Grumman 236 wrote:

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.

Interestingly, when I flew the plane to the airport with the
avionics shop today, the transponder worked perfectly. No
tower complained, and the reply indicator was well behaved.
That would seem to imply an intermittent antenna problem
(unless there's something in the radio that will do that.)

I took a look at the antenna connection before the flight.
It looked (to me) like a hastily done coax connection. I
know almost nothing about avionics, so I don't even know
if it is coax, but the mesh sleeve of the cable was exposed.
I thought it was supposed to be covered by the metal sleeve
of the connector itself... Maybe the installer took off too
much plastic, or maybe it got pulled out when the guy did
the altitude encoder inspection...

Anyways, I'll let the shop sort it out.