Keithley 630 Instruction Manual page 22

MODEL 630 POTENTIOMETRIC ELECTROMETER SERVICING
Difficultv
Measurements are Out of
specifications
on all ran-
ges other than 500-volt
Excessive ac interference
Non-symmetrical response
on polarity switching and
low sensitivity on most
sensitive ranges
Instrument does not respond
to a signal and offset on
all ranges
Greatly increased sensiti-
vity on 10 and 100 null
ranges and 5, 50 and 500V
VTVM ranges
500V supply operates but
null detector is completely
inoperable
Keasurements are Out of
specifications on any range
tihen the Reference Voltage
Dials are at anv settine
other than 4 9 9 9 10 -
Probable cause
Resistor R333 is faulty
R131 out of adjustment
Oscillator frequency has
drifted
Relay K102B is defective
Relay KlOlB is defective
Fuse F201 is blown
One of the Kelvin-Varley
divider resistors is
faulty
Solution
Replace the faulty resistor
Adjust oer oaragraoh 4-8
Check frequency.
Reset to
approximately 94 cps. (Refel
to paragraph 5-7).
Check operation of relay. 11
faulty replace with Keithley
relay or return t0 factory
Check operation of relay. If
faulty replace with new relay
from Keithlev or return to
Check for possible short in
-6v supply (~~104).
See paragraph 5-3
TABLE 4.
(Sheet ).
Model 630 Troubleshootir :.
b. Paragraph 6-2 describes how to remove the Model 630 :over.
Before proceeding with
the troubleshooting, check the vacuum tubes. Normally,
replacing tubes will clear up the
difficulty.
All tubes can be readily tested on a grid-modulated tube tester.
If replac-
ing a tube does not correct the trouble, continue the procedures.
Replacing tubes does
not necessitate recalibration of the instrument.
c. Table 4 contains the mwze ccnnm~n troubles which might occur. If the repairs indica-
ted in the Table do not clear up the trouble,
the difficulty will have to be found through
a point-by-point check of the circuits. Refer to the circuit description in Section 3
to find the more crucial components and to determine their function in the circuit. The
complete circuit schematic, 18261E, is found in Section 7,
4-5.
ADJUSTING GRID BIAS OF CATHODE FOLLOWER. The grid bias of the cathode follower,
V1003 (Figure 15), should be -3 volts. Use the Keithley Model 610B to check this value
i5"/,.
If necessary, adjust the DC BAL potentiometer, R157 (Figure 16), to this value.
0166R
19