[R-390] R-390 Digest, Vol 81, Issue 10

[Dennis Gibbs]

Norman,

Congratulations on correcting this problem.

I have run into this many times on R-390 (non-'A') models.  I have also
found the cause to be a leaky C-608 as well, with the same exact symptoms.

Dennis

==========================================

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