[Collins] KWM2A cap can replacement

[Dr. Gerald N. Johnson]

On Sun, 2008-03-30 at 19:29 -0700, Rick Gouge wrote:
> Is there a source for the C106 can capacitor? I have a slight audio hum when the af gain is turn down.  Rick VE7RiK

Possibly Antique Electronic Supply or Surplus Sales of Nebraska. But a
NOS electrolytic is NOT a good buy. New production is important.

The capacitor in the radio is the least likely source of hum. Its most
often the cause of motor boating or squealing audio feedback. The power
supply is the source of 120 Hz sawtooth hum. At least the low voltage
plate supply has 120 Hz sawtooth ripple, that gets worse as the filter
capacitors age. And they do age, faster when not used than when used.

Tubes supply 60 Hz sinewave hum from heater to cathode leakage. A
significant fraction of tubes fail from that. There's a 100 uf capacitor
from the first audio cathode to ground that when it goes open makes the
first audio tube more sensitive to heater cathode leakage. Its a
miniature electrolytic that ages faster than big electrolytics.

The bias supply also can supply hum because there's some negative bias
on the audio output tube. Either filter, or the more vintage 516F-2 used
a selenium that is way beyond its life expectancy today and contributes
both excessive forward drop and excess reverse leakage to damage the
bias supply filter capacitor. Forward drop and a damaged filter (or just
poor from old age) bias filter capacitor will contribute 60 Hz sawtooth
ripple, but the back leakage may modify that wave form with a 60 Hz
sinewave component.

Tubes are in sockets because their lifetimes are shorter than
electrolytics, though in some radios electrolytics were also in sockets.

The power transformer may also introduce hum into the speaker when the
speaker is in the power supply case. The power transformer in my 75S-3B
introduced hum modulated noise in the plastic cased mechanical filter,
that I solved with a jacket of mumetal and the factory later added a
steel filter shield.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer