Another thing I've run into in refurbing a bunch of TR-4s and
TR-4Cs is intermittants in the tube sockets.
The tube pins get oxidized and this causes no end of troubles.
Even in the final cage- I had one that kept acting weird until
I noticed that one of the finals no longer had a filament!
On some of the sockets- especially in the receiver front end
and the modulator- it took De-Ox-Id on the tube sockets
and tube pins to fix the problem.

Dave S.