[Boatanchors] Hum problem solved!

[David]

Wanted to share my troublesome hum problem discovery on my 75S3-B =
receiver, which may be applicable for other BA's

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 =
section 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 (for the 75S3X)
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



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