On Jun 15, 8:30 am, Road Dog wrote:
My transponder can't be seen by ATC anymore and I'm
trying to narrow down the cause and plan for the
repair. If it can be done simply by removing the
radio and taking it somewhere to be bench tested,
that'd be much easier for me. It's some vintage
of a KT76.

It can. Most shops charge for a bench test/

A couple of months ago, the altitude encoder was
inspected and found to be mostly fine. The inspector
did warn me about the lifespan of something but I
don't remember if it was the radio or encoder.
It seemed to run fine after this inspection but
I think a couple towers mentioned it was weak.

The cavity tube, probably


Recently, after a 13 minute flight, it still wasn't
visible to ATC so I don't think it's a warm-up problem.

Another symptom is that the light is constantly
flickering (although it goes solid when I press
ident.)

Sounds like the antenna's unplugged. Any chance it happened
accidentally when seat was removed for anual or something like that?
Noise will light the interrogation light if the antenna's unplugged.