AR 10009284, Operating And Service Manual

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Model 40T26G40A
Rev A 17
Push S4 (MORE) three times to go to MENU 4. Verify that the heater voltage and current are near their
nominal levels. The values of these parameters at the time the TWTA left the factory are logged on the test
data sheet.
Push the OPERATE push-button. You will now see the cathode and the collector voltages rise. Verify that
the collector and cathode voltages are near nominal. The helix current should be close to the nominal value
for no RF drive. Then push MORE or EXIT to go back to MENU 1.
Set the TWTA gain to maximum. Adjust the RF generator to slowly increase the RF drive toward 0 dBm to
reach the desired FWD PWR on the display and power meter (connected to sample port). The forward power
bar graph will become active, with maximum length when peak power output is achieved. Best performance
is obtained when the input RF drive is set at or just below the level, which causes peak power output. Do not
set the input drive level above 0 dBm; input drive levels above +13 dBm may damage the unit. The reflected
power level should remain below 10% of the forward power, assuming that the load is properly matched.
An alternate procedure is to pre-set the TWTA gain to minimum, set the RF generator to 0 dBm and then
slowly increase the TWTA gain to set the desired RF output level.
Observe that the helix current is sensitive to the RF drive level of the TWT. It is at a minimum with no RF
drive. The helix currents with no drive and with rated RF output mid-band are logged on the test data sheet.
The value of the helix current is a good qualitative indicator of RF drive present.
To shut the system down, turn down the RF generator level below -50 dBm and press STANDBY. Allow the
TWTA to cool down until the TWT temperature drops below 60°C., then turn off main power
3.7 EMERGENCY BYPASS OPERATION
For reference, see Schematic Diagram No. 10-24830 in section 5.2. The TWTA is provided with a means of
operating the amplifier manually if there is a failure of the control module and it is imperative that the
amplifier remains on line.
CAUTION:
Emergency bypass operation disables certain protective and diagnostic
features. For this reason, the emergency bypass mode of operation should
be used only when the control unit fails and when it is essential to remain
on line.
To access the manual controls, remove the two 4-40 screws securing the emergency switch cover on the front
panel. Emergency bypass mode is selected by pushing the left-hand switch (S1) to the left. The center switch
(S2) toggles between high voltage on (left) and high voltage off (right). The right-hand switch (S3) selects
between solid-state amplifier (SSA) active (RF on) in the left-hand position, and SSA off (RF off) in the
right-hand position. There is also a manual control for the gain adjustment; it is a flat, square, single-turn pot
(R1).
CAUTION:
Do not adjust 20-turn pot R11; its function is to set the RF foldback level,
and it is calibrated at the factory.