William Mutch wrote:

Does the loudness of the hum vary with the volumn control?

only slightly. the main hum injection seems to be after the
pot... still there even with audio gain all the way down.

Before doing anything else, substitute the audio output tube.
As I recall, that's a hot chassis radio with a 50C5 output tube, so
also check the power line filter caps and the the rubber isolation mounts
that keep the chassis isolated from the case.

I can't seem to get the schematic off of BAMA... it tells me that I don't
have permission. So I am not sure if the audio gain pot tap goes right
to the grid of the 50C5 or not... but it might well.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."