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Section V
Paragraphs 5-30 to 5-31
d. Connect an ac voltmeter to the output of the
power supply, at pin 3 of VI.
e. Adjust the line voltage from low to high line
(103 -
127 volts).
Ripple voltage should not be
greater than 7 mv.
5-30. TROUBLESHOOTING.
5-31. If the twt amplifier is suspected of unsatisfac-
tory operation, proceed as follows:
Model 492A/494A
a. Remove all the external connections to the in-
strument and set GRID BIAS to zero.
b. Measure the electrode currents of the twt using
the front panel meter.
c.
If
the electrode currents are abnormal, remove
the instrument from the cabinet (see paragraph 5-8)
and with the indication observed, determine the
possible cause and action from table 5-3.
Table 5-3. Troubleshooting Chart
Indication
Possible Cause
Action
Check
Cathode current low
Regulated voltage low
Replace VI
Table 5-2
Check regulation
Para. 5-28
Low or no helix voltage
Replace V5
Table 5-2
Low anode voltage
Adjust anode voltage
Para. 5-26
Low twt emission
Replace capsule
Para. 5-12
Cathode current high
High anode VOltage
Adjust anode voltage
Para. 5-26
Gassy twt
Replace capsule
Para. 5-12
Anode current high
Low helix voltage
Adjust chassis helix voltage
Para. 5-24
Replace V5
Table 5-2
Gassy twt
Replace capsule
Para. 5-12
High anode voltage
Adjust anode voltage
Para. 5-26
Helix current high
Misalignment of the twt
Align capsule
Para. 5-16
Collector current
Low, or no twt magnet current
Replace F2 (iffan does not turn)
low
(low field).
Replace defective magnet power
supply components.
No voltage on collector (discon-
nected at connector), Fig. 5-3
No or little control of
High impedance path between the
Replace capsule
Para. 5-12
Cathode current with
anode and grid. May be 40 meg-
GRID BIAS control.
ohms or more.
Low gain.
Misadjustment of anode voltage.
Adjust anode VOltage
Para. 5-26
The anode may have been ad-
justed to obtain the COrrect
cathode current with some grid
bias on the tube.
Wrong value of helix voltage
Adjust chassis helix voltage
Para. 5-24
Low cathode current
See cathode current low
(in indication column)
Oscillation
Defective Helical Attenuator
Replace capsule
Para. 5-12
Excessive cathode current
See cathode current high
(in indication column)
5-6
00144-2
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