Aaron Coolidge writes:

Jay Honeck wrote:
: The third digit on my formerly bullet-proof Narco AT-50 transponder has
: suddenly started displaying incorrectly for ATC. Specifically, it seems to
: be showing as a "4" on their screens, regardless of what we dial it to in
: the plane. Thus, instead of squawking "1200" when we're VFR, we're
: squawking "1240"...

: What could cause this? Any recommendations on repair versus replacement?

Jay, as others have said you've probably got a problem with the wafer switch
that the knob actually turns. A decent avionics shop will be able to take
a wafer switch out of a junk AT50 (oxymoron?) and replace yours.

If Jay owned an experimental airplane, I'd suggest he got some contact
cleaner at Radio Shack, take the xpndr out of the plane, open it, and
spray some cleaner onto the suspect switch, and then go fly test it.

But I can't, since he owns a certificated airplane (which makes him
certifiable).

-jav