[Collins] 75S3B hum solved!

[David]

Wanted to share my troublesome hum problem discoer on my 75S3-B receiver.
Recently acquired, it came with a disturbing 60 Hz hum, even with the audio
controls turned down.  I originally suspected the known heater to grid
leakage problems with some of the tubes or bad filter caps, but neither tube
replacements nor paralleling good electrolytics with the power supply caps
did the trick.

I finally discovered that by rocking the twistlock power supply cap C59, I
could induce a variation on the hum, to point of completely eliminating it
or severely aggravating it..  On a scope, the hum component was weird, at
times it almost looked like a coherent 120 Hz parasitic on a residual ripple
waveform.  This was apparent on 2 of the three elements on the multi cap
electrolytic can.

Using my trusty ESR meter I found I could induce a variation of the reading
(and audio buzzing from the speaker) by rocking the cap (of course with the
power off).  Upon disassembly, I found evidence of very slight electrolyte
leaking at the point where the internal common capacitor foil is folded
under the outside metallic shield of the capacitor assembly (see
http://home.houston.rr.com/ddlee/cap/photo2.jpg).  I can induce degraded
capacitance effects (again by measurement with the ESR meter) by rocking the
solder tab located immediately adjacent to where the internal foil meets the
external case.

This sure was a toughie especially since paralleling known good
electrolytics with this can had no affect on the hum.

I just order one of Antique Equipment Supply's (tubesandmore.com) just
released new Sprague Twistlock multisection cans (also 40/40/40 uF , but is
1 3/8" dia vs 1") to bring the RX back to health.   Supposedly this new cap
is has just become available from AES since last month.  Hope it fits in
spite of the 3/8" larger dia.  Guess I could have also opted for a Frontier
Cap rework job...

Hope this post might be helpful to any other with this same problem

73s
dave
W5OC