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?

Thanks!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"