RST Engineering wrote:

The encoder working or not working has absolutely nothing to do with the
reply light.

That's why I'm thinking it's the antenna.

Why don't you consider running some tests before shotgunning parts that may
or may not be bad?

Like what? I had the King KT76A benched-checked and it came through
with flying colors. They checked the impedance of the antenna and found
that it was low (high?) but within tolerances. Cleaning some contacts
improved it slightly. That's probably why they suggested I replace the
antenna.

Would you know if a bench check would tell them if the transponder tube
took too long to warm up? Since I had to fly the plane there, it would
have been "warm" already.

Marco