[R-390] R-390 (non-A) Audio Hum

[nryan at mchsi.com]

Gentle Readers,

An annoying hum, audible especially with strong signals, had bugged me for some time, so tonight I took action to resolve it.  Tweaking the hum balance control R-614 did not help.  Swapping with a known good audio/voltage regulator deck narrowed the problem to that module.

Tubes on the faulty module were good, but 180 V B+ was low at around 125 V.  Some time ago I had replaced out of spec resistors, so I turned to checking capacitors.

C-607, a leaky mica 100 pF cap was at fault.  Who knew it would come down to a seemingly minor component like that?  I adjusted the hum balance with a sensitive AC meter to get the reading close to zero.

The R-390 sounds great now with no hum.

73 de Norman, KG4SWM

-- Any activity becomes creative when the doer cares about doing it right, or doing it better. -- John Updike