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Avionics transponder aircraft instrument. tube tester
This is a new test set is designed to test and evaluate the transmitter tube found in most existing general aviation transponders. Except for some modern all solid state units, most transponders rely on a vacuum tube output stage. This tube, known as a cavity, is expensive to replace. It may cost more than the value of the transponder.
This test set will enable the technician to evaluate the tube, even in an otherwise dead transponder. This saves time spent fixing a transponder which turns out to have a weak cavity. Some tubes may work, but be close to the end of their life. These weak tubes can be detected easier and with more certainty, using the test set, than with the usual bench test proceedure. This test set will help avoid returning these unreliable units to service. Valuable, usable tubes may be lurking in junk transponders. These can be identified, salvaged, and used to repair other units.
I designed this test set because I needed it. I was always surprised at the way transponder cavity tubes seem to die suddenly, sometimes just a few days after bench testing OK. You would think that the tube would gradually lose power over a period of time, but this is not the case. In the transponder, you cannot judge the condition of the tube. It keeps pumping out power until the end.
After some research and experiments, I came up with a method of evaluating the cavity tube and its likely remaining life. This test set allows you to manipulate the operating voltages of the tube while observing the resulting output on a scope. Using this setup, you compare a new good tube with the questionable tube and see the difference. Poor tubes are easy to spot. Similar methods are used to evaluate high power tubes used in industry. Why not apply these methods to avionics tubes?
