<VV> FW: Radio problem

Tony tony.underwood at cox.net
Wed Dec 26 11:32:29 EST 2007

At 11:23 AM 12/24/2007, Dennis PLEAU wrote:
>To: ddpleau at msn.comSubject: Radio problemDate: Mon, 24 Dec 2007 
>10:55:10 -0500From: whaledrvr at aol.comIt sems that your volume 
>control is the problem. Squirt the inside of the control with 
>electronic cleaner. The control uses a carbon based resistance which 
>gets dirty with age.Ralph

For what it's worth:

There's a capacitor attached to the center tap (wiper) of the volume 
control.   It's a blocking capacitor, there to keep DC off the 
control wiper seeing as how the wiper contact is feeding audio to the 
base lead of a transistor which has some DC potential on 
it.   Sometimes this capacitor, after a few decades, tends to become 
internally "leaky" and can cause a small DC potential to find its way 
into the volume control which can aggravate the scratchy scrunch noise.

If for some reason the radio is on the bench apart and you have the 
knowledge and expertise to replace the capacitor (along with any 
other original electrolytic capacitors while you're at it) you should 
do so.    These radios are really getting old now, and the 
electrolytic capacitors as installed in the 1960s are certainly 
getting long of tooth and they do not last practically forever, like 
resistors and ceramic disc capacitors tend to do.

...when I do one of these original Delco radios, I generally 
arbitrarily replace every electrolytic cap I can find, jus' 'cause.

The bigger can-type multiple segment capacitor is severely solidly 
mounted on the metal chassis and usually too much trouble to simply 
replace, so I shunt it with tubular caps, which works in most cases 
since this can-type capacitor generally tends to just dry out and 
lose capacity, rather than develop "series leakage" the way some of 
those smaller electrolytic caps sometimes do.

An easy way to find out for sure if the volume control coupling 
capacitor is leaking is to check for DC voltage on the wiper contact 
(the middle one) of the volume control while turning the volume 
control up and down, peg to peg.   If there's any DC voltage showing 
up, any at all, and a fraction of a volt is too much, there's a leaky 
capacitor somewhere.

Now, it's a ganged control, with two segments on the same assembly... 
the volume control segment is the one at the back end; the one closer 
to the front of the control is the tone control segment.    While 
you're at it, shoot some light oil into the tone control segment as 
well...  WD-40 will work as well as most anything else calling itself 
"contact cleaner" these days.

Now:   If a shot of WD-40 fixes the scratchy control noise, you're 
probably OK for now.   If the shot of cleaner-lube does NOT get rid 
of the control noise, it's not unlikely that there are some 
deteriorating capacitors in there somewhere... and it's probably best 
to replace them while you're there.   Now, it does require that you 
have some electronics experience, particularly in the area of 
component identification and replacement...  don't forget that part.


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