I recently bought a Drake TR-4C / RV-4C / AC-4 station
which came from an estate. The gear is nice and clean
and appears to be 100% stock.

After I rebuilt the power supply and replaced a couple
of tubes, the station is operational with the exception
that I have no push-to-talk (PTT). VOX works fine on CW
and SSB. I replaced the 6EV7 tube, and that didn't help.
I tried two different microphones and got the same result
with both of them. Both mics work fine with my B-Line,
so it's not a mic wiring or switching problem. Before I
dive in with a voltmeter, does anyone have any suggestions
on what may be wrong, perhaps based on personal experience?
And yes, I'm aware that the CW key must either be closed
or unplugged for PTT to function.

Thanks and 73,
Dean K5DH