I have an old TRT-250 transponder maintenance manual, for the ORIGINAL
analog TRT-250 (not the 250D digital version). This transponder is no longer
supported by Free Flight Systems and I suspect few shops take them in for
repair, as some of the parts are simply not available (most importantly, the
thumbwheels). If there is interest in this manual, I will post a PDF of it
on my web site. It's got all the schematics, etc.

Juan