wrote:

Hey all... just got official confirmation that my KT76's power output is way
low.... classic sign of weak/dying/dead cavity tube. Just looking for suggestions on
what might be a good, cost-effective replacement. I'm tired of futzing with
transponders so I'm pretty sure I want a solid-state replacement.

I just had a thought too (for any avionics techs reading)... perhaps the lack
of DME suppression lead to the premature demise of the KT76? I was going to wire it
up when I installed the DME, but the KT76 doesn't have that capability. Maybe the DME
cooked it?

-Cory

Nope, the external suppression, on transponders that have it, just
blocks off the transponder reply so that it doesn't send out a reply in
response to the DME interrogation pulses. Its only purpose is to cut
down on unsolicited replies going back to the ATC interrogator.

More likely, your cavity just got tired. The cavity is a tube used to
obtain the required transmit power. When these transponders were new,
it was the only reasonably priced technology available for generating
200W RF power in the frequency range. Being a tube, it eventually
weakens with age.

Most of the new transponders have semiconductor RF amplifiers in the
transmitter. The reply circuits are also completely digital, using
quartz crystals to derive the pulse timing. Transponders of the Narco
AT150 vintage which used resistor-capacitor combinations to generate the
pulse timing for both decode and reply generation, and therefore require
calibration and are prone to drifting out of calibration. Won't happen
with the newer transponders.