"No production shortcuts here" as the old ad says.
100% hand wired.
It sounds like a filament is dying. The set is built on terminal strips & tube sockets. Zenith mounted them 2 ways. Either by soldering 2
extra pins on the sockets & strips to the chassis or rivets. When
soldered in they use that as a GND & they can break loose.
For now run the set & watch all the audio tube filaments for
going out or dimming when the set acts up. If thats not it
follow the squirrels sage advice. Beond that there are a few
crude tests u can do without equiptment.

73 Zeno:smoke:

I'd suspect the 6BN6 tube.