Today's Mystery Guest (RCA CTC-4)

[ChrisW6ATV]

I wonder if the capacitors in T115 could have gone bad ("silver mica disease" or similar)?

Or, if that transformer was sufficiently mistuned enough, would it kill the oscillator? Maybe, count turns (so you know where you started) and run its slug all the way from end to end and see what you get. I think if you got your scope connected to that circuit's output, you would see quite clearly if/when the oscillator started running.

Among the repairs I did on a CTC-4 several years ago, I replaced two 6AZ8s. You already tried that, though.