Before getting started, put the transmitter on "STANDBY" and verify that:
· 1) Heater Voltage is approx. 7.0V at 17 A nominal (or factory specified setting)
· 2) Ion Pump is approx. 3.3KV at 0
· 3) Focus is set to approximately 11 Amps nominal
· 4) Set Re-Focus to 7.25A nominal
· 5) Make SURE that you test all of the Safeties and Interlocks for proper operation before continuing.
Course Tuning Procedure: Before beginning normal tuning, perform the following on ALL Cavities
· Turn the Tuning Knob FULLY counterclockwise until it comes to a stop
· Make sure that the counter reads ZERO. If it doesn't, set it now and lock it down so it tunes properly. On Varian Transmitters, the counter is "gravity" centered. So you zero them with the cavity in an upright position, then lay the cavity on its side (normal position).
· Turn the knobs on ALL cavities till the counter reads 10 turns.
WARNING: If during tuning, the penultimate (3rd) cavity EVER reaches the carrier resonant frequency*** you will probably destroy the tube.
(I strongly recommend that you don't let this happen.)
Normally, the third cavity is kept 6MHZ up from the carrier frequency.
Tuning Procedure: Once the course tuning is accomplished, and all the prerequisite voltage and current levels have been checked, you can begin the Klystron tuning as follows:
1) Tune the Ultimate Cavity (the driver) to Fc (frequency of the video carrier)
2) Tune the Penultimate Cavity to AT LEAST 8 MHz above Fc.
3) Tune the 2nd Cavity 1 1/2 MHz below Fc
4) Tune 3rd Cavity (if it exists) to 4 MHz above Fc
Note: this is only done on 5 cavity Klystron tubes. Disregard this step on a 4-cavity tube.
5) Tune the 1st Cavity to 2MHz above Fc.
6) Raise the Beam Voltage to a normal level
7) Increase the Drive to normal
8) Adjust the Loading and Coupling of the cavities for slope and edge effect.
At this time you need to make sure that the following guidelines are followed exactly:
· Anode Voltage is a low as necessary
· Filament Voltage is still at the factory specified setting (7.0V nom)
· Tube Seal temperatures are low
· Ion Pump is functioning, but not overworking
· High Tube gain
· Low VSWR
· Bandwidth is just enough to do the job ( within FCC tolerances).
· Total Currents 3.5KA (A Nominal Amount)
· Just Enough Headroom
Apply filament power in the usual manner. Allow the klystron to warm up for a minimum of five minutes. Do not leave it for longer then fifteen minutes before applying high voltage. When applying high voltage, start from the lowest value possible. Set body current trip level to 20mA, as a precaution, while running up. There may be intermittent bursts of body current as gas is cleared. This should clear fairly quickly, so that full operating voltage is reached in a few minutes. Once full voltage is attained, RF may be applied. The klystron should be run both with and without RF input for a minimum of a half hour after the beam current has been seen to stabilize. The time to stabilize will vary with the age of the klystron, storage time, storage conditions, etc.
Once the klystron is in service, it is extremely important to operate the klystron correctly with adequate maintenance. In achieving this, three items should be stressed:
i) Stand-by mode should be with beam switched on. Leave the klystron on stand-by with filament on but beam off, for extended periods (days rather than hours), can cause damage to the cathode, resulting in premature failure of the klystron.
ii) It is most important that the collector cooler is kept clean. If the cooler becomes clogged, the collector can become overheated. Over a sustained period, gas can evolve causing damage to the cathode. Maintenance of cooling air filters is similarly important.
iii) The filament should always be operated at the nameplate voltage for a particular klystron. Operating the filament too high or too low can cause premature klystron failure. When the klystron is old, and the beam current drops due to cathode depletion, the filament voltage can be raised 0.5V without causing any harm. This will temporarily extend the life of the klystron for a short period of time until a replacement klystron can be obtained. There is, however, no guarantee as to the period of extended klystron life, if any, as this does vary from klystron to klystron depending on the condition of the cathode.
