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Adjustments; Excessive Helix Current; Chassis Helix Control; Anode Voltage Control - HP 492A Operating And Service Manual

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)
Model 492A/494A
k. Remove the meter plate which indicates the
normal cathode current for the old tube and install
the new plate.
m. Perform the chassis Helix control adjustment,
see paragraph 5-24.
n. Turn the power off and install the cabinet.
p. Set the meter switch to HELIX and turn the
power ON. The meter reading should be the same as
in step j; if not, the setting of the capsule was dis-
turbed when the cabinet was replaced. Turn off the
power, remove the cabinet, and reposition the capsule
as before, starting with step f. Replace cabinet; again
measure the cathode and helix currents, which should
be the same as with cabinet removed.
5-17. CHANGI G THE FREQUENCY RANGE OFTHE
492A OR 494A.
5-18. The t$j) Models 492A and 494A are identical ex-
cept for the traveling wave tube type supplied and the
operating voltage adjustments.
The proper range of
dc operating voltages for one tube type or the other is
selected by S2 on the instrument chassis (figure 5-1).
5-19. To change a 492A to a 494A proceed as follows:
a. Remove the cabinet; see paragraph 5-8.
b. Remove the twt capsule; see paragraph 5-15.
c. Turn S2 to the 494A position; see figure 5-1.
d. Install the typeM2201-K(HPI952-0009)installa-
tion procedure is the same as type M2207-A except
S2 should be in 494A position. (See paragraph 5-16.)
5-20. To change a 494A to a 492A proceed as follows:
a. Remove the cabinet; see paragraph 5-8.
b. Remove the twt capsule; see paragraph 5-15.
c. Turn S2 to the 492A position, see figure 5-1.
d. Install the typeM2207-A (HPI952-0012) installa-
tion procedure is the same as type M2201-K, except
S2 should be in 492A position. (See paragraph 5-16.)
5-21. ADJUSTMENTS.
5-22. EXCESSIVE HELIX CURRENT.
5-23. If excessive helix current is indicated on the
front panel meter with normal currents in the other
twt electrodes, the electron beam is not being focused
accurately through the center of the helix. Measure
the dc voltage and current supplied to the magnet at
115 volt line, which should be approximately 120 volts
at 700 mao The ac ripple on the magnet supply should
be less than 1 volt rms.
If
these measurements do
not reveal the cause of the trouble, check the align-
ment of the twt in the magnet. See paragraph 5-16.
00144-2
Section V
Paragraphs 5-23 to 5-29
5-24. CHASSIS HELIX CONTROL.
5-25. The chassis Helix control, R25, determines
the range of the front panel HELIX control by govern-
ing the output voltage of the regulated power supply.
This adjustment should be performed whenever a new
twt capsule is installed or the gain of the amplifier is
less than 30 db.
To adjust, proceed as follows:
a. Remove the cabinet, see paragraph 5-8.
b. Set twt HELIX control to position 5, and adjust
GRID BIAS control to obtain normal cathode current.
c. Connect a test setup as shown in figure 5-4.
d. Set the generator for the Model 492A to 4 gc,
for the Model 494A to 7 gc. For both models set the
generator attenuator to -30 dbm, in cw operation.
e. Adjust the chassis Helix control, R25, for maxi-
mum gain, as read on the 430C.
f. Change the frequency of the generator, for the
Model 492A to 8 gc, for the Model 494A to 12.4 gc,
maintaining the same power level.
g. The 430C should read greater than 0 dbm.
If
the 430C doesn't read greater than 0 dbm, readjust
the chassis Helix control, R25, to obtain from 0 dbm
to +2 dbm.
Recheck the gain for the Model 492A at
4 gc, for the Model 494A at 7 gc, and readjust R25,
if necessary, to obtain nearly equal gain at both fre-
quencies.
5-26. ANODE VOLT AGE CO TROL.
5-27. The Anode control, R30, on the chassis of the
instrument, (see figure 5-1), is provided so that the
two cathode current can be maintained at its normal
level (as indicated on the plate attached to the meter
face) as the tube ages. To adjust proceed as follows:
a. Place meter selector switch to CATH. position
and GRID BIAS control to O.
b. Rotate Anode control adjust, R30, to obtain
normal cathode current.
Also note that other tube-
element currents do not exceed their safe maximum
limits (see table 3-1).
5-28. REGULATED POWER SUPPL Y.
5-29. To check the regulated power supply, proceed
as follows:
a. Set GRID BIAS control to O.
b. Connect a dc voltmeter to the output of the
power supply, at pin 3 of VI.
c. Adjust the line voltage from low to high line
(103 - 127 volts).
Power supply voltage should not
change more than 10 volts.
5-5

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494a