[Tom Hamilton]

Hey all,

I have some questions about silver mica capacitors. How do you know when your radio is using silver mica capacitors? Does every radio use silver mica caps or is it only certain radios? Can you tell from looking at the schematic that silver mica capacitors are used? Is there a certain value that means it's a silver mica? Is there a specific stage or application in a radio that only uses silver mica capacitors? Is there any way to tell by looking at a mica capacitor that it's a silver mica?

I think that's enough questions about silver mica capacitors.

Thanks a lot,

Tom

[Jamie]

This should answer some of your questions..

http://www.ppinyot.com/if_transformers.htm
The dreaded "Silver Mica Disease"..

[morzh]

Most radios will use them.

Mostly they are used in LC filter like IF filter. Those are typically low value (tens to a few hundreds of pF, or as some folks say uuF) high voltage.
They look like square flat potted greenish-brownish things, 3-4mm thick.
Most times they do not need to be changed as they are fairly stable and reliable except those very old "compressed micas" which none of my radios have.
They are not used as bypass caps.

PS. There are sometimes potted paper caps that look like mica caps. But they are larger values.

[exray]

Guess we need to talk about construction of caps.

In the older days there were mica caps. Leaves of real mica material interleaved with a thin metallic foil not unlike any other textbook cap.
Later on they began to 'spray' the metal onto the mica dielectric instead of having metallic leaves. "Silvered" has to be taken with a grain of salt but a silver compound seems to have been the popular choice.

Silver tends to wander away due to natural electrolysis. (If someone can describe that better feel free to do so). Sort of like that photo that you printed on an ink-jet printer ten years ago that has gotten blurry! On an individual component that's not such a big deal although I have seen open-air trimmers where the 'silver' has simply oxidized away leaving an open circuit.

The most common problem we find nowadays is in IF transformers that were constructed with a mica sheet and little pads of "silvering" to construct two or more capacitors into the circuit. The silvering bleeds across microscopically and creates havoc.

[Tom Hamilton]

Hi Everybody,

Thanks a lot for the answers. The reason I was asking the question is that I have a Philco 39-30 that has a couple silver mica capacitors and a 41-256 that has a couple as well. At least I think it's a couple. I've read that these 370 pf silver mica capacitors on my 39-30 need to be replaced with 390 pf as the 370 pf value is no longer available. I see on the parts list for this radio that this capacitor is described as silver mica. Is that usually the case with Philco radios? That silver mica capacitors are described as such on the parts list?

Sorry if I'm making anyone crazy with all the questions.

Thanks a lot, Tom

[Arran]

The mica caps in question are not silver mica capacitors but cheaply made conventional units made in house by Philco from 1939-42. They are larger, physically and in capacity, to the usual domino coded ones molded in Bakelite, and they are sealed with wax. What usually happens is that these style mica caps become leaky and prevent the push button banks from working properly.
Regards
Arran

[codefox1]

So change' em already and lets go from there.

[morzh]

If you want to remove the question of 370pF,

Buy this

http://www.mouser.com/ProductDetail/Corn...QmarKbk%3d
or
http://www.mouser.com/ProductDetail/Corn...LQc3Pn8%3d

and this

http://www.mouser.com/ProductDetail/Corn...D25dZAA%3d


And parallel them. You will get exactly 370pF.

[Tom Hamilton]

Thanks everybody for all the information. I appreciate all the knowledge and help from the phorum.

Tom