Tried that today Walt. No joy!
Anyone got a schematic in e-format?
I may take you up on that Walt. Problem is I don't want to spend much on a repair of a $500 box.
I think the tube is good if what Gil says is true. I get a really good bright reply light when I press the ident button.
would still like to have a schematic and even better the maintenance manual if for nothing more than education.
The little orange "I am being interrogated" light is activated solely from pick up of the RF component of the output circuit. There is no physical connection to the output circit
If that is pulsing in flight then you are at least generating some RF signal. If it lights when Ident is pressed then the tube is not totally dead.
If the tube is dead, then replace the antique technology with a new solid state one...
The reply light will light when it receives an interrogation and is an indication that the transponder has decoded the signal and is replying. However, it does not indicate if the transmitter is actually transmitting anything. In fact, the reply light will still function with the cavity tube removed. Same situation with the Ident.
Hearing the T-whine from the power supply is good but not a guarantee that all the proper voltages are present.
If the antenna and coax are OK, and the power supply voltages are correct, I'd suspect a problem with the receiver front end or the LSI chip.
On the schematic I saw the reply light was driven by a small wire pick up next to the RF cavity. A diode turned it into a dc signal to drive a physical lamp, with a light sensitive resistor providing the variation due to ambient light. I remember being impressed by the simplicity of the circuit.
Are all generations of the KT-76A wired as you say, or did I read the schematic wrong?
Isn't the KT-76A transponder old enough to have early versions pre-LSI chips?
Sounds like an Encoder problem.