[Hallicrafters] Ho Hum.....

[Mike Peron]

I experienced a similar puzzling receiver audio hum from my vintage SR-150 a couple of years ago. Noticed it most with the RF gain turned all the way down and AF all the way up. I traced it to a bad product detector tube (V6: 12BE6) that feeds the AF gain control. Since swapping it with another tube fixed the problem, I was fat, dumb and happy until I swapped a couple more well tested tubes. These other replacement tubes ended up with the same 60 Hz hum so I probed a bit deeper. Turns out V6's cathode bypass capacitor (a small unassuming 5 uf electrolytic) was shot. Replacing it solved the problem for good and now I have no hum with any of the tubes. 

A good subtle lesson that the first fix isn't always the right one. 

Mike 
WB6UTW