6-3 Output 1 & 2 Board, Schematic Diagram .....…………………………………………………………………………………..…6-8

6-4 Output 3 & 4 Board, Component Location ..……………………………………….…………………………………………..6-13
6-4 Output 3 & 4 Board, Schematic Diagram .………………………………………….…………………………………………..6-14

iii

LIST OF TABLES

Table Page

3-1 Test Equipment Required for Verification ………………………………………………………………………………………… 3-2
3-2 Low Range Voltage and Current Values …………………………………………………………………………………………… 3-4
3-3 Performance Test Record for Agilent 6625A and 6628A ...………………………………………………………………………. 3-15
3-4 Performance Test Record for Agilent 6626A and 6629A ………………………………………………………………………… 3-16
4-1 Test Equipment Required for Troubleshooting …………………………………………………………………………………… 4-5
4-2 Fuses …………………………………………………………………………………………………………………………………….4-6
4-3 Tests Performed at Power-On ..……………………………………………………………………………………………………… 4-8
4-4 Power-On Self Test Error Message ………………………………………………………………………………………………….. 4-9
4-5 ERROR Codes and Messages.………………………………………………………………………………………………………. 4-10
4-6

GPIB Board S.A. Test No. 1 ………………………………………………………………………………………………………… 4-18
4-7 GPIB Board S.A. Test No. 2 ………………………………………………………………………………………………………… 4-19
4-8 GPIB Board S.A. Test No. 3 ………………………………………………………………………………………………………… 4-20
4-9 GPIB Board S.A. Test No. 4 ………………………………………………………………………………………………………… 4-21
4-10 GPIB Board S.A. Test No. 5 ………………………………………………………………………………………………………… 4-22
4-11 GPIB Board S.A. Test No. 6 ………………………………………………………………………………………………………… 4-23
4-12 GPIB Board S.A. Test No. 7 ………………………………………………………………………………………………………… 4-24
4-13 GPIB Board S.A. Test No. 8 ………………………………………………………………………………………………………… 4-25
4-14 Keyboard Signal Paths ………………………………………………………………………………………………………………. 4-27
4-15 Microcomputer (U312) Signal Measurements During the Self Exercise Routine …………………………………………… 4-44
4-16 U368 Signal Levels …………………………………………………………………………………………………………………… 4-45
4-17 Signal Processor (U327) Signal Levels …………………………………………………………………………………………….. 4-47
4-18 Typical Power Module Voltage Levels ……………………………………………………………………………………………. 4-49
4-19 Miscellaneous Trouble Symptoms …………………………………………………………………………………………………. 4-50
5-1 Output Board Configurations ……………………………………………………………………………………………………….. 5-1
5-2 Reference Designators ………………………………………………………………………………………………………………… 5-1
5-3 Abbreviations …………………………………………………………………………………………………………………………...5-2
5-4 Federal Manufacturer Codes ………………………………………………………………………………………………………… 5-3
5-5 Chassis Parts …………………………………………………………………………………………………………………………… 5-4
5-6 Output Board Replacement Part List …………………………………………………………………………………………………5-7
5-7 25W/.5A Replacement Parts List ……………………………………………………………………………………………………. 5-9
5-8 50W/2A Replacement Parts List …………………………………………………………………………………………………… 5-18

iv

Section I

INTRODUCTION

1-1 SCOPE

This manual contains principles of operation, verification,
and troubleshooting information for the power supply.
Replaceable parts lists and circuit diagrams are also
provided. Installation, operation, programming, and
calibration procedures as well as detailed specifications are
given in a separate Operating Manual, Agilent Part No.
06626-90001.

Wherever applicable, the service instructions given in this
manual refer to pertinent information provided in the
Operating Manual. The information in each manual covers
model 6625A, 6626A, 6628A, and 6629A. The main
differences between the models are the number and type of
outputs each model contains. These differences are specified
in each of the manuals.

The following is a listing of the information contained in this
manual with a brief description concerning its scope and
purpose.

Principles of Operation: Section II provides block diagram
level descriptions of the supply’s circuits. The GPIB
interface (digital circuits), the power control (analog and
digital circuits), and power output (analog circuits) are
described. These descriptions are intended as an aid in
troubleshooting.

Verification: Section III contains test procedures that check
the operation of the supply to ensure that it meets the
specifications given in Section I of the Operating manual.

Troubleshooting: Section IV contains board level
troubleshooting procedures to isolate a malfunction to a
defective board (GPIB or output board) or assembly (front
panel, power transformer, or cable assembly). Additional
troubleshooting procedures are provided to isolate the fault
to a defective component on the board. Board and assembly
level removal and replacement procedures are also given in
this section.

NOTE

Calibration is generally required after a repair is made.
Software calibration procedures are given in Appendix A
of the Operating Manual. After calibration is completed,
perform the applicable test(s) given in Section III of this
manual to ensure that the supply meets all specifications.

Replaceable Parts:

replaceable parts for all electronic components and
mechanical assemblies.

Section V provides a listing of

Circuit Diagrams:

and component location diagrams . The names that appear
on the functional schematics also appear on the block
diagrams in Section II. Thus, the descriptions in Section II
can be correlated with both the block diagrams and the
schematics.

Section VI contains functional schematics

Logic Symbology:

the logic symbols used on the functional schematics.

Fault Indicator (FLT) and Remote Inhibit (INH): A fault
indicator and remote inhibit circuit, which provide
additional shutdown protection should either the GPIB
and/or controller fail, are available optionally. See a
separate document entitled, "Appendix E Option 750
Operating Instructions" for the Multiple Output Linear
System DC Power Supply Agilent Models 662xA (Agilent
P/N 5957-6372).

Appendix A gives a brief description of

1-2 SAFETY CONSIDERATIONS

This product is a Safety Class 1 instrument, which means
that it is provided with a protective earth terminal. The
instrument and this manual should be reviewed for safety
markings and instructions before operation. Refer to the
Safety Summary page at the beginning of this manual for a
summary of general safety information. Safety information
for specific procedures is located at appropriate places in the
manual.

1-3 INSTRUMENT AND MANUAL

IDENTIFICATION

Agilent Techonologies instruments are identified by a two­part Serial number, i.e. 2601A-00101. The first part of the
serial number (the prefix) is a number/letter combination
that denotes either the date of manufacture or the date of a
significant design change. It also indicates the country of
manufacture. The first two digits indicate the year (25 =
1985, 26 = 1986, etc), the second two digits indicate the week,
and “A” designates the U.S.A. The second part of the serial
number is a different sequential number assigned to each
instrument.

1-1

If the serial number prefix on your power supply differs
from that shown on the title page of this manual, a yellow
Manual Change sheet that is supplied with the manual
add/or manual backdating changes in Appendix A of this
manual define the differences between your supply and the
supply described in this manual. The yellow change sheet
may also contain information for correcting errors in the
manual.

The serial number prefixes listed on the front of this manual
indicate the versions of the supplies that were available
when the manual was issued. If the serial prefix of your
supply is not listed in this manual, the manual may include

a yellow “Manual Changes” sheet. That sheet updates this
manual by defining any differences between the version of
your supply and the versions included here, and may also
include information for correcting any manual errors. Note
that because not all changes to the product require changes
to the manual, there may be no update information required
for your version of the supply.

1-4 FIRMWARE REVISIONS

The Read Only Memory (ROM) chip inside of your supply is
identified with a label that specifies the revision of the
supply’s firmware, see paragraph 4-24

1-2

Section II
PRINCIPLES OF OPERATION

2-1 INTRODUCTION

The following paragraphs provide block diagram level
descriptions of the power supplies. Differences between the
models are given as required. The descriptions provide a
basic understanding of circuit operation and are intended as
an aid in troubleshooting . It is assumed in the following
discussions that you are familiar with the operating and
programming instructions presented in the Operating
Manual (Agilent Part No. 06626-90001).

2-2 OVERALL BLOCK DIAGRAM DESCRIPTION

(FIGURE 2-1)

Figure 2-1 is a block diagram that illustrates the major
assemblies contained within the power supply. As shown in
the figure, each supply includes ac input circuits, an GPIB
board, front panel display and keyboard, and two or more
output boards.

2-3 AC Input Circuit

The ac input circuit consists of a line module on the rear
panel of supply, front panel ON/OFF switch S1, power
transformer (T1), located in the front of the chassis, and a
cooling fan located in the rear of the chassis. The line module
contains a voltage selector card that selects the applicable ac
input voltage: 100 Vac, 120 Vac, 220 Vac, or 240 Vac. The
voltage card selection must match the nominal line voltage
that is connected to the unit. The line module also contains
the main fuse F1. An 8 A fuse (normal blow) must be
installed for a 100/120 VAC input; a4 fuse (normal blow)
must be installed for a 220/240 VAC input. The ac input is
applied to the power transformer when S1 is ON.
Depending on the line module setting, the 120 Vac cooling
fan either runs directly from the line module setting, the 120
VAC cooling fan either runs directly from the line or from
the appropriate transformer tap. The power transformer
provides the main ac inputs to the output boards and also
provides the ac inputs for the bias voltage supplies located
on the GPIB board and each output board. Ac power
distribution is shown in detail in Figure 6-1 in the rear of this
manual.

2-4 GPIB Board

The GPIB board contains the GPIB interface, system
microcomputer, output boards interface, and front panel
interface. These circuits provide the interface between the
user and the multiple outputs of the power supply. Each
output board is actually an output channel that can be
individually selected and controlled over the GPIB or from
the supply’s front panel. The GPIB board inter prets
commands from the GPIB or from the front panel to
control the selected output. The GPIB board also processes
measurement and status data received from the output
boards. This data may be read back to the controller over

the GPIB and/or displayed on the supply’s front panel.
Also, each output board can be individually calibrated over
the GPIB using calibration commands (See Appendix A in
Operating Manual). Correction factors are calculated during
calibration and stored in non-volatile memory on the GPIB
board. The GPIB board is described in greater detail in
paragraph 2-7.

2-5 Front Panel

Most of the remote operations that can be performed via the
GPIB can also be performed from the supply’s front panel.
In addition to the ON/OFF switch already mentioned, the
front panel contains an LCD display and a keypad. The LCD
display consists of an alphanumeric display and status
annunciators. The LCD normally displays the measured
output voltage and current of the selected output. When
programming an output from the front panel keypad, the
selected output channel, the function being programmed,
and the present value will be displayed. The annunciators
indicate which output channel has been selected and give
GPIB and power supply status information. The keypad
allows control of the supply’s system functions as well as
individual control of each output channel. Detailed
instructions on using the front panel’s display and keypad
are given in the Operating Manual.

2-6 Output Boards

The Agilent 6625A and 6628A contain two output boards
and the Agilent 6626A and 6629A contain four output
boards. The output combinations that correspond to each
model are shown in Figure 2-1. Each isolated output can
supply power in two ranges as shown in Figure 2-2. This
flexibility allows you to use the same output to power loads
with different voltage and current requirements. The output
ranges and operating characteristics of each output are
described in greater detail in Section IV of the Operating
Manual.

As shown in Figure 2-1, each output board contains a
rectifier/filter, power module, control circuit, secondary
interface circuit, and bias supplies.

The ac input to each output board is rectified, filtered, and
applied to the power module regulator. Each output board
employs series regulation techniques. The regulator element
is connected in series with the load and operates in the linear
region (between saturation and cutoff) of the transistor
characteristic curve. Regulation is achieved by varying the
conduction of the series element in response to changes in
the line voltage or the load. The constant voltage CV control
circuit compares the voltage at the output with a reference
voltage and generates a control signal which varies the
conduction of the series regulator to raise or lower the
output voltage as required. The constant current CC control

2-1

2-2

Circuit compares the voltage at the current monitor resistor
with a reference and likewise varies the conduction of the
series regulator.

The interface circuit on the output board receives digital
signals from the GPIB board and converts them to analog
signals (reference voltages) which are sent to the control
circuit to program the output voltage and current.

The output boards can be commanded to send measurement
and status data back to the GPIB controller and/or to the
display on the front panel. The data is sent back via the
secondary interface circuit and the appropriate circuits on
the GPIB board.

The output board is able to sink current as well as source
current. Current sink limits are fixed at values slightly
higher than the maximum current source limit for the
particular output voltage operating point. See Figure 2-7 for
typical current source and sink characteristics. The output
board circuits are described in greater in paragraph 2-24.

2-7 GPIB BOARD (FIGURE 2-3)

Figure 2-3 illustrates the major circuits and signal flow on
the GPIB board. Complete circuit details are shown on the
functional schematic in the rear of this manual.

The functional names on the block diagram correspond with
those on the schematic so that the diagrams can be
correlated. As shown in Figure 2-3, the major circuits consist
of the GPIB interface, the system micro-computer, the
output boards interface, and the front panel interface circuit.

2-8 GPIB Interface

These circuits consist of the GPIB bus connector (J201),
transceivers (U203) for the 8 data lines and 8 control lines,
and the GPIB talker/listener chip (U202). All GPIB (IEEE-

488) functions are implemented by the GPIB chip which
handles data transfer between the microprocessor and the
GPIB, handshake protocol, and talker/listener addressing
procedures. The GPIB talker/listener chip is connected to

the data bus and appears as memory locations to the
microprocessor.

The eight data lines (DIO1-DIO8) of the GPIB are reserved
for the transfer of data and other messages in a byte serial,
bit parallel manner. Data and message transfer is
asynchronous, coordinated by the three handshake lines
(DAV, NRFD, and NDAC). The power supply can be a talker
or a listener on the GPIB. The controller dictates the role of
an GPIB device by setting the ATN (attention) line true and
sending talk or listen addresses on the data lines (DIO1­DIO8). The power supply’s GPIB address is stored in the
EEPROM (electrically erasable programmable memory) chip
along with other system variables. You can find out your
supply’s GPIB address by using the front panel ADDR key
as described in the operating manual. As shipped from the
factory, the power supply’s address is set to 5. Any address
from 0 through 30 is a valid address.

There are five GPIB control lines: ATN, IFC, REN, SRQ, and
EOI (IEEE-488). When the controller sets the ATN line true,
all devices on the bus must “listen” to the addresses and
universal commands placed on the bus. When ATN is false,
only devices that are addressed will actively send or receive
data. All unaddressed devices will ignore the data lines
when ATN is false.

2-3

2-9 System Micro-Computer

The system micro-computer decodes and executes all
instructions, and controls all data transfers. It consists of a
microprocessor, an address decoder, RAM and ROM
memories, data buffers/latches, and a real time clock as
shown in Figure 2-3.

2-10 Microprocessor and Clock Circuits. These circuits
contain a high performance 8-bit microprocessor(U201) and
associated clock circuits. The microprocessor operates on a 1
MHz cycle, which it derives from a 4 MHz ceramic resonator
oscillator(Y201). The 1 MHz Q signal is generated by the
microprocessor for use by other circuit.

A 4 millisecond (approximately) clock signal, applied to the
microprocessor interrupt input, enables the microprocessor
to keep track of real time. This allows the microprocessor to
form necessary tasks on a regular basis. The real time clock
signal is also used to keep track of the time that has elapsed
since the output was last changed. This enables
microprocessor to determine if a CV/CC mode change
occurred before the selected time delay (see Reprogramming
Delay discussion in Section V of the Operating Manual). The
microprocessor inhibits the OCP function until the delay is
over.

The microprocessor also uses the 4 millisecond clock to
determine when to refresh the front panel display and to
perform other regularly scheduled jobs.

The R/W (read/write) output from the microprocessor
indicates the direction of flow on the data bus, either to or

from the microprocessor. A low level R/W signal indicates
that the microprocessor is writing data onto the data bus. A
high level R/W signal indicates that the microprocessor is
reading data that was placed on the bus by the addressed
circuit. The microprocessor uses the address decoder circuit
and the address bus to specify the data transfer locations.
Addresses are valid on the rising edge of the Q signal.

2-11 Data Bus latches (U217) and Buffers (U216). The
timing sequence of the microprocessor is such that the
circuits providing data for the microprocessor are de­selected (address disappears) before the microprocessor can
read the data. The data bus latches (U217) latch the data to
be read by the microprocessor. The data is updated on every
falling Q pulse. Data put on the data bus by the
microprocessor goes around the latches though buffers
(U216).

2-12 Free-Run and Signature Analysis Jumpers. The data
bus is connected to the microprocessor through a jumper
pack (W202). For some signature analysis tests of the
microprocessor kernel (microprocessor, RAM, ROM), the
data bus is broken by moving W202 from the NORMAL
position to the NOP position (see paragraph 4-23). This
connects a NOP (no operation) code (free run) to the
microprocessor data inputs. The NOP code does not contain
an address for the next instruction so the microprocessor
goes to the next highest address. Therefore, the address bus
looks like a 16-bit counter that continuously rolls over and

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starts at zero. The contents of each address appear
sequentially on the data bus (other side of the break) In
addition, for all signature analysis tests, jumper W201
must be moved from the NORM RUN position to the SIG
ANALYSIS position (see paragraph 4-23).

2-13 Address Bus and Address Decoder. The
microprocessor has 16 address lines (A0-A15) allowing it to
address 65,536 locations. The address decoder (U208) allows
each addressable circuit to look at a shorter address. The
chip select signals (CS0-CS8) are decoded from the higher
order address lines (A12-A15). When a data buffer’s CS is
decoded, it places its data on the data bus lines. When a data
latch’s CS is decoded, the output of each latch will be set to
the logic state that is present on the associated data bus line.
If the chip select for the RAM (random access memory),
ROM (read only memory), or talker/listener chip is
decoded, the selected circuit will decode the lower order
address bits supplied to it on the address bus.

2-14 Memory (ROM and RAM). The system microcomputer
contains both ROM (U206) and RAM (U207) devices. The
32KK non-volatile ROM contains the operating program and
parameters. The 2 K static RAM stores variables voltage to
be programmed, output current readback, etc. A third
memory chip, shown in the output board interface block of
Figure 2-3, is the EEPROM (electrically erasable
programmable memory). The EEPROM (U230) stores all of
the system constants including calibration constants, the
supply’s GPIB address, and model number (see paragraph
2-19).

2-15 Real Time Clock. The real time clock (U209) consists
of a 14-stage ripple counter that divides the 1 MHz Q clock

signal from the microprocessor to produce a pulse every 4
milliseconds. The real-time clock is used by the
microprocessor to schedule regular jobs as described
previously. The TIMER ENABLE signal resets the counter to
zero.

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2-16 Output Boards Interface

This circuit provides the interface between the system
microcomputer and each of the output boards (up to 4) in
the power supply. Data is transferred serially one bit at a
time between latches/buffers on the GPIB board and
optoisolators on the output boards. As shown in Figure 2-3,
the latches/buffers use data bus lines D0-D3 to send/receive
data from the applicable output. Data line D0 is used for
output board 1, D1 for output board 2, D2 for output board
3 (if present), and D3 for output board 4 (if present). A
controlled and regulated 5 volt line is also generated on the

GPIB board to operate art of the opto-isolators on the

output boards. In addition to interfacing with the output
boards, the latches/buffers interface with the 4 K bit serial
EEPROM in which system constants are stored.

2-17 Data Buffers. These 3-state buffers (U212) place the
serial data from each output board and the EEPROM on the
supply’s system microcomputer data bus lines when chip

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select CS3 is decoded. Serial data from output boards 1-4
appears on data bus lines D0-D3, respectively, and EEPROM

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2-4

serial output data appears on data bus line D7. Logic 0’s will
always appear on data bus lines D4-D6 when CS3 is
decoded because these buffer inputs are connected to
COMMON. All buffer output are held in the high

impedance state when CS3 is not decoded.

2-18 Data Latches. These stages (U213) are edge-triggered
D-type flip-flops. On the rising edge of the CS2 chip select,
the output of each stage will be set to the logic state that is
present on the associated data bus line. Data bus line D0-D3
are the serial data input lines for output boards 1-4,
respectively. Data bus line D4 controls the TIMER ENABLE
signal line to the real time clock circuit; D5 is the chip select
line for the EEPROM; D6 is the clock signal for the
EEPROM; and D7 is the data input line for the EEPROM.

The data that is transferred between the GPIB board and
the output boards (up to 4) passes through optical isolators
located on each output board.

2-19 EEPROM. This 4 K bit serial EEPROM (electrically
erasable programmable memory) stores the power supply’s
GPIB address and model number as well as the constants
used in calibrating the supply. The EEPROM (U230) is
nonvolatile allowing it to retain the stored information after
power is cycled off and on.

Because the RAM operates faster than the EEPROM, at
power on, the stored data is read into RAM in the system
microcomputer via data bus line D7.

The EEPROM’s 4096 bits of read/write memory are divided
into 2 pages of 8 X 256 each. Each register can be serially
read from or written to using data bus line D7. Input data is
received via a data latch and output data is sent via a data
buffer.

Data written to the EEPROM is stored in a location until it is
updated by a write cycle. The CHIP SELECT and CLOCK
signals are use by the microprocessor to control the
EEPROM’s programming modes. AT power on, the
EEPROM signal holds the EEPROM’s CLOCK signal off to
protect against accidental data writes when power is initially
applied.

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2-20 Front Panel Interface

These circuits provide the interface between the supply’s
system microcomputer and the front panes (keyboard and
LCD display). The microprocessor uses the data latches
(U210 ) and data buffers (U214) to transfer data between the
supply’s system microcomputer and the front panel.

2-21 Data Latches. On the rising edge of the CS5 chip select
these D-type flip-flops will be set to the logic states that are
present on the data bus lines.

Data bus lines D2-D7 are fed directly to the front panel
display to indicate power supply conditions The LCD
display may indicate the output voltage and current for a
selected output board, the present function being
programmed, a programmed message, or an error message.

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The annunciators provide operating and status information.
The microprocessor uses the real time clock to determine
when update/refresh the display.

Data bus line D0-D2 are fed to the 3 to 8 line keyboard
decoder (U211). The microprocessor successively drives
each of the eight open collector outputs of the decoder and
monitors the four readback lines from the keyboard to
determine which key was pressed. The readback lines are
held high until a depressed key pulls the line low.

2-22 Data Buffers. These 3-state buffers place the keyboard
readback data on data bus lines D4-D7 when chip select CS4
is decoded. As stated above, the microprocessor will use this
information to determine which key was pressed. In
addition buffers provide the following data on bus lines D0­D3 when CS4 is decoded:

or
a logic 0 (Jumper W201 is installed in the Skip

D1 - A logic 1 (Jumper W201 is not installed in the

or
a logic 0 (Jumper W201 is installed in the Cal

D2 - A logic 0 indicates Remote Inhibit is true

D3 - A logic 1 indicates OPTION 750 is installed in

All buffer outputs are held in the high impedance state
disconnecting it from the data bus when CS4

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D0 - A logic 1 (Jumper W201 is not installed in the

Skip Self Test position) – tells the
microprocessor to perform the self test at
power on;

Self Test position) – tells the microprocessor
not to perform self test at power on.

Cal Lockout position) – tells the
microprocessor to respond to calibration
commands;

Lockout position ) – tells the microprocessor
to ignore calibration commands. This jumper
provides security against unauthorized
calibration.

(OPTION 750).

power supply.

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is not decoded.

2-23 Bias Supply and Start-Up Circuit

The bias supply (U218) provides + 5 V bias power to operate
the circuits on the GPIB board. The start-up circuit (U220,
U222) generates the OPTO PON signal (delayed +5 V) which
is used to power the optical-isolators on the output boards.
The OPTO PON signal is initially held low for
approximately 100 ms to prevent the erroneous transfer of
data at power on. The start-up circuit also generates PCLR
(power clear) and EEPON (EEPROM power on) signals
when power is turned on. The PCLR signal is held low at
power on to initialize the talker/listener and microprocessor

chips. The EEPON signal is held low at power onto disable
the EEPROM clock. Thus, the start up circuit delays turning
on the microprocessor and optoisolators until the bias
voltages have stabilized. If the line voltage drops after the
unit has been turned on, the start-up circuit will again
generate the low level signals to disable the interface and
remove power from the supply’s outputs.

2-5

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2-6

2-24 OUTPUT BOARD

The following paragraphs provide block diagram level
descriptions of the output board. The descriptions cover the
two output board types. Differences between the board
types are given as required. Figure 2-1 shows which output

board types are used in the power supplies.

The descriptions that follow are divided into two main block
diagram discussions: Secondary Interface Circuits and
Control Circuits. The block diagrams illustrate the major
circuits and signal flow on an output board. Complete
circuit details are shown on the output board functional
schematic Figure 6-3 in the rear of the manual. The
functional names on the block diagrams correspond with
those on the functional schematic.

2-25 Secondary Interface Circuits (Figure 2-4)

These circuits receive digital signals from the GPIB board
and convert them to analog signals (voltages) which are sent
to the power mesh and control circuits to program the
output voltage, output current, and overvoltage.

Measurement and status signals are sent back to the
secondary interface circuits from the power mesh and
control circuits to be processed before they are sent on to the
GPIB board and then to the GPIB controller and/or the
front panel. The following paragraphs describe the interface

circuits shown in Figure 2-4.

2-26 Microcomputer. This 8-bit microcomputer (U312)
contains a CPU, ROM, and RAM. These internal circuits

process all data that is transferred between the GPIB board
and the power mesh and control circuits on the output
board. GPIB board data is transferred serially via optical
isolators which connect incoming data to an input port on
the microcomputer and outgoing data to an output port on
the microcomputer.

On the output board side, the microcomputer uses an 8-bit
parallel bi-directional data bus to program DACs which
control the output voltage, output current, overvoltage
setting, and sets the readback DAC. Various status and
operating conditions are read back on the data bus. The
microcomputer also generates address and control signals
which are used by other interface circuits. The interrupt
input to the microcomputer is used in conjunction with
readback monitor switches (U365, U366, and U368) analog
multiplexer (U323) and DAC (U321) to perform a successive
approximation A/D conversion in order to readback output
voltage and current values as well as various test point
voltages.

2-27 Address Decoder. This circuit (U320) decodes
addresses sent by the microcomputer and generates the
appropriate chip select signal (CS0 – CS6) to select which
circuit sends or receives data. CS0 selects the status monitor
(part of U327) to send status data back to the microcomputer
on data bus lines D0-D5. CS1-CS4 determine which DAC
will receive data. CS1 selects the 14-bit CV (Constant
Voltage) DAC, CS2 selects the 14-bit CC (Constant Current)
DAC, CS3 selects the 14-bit Readback DAC and CS4 selects

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the 8-bit OV (Over Voltage) DAC. C55 selects the
programming latches (U367), and CS6 selects the readback
monitor switches (U365, U366, and U368). The digital inputs
(D0 – D7) to the DAC’s are derived from the GPIB
controller or from the front panel depending upon whether
the supply is in the remote or local mode.

2-28 CV DAC. The 14-bit CV DAC (U313) and amplifier
(U360) convert the digital input signal from D0 –D7
supplied through latches (U369) into an analog signal (CV
PROG) in the range of 0 to – 10 Volts. This output signal is
used as a reference voltage and is send to the voltage control
circuits (see paragraph 2-46) to set the output voltage to the
programmed value.

The most significant bits (MSB’s) are loaded into the input

register of U313 from the data bus when: address line A3
goes high, address line A4 goes low, and CS1 goes low. The
least significant bits (LSB’s) are loaded into the input register
of U313 from the data bus when: address line A3 goes low,
address line A4 goes high, and CS1 goes low. The data in the
input register in transferred to the DAC of U313 when:
address line A3 is high, address line A34 is high, and CS1 is
low.

CV PROG is also sent to the analog multiplexer so that it can
be measured during power on self test.

U369 and U370 provide isolation between the 8-bit data bus
and the CV/CC DAC’s. This isolation assures that signals on
the data bus will not be capacitively coupled through the CV
and CC DAC’s as noise.

2-29 CC DAC. The 14-bit CC DAC (U314) and amplifier
(U361) convert the digital input signals in a similar manner
as the CV DAC into a analog signal (CC PROG) in the range
of 0 to - 10 Volts. This signal is used as a reference voltage
and is sent to the current control circuits (see paragraph 2-

47) to set output current to the programmed value.

The most significant bits (MSB’s) are loaded into the input

register of U313 from the data bus when: address line A3
goes high, address line A4 goes low, and CS1 goes low. The
least significant bits (LSB’s) are loaded into the input register
of U313 from the data bus when: address line A3 goes low,
address line A4 goes high, and CS2 goes low.

This data in the input register is transferred to the DAC of
U314 when: address line A3 is high, address line A4 is high

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and CS2 is low. CC PROG signal is also sent to the analog
multiplexer (U323) seo that it can be measured during
power on self test.

2-30 OV DAC. The 8-bit OV DAC (U363) and amplifier
(U319) convert the digital input into an analog signal (OV
DAC) in the range of 0 to – 10 Volts. This signal is compared
with the output voltage exceeds the programmed OV
setpoint (see paragraph 2-44).

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2-7

The OV signals is also sent to the analog multiplexer so that

it can be measured during power on self test.

2-31 Readback Amplifier and Analog Multiplexer. The

analog multiplexer (U323) selects one of eight inputs (value

of these inputs are from 0 to 10 Volts) to be applied to the

readback signal comparator (U324) for the A-to-D converter.

The selected signal is determined by address lines (A0-A2)

which are received from the microcomputer. The analog

inputs to the multiplexer indicate the following:

COM - hardwired to common to reduce noise when no

signals are being sampled.

FUSE - output board’s return fuse status (read back

during power-on self test)
VFS - Readback amp output (U315A)
V/I MUX - Range amp output (U315C)
CV DAC - voltage DAC output
CC DAC - current DAC output
OV DAC - overvoltage DAC output

U315C can be configured as an inverting or non-inverting
amplifier. Swiches (U365) determine it’s configuration as
well as the input to amplify. U366 is use to determine the
gain of the amplifier.

U315B is used as a buffer. For current readback, inputs from
the 4 terminal shunt resistor R408 are select via U365. For
voltage readback low range, U366 (D) is used as the input to
U315C.

2-32 Readback DAC and Signal Comparator. The
readback DAC (U321), amplifier (U362), readback signal
comparator (U324), and analog multiplexer (U323) along
with the microcomputer (U312) form an analog-to-digital
converter (ADC) which monitors the output board signals
sent to the analog multiplexer.

The readback DAC (U321) and amplifier U362 convert the
digital input signal from the microcomputer to an analog
signal in the range of 0 to – 10 volts. The DAC internally
formulates the 14-bit DAC data from the 8-bit (DB0-DB7)
data bus (same as the CV DAC described above).

The output of the DAC and the output of the analog
multiplexer are applied to the signal comarator U324. The
readback DAC, under the control of the microcomputer,
successively approximates the value of the multiplexer’s
output to a 14-bit resolution,. Starting from the most
significant bit, each bit is successively compare to the
multiplexer’s output and is kept or discarded depending on
whether its value is less than (kept) or greater than
(discarded) the multiplexer’s output.

Each comparison (successive approximation) is evaluated by
the microprocessor via its INT input. The microcomputer
maintains a running total of the approximations (sum of the
kept bits) which, when complete, represents the value of the
analog multiplexer’s output.

2-33 CV and CC Programming Range Switching. U367,
U364, and resistor pack U381 determine the attenuation
factor for the CV and CC signals. Programming range
latchU367 receives information via the data bus (DO0 and
DO1), which determines if the power supply will operate in
the low or high voltage and current ranges. Using this
information, U367 sets analog switches U364 for the proper
divider tap for the desired range (full DAC output O to – 10
V for high range, or a portion of the 10 V for the low range).

2-34 Readback Range Switching. U365, U366, and U368
provide readback of the output of the power supply to the
analog multiplexer (U323), except for the 50 V range (VFS).
Readback latch U368 receives information via data lines
DO0 and DO1 which set up monitor switches U365 and gain
select switches U366 to readback the output parameters.

2-35 Signal Processor. This special purpose IC (U327)
processes both analog and digital signals to interface the
microcomputer with the power mesh and control circuits.
The circuits can be functionally divided into status monitor,
overvoltage detector and driver, and power-on/start-up
circuits.

Status Monitor

the control loops, logic to decode these input line, and flip­flops to catch and hold changes. The inputs to the status
comparators are the CV LOOP, + CL LOOP, and – CL LOOP
signals from the power control circuits (see Figure 2-5). The
outputs of the comparators are combined in logic circuits
which then go into the set inputs of flip-flops which hold the

status of CVO, + CLO, - CLO, and UNREG outputs. UNREG
is decoded if the output is not regulated by a CV or CL
control loop.

The flip-flops are set by any transition into a decoded state.
This generates a record of whether any of the conditions
(CV, + CL, - CL, UNREG) existed since the last time the flip­flops were reset. The STATUS RESET input line from the
microcomputer resets the flip-flops.

The status monitor circuit also receives OV SENSE and

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THERM inputs. The THERM signal is received from the
power module(s) in the power mesh (see Figure 2-5) and
indicates when an overtemperature condition exists. Note
that when the microcomputer senses the overtemperature
(OT) condition via data bus line D4, it shuts down the
output. This circuit resets automatically and restores the
output approximately 30 seconds after the temperature
drops sufficiently for safe operation.

________

The OV SENSE input signal indicates when the output’s
overvoltage detector circuit has been tripped and the output
has been shut-down (see overvoltage detector description
below). The THERM and OV SENSE inputs control the OT
and OV outputs of the status monitor. Note that the OT and
OV status are not held in flip-flops. All of status monitor’s
outputs (CVO, CLO, - CLO, OV, OT, and UNREG are
returned to the microcomputer via data bus lines D0-D5
when chip select CS0 is decoded.

– this circuit consists of comparators to monitor

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2-8

2-9

Overvoltage Detector

signal which shorts the output by firing the SCR crowbar
(within the power module) on the output if any of the
following conditions are present:

1. The output at the + V terminal exceeds the
programmed OV trip point (OV PROG). Note that the
+I READBACK signal provides an offset to
compensate for the voltage drop across the current
monitor resistor. The POV DISABLE signal inhibits
the programmable OV function from affecting the OV
DRIVE signal.

2. The voltage from the + V output terminal to the +S
terminal or from the –S terminal to the –V output
terminal exceeds 10 V (applies to remote sensing
only).

3. A trip signal is received on the output’s OV terminal.

4. The output’s fixed overvoltage circuit is activated.

Power-On/Start-Up

comparator circuit (BIAS TRIP input signal to U327) is
initially low which holds the PCLR and ON/OFF signals
low. With PCLR low, the microcomputer is held in the reset
state.
off preventing any power from reaching the output
terminals.

The turn-on comparator circuit (part of U325) monitors the
unregulated bias supply to determine if it is high enough to
guarantee regulation by the three-pin regulators. The
medium rail voltage is also monitored to ensure that it is
above the minimum level required for proper operation of
the power module. When these two conditions are met, the
BIAS TRIP line is allowed to go high (approximately 0.7 V).
Then, after a delay of approximately 0.3 seconds (provided
by an external delay capacitor, C346), the PCLR signal goes
high allowing the microcomputer to complete its
initialization routine and set the OUTPUT ENABLE line
low. This allows the ON/OFF signal to go high (+ 2.4 V)
enabling the control circuit and current sources which allow
power to reach the output terminals. Note that whenever the

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OUTPUT ENABLE signal is high, the ON/OFF signal is low
and turns off the control circuit thus preventing power from
reaching the output terminals.

If the line voltage drops below a minimum level, the
comparator described above will shut-down the output
(remove power from the output terminals) until normal line
voltage is restored. This resets the microcomputer and sets
the output to the turn-on state.

2-36 Bias Supplies and Precision Reference Voltage. The
bias supplies (U300-U303) generate the voltages required to
operate the circuits on the output board. The precision
reference voltage circuit (U318, U319A/B/C) operates from
the + 15 V bias and generates the VREF outputs (10 V ±

0.5%) which are used by the DAC’s and the control circuit.

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With ON/OFF low, the power control circuits are held

– This circuit generates the OV DRIVE

– At power-on, the output of the turn-on

________

____ ___

____

_____________

___

2-37 Power Mesh and Control Circuits (Figure 2-5)

The power mesh circuit in the upper half of Figure 2-5
converts the AC from the power transformer to regulated
DC output power. The primary power control element is the
power module hybrid (U338). The power mesh circuit
generates a constant voltage or constant current output
under control of the control circuits shown in the lower half
of Figure 2-5. In addition to controlling the power mesh
circuit, the control circuits send measurement and status
data back to the GPIB controller and/or front panel via the
interface circuits on the output board and the GPIB board.

2-38 Rectifier and Filter. These circuits consist of two full
wave bridge rectifier circuits with filter capacitors connected
across the rectifier outputs. The proper ac voltage levels are
applied to the rectifiers via secondary windings of chassis
power transformer T1 (see Figure 2-1). The rectifiers provide
raw dc o the power module at three different levels (high,
medium, and low power rails). The return line for the
rectifier circuits is fuse protected. If this fuse opens, the
power supply will fail self test, all outputs will be disabled,
and the error message “FUSE CH <n>” will be displayed
(where n specifies the particular output board, 1-4).

2-39 Power-On Circuit and Current Sources. The power­on circuit (Q318 and Q319) is used to turn on the current
source transistors and the bleed circuit (see paragraph 2-43)
which is connected across the output of the supply. The
power –on circuit is activated when it receives the ON/OFF
signal (2.4 V level) from the signal processor (U327).

The current sources U336) are a series of transistors
connected to the high rail. When activated by the power-on
circuit, the current sources supply a few milliamps to the +
BASE DRIVE, in conjunction with the – DRIVE signal (see
BASE DRIVE CIRCUIT description below), control the
conduction of the series pass elements and shunt ( - CL) in
the power module.

2-40 Power Module Reference Voltage. When the current
sources have been turned on, this circuit (P/O U340 and
U337) provides a reference voltage (about 2 V above + V) to
the power module REF input. The REF input is used by an
internal control circuit that allows switching between the
low, medium, and high rails. The reference circuit includes a
transistor (P/O U340) that turns on when the current
sources apply power, a programmable reference (U337)
which provides the reference voltage, and bypass capacitors
C366 and C367.

2-41 Power Module. The power module hybrid U338
receives three unregulated DC voltage levels on its high,
medium, and low voltage input rails. The power module
contains series regulator stages, an SCR overvoltage circuit,
a down programmer, a built-in overtemperature thermistor,
and a reverse output voltage protection diode.

2-10

Series Regulators –

pass transistors which regulate the voltage received from the
selected power rail. The power module automatically selects
the proper input rail depending upon the output voltage
required. For example, if the low rail is supplying current
and the output voltage exceeds the low rail minus about 2.5
V, the medium rail begins to raise the voltage on the
BYPASS input and supply current. Finally, if the output
voltage exceeds the medium rail minus about 2.5 V, the high
rail will begin to supply current.

As stated previously, the conduction of the series pass
transistors is controlled by the + BASE DRIVE and – DRIVE
inputs. Normally there is about a diode drop between these
two input pins. The current sources drive the series
regulator into conduction via the + BASE DRIVE input. The
– DRIVE input from the base drive circuit (see paragraph 2-

42) controls the amount of + BASE DRIVE current that
drives the series regulators in order to maintain a regulated
output. Any + BASE DRIVE current from the current source
that is not required by the series pass transistor to regulate
the output is drawn away by the control circuit through the
– DRIVE input via Q335. A level of current through Q335
that exceeds the + BASE DRIVE current can turn on the
power module current sink transistors to sink output
current up to the negative current limit value.

SCR Overvoltage Circuit –

SCR whose gate input is capacitively coupled to the OV
GATE pin. The OV GATE signal can fire the SCR for a
number of reasons which are described later under the
“Overvoltage Protection Circuit” paragraph. In addition to
shorting the output, the fired SCR will cause the OV SENSE
signal to go low signaling the microcomputer to program
the output to zero. The output will remain shorted and
programmed to zero until the circuit is reset. The SCR circuit
is reset when the POV DISABLE signal (OVRST command)
is received by the OV reset circuit (Q320). The condition that
caused the overvoltage must be removed in order for the
circuit to remain reset. If the condition is not removed, the
OV GATE signal will again fire the SCR and disable the
output. Note that in addition to resetting the SCR, the
OVRST command will program the output to the settings
that existed before the OV occurred.

Down Programmer

are used to sink output current and are capable of rapidly
down programming the output voltage to about 2 V. An
external FET down programmer circuit (see paragraph 2-49),
is connected across the output to continue down
programming the output voltage below 2 V.

Overtemperature Protection

overtemperature circuit that consists of a negative
temperature coefficient thermistor that senses the power
module's temperature. When the power module's
temperature rises enough to reduce the THERM input
resistance to about 8 K ohms, the thermistor drops below 2.5
V (approximately) notifying the signal processor that an
overtemperature (OT) condition has occurred. The signal

The series regulator stages consist of series

The power module has an internal

_______

- Separate transistors in the power module

- The power module also contains an

_____

processor then relays this information to the microprocessor
which will shutdown the particular output with the
overtemperature condition. The output will be restored 30
seconds after a safe operating temperature is reached.

Reverse Output Voltage Protection Diode

contains a diode with its cathode connected to the
COLLECTOR output and its anode connected to the power
module COMMON. This diode is essentially connected
across the power supply's output terminals to protect the
output from having reverse voltages applied.

2-42 Peak Current Limit. This circuit Q321, Q322, R407, P/0
U340, quickly limits the amount of current through the
series regulator elements in the power module. It is
activated when the output current exceeds the full scale
value + about 75% in either the sourcing or the sinking
direction.

The series pass regulator in power module U338 is
connected in series with an external resistor (R407). When
the voltage across R407 exceeds a diode drop in either
direction, the peak current limit circuit is activated and
limits the conduction of the series pass transistor element or
current sink transistor. This circuit reacts much faster than
the + or--current control circuits (see paragraph 2-47).

When the peak current limit circuit is activated in the
current source direction, not only will the conduction of the
series regulator be limited, but the current control circuit
(U376) will be quickly activated through P/O U340, CR341,
U376 and R405 to take control of the current limiting action.

2-43 Bleed Circuit. This circuit (Q341, R456, etc.), connected
from + V to - 7 V, provides a fixed current of about 15 mA
through the series pass elements in the power module so

that they are never completely turned off. The bleed circuit
is activated via the power-on circuit when the ON/OFF
signal is high. The bleed circuit maintains stability with
large output capacitors under light loading conditions and
helps to keep the output impedance constant.

2-44 Sense Protect Circuit. This circuit (P/O U375 and
P/O U351) monitors the voltage from + V to + S and from ­S to - V. If either of these voltages exceeds 1.0 V, the sense
protect circuit will generate a signal which will fire the
overvoltage protection circuits and shut down the output
(see paragraph 2-50). This circuit prevents the output
voltage from being regulated at a value higher than the
maximum value for which it was designed.

2-45 Base Drive Circuit. When activated (ON/ OFF is at
approximately 2.4 volts), this circuit (Q335 and U348)
provides the - DRIVE input to the series regulator and
current sink transistors in the power module. The - DRIVE
input determines how much drive current (+ BASE DRIVE)
the power module will receive. The -DRIVE input is
controlled by either the voltage control (CV), current control
( + CL), or negative current limit circuits ( - CL).

- The power module

2-11

2-12

The CV or + CL signal controls the base drive circuit via OR
gate diodes CR351 or CR348 to generate the – DRIVE signal
in order to control the conduction of the series regulators in
the power module and provide a regulated output. If the
output is less than the programmed value, the – DRIVE
signal will allow more + BASE DRIVE current causing the
series regulators to conduct more and raise the output. If the
output exceeds the programmed value, the – DRIVE signal
will divert current through Q335 and U348 of the base drive
circuit and away from the + BASE DRIVE power module
input causing the series regulators to conduct less thereby
reducing the output The voltage control (VV) circuit and the
current control (+CL) circuit is described in paragraphs 2-46
and 2-47, respectively.

When the output is operating in negative current limit, the –
CL signal controls the base drive circuit via diode CR354 so
that the – DRIVE signal controls the conduction of the
current sink transistors in the power module. The negative
current limit circuit which generates the – CL signal is
described in paragraph 2-48. The maximum – Base Drive is
reached when the drop across R428 reaches. 6 volts, turning
on part of U348 which limits the base drive to Q335.

2-46 Voltage Control Circuit. When the output is operating
in the constant voltage mode, this circuit generates the CV
control and CV LOOP signals. The CV control signal is
applied through OR gate diode CR351 to control the base
drive circuit in order to regulate the output voltage. The CV

2-13

LOOP signal is sent back to the secondary interface circuit to
indicate that the output is in the constant voltage mode of
operation. The voltage control circuit compares the output
voltage to the programmable reference voltage CV PROG to
produce the CV signal. As shown in the simplified
schematic of Figure 2-6, the major components in the voltage
control circuit are: unity gain buffer (U378 and inverter
amplifier U372), output sence buffer U373 (who used for – 5
guard), inverting differential amplifier U352A, and CV error
amplifier U347. The reference voltage (CV PROG, 0 to –10 V)
is applied to U372 which produces a 0 to + 10 V signal
feeding into the summing junction S1 (U347-2). The output
voltage is monitored by U352A which produces a 0 to - 10 V

signal that represents the output voltage magnitude which is
also fed into S1. The 0 to –10 V signal is also sent back (V
READBACK) to the secondary interface to indicate the
magnitude of the output voltage.

If the output voltage exceeds the programmed voltage, the
summing junction goes negative, causing U347/U377 to
produce a positive going CV control signal. For this
condition, the base drive circuits will conduct more and pull
current away from the power module’s + BASE DRIVE
input via the – DRIVE input line. This will cause the power
module’s series regulators to conduct less and thus reduce
the output voltage.

2-14

If the output voltage is less than the programmed voltage
,the junction goes positive causing U347/377 to produce a
negative going CV control signal . For this condition ,the
base drive circuit will conduct less allowing more current to
flow into the +BASE DRIVE input. This will cause the power
module’ s series regulator to conduct more and thus increase
the output voltage.

2-47 Current Control Circuit. When the output is operating
in the constant current mode, this circuit generates the +CL
control and the +CL LOOP signals. The +Cl control signal is
applies through OR gate diode CR348 to control the base
drive circuit in order to regulate the output current. The +CL
LOOP signal is sent back to the secondary interface circuit to
indicate that the output is in the constant current mode of
operation.

The current control circuit compares the output current to a
programmable reference voltage (CC PROG). This
comparison produces the + CL control signal. In order to
make this comparison, the circuit monitors the voltage (+
SHUNT) across current monitoring resistor R408. This
voltage drop is proportional to the amount of output
current. The +SHUNT and +CL PROG signal are connected
through scaling resistors to summing point S2 for
application to U346 (CC Error Amplifier) as show in figure
2-6. Based on this summing action, U346 generates the +CL
control signal which is applied to the base drive circuit via
buffer amplifier U376 and OR gate diode CR348 to control
conduction of the series regulators in the power module in
the same way as described above for the voltage control
circuit. The +SHUNT signal is also sent back to the
secondary interface to indicate the magnitude of the output
current.

The current control circuit receives an input from peak
current limit circuit (in the current sourcing mode only) as
shown in figure 2-5. When the peak current limit circuit (see
paragraph 2-42) is activated, it immediately limit the
conduction of the series regulators in the power module.

2-48 Negative Current Limit Circuit. This circuit provides a
limit to the amount of current that the supply can sink. The
circuit may be activates, if a current source such as another
power supply (or energy storage capacitor) is connected
across the output terminals and its voltage is greater that the
programmed output voltage.

When the output is in negative current limit, this circuit
generates the – CL control and the – CL LOOP signal. The –
CL control signal is applied through diode CR354 to the base
drive circuit. The – CL LOOP signal is sent back to the
secondary interface to indicate that the output is in the
negative current limit mode.

As shown in the simplified schematic of figure 2-6, the
negative current limit circuit consist mainly of an open
collector toggle comparator (part of U351) and –CL error
amplifier (U350).

U375 acts as a clamp to ensure the –CL Summing junction
(S3) does not exceed +10mV.

_______

_______

The voltage drop ( +SHUNT-which is a negative voltage
when sinking current) across the current monitoring resistor
R408 is applied to summing junction S3 along with a
reference voltage. Based on this summing action, error
amplifier U350 generates the –CL control signal which is
applied through diode CR354 to control the base drive
circuit.

For the 50W outputs, comparator U351 toggles the reference
voltage between the 1.1 and 2.2 Amp range levels. This is
required because the output board has two fixed ranges (a
high voltage/low current and a low voltage /high current).
As you can see in Fig.2-7, a 50W output can sink up to 2.2A
when its output is below 26V, and up to 1.1 A when its
output is approximately 26V.

U351 constantly monitors the output voltage in order to
provide the proper reference voltage to the summing
junction of U350. If the output voltage is in the high range,
the open collector output of U351 will be near ground;
thereby dividing down the VREF voltage to summing
junction S3 resulting in a lower sink current limit of –1.1A. If
the output voltage is in the low range, the collector output of
U351 will be 15V, resulting in a higher sink current limit
(about –2.2A). R476 provides a small amount of positive
feedback (hysteresis) to prevent “jitter” at the switch point

2-49 FET Downprogrammer. When the output voltage
drops below approximately 4V (approximately 2 volts for
the 50W outputs), the Down Programming circuit comes on
(current sinking characteristics are shown in Figure 2-8). The
FET Downprogrammer circuit (part of U351A/Q342,R456),
is connected across the output. Divider R464/R462 senses
when the output falls below 4 Volts (approximately 2V for
the 50W outputs). This turns on U351A and FET Q342 and
connects R457 across the output to aid downprogramming.

Notice in Figure 2-8 on the 25W/.5A graph, the 15 ohm
slope {approximate} (11 ohms for the 50W/2S outputs), is
due to the resistor R457 in series with FET Q342 and the –

0.01 Amps at VOUT equals zero volts, represents the bleed
current in Q341.

2-50 Overvoltage Protection Circuits. These circuits
generate the OV GATE signal which fires the SCR in the
power module and shuts down the output. Figure 2-9 is a
simplified schematic of the overvoltage protection circuits
which are comprised mainly of: a fixed overvoltage sensing
circuit (U354) and divider (R357/R359) that compares the
OV DAC signal to the output voltage, signal processor U327,
diodes CR356-CR360, and pulse transformer (T301) that
couples CR356-CR360, and pulse transformer (T301) that
couples the remote trip signals that are sent/received via the
+OV and –OV terminals.

As shown in Figure 2-9, the main input to the overvoltage
protection circuits in the OV DRIVE signal which is received
from the overvoltage detector (P/O U327, see paragraph 2-

32). The OV DRIVE signal goes high to activate the OV
GATE signal which is sent via diode CR357 to fire the SCR
in the power module. The conditions which activate OV
DRIVE are described in the following paragraphs.

2-15

If the output voltage from +V (R359) exceeds the
programmed overvoltage setting (derived from OV REF
through R357), the overvoltage comparator signal (OV
COMP) goes positive and activates the OV DRIVE and will
fire the SCR provided that the POV DISALE signal is low.
The CURRENT COMP signal is included in the comparison
to compensate for the voltage drop across the current
monitoring resistor and permit an accurate comparison. The
POV DISABLE signal is high only during power on and for
a brief time during an overvoltage reset.

Note that the OV DRIVE signal is also sent to the ± OV
terminals via diode CR356 and transformer T301 to either
notify a remote circuit that the overvoltage circuit was
tripped or alternatively to fire other output boards (up to
eight) by paralleling the external OV lines. The OV TRIP
signal can activate the OV DRIVE and shut down the supply
regardless of the state of the POV DISABLE signal. As
shown in Figure 2-9, OV TRIP is the output of a wired OR
gate and can be activated by either the SENSE PROTECT
signal (as described in paragraph 2-44) or by the REMOTE
OV TRIP signal. The REMOTE OV TRIP signal can be
generated by the fixed OV sensing circuit or by a remote
signal connected to the output’s + OV and – OV terminals.

Fixed Overvoltage Sensing Circuit –

circuit (U354) continually monitors the voltage across the

The fixed overvoltage sensing

output terminals. Because it is biased by the voltage at the
output terminals, it can be activated and provide protection
even when the supply is not connected to the ac power line.

The fixed overvoltage sensing circuit will activate when it
senses a voltage that is approximately 120% of the
maximum rated output voltage for the associated output. If
the output voltage exceeds this threshold, the OV GATE
signal is generated via diode CR358 and fires the SCR. Note
that the fixed overvoltage sensing circuit will also activate
the OV DRIVE signal via diode CR359 (REMOTE OV TRIP).
The OV DRIVE signal then transmits the overvoltage
condition to the ± OV terminals via diode CR356, and
transformer T301 as previously described.

As shown in Figure 2-9, the trip signal enters at the ±OV
terminals and is coupled through pulse transformer T301,
diode CR360, and the overvoltage detector circuit to
generate OV GATE and shut down the supply.

2-51 Guard Bands. Guard Bands are employed on the +
Shunt track, the – Sense track, and Common track (pin 3 of
R408) on the PC board. The guard bands shield these traces

ensuring they are not affected by DC leakage currents from
adjacent tracks.

2-16

2-17

Section III

VERIFICATION

3-1 INTRODUCTION

This section contains test procedures that check the
operation of the power supply. Four types of procedures are
provided: Operation Verification Tests, Performance Tests,
Extended Tests, and Temperature Coefficient Tests.

The Operation Verification Tests comprise a short procedure
to verity that the supply is performing properly, without
testing all specified parameters. The Performance Tests
provide a more complete test of the supply by testing most
of the specifications listed in Table 1-1 in the Operating
Manual (Agilent Part No. 06626-90001).

The Extended and Temperature Coefficient tests are similar
to the Performance tests except that they are conducted in a
controlled environment and require a longer period of time
to complete each test.

If failures are encountered or out of specification test results
are observed, refer to the Troubleshooting Procedures in
Section IV in this manual. The troubleshooting procedures
will determine if repair and/or calibration is required.
Calibration procedures are provided in Appendix A of the
Operating Manual.

For mechanical reasons, there are two different
assemblies for the 50W/2A output boards
When ordering replacement output boards,
determine the appropriate assembly number
from “Chassis Boards” section of Table 5-5.
Note that the replaceable parts on both
50W/2A assemblies are the same (see Tale 5-7).

3-2 TEST EQUIPMENT REQUIRED

Table 3-1 lists the equipment required to perform the tests in
this section. The tests are performed by sending commands
to and receiving data from the supply under test via the GP
IB. An Agilent Series 200 or 300 computer is used as the GP
IB controller. Tests that do not verify readback via the GPIB
can also be performed manually from the supply’s front
panel and, consequently do not require use of a controller.

NOTE

The tests should only be performed by qualified
personnel. During the performance of these tests,
the output of the supply being tested may voltage
levels above safe levels.

3-3 OPERATION VERIFICATION TESTS

To assure that all outputs of your supply are performing
properly, without testing all specified parameters, perform
the test procedures outlined.

a. Perform the turn-on and checkout procedures

given in Section III, paragraphs 3-3 thru 3-9, of the

Operating Manual. These procedures include a
power-on self test.

b. Perform the performance tests listed below on each

output of your supply.

Voltage Programming and Readback Accuracy
(paragraph 3-12)
CV Load Effect (paragraph 3-13)
CV Noise (paragraph 3-15)
Overvoltage Protection Tests (paragraph 3-18)
Current Programming and Readback Accuracy
(paragraph 3-21)
CC Load Effect (paragraph 3-23)

3-4 PERFORMANCE TESTS

3-5 Introduction

The following paragraphs provide test procedures for
verifying the supply’s compliance with the specifications
listed in Table 1-2 of the Operating manual. The procedures
cover all models in the series of Multiple Output Power
Supplies. The performance test procedures must be
performed on each output. Figure 3-1 shows the outputs
present and output ranges on each model.

The test procedures that follow give settings and results for
each type of output that may be tested. There are two types
of outputs: 25 Watt and 50 Watt. Make sure that you use the
test settings and results listed for the particular output

3-1

3-2