Keithley 610B Instruction Manual page 35

MODELS 6108, 610BR ELECTROMETERS MAINTENANCE
1. Check the filaments of the electrometer tubes, VLOL and V102, by measur-
ing the voltage drop across the filament dropping resistor, Rll9 (Figure 22).
IF both filaments are operating properly,
the voltage will read 2.0 volts dc
?lO%.
2.
Check the output tube, V103 (Figure 23 ), to see that both filaments
are lit.
3.
Inspect the leads from the shock-mounted input printed circuit board,
PC-84, to the amplifier printed circuit board, PC-85, for possible breaks.
d. Su~oly. Power
1. Set the FEEDBACK Switch to FAST to connect the instrument's chassis to
floating ground.
Connect a 100~megobm dc voltmeter between the -10 volt
supply and the chassis.
(-10 V is indicated on PC-86, Figure 18.) The
voltage should be -10 volts dc 220%.
2. Connect the voltmeter between the +21 volt supply and the chassis.
(~21 V is indicated on PC-86, Figure 18.) The +21 volt supply can,not
operate unless the -10 volt supply is operating.
The voltage should be
~'21 volts dc 210%.
A faulty ~21 volt supply can indicate a faulty zener,
D210 (Figure 23).
3.
Check the +2SO end -265 volt supplies; these should be correct within
10%.
For this measurement, use output ground as the co@anon connection for
both supplies.
e.
Amplifier.
1. To check the amplifier, disconnect the feedback 150~ by removing the
output tube, V103. This allows each stage of the amplifier to be individual-
ly checked.
It also eliminates the possibility of applying excessive voltage
to the electrometer tube grids, causing serious damage.
NOTE
Use a LOO-megohm dc voltmeter as a null detector to check the amplifier
stages.
Do not ground its low side.
2.
Connect the voltmeter between the plates of VlOl and VlO2 (Figure 22).
Adjust the COARSE and MEDIUM ZERO Controls for null.
If a null cannot be
reached, check VlOl, V102, the COARSE end MEDIUM ZERO Control circuits
(resistors R128 to RlSl), and transistors QlOl and Q102. Check the tran-
siscors by removing them and adjusting for null again.
lf null is now reach-
ed,
replace the transistor pair with a new pair.
3.
Check the next stage by connecting the voltmeter across the emitters
of QlOl and 9102 (Figure 21) end adjusting the COARSE and MEDIUM ZERO
Controls for null. If a null is not reached, check this stage end the base
circuit of the next stage. Check the base circuit by removing transistors
9103 and 4104 end again adjust for null. If null is now reached, replace
the transistor pair with a new pafr.
1064~ 31