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Model 721A
Sect. IV Page X
MAINTENANCE
4-1
CONTENTS
This section tells how to make internal adjustments,
locate trouble and how to check over-all perform¬
ance.
Paragraphs 4-4 and 4-5 may be used as a
rapid performance check to certify that the power
supply is operating within published specifications.
These tests can be made with the instrument in its
cabinet.
The power supply has no parts which have a definite
limited life. The instrument should operate indefi¬
nitely with no routine maintenance, if any parts are
replaced you should recheck the settings of the
screwdriver controls which set the maximum output
voltage (R21) and maximum short circuit current
(R8).
Variations among parts may make it neces¬
sary to readjust these controls slightly.
Reseal
the control with duco cement after adjustment,
otherwise the setting will change with shock and
vibration.
A list of possible troubles and the probable cause
are tabulated in paragraph 4-3. In each case, curing
the trouble involves replacing the defective parts,
except loose end-clips on the silicon rectifiers may
be repaired by slightly crimping the fitting.
Be careful when soldering on the etched circuit
board.
You can cause damage by excessive heat
or improper technique. Figure 4-1 explains some
of the proper techniques to follow.
Paragraphs 4-4 and 4-5 show suitable set-ups for
checking power supply performance. The equipment
and connections should be followed carefully to avoid
false results. This is especially true when making
ripple and dynamic ac impedance measurements.
Stray ground loops are easy to establish if equip¬
ment grounding techniques are not carefully con¬
trolled.
The relative position of each instrument
in the set-up should be followed.
If equipment
other than that shown is used, it should be of equiv¬
alent performance and input characteristics. Avoid
long leads to prevent stray pick-up.
The procedure given does not account for power
supply noise which adds to the voltmeter reading
due to internal impedance.
The residual power
supply
noise represents a small error at low
audio frequencies when calculating the internal im¬
pedance.
However, the value obtained will be well
within
rated
performance
even neglecting* this
error.
Standard components are used for manufacture of
^instruments whenever possible.
Special'com¬
ponents are available directly from the <$ factory.
Perhaps your most convenient source for spare
or replacement parts is your local^ Representative
who maintains a parts stock for your convenience.
When ordering parts, please specify instrument
model and serial number plus the component des¬
cription and stock number appearing in the Table
of Replaceable Parts.
Your local ($ Representative also maintains com¬
plete service facilities and specially trained per¬
sonnel to assist you with any engineering, appli¬
cation, test, or repair problems you may have
with ^instruments.'
4-3
TROUBLE LOCALIZATION
Table 4-1 lists some possible troubles and their
causes.
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