I would be looking for rust or water in the RF connectors and the
antenna ground connections of the transponder antenna. The tube that
is used for the older transponders is affected by the load impedance
of the coax and antenna. If the impedance changes then the TX
frequency changes. Mine is still on frequency even after 27 years
without adjustment.
John

On Thu, 11 Mar 2004 00:49:41 GMT, Ray Godwin
wrote:

I just had a VFR Cert. done and the avionics shop said that the antenna
of coax was causting the frequency of the transponder to be out of
limits. Of coarse they had to bench adjust the frequency of the
transponder to compensate. Has anyone ever had this problem before?
Thanks,
Ray