HP 722A Service manual

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HEWLETT-PACKARD COMPA Y

01155·1
OPERATING AND SERVICE MANUAL
MODEL 722A/AR
SERIALS PREFIXED: 105-
DC POWER SUPPLY
Copyright HEWLETT.PACKARD COMPANY 1961
1501 PAGE Mill ROAD. PALO ALTO, CALIFORNIA, U.S.A.
P rioted: NOY 1961

Model 722A
TABLE OF CONTENTS
Table of Contents
List of Illustrations and Tables
Section
,Page
VMAINTENANCE
5-1
5-1. Introduction
5-1
5-3. Maintenance Procedures
5-1
5-4.
Dustcover Removal .
5-1
5-6. Zero-Setting the Meter
5-1
5-8.
Printed Circuit Board Removal
5-1
5-10. Test Equipment Required
5-1
5-12.
Performance Check
5-1
5-14.
Troubleshooting
. 5-1
5-15.
Introduction
5-1
5-17.
Trouble Analysis •
5-1
5-20.
Detailed Test Procedure
5-5
5-23.
Reference Voltage Adjustment
5-6
5-25.
Monitoring
5-6
5-27.
Voltage Range
5-6
5-29.
Current Limit
5-6
5-32.
Performance Test
5-6
5-34.
Voltage Range
5-6
5-36.
Current Limiting.
.
5-7
5-38.
Operation Characteristics •
5-7
5-39.
Ripple Characteristics
5-7
5-40.
Load Regulation
5-7
5-41.
Line Regulation
5-8
Section Page
IGENERAL INFORMATION .... 1-1
1-1. Introduction . . . . . 1-1
1-4. Description . . . . . 1-1
1-8. Differences in Equipment . . .. 1-1
1-10. Equipment Supplied . . . . 1-1
1-12. Other Modes of Operation . 1-1
II PREP ARATION FOR USE
2-1
2-1.
Unpackingand Mechanical Inspection
2-1
2-4.
Site Selection and Installation
2-1
2-6.
Line Power
2-1
2-8.
Operation from 230-Volt Supply
2-1
2-11.
Preparation for Storage and Shipment
2-1
2-14.
Storage
2-1
2-16. Incoming Inspection &Perform. Test
2-1
III OPERATING INSTRUcrIONS
3-1
3-1. Operation
3-1
3-3. Current Limiting .
3-1
3-6. Remote Sensing
3-1
IV
PRINCIPLES OF OPERATION
4-1
4-1.
Overall Block Diagram
4-1
4-3.
Rectifyingand Filtering Circuits
4-1
4-6.
Series Regulator
4-2
4-8.
Output Circuit
4-2
4-10. Voltage Regulating Circuit .
4-2
4-14. Current- Power Limiter .
4-2
4-18. Protection Circuits .
4-3
VI REPLACEABLE PARTS
6
-1. Introduction . . .
6-5. Ordering Information . .
6-1
6-1
6-1
01155-1
LIST OF ILLUSTRATIONS
Number Title
1-1. Model 722AR DC Power Supply.
3-1. Operating Controls .........
3-2. Remote Sensing. . .
3-3. Charging Batteries .
3-4. Parallel Operation
4 -1. Simplified Schematic
4-2. Block Diagram. . . .
4-3. Current Power Limiter
5-1. Model 722A Bottom View
5-2. Model 722A Top View
5-3. Servicing Etched Circuit Boards .
5-4. Reference Power Supply (Schematic) .
5-5. Power Supply (Schematic) . . . • . .
LIST OF TABLES
Number Title
1-1. Specifications . . . . • . . .
5-1. Recommended Test Equipment
5-2. Troubleshooting Guide
5-3. Replacement Guide . . . . • . . • .
6-1. Reference Designation Index
6-2. Replaceable Parts ....
Page
1-1
3-0
3-1
3-1
3-1
4-0
4-1
4-3
5-2
5-3
5-4
5-9
5-10
Page
1-0
5-0
5-5
5-5
6-2
6-6
iii

Section I
Table 1-1
Model 722A
Table 1-1. Specifications
RATED OUTPUT: o to 60 volts dc; 0 to 2 amperes dc
LINE REGULATION: Less than 2.5 mv change for ± 10% line voltage change; output set
between 0 to 60 volts
LOAD REGULATION:
Less than 5 mv change at output terminals for 0 to 2 amperes change;
output
set between 0and 60 volts
TRANSIENT RECOVERY TIME: Less than 200 11 sec for recovery within 5 mv for a change from 0 to
full load or full load to 0 at any rated output or line voltage
NOISE AND RIPPLE:
Less than 250 11 v rms
TEMPERATURE STABILITY:
Better than 0.02%j
OC
or 5 mv JOC, whichever is larger
TEMPERATURE RANGE: o
to 550 for operation within specifications
OUTPUT IMPEDANCE: DC:
Less than 2.5 milliohms
AC: Less than 5 milliohms in series with 10 Ilh
OUTPUT METERS: Voltage: 0
to 60 volts, one range
Current: 0 to 2.5 amp, one range
PROTECTION: Output
current limiter continuously adjustable from 100 rna to 2.2 a
COOLING:
Forced air
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722AR
Rack Mount:
DIMENSIONS:
WEIGHT:
Rack Mount: Net 34 lb
POWER: 115
or 230 volts ± 10%, 50 to 60 cps, 40 to 250 watts depending on
line and load conditions
1-0
01155-1

Model 722A
SECTION I
GENERAL INFORMATION
Section I
Paragraphs 1-1 to 1-13
1-1. INTRODUCTION.
1-2. This is an operating and service manual for the
Model 722A DC Power Supply. This manual is appli-
cable only to instruments with the prefix shown on
the front cover except as modified by change sheets.
1-3. Hewlett-Packard instruments use a two-section
eight-digit serial number, that is, 000-00000. The
first three digits are an identification number; the
last five digits are the instrument serial number. If
the identification number on the instrument does not
agree with the identification number shown on the
manual title page, there are differences between the
manual and instrument. These differences are de-
scribed in manual change sheets having the proper
identification number.
1-4. DESCRIPTION.
1-5. The C€tJ Model 722A DC Power Supply is a com-
pletely transistorized, regulated power supply. It
will supply up to 60 volts at 2 amperes with continuous
adjustment of voltage and current limiting. The cur-
rent limit c~n be set at any current between 100 ma
and 2 amperes to protect circuit elements, such as
transistors, under test.
1-6. Two meters measure both current and voltage
drawn by the load continuously so that you may monitor
power supply drain conveniently. The output impedance
is low for both dc and ac so that a minimum of de-
coupling is required in cascaded circuits. Terminals
for remote sensing are provided on the rear of the
chassis so that the ohmic resistance of the supply
leads is minimized.
1-7. The usefulness of the Model 722A is not limited
to transistor applications, but is useful wherever
high stability is required. For instance, Model 722A
is an excellent source for regulating the filament
voltage of vacuum tubes.
1-8. DIFFERENCES IN EQUIPMENT.
1-9. This manual has been written and illustrated for
the Model 722AR (rack-mounted instrument) since this
is the only style of instrument available at the time
of writing. A cabinet-mount model may become avail-
able in the future. This manual will also apply to the
electrical details of the cabinet-mounted model even
though the physical configuration may be different.
1-10. EQUIPMENT SUPPLIED.
1-11. The equipment supplied consists of only the
Model 722A DC Power Supply as this supply is com-
plete in itself.
1-12. OTHER MODES OF OPERATION.
1-13. Two Model 722A's may be connected in series
for greater voltage. Do not connect more than two
722A's in series. Refer to section III for instructions
before attempting to operate these supplies in parallel.
01155-1
Figure 1-1. Model 722AR DC Power Supply
1-1/1-2

Model 722A
SECTION II
PREPARATION FOR USE
Section II
Paragraphs 2-1 to 2-17
,i
2-1. UNPACKING & MECHANICAL INSPECTION.
2-2. Inspect instrument for signs of damage incurred
in shipment. This instrument should be tested as
soon as it is received. If it fails to operate properly,
or is damaged in any way, a claim should be filed
with the carrier. Afull report of the damage should
be obtained by the claim agent, and this report should
be forwarded to us. We will then advise you of the
disposition to be made of the equipment and arrange
for repair or replacement. Include model number
and serial number when referring to this instrument
for any reason.
2-3. Hewlett-Packard Company warrants each in-
strument manufactured by them to be free from defects
in material and workmanship. Our liability under this
warranty is limited to servicing or adjusting any in-
strument returned to the factory for that purpose and
to
replace any defective parts thereof. Any damage
to the instrument upon receipt is due to the carrier.
File a claim with the carrier as instructed in the pre-
ceding paragraph.
2-4. SITE SELECTION AND INSTALLATION.
2-5. The Model 722A should be mounted securely in
a rack. In environments of severe vibration the rear
of the chassis should also be fastened securely. There
should be sufficient space to the rear and along the
sides of the instrument to permit free flow of cool air
for cooling. This instrument will fit any standard
19-inch rack which has a space 5-1/4 inches high. If
the rack is not accessible from the rear, plug the
power cable in before sliding instrument into rack.
Also make any connections to the rear terminal strip.
If the rack is accessible from the rear, these con-
nections may be made at any time. Secure the instru-
ment to the rack with screws and cup-washers. Con-
nect the power cable and connections, if any, to the
rear terminals. Connect the power cable to the power
source.
2-6. LINE POWER.
2-7. The three-conductor power cable supplied with
the instrument is terminated in a polarized, three-
prong male connector recommended by the National
Electrical Manufacturers' Association (NEMA).
WAR ING
The third conductor grounds the instrument
chassis for the PROTECTIO OF OPER-
ATING PERSONNEL. When using a three-
prong to two-prong adapter ground third lead
(green wire) externally.
2-8. OPERATION FROM 230-VOLT SUPPLY.
2-9. This instrument may be used with either a 115-
volt or 230-volt supply with a frequency from 50 to
60
cps, single-phase. This instrument is shipped
01155-1
from the factory ready for operation from a 115-volt
source unless otherwise indicated. Operation from
a 230-volt supply is possible by changing jumper con-
nections or by flipping a switch if the instrument is
equipped with the lIS-volt to 230-volt switch option.
To change the jumpers refer to the schematic for
details (see also figure 5-5).
WAR I G
Remove the power cable from the wall re-
ceptacle before removing the dust cover.
Dangerous potentials are exposed inside the
instrument.
Replace the fuse with the one called out for
230-volt operation in the table of replaceable
parts (section VI).
2-10. If your instrument has the lIS-volt to 230-volt
switch option the input voltage may be changed without
removing the instrument from the rack. First turn
the instrument off or remove the power cable. Then
with a pointed tool, such as the point of a pencil, flip
the lIS-volt to 230-volt switch on the rear apron of
the instrument to 230 volts. Replace the fuse with
the one called out for 230-volt operation in the table
of replaceable parts (section VI).
2-11. PREPARATION FOR STORAGE
AND SHIPMENT.
2-12. The best method of packing the instrument is
in the original shipping carton with the original fillers
packed in the same manner as when received from the
factory. Therefore, when unpacking note carefully
the method of packing and save the original packing
material for possible future re-use.
2-13. If the original packing material is not available
and it is desired to package the instrument for storage
or shipment, first wrap the instrument in heavy kraft
paper to avoid scratching the paint. Then pack in a
cardboard carton with a bursting strength of at least
150 lb per square inch. Pad the instrument on all
sides with at least 2 inches of rubberized hair or at
least 4 inches of tightly packed excelsior.
2-14. STORAGE.
2-15. No special precautions are necessary in storage
except the usual protection against mechanical damage,
salt air, etc.
2-16. INCOMING INSPECTION AND
PERFORMANCE TEST.
2-17. This instrument should be checked as soon as it
is received to determine that its electrical character-
istics have not been damaged in shipment. Refer to
paragraphs 5-32 through 5-41.
2-1

Section III
Figure 3-1
Model 722A
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CURRENT LIMIT +
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POWER SUPPLY e
MOOR r5j) 1ZZAR
HEWLETT PACKARD
Nl.OAlTO (AUro ..... ,.
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1. Turn AC POWER switch ON.
2. Adjust COARSE and FINE VOLTAGE
controls until the voltage on the OUTPUT
VOLTAGE
meter is of the desired value.
3. Adjust CURRENT LIMIT control until the pointer indicates a current value some-
what greater than the expected value.
4. Turn DC POWER switch ON.
If the OUTPUT VOLTAGE drops when DC POWER is turned on, the current limit
is probably being exceeded.
Figure 3-1. Operating Controls
3-0
01155-1

Model 722A Section III
Paragraphs 3-1 to 3-11
SECTION III
OPERATING INSTRUCTIONS
3-1. OPERATION.
3-2. Refer to figure 3-1 for a complete illustrated
description of all operating controls. No preliminary
adjustments are necessary before turning on this in-
strument. Connect the wires from the load to the
output terminals either in the front or back of the in-
strument. If these leads are long or run past a source
of interference, twist them if they are open-wire
leads or run a shielded cable. Figure 3-1 explains
step by step how to operate this instrument.
- SENSE
-POWER
SW
FRONT
RL
REAR OR
REAR
SW
+POWER
+SENSE
RO
Figure 3-2. Remote Sensing
722A 1*2
CR CR
- O~----+lII~ ~J----l
J
+0------ RO
722A
722A .. ,
3-10. If the load must be removed with the supply on
be careful where you break the leads going to the load.
Referring to figure 3-2 you may break the circuit at the
point(s) marked SW. Do NOT break the circuit at the
point(s) marked X as the supply will be damaged.
Figure 3-3. Charging Batteries
3-11. If this supply is to be connected to an active
load (one which supplies voltage) further precautions
must be taken. The load must not be allowed to run
current through the supply in the reverse direction.
WHEN CONNECTI GTHIS SUPPLY TO AN ACTIVE
LOAD ALWAYS MAKE SURE THAT THE VOLTAGE
OF THE MODEL 722A SUPPLY IS GREATER THAN
THE VOLTAGE ON THE TERMINALS WHERE THE
SUPPL Y IS TO BE CONNECTED. The recommended
way of protecting the supply is to use diodes. Figures
3-3 and 3-4 illustrate the proper way to connect these
diodes when connecting this supply to a battery or
when connecting two supplies in parallel. These diodes
must be able to carry 2 amperes current and have a
peak inverse voltage greater than 50 volts. A suitable
diode may be ordered from Hewlett-Packard as stock
number 1901-0019.
3-3. CURRENT LIMITING.
3-4. The maximum current delivered by this sup~ly
may be set to any value between 0.1 and 2 amperes.
This will limit the current to the set value and protect
the equipment under test in addition to protecting the
transistors in the Model 722A from dissipating too
much power.
3-5. Occasionally when the current-limit control is
set to maximum current and maximum current is
drawn the instrument will not adjust down later to
a smaiIer current limit. In this case switch the power
switch to OFF-RESET position momentarily and then
back ON. The instrument will now be reset and will
limit at lower levels of current limit.
3-8. To minimize this effect a separate set of termi-
nals for the sensing voltage is provided on the rear
of the instrument. These terminals permit a separate
pair of leads to connect at the load to supply the sens-
ing voltage. These leads carry no load curre~t. but
are actually inside the regulating loopof the amplIfIer.
3-9. To use remote sensing run a separate set of
leads from the load to the supply. These leads do not
need to be as heavy as the supply leads but they must
be protected from hum pick-up. Run either twisted
pair open-wire leads or shielded leads if hum pick-up
is severe. Connect the leads to the terminals marked
REM+ and REM- on the terminal strip on the rear of
the instrument. At the load connect these leads to
the load (see figure 3-2).
3-6. REMOTE SENSING.
3-7. When the Remote-Local switch on the rear of
the instrument is in the Local position the voltage for
regulation control (sensing voltage) is taken off the
output of the supply at the front panel. This is not
always the best point to obtain this voltage because
there is a drop in the supply leads between the load
and the supply.
CAUTION
Be sure to observe polarity when making
these connections. Wrong connections may
damage the supply.
+ o---~~---o+
RO
Figure 3-4. Parallel Operation
01155-1
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Model 722A
SECTION IV
PRINCIPLES OF OPERATION
Section IV
Paragraphs 4-1 to 4-4
"
4-1. OVERALL BLOCK DIAGRAM.
4-2. Referring to figure 4-2, block diagram, note
that the power-line voltage passes through a power
transformer, a variable transformer, and then to a
bridge rectifier circuit. The COARSE VOLTAGE
control varies the voltage from the variable voltage
transformer delivered to the bridge rectifier. The
rectified ac from the rectifier is filtered and supplied
to the series regulator. The current goes through
the series regulator to the load. A voltmeter across
the output and an ammeter in series with the output
monitors the direct voltage and current fed to the load.
Current and voltage sampling circuits feed the current
limiter. The current limiter enables the operator to
set the maximum current the Model 722A will deliver
to the load to any value between 0.1 amp and 2.0 amps.
The current limiter also serves to limit the maxi-
mum power dissipated in the series regulator to a
safe value. The amplified dc voltage change at the
output is applied as a voltage to the series regulators
which tends to counteract the change in output voltage.
The reference power supply furnishes constant volt-
ages which are used throughout the instrument for
reference and supply purposes. Terminals are pro-
vided for remote sensing so the sensing voltage can
be connected to the output externally.
Note
In
discussions that follow, the positive side
of the output voltage is considered circllit
ground.
4-3. RECTIFYING AND FILTERING CIRCUITS.
4-4. Referring to figure 4-1 note that alternating cur-
rent is fed through aconventional power transformer
Tl into a variable autotransformer T2. The purpose
of T2 is to keep the voltage across the series regu-
lators Q1 and Q2 more or less constant regardless
of the output voltage. Transformer T2 is ganged with
the output voltage control (COARSE VOLTAGE) R25.
If the output voltage were reduced toalow value with-
out reducing the input voltage to the voltage regulator,
excessive power would have to be dissipated in the
transistors of the series regulator at maximum output
current. By the use of a variable autotransformer·
this dissipation is kept to acceptable values.
TI
POWER
TRANS-
FORMER
T2
VARIABLE
TRANSFORMER
VOLTAGE
SAMPLING
POINT")
SERIES
REGULATOR
CURRENT
SAMPLING
POINT)
RIB
AC
IN
CURRENT-
POWER
LIMITER
OUTPUT
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L...---------+---------------_ ......... --+--0+
01155-1
REFERENCE
SUPPLY
Figure 4-2. Block Diagram
BD-L-Z47B
LOCAL
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iL-
lf
L...-_;'-' OREMOTE
SENSING
REMOTEU.
4-1

Section IV
Paragraphs 4-5 to 4-17
4-5. Alternating current from the autotransformer
is rectified in a bridge rectifier CRI-4. The output
of the bridge is a full-wave rectified ac voltage. This
voltage is filtered by the large capacitor C1. Thus
the voltage supplied to the series regulator is almost
pure direct current.
4-6. SERIES REGULATOR.
4-7. The series regulator consists of two transistors
in series, one of which is driven by the series regu-
lator driver Q3. The other transistor is driven by
Q11, the third stage in the amplifier. The action of
these circuits is explained in paragraphs under these
titles.
4-8. OUTPUT CIRCUIT.
4-9. Resistor R18 is connected between the output of
the series regulator and the negative output terminal.
An ammeter connected across R18 is calibrated to
read the current to the load. Voltmeter M2 is con-
nected across the output and reads the output voltage.
The output is available on the front panel terminals
and on the rear terminal strip for remote application.
4-10. VOLTAGE REGULATING CIRCUIT.
4-11. The voltage regulating circuit consists of the
FINE VOLTAGE adjustment R24 (refer to figure 4-1),
the COARSE VOLTAGE adjustment R25, the amplifier
circuit (QI3, Q12 and Q11 and associated circuitry)
and the series regulator Q2. The diode limiters,
CR20 and CR21, limit the input to the amplifier in both
the positive and negative directions. Consider what
happens when the output voltage drops. This drop may
be aslow shift in output voltage or an ac signal on the
output. AC signals are fed to the amplifier through
C5 and R40. This low-impedance path increases the
loop gain for ac signals. When the voltage on the base
of Q13 (see figure 4-3) becomes more positive (less
negative as when the output of the supply drops) Q13
draws more base current since the emitter-base bias
is increased (Q13 is a npn-type transistor). This
signal is amplified and reversed in phase. When
coupled to Q12, this reversed signal again causes
more current to flow since the forward bias is again
increased (Q12 is a pnp-type transistor).
4-12. Diodes CR18 and CR19 are zener diodes which
provide direct coupling between the stages in the
amplifier and maintain proper collector voltages on
the transistor elements. These diodes are kept in the
zener breakdown condition by the currents flowing in
resistors R35, R36, R38, and R39. The signal is
amplified and inverted in Q12. Capacitor CIS, in-
ductance Ll and resistor R31 in the emitter circuit
of Q12 help stabilize the loop. The emitter of Q12
is connected through these components toapointwhich
is separated from the positive output terminal by re-
sistor R33. Load current through this resistor injects
current feedback into the loop which improves the load
regulation. The inverted signal from Q12 tends to
4-2
Model 722A
reduce the base current in Q11 (a pnp-type transistor).
The signal is amplified and inverted in Q 11 and fed to
the base of Q2. This signal tends to increase the
current through Q2 (another pnp-type transistor).
Increasing the current through Q2 increases the volt-
age out of the supply. Since the original action was
a decrease in voltage this action will tend to restore
the original conditions, regulating the supply.
4-13. The dc output voltage is set by varying the
COARSE and FINE VOLTAGE
controls R24 and R25.
In explaining this circuit, resistor R51 can be thought
of
as a constant-current source. Most of this cur-
rent normally flows out through R24 and R25 to the
series regulator with only an insignificant amount
going to the base of Q13. Thus as the resistance of
R24 and R25
is increased the voltage on the negative
output lead will go further negative since the constant
current through these resistors will developa greater
voltage drop.
4-14. CURRENT-POWER LIMITER.
4-15. Transistor Q4 plus associated circuitry forms
a protective circuit which limits the current and power
dissipation in the series regulators and load. Refer-
ring to figure 4-3, no current flows through Q4 under
normal conditions since the base is biased positive
with respect to the emitter. The maximum current
available for the base of Q2 is fixed by the resistors
RIO and Rll and the reference supply. Normally, the
current not used by the base of Q2 to supply a certain
load current passes through the collector of Ql1.
4-16. When the instrument is in the current-limiting
condition, it is essentially a constant-current supply
rather than a voltage supply. The voltage derived
across R18 is coupled to the base of Q4 through CR8
and R14. As the
load current increases, this voltage
increases to a level where Q4 begins to conduct. Any
attempt to increase the load current further merely
turns Q4 on harder, depriving Q2 of the necessary
base current to sustain this larger load current. This
action tends to keep the load current constant. Since
the output voltage is below the desired output voltage
in the current-limiting condition, the amplifier turns
off Ql1. With Q11 turned off the only paths for the
current through RIO and R11 are into the base of Q2
and the collector of Q4.
4-17. Referring to figure 4-3, as the load resistance
is further decreased the output voltage decreases
since the current is held constant. This tends to in-
crease the voltage across the series regulator. When
this voltage reaches a certain value, the zener diode
CR6 will
start to conduct. This current appears to
Q4 to be due to increasing load current and Q4 tends
to conduct more current away from the base of Q2 thus
decreasing the load current and limiting the maximum
dissipation in the series elements. Diode CR8 acts
as a0.7 volt battery since it is forward biased at all
times by the current through R12. Resistor R14 ad-
justs the lower limit of the current limit range while
R20 adjusts the upper limit. The CURRENT LIMIT
potentiometer R5 adjusts the current limit to any value
between the upper and lower current limits.
01155-1
•

Model 722A Section IV
Paragraphs 4-18 to 4-20
!Vc
FROM
RI8
- ----+
RECTIFIERS
Q2
RSA
R7
+
RI2
CI
LOAD
R33
FROM
+ --+ -..>---- ....... ..JI,/I"f'y--- ....
RECTIFIERS
CIS LI R37
RSI
50-11-868
+
Figure 4-3. Current Power Limiter
4-18. PROTECTION CIRCUITS.
4-19. Referring to the schematic in the back of the
manual, note that relay Kl is shown in the normal
energized position. When the instrument is turned
off, or if the power fails, the relay will return to the
opposite position immediately. The center contacts
of the relay Kl on the schematic then apply afixed
positive bias to the base of Q2 tending to turn it off.
If this were not done the output voltage would have a
transient impressed upon it due to the fact that the
amplifier loses control of the series elements faster
than the large input capacitor C1 can be discharged
through R3. Note that the relay switches R3 across
Cl when a power failure occurs to help discharge Cl.
4-20. When the instrument is first turned on the relay
Kl is in the opposite position to that shown in the
schematic at the end of this manual. The voltage applied
01155-1
to the coil of Kl must first charge capaCitors C 13
and C14 through R31 before the relay coil becomes
energized. When these capacitors charge up, the
relay coil will become energized and the relay
contacts will switch, providing a time-delay in turn-on.
The top set of contacts switch R2 out of the circuit.
This resistance is used during the initial time-delay
period to limit the in-rush of current through the
bridge rectifiers CRI-4. The middle set of contacts
switch the base of Q2 from afixed positive voltage to
its normal operating condition. This positi-ve voltage
keeps Q2 turned off during the initial turn-on delay,
giving the amplifier time so its voltages can reach
their operating levels and gain control of the feedback
loop. This action will prevent output voltage transients
which could exceed the desired output voltage. The
bottom set of contacts discharge the capacitors C13
and C14
through R32, preparing the circuit for the
next turn-on cycle.
4-3

Section V
Table 5-1
Table 5-1. Recommended Test Equipment
Model 722A
Instrument Type
Required Characteristics
Use
Instrument Recommended
DC Voltmeter Accuracy of ± 1%
Measure voltages
Analog,
rfiJ Model 412A/ AR
Digital,
1j; Model 405A/B/C
AC Voltmeter
Accuracy of ±3%
Measure ripple
~ Model403A
Floating input
Battery operated
Oscilloscope
5 mv-cm sensitivity
Measure ripple peaks
~ Model 130B, 150A with
Model 151B
~ Model 160B /1 70A with
Model 162D
Variable
Monitor meter 1 volt res-
Change ac input voltage Superior type UC1M
Transformer
olution and ±1% accuracy
Low-Heat
23-1/2 watt element
Solder printed circuit boards
Soldering Iron
( connect iron to 75 volt line) Ungar Model 776
Soldering Iron Handle
Ungar Model 535
Heater Element
Ungar Model PL333
Soldering Iron Tip
Stable Voltage
oise and ripple less than Reference voltage to test Another ~ Model 722A;
Source
250 microvolt at 60 volts regulation 45 volt and 7 volt battery
connected in series
Load 25 ohms at 150 watts
Load for measuring ripple,
Any resistor or combination
regulation, etc.
of resistors of suitable value
(may use wirewound resistors,
supply not affected by in-
ducti ve load)
Precision 1 ohm ± 1%, 5watt resistor Measure current with
~ stock number 0811-0040
Resistor
voltmeter
5-0
01155-1

Model 722A
SECTION V
MAINTENANCE
Section V
Paragraphs 5-1 to 5-18
5-1. INTRODUCTION.
5-2. This section contains maintenance and service
information for the Model 722A DC Power Supply.
A performance check is included at the end of this
section to be used to verify instrument operation. This
check can be made with the instrument in its cabinet
and is agood test as part of preventive maintenance
and incoming quality control inspection.
5-3. MAINTENANCE PROCEDURES.
5-4. DUSTCOVER REMOVAL.
5-5. Remove the eight screws (four on top and four
below) holding the dustcover on the instrument. Pull
the dustcover from the instrument.
5-6. ZERO-SETTING THE METER.
5-7. The meter pointer must rest on the zero cali-
bration mark on the meter scale when the instrument
is at normal operating temperature, resting in its
normal operating position, and the instrument is
turned off. To zero-set the meter proceed as follows:
a. Turn on instrument and allow it to come up to
normal operating temperature (about 20 minutes).
b. Turn the instrument off. Wait two minutes for
power-supply capacitors to discharge completely.
c. Rotate adjustment screw on front of meter clock-
wise until the meter pointer is to the left of zero and
further clockwise rotation will move the pointer up-
scale towards zero.
d. Turn the adjustment screw clockwise until the
pointer is exactly over the zero mark on the scale.
If the screw is turned too far, repeat steps cand d.
e. Turn meter adjustment screw counterclockwise
about 15 degrees to break contact between adjustment
screw and pointer mounting yoke, but not far enough
to move the pointer back downscale. If screw is
turned too far, as shown by the needle moving, repeat
the procedure. The meter is now zero-set for best
accuracy and mechanical stability.
5-8. PRINTED CIRCUIT BOARD REMOVAL.
CAUTION
Be sure to turn off the instrument and allow
time for the capacitors to discharge before
changing the printed circuit boards. Do not
attempt to operate this instrument with any
of
the printed circuit board missing.
5-9. The printed circuit boards are held in by being
plugged
into sockets. Since these sockets have many
01155-1
contacts the board is held in quite firmly withoutbeing
fastened in any other way. To remove these boards
some additional help may be needed. Place a screw-
driver (right-angle screwdriver with boards A &: B)
under the end of the board nearest the chassis and
pry up.
5-10. TEST EQUIPMENT REQUIRED.
5-11. Test equipment required to test this instru-
ment is listed in table 5-1. The necessary specifi-
cations required to test this instrument are listed so
that other equipment with equivalent specifications
may be used.
5-12. PERFORMANCE CHECK.
5-13. Before attempting to troubleshoot this instru-
ment make sure the fault is with the instrument and
not with
the associated circuit under test. The per-
formance check will enable you to determine this
without having to remove the instrument from the
cabinet. BE SURE TO PERFORM THIS TEST BE-
FORE DISTURBING ANY OF THE INTERNAL AD-
JUSTMENTS OF THE INSTRUMENT. This test may
also be used as an incoming inspection test to make
sure the instrument has not been damaged in shipment,
for periodic maintenance, or to check operation of
the instrument after repairs. The performance test
will be found in paragraph 5-32.
5-14. TROUBLESHOOTING.
5-15. INTRODUCTION.
5-16. Components within Hewlett-Packard instru-
ments are conservatively operated to provide maxi-
mum instrument reliability. In spite of this, parts
within an instrument may faiL Usually, the instru-
ment must be immediately repaired with a minimum
of IIdown time". A systematic approach, such as is
given later in this section, can greatly simplify and
thereby speed up the repair.
CAUTION
Be careful not to short voltages across the
transistors. Small bias changes may ruin a
transistor due to excessive dissipation. BE
SURE TO TURN THE INSTRUMENT OFF
BEFORE DOING ANY SOLDERING. A small
leakage current from the soldering iron ap-
plied at the input being amplified may exceed
ratings on the output transistors.
5-17. TROUBLE ANALYSIS.
5-18. Some systematic troubleshooting can be done
with
this instrument. For instance, if there is no
voltage throughout the instrument check the fuse and
5-1

Section V
Figure 5-1
REMOTE -LOCAL SWITCH
/S3
TERMINAL STRIP FOR
230V CONVERSION IF
UNIT
DOESN THAVE
115V-230V
SWITCH.
Model 722A
MP-S-894
5-2
Figure 5-1. Model 722A Bottom View
01155-1

Model 722A
Section V
Figure 5-2
1")1155-1
MP-S-895
Figure 5-2. Model 722A Top View
5-3

Section V
Figure 5-3
SERVICING ETCHED CIRCUIT BOARDS
Model 722A
Excessive heat or pressure can lift the copper strip from the board. Avoid damage by using alow power
soldering iron (50 watts maximum) and following these instructions. Copper that lifts off the board should
be cemented in place with a quick drying acetate base cement having good electrical insulating properties.
A break in the copper should be repaired by soldering a short length of tinned copper wire across the break.
Use only high quality rosin core solder when repairing etched circuit boards. NEVER USE PASTE FLUX.
After soldering, clean off any excess flux and coat the repaired area with ahigh quality electrical varnish
or lacquer.
When replacing components with multiple mounting pins such as tube sockets, electrolytic capacitors, and
potentiometers, it will be necessary to lift each pin slightly, working around the components several times
until it is free.
WARNING: If the specific instructions outlined in the steps below regarding etched circuit boards without
eyelets are not followed, extensive damage to the etched circuit board will result.
1. Apply heat sparingly to lead of component to be
replaced. If lead of component passes through
an eyelet in the circuit board, apply heat on com-
ponent side of board. If lead of component does
not pass through an eyelet, apply heat to CQii':'
ductor side of board.
3. Bend clean tinned leads on new part and care-
fully insert through eyelets or holes in board.
2. Reheat solder in vacant eyelet and quickly in-
sert a small awl to clean inside of hole. If hole
does not have an eyelet, insert awl or a#57
drill from conductor side of board.
4. Hold part against board (avoid overheating) and
solder leads. Apply heat to component leads on
correct side of board as explained in step 1.
: ;: ;: ::;:;:;:;:;:;:::;:;:::;:; ,',J
_:;;.-.:::;;0-'"
In the event that either the circuit board has been damaged or the conventional method is impractical, use
method shown below. This is especially applicable for circuit boards without eyelets.
1. Clip lead as shown below.
~,-- .... - .. :.:- ..... -:: .. - ..... --r-----~~~E
2. Bend protruding leads upward. Bend lead of
new component around protruding lead. Apply
solder using a pair of long nose pliers as a
heat sink.
APPLY~
SOLDER ~~
This procedure is used in the field only as an alternate means of repair. It is not used within the factory.
Figure 5-3. Servicing Etched Circuit Boards
5-4
01155-1

Model 722A Section V
Paragraphs 5-19 to 5-21
CAUTION
Table 5-3. Replacement Guide
CR5
Regulation
CRl, 2, 3, or 4 Line fuse, brush and winding on T2
Check these items:
All transistors and diodes on the chassis have an
anodized aluminum washer and silicon grease be-
tween the transistor and the chassis. The transis-
tors on the fan heat-sink have grease only. When
replacing transistors or diodes be sure to replace
the insulating washer (if any) and the silicon
grease so that these components will have good
heat conduction to the chassis. Use agood grade
of heat-conducting grease, such as Dow Corning
silicon grease #3 or Compound 5.
If you change:
primary circuit. Transistors QI-3 should share the
outP\lt voltage (a voltage drop should appear across
each of these transistors). If there is no voltage
drop across one of these transistors, it is most likely
shorted. If some component on an etched circuit
board is suspected and you have another instrument
that is functioning properly, substitute the board con-
taining that component from the good instrument.
5-19. The circuit consisting of the series regulators
and amplifiers presents a difficult problem for sys-
tematic trouble analysis. This is a feedback circuit
in which afaulty component anywhere will affect the
entire loop. In this case symptomatic trouble analysis
may be easier. Refer to the symptom column in
table 5-2, Troubleshooting Guide and test the cor-
responding transistor or diode. If you have to re-
place any transistors or diodes refer also to table
5-3, Replacement Guide for additional tests which
may be necessary.
Table 5-2. Troubleshooting Guide CR6
Current limiting
Symptom
Blown
line fuse
Check
-
CRl, CR2, CR3, CR4, CRI2,
CRI3, CRI4, CRIS, CRI6,
CR22, CR23, shorted winding
on T1, or brush and winding
on T2.
CR7
CR8
CR9
CRI0
Voltage CR7 (should be about
0.8
volts)
Current limiting, recalibrate cur-
rent limits
Current limiting
Output C6
Poor load
regulation
CR5, CR11, CRI7, Q2, Q4, Q11,
Q12, Q13, R33 and
S3
CRII
Load regulation and transients
with dc switching
1) Loosen fan by removing four allen-head screws
holding fan to spacers. Slide fan to one side.
Note
Ql and Q2 are located on the fan heat-sink. To
remove these transistors proceed as follows:
See table 5-3, Replacement Guide, for additional
tests which may be necessary if you replace the
above components. See figure 5-3, Servicing
Etched Circuit Boards.
Poor current
limiting
CR6, CR8, CRI7, Q4, R9
CR12-16
CR17
CRI8, 19
Ql, 2, or 3
Q4
Q11
Ripple,
voltage output, maximum
current, relay operation
Current limiting, transient re-
sponse to loading
Collector voltage on Q12 and Q13
Check for voltage drop across
transistors; regulation at 103
volts in, 60 volt to 2 amp out
Current limiting, R14, R20
CRI7, Q4, output voltage; regu-
lation at 127 volts in, low output
voltage
5-20. DETAILED TEST PROCEDURE.
5-21. The following test procedure should be per-
formed only after the performance test (paragraph
5-32) has shown that this instrument is faulty. DO NOT
PERFORM THIS PROCEDURE AS AN INCOMING IN-
SPECTION
OR PROOF OF PERFORMANCE CHECK.
The specifications for your instrument are given in
2) Unsolder wires going to transistor. Note that
the heavy wire goes to the emitter and the finer
wire goes to the base (collector is connected to
case).
3) With a wrench remove the screw holding the
transistor. Remove transistor.
Installation of anew transistor is the reverse of
the above procedure. Be sure to note which is
the emitter and which is the base pin before
mounting the transistor.
Q12, 13
Voltage control and regulation
01155-1
5-5

Section V
Paragraphs 5-22 to 5-35
the front of this manual (table 1-1). The following
test procedure contains extra checks to help you
analyze the troubles in this instrument. These extra
checks and the data they contain cannot be considered
as specifications.
5-22. Do not perform all the tests in this procedure.
Do only those tests associated with the particular
sections of the instrument shown to be faulty by the
performance test. Indiscriminate adjustment of the
internal controls to "refine" the settings may actually
cause trouble.
5-23. REFERE CE VOLTAGE ADJUSTME T.
5-24. To set the reference voltage proceed as follows:
a. Connect a voltmeter, such as the r.5f; Mode1405A/
B/C across C24 (reference supply filter capacitor
on board A).
b. Adjust R48
(Reference Voltage Adjust potentiom-
eter) until the meter reads 13.2 ±0.2 volts.
c. Remove meter.
5-25. MONITORI G.
5-26. For all following tests (subparagraphs 5-27
and 5-29) the output voltage should be monitored. The
positive output terminal of the Model 722A is con-
sidered circuit ground. To monitor the output con-
nect a voltmeter, such as the r.5f; Model 405A/B/C
as follows:
a. Turn AC and DC switches to OFF. Remote-
Local switch on Local.
b. Turn COARSE VOLTAGE potentiometer R25
fully counterclockwise.
c. Attach voltmeter to the output terminals (front
or rear) of the Model 722A "with the black terminal
of the voltmeter connected to the red (positive) termi-
nal of the Model 722A. (In this test procedure the
positive side of the Model 722A output is considered
circuit ground.)
5-27. VOLTAGE RANGE.
5-28. To check the voltage range of the Model 722A
proceed as follows:
a. Turn the AC and DC POWER switches to ON.
b. Tum the COARSE and FINE VOLTAGE controls
on the front panel fully clockwise. The voltmeter
across the output should indicate 60.1 to 60.3 volts.
If not, adjust R48.
c. Turn the COARSE and FINE VOLTAGE controls
on the front panel fully counterclockwise. The volt-
meter across the output should indicate 0.05 to 0.02
volt (positive terminal on the Model 722A is considered
ground).
5-6
Model 722A
5-29. CURRENT LIMIT.
5-30. A simple way to measure current with a volt-
meter is to use a load made up of a25 ohm, 150 watt
resistor in series with a precision 1%, 1 ohm,S watt
resistor, r.5f; stock number 0811-0040. Connect a volt-
meter in shunt with this precision resistor. In the
following
procedure the term shunt voltmeter will be
used to distinguish this voltmeter from other volt-
meters.
5-31. To adjust the current limiting proceed as follows:
a. Turn the DC POWER switch to OFF.
b. Connect the load mentioned above the OUTPUT
terminals.
c. Connect the shunt voltmeter across the 1 ohm
precision resistor.
d. Turn the DC POWER switch to 0 •
e. Turn the CURRE TLIMIT potentiometer on the
front panel fully clockwise and the COARSE VOLTAGE
control fully counterclockwise.
f. Turn the COARSE VOLTAGE control slowly
clockwise until the current appears to limit (volt-
meter on panel stops rising). The voltage on the
shunt voltmeter should be 2.3 volts. If not, adjust R20.
g.
Adjust the COARSE and FINE VOLTAGE control
until the shunt voltmeter reads exactly 2.00 volts (in-
dicating 2 amps current).
h. The OUTPUT CURRENT meter should read
exactly 2.00 amps. If not, adjust Ammeter Calibrate
potentiometer (R16) until the OUTPUT CURRENT
meter does read exactly 2.00 amps.
i. Turn COARSE VOLTAGE control until the output
voltage is about 10 volts and tum CURRENT LIMIT
control fully counterclockwise.
j. The voltage on the shunt voltmeter should be
0.08
volts (0.08 amp) or less. If not, pad R13.
5-32. PERFORMANCE TEST.
5-33. Before attempting to troubleshoot this instru-
ment make sure the fault is with the instrument and
not in the associated circuit under test. This proce-
dure will enable you to determine this without having
to remove the instrument from the cabinet. BE SURE
TO PERFORM THIS TEST BEFORE DISTURBING
A Y OF
THE INTERNAL ADJUSTMENTS OF THE
INSTRUMENT. This test may also be used as in in-
coming inspection test to make sure the instrument
has not been damaged in shipment, for periodic main-
tenance, or to check operation of the instrument
after repairs.
5-34. VOLTAGE RANGE.
5-35. To check the voltage range proceed as follows:
01155-1