Thanks for all the tips folks, I actually found the problem and was returning to post the answer. The IEC electrical cord socket mounts directly to the PCB on this amp (actually a small PCB dedicated to the mains switch and such). The board mounted IEC receptical must have been knocked around some, no doubt from strain on the end of the electrical cord plugged into the amp while the amp was transported and gigged for years. One of the solders or traces lifted on the IEC socket and blew a trace right there at the socket. I couldn't see it until I started removing all of the boards. J M Fahey's suggestion of tracing out the path starting from the electrical cord pin would have worked perfect to diagnose this.