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Heater voltage backed off, beam off during downtime
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Heater voltage full, beam off during downtime If TWT tubes are stored for extended periods
of time, the TWTs should be operated at least every six months using the following
procedure.
Note: Running the TWT at 0.3 V above the heater voltage on the label can
reduce the TWT life by as much as one half.
Note: If the spare TWT is to be tested, follow the instructions in TWT
Replacement. If the spare TWT is to be tested, follow the instructions in
"TWT Replacement" later in this chapter.
1. The amplifier should be turned on with no RF drive applied.
2. Allow the TWT to warm up for at least 5 minutes, but not longer than 15 minutes, before
applying operating voltages.
3. Enter standard operating mode on the HPA control panel.
4. Put the amplifier in Transmit.
5. If the TWT will operate in Transmit without a limit trip, continue this procedure. If not refer
to "Maintenance and Troubleshooting" in the CHPA Service Manual for V2.
6. The TWT should be run without RF until the helix current has stabilized.
7. Apply RF slowly up to the point of rated power.
8. Run with and without RF input for a minimum of 30 minutes after the body current has
stabilized. The time required to stabilize varies with the age of the tube, storage time, and
storage conditions.
9. Follow the instructions in "Performance Testing".
10. Go through normal shutdown.
Once the TWT is in service, it is important that the tube is operated correctly. There are three
guidelines to follow:
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In Redundant Mode (1:X configuration), the amplifier should be left in Transmit. Leaving
the TWT in Standby with the heater on for several hours can cause the cathode to age more
quickly. This reduces the life of the TWT (even with heater back off) and may create gas that
will poison the cathode and reduce the emission.
•
The collector radiator must be kept clean. If the radiator becomes clogged, the collector can
overheat and over a long period can produce gas that can poison the cathode and affect the
performance. Cleaning the external cooling air filters is also important.
•
The heater must be operated at the nameplate voltage for the particular TWT. A voltage
setting too high or too low can cause premature failure. However, when the TWT is old and
the beam current drops because of cathode depletion, the heater voltage can be raised 5
percent without causing any harm. See heater adjustment procedures in the CHPA Service
Manual.
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REPRODUCTION OR RELEASE WITHOUT EXPRESS PERMISSION IS STRICTLY PROHIBITED
CHPA SERIES
6-7
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