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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 a good 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 c and 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 REMOV AL.
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 without being
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
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