[Boatanchors] Hum sources?

[Ian Wilson]

I'm working on an AN/TRR-5 (Espey R-366) morale radio.

The otherwise rather nice audio system is outputting more hum
than I think is normal. The level of hum is independent of the AF
gain control setting.

The B+ filtering - my first suspect - is very clean, so it appears
that the electrolytics are in good condition. There is a 3rd stage
of B+ filtering going to the low-level gain stages in addition.

Looking at the schematic, the only places where I can see hum
being injected are the first audio amplifier (1/2 12AT7), the second
audio amplifier/phase splitter (12AU7), or possibly the muting
switch from the bandchange switch to the o/p tube grid. Possibly
a screened wire has become detached from ground.

Anywhere else? For some reason the power cable has a 2-pin
plug on it, but the receiver schematic shows a ground connection
(and I can see the wiring from power socket to chassis ground).

I can try substituting the double triodes this evening (would this
mean heater/cathode leakage?) but wondered what the most
likely source is - and whether I am overlooking something.

73, ian K3IMW