[R-390] bfo problems

[Kenneth G. Gordon]

> My r-390a has a slight hum when the bfo is on. Cw and ssb signals have
> a slight "vibrato" sound. Cw notes are not clean and crisp with any
> setting of the mgc level. I suspected one of the filter caps, but they
> are ok. The decoupling cap on the bfo 150v b+ is ok , there is no
> cathode to filiment short. I turned off the bfo switch, and injected a
> 455 khz carrier from my urm-26d signal generator to pin 5 (plate)of
> the tube socket. I still had the hum with the signal generator acting
> as the bfo. The radio sounds fine listening to a m broadcast. Any
> ideas ???  Thanks .......Steve.....KJ8L

Substitue the HF oscillator tube and/or any heterodyne/conversion 
oscillator tube. I have an HQ-110-C which did the same thing. 
BOTH the HF (tunable) oscillator tube and the 2nd conversion 
(crystal) oscillator tubes had high cathode to filament leakage. My 
TV-2 said they were both all right. Cured it by substituting new 
tubes. Afterwards, I found a service bulletin from Hammarlund that 
explained the problem, which was apparently not uncommon.

Ken Gordon W7EKB