Earlier, (Jay) wrote:

Whatever option you pick, don't buy a
piece of avionics with a "valve" in it.
Its got a limited life for sure. Do
they even make those anymore?

I disagree with that advice. It excludes the vast majority of
transponders on the used avionics market, many of which can provide
excellent value.

All the old Narco AT-50, King KT-76, and Cessna transponders have
cavity tubes. Yes, the tube has a limited life. And, yes, cavity tubes
are expensive. However, for most units that lifetime is several
thousand hours.

But when your choices are between a $1100 used solid state transponder
and a $500 used KT-76, you have to think carefully about what that
extra $600 is going to buy you. Spending it only because solid state
stuff is "cool" is sort of silly. Spending it because you actually
have a requirement for the benefits of solid state design can be
smart.

I've bought two used transponders in the last five years, and both
have worked out very well. The first unit was a used but nice looking
KT-76 which I bought on the Internet for a beater airplane. We closed
the sale at an avionics shop in San Jose after a bench check looked
good. The other unit was a used Garmin GTX-320, which I also bought on
the Internet. I chose it for my glider because I specifically wanted
its low power consumption, and was willing to pay extra money for it.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24