5-8. CABINET REMOVAL.
5-9. To remove the instrument chassis from the
cabinet proceed as follows:
a. Place the twt panel down on a soft pad.
b. Remove the two 10-32 round head machine
screws which secure the rear of the cabinet to the in-
c. Lift the cabinet up and off the chassis. The
panel bezel remains attached to the panel.
5-10. TUBE REPLACEMENT.
5-11. A list of tubes in the 492A and 494A and the
checks and adjustments that must be made following
replacement, are in table 5-2.
Table 5-2. Tube Replacement List
Check voltage range
of HELIX control
Adj. R25 if necessary
see paragraph 5-24.
Same as VI.
Check range of GRID
Same as VI.
a. Helix Modulator
Same as VI.
b. Discharge Tube
Traveling Wave Tube See para. 5-12.
5-12. TRAVELING WAVE TUBE CAPSULE
5-13. Because the traveling wave tube is fragile and
adjustment of the coupling helices is critical, re-
placement tubes are furnished in the complete cap-
sule assembly which includes the tube, coupling hel-
ices, coaxial cables, and panel connectors complete
in a single unit.
Refer to the "Conditions of War-
ranty" page in the manual.
5-14. Special traveling wave tubes in which gain has
been sacrificed to obtain a more constant output over
the full frequency range and a very low value of inci-
dental amplitude modulation during phase modulation
are available on special order.
traveling wave tube from the 492A or 494A proceed
Paragraphs 5-7 to 5-16
a. Remove the instrument from the cabinet. (See
b. Disconnect the twt collector lead connector at
the rear of the capsule, shown in figure 5-3.
c. Remove the screws mounting the INPUT and
OUTPUT connectors to the panel.
d. Loosen all the capsule alignment screws. The
front alignment screws are shown in figure 5-2 and
the rear alignment screws are shown in figure 5-3.
e. Remove twt plug from socket.
This plug is
located on bottom of instrument. Refer to figure 5-2.
Grasp the rear of the capsule and carefully pull it out
of the magnet.
5-16. INSTALLATION AND ALIGNMENT. To install
and align a traveling wave tube capsule proceed as
a. Hold the twt capsule so the recessed hole in
collar is turned upward; insert the capsule into the
magnet and be sure the spring-loaded ball (bullet
catch) seats into the recessed hole.
b. Plug the collector lead into its connector as
shown in figure 5-3.
c. Reconnect twt plug to socket as shown in fig-
d. Turn the GRID BIAS control completely clock-
wise for the most negative bias voltage (cathode posi-
tive); turn the power ON.
necessary adjust R47 to
obtain correct heater VOltage. Refer to twtdata sheet
(supplied with new tubes) and figure 5-1.
e. The alignment screws determine the position of
the capsule in the magnet. When the capsule is cor-
rectly positioned the helix current will be minimum
(near zero). Therefore the relative level of the cur-
rent flowing in the helix is an indication of correct
f. Set meter switch to HELIX.
the meter indi-
cation is near zero, there is insufficient current flow
to check the alignment. Reduce the grid bias voltage
until the meter indicates some small readable helix
current; at the same time be sure that the cathode
current is below its safe maximum for continuous
operation after warmup, see table 3-1.
g. Adjust the alignment screws to the setting which
minimizes the helix current.
h. Reduce the cathode current with the anode con-
trol and turn the GRID BIAS control to 0, then adjust
the anode control to obtain the normal cathode current
(see the meter plate that came with the new tube).
i. Readjust the alignment screws; note the elec-
any currents are abnormal see
j. Lock the alignment screws, at the same time
watching the meter to see that the helix current does
not increase. Note the meter reading.