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Originally Posted by JimWoo50
I bought a King Kt-76tso transponder condition unknown and am wondering how to test it. It has a wire harness with it and I tried to "power it up" by connecting the black wire to a battery through the aircraft electrical sys. and grounding the other wire. No result. I had the harness attached to the encoder. Is this unit so obsolete as to render the price of testing it (shipping et al.) prohibitive? Is it even legal?
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Be careful with no antenna and powering it up, that is not advised. Also unless you really know where the wires go, DON'T connect them. You can fry a good transponder. They are very rugged and reliable but make sure you know what you are doing. A better idea is what Gill suggested.
Gills suggestion is a good one. Bring it to a shop, tell them you bought it and want to see if it just works. Chance is they will give it a quick test, check sensitivity and power with out charge.
I have bought two used Transponders (primary and backup) for 2/3rds the cost of one new transponder, and my local shop has helped me out with a quick check post-purchase. I usually slip the tech $5 for his effort. Like gill said it takes less than 5 min. Its not a certification or full test, just a functional "warm fuzzy" test. Be care full with casual homemade test as I said. There are several things that is checked but its all basic, is it making power (tube output), is it sensitive to interrogation (receiver), transmits on the right frequency and transmits the proper pulse and that's about it.
The KING KT-76 is a very popular transponder, a good design with a good reputation. Chance is if it passes a quick test, its good. The "Tube" (actually a metal cylinder) is the part that cost $600-$700. Its the thing that makes the microwave signal.
If it (the tube) works, the transponder usually works. If the tube burns out its obvious, and usually it's not worth fixing an old transponder with a bad tube, since the tube cost almost as much as a new transponder and more than a working used unit. Also once you have the new tube the unit is still old with other components which might give out, like the switches. Although not expensive, labor and parts add up.
Nothing wrong with tube transponder and that is all we had for the last 30-40 years, until the new crop of "solid state" (tubeless) units came around in the last 5-10 years. Most tubes can last +20 years or more.
Transponder questions
Try an avionics shop
Linear Mode
