Correcting For No Output; Correcting For Inaccurate; Voltage Output - Keithley 242 Instruction Manual

MODEL 242 VOLTAGE SUPPLY SERVICING
shorting cap on the - OUTPUT Receptacle.
Set the Voltage Switches for a 300-volt output;
make sure the TRIM Control is off. Put on the POWER and HIGH VOLTAGE Switches, as outlined
in paragraph 2-2.
NOTE
Be careful working inside the chassis. If the POWER Switch is on, up to 200 volts
may appear at points in the control circuits. High voltage appears on both sides
of the chassis. Before working on a faulty Voltage Supply, disconnect the power
cord and discharge all filter capacitors, as Cl06 and C115.
4-6.
CORRECTING FOR NO OUTPUT.
Check both front panel fuses. The HIGH VOLTAGE Pilot Light indicates when it is
pozsible to have a high voltage output ; however, it can light even when the fuse is blown.
Replace defective fuses. If the fuse again blows, check further for the cause.
b.
Measure the plate voltage of series regulator tube V12. Use a dc voltmeter which
measures to 4000 volts. Make sure the Voltage Switches are set at 300 volts before making
this measurement.
The plate voltage should be 2000 volts f20%.
NOTE
Discharge high voltage from tube V12 through a bleeder. Discharging the cap
directly to the chassis may damage resistor R113 or diode DlOl.
c. If the plate voltage is not correct, check rectifiers VlO and Vll, the series resis-
tor R113 (Figure ll), the filter capacitor Cl06 (Figure 8) and the primary winding of
transformer T2 (Figure 8) for 117 volts.ac. If no voltage appears on the transformer pri-
mary, check the overload relay, Kl (Figure lo), and the control relay, K2 (Figure 8).
d. If the plate voltage of tube V12 is correct, check the cathode voltage across pin 1
or 7 to ground. The voltage should be 300 volts *LO%.
e.
Incorrect voltage at the cathode indicates tube V12 is faulty or incorrect biasing
conditions exist. If the voltage is still not correct after tube V12 is replaced, check
tube V4 (Figure lo), the dc amplifier tube V3 (Figure lo),
the ac amplifier tube V2 (Fig-
ure lo), and the diodes in the demodulating bridge, D106 to DlC9 (Figure 10).
f. If the plate voltage of V12 is correct, check the overload relay Kl. Refer to par-
agraph 4-9.
4-7. CORRECTING FOR INACCURATE VOLTAGE OUTPUT. Poor regulation or excessive drift will
affect the accuracy of the voltage output. Several factors cause this; check the following.
a. A defective capacitor Cl15 (Figure 8) will cause the output to be unstable.
b.
Although it normally does not need replacement, check the zener diode D103 (Figure
8).
Since adjusting the diode circuit requires recalibrating the entire instrument, check
the zener diode only after all other possible trouble sources are eliminated.
1. Remove the jumper in series with diode D103 (the jumper is next to R133, Figure 9).