[R-390] Hum

[Jim Brannigan]

I have a '67 EAC R-390A with a low level hum that is making me crazy.

The hum is controlled by the AF gain control and therefore before it.  It is
influenced by the 800cps bandpass filter and disappears when V601 (1st AF
amp) is removed.

The plug in Capacitors have been replaced with new electrolytics.  C609 in
the cathode circuit has been replaced and the Rippel audio mod installed.
The tube has been swapped for another and the hum still persists.

Short of replacing every component in the 1st AF amp, I'm stumped!!

Any suggestions?

Thanks in advance,

Jim