We had the same problem on our AT-150. What we finally found was that there
was a wire chafed from the altitude encoder that had been remotely mounted.
Over the years the wire chafed due to vibration and was grounding out,
causing a loss of signal to the transponder. A wire reroute and
re-insulating solved the problem.

Good luck


"Ray Bengen" wrote in message
...
What can make my xponder only give out mode-a and only mode-c
intermittenly.

Thanx in advance.

Ray