RadioBanter - View Single Post

Paul P[_2_] · October 30th 07, 02:44 PM posted to rec.radio.amateur.boatanchors

"Richard Knoppow" wrote in message
...

"COLIN LAMB" wrote in message
...
And, Phil's website is as good and concise as it gets about replacing
capacitors. See http://antiqueradio.org/recap.htm

Even if you have been doing it for years, you will appreciate Phil's nice
writing style and useful information.

And, a big fat A for taking the time to help spread his wisdom, learned
from years of experience.

Even after years of repairing radios, we learn that we can do it better.
A few years ago I was going to replace a paper cap in a 17 tube receiver.
I do them one at a time. I clipped the old one out, got a phone call and
dinner call and shut the soldering iron off and left. I was not able to
return to the work for about a month. By then, I knew there was a
capacitor out - but I had clipped the old leads off and it took me an
hour of tracing and schematic reading before I could determine where the
missing capacitor was. Now, I am more methodical and make notes before I
clip the capacitor. A notebook and pencil are always nearby to document
before I start work. Life has been much simpler since then. And, it
really is a good idea to test the radio after a maximum number of
changes.

Colin K7FM
I will add to what is on this page that the flat molded caps are often
also mica types, however, they are not silvered micas. They are simply
stacks of alternating mica dielectric and foil plates. They are not as
reliable or high performance as silvered micas and should be checked.
These were used mostly in high voltage applications or where waxed paper
had too much loss.
Black Beauty caps are not exactly paper dielectric but used paper
impregnated with a polyester. They _should_ have been low loss and quite
reliable but became notorious within a few years of their manufacture for
developing high losses. I suspect this may have been due to some problem
with the molded casing because Sprague sold capacotors with a similar
construction but in dipped epoxy casing as Orange Drops and I have not
heard that they shared the same short life problems.
BTW, while silvered mica caps are generally very reliable they can
still go bad. I've had to replace quite a few in rebuilding a couple of
SP-600-JX's due to instability. Here again all were molded bakelite cases
so I suspect either the case or perhaps end connections. Again, the dipped
epoxy mica caps do not seem to suffer from this problem.

--
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Richard Knoppow
Los Angeles, CA, USA

I discovered that any of the flat "Domino" style caps with Micamold stamped
on them are best replaced.

Paul P.