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Capacitor question - molded paper vs Molded Mica - Printable Version

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Capacitor question - molded paper vs Molded Mica - wakani - 11-28-2011

A friend, much smarter than I insists this is a molded paper capacitor and must be changed in my quest to re-cap a 46-1213. I think not, rather a molded mica and it should be ok, emphasize should. Who is correct.

Second question: As a kid I was always told that if a schematic listed a capacitor in MMF it was probably a mica. Thus a.0006 mf would be paper and the same value 600 mmf would probably be mica. Not anywhere near true?


[Image: http://i58.photobucket.com/albums/g271/wakani/cap.jpg]

Re: Capacitor question - molded paper vs Molded Mica - codefox1 - 11-28-2011

You're both right. Can't see which way the arrow is pointing on your photo, so not sure of the value. A good schematic and parts list will be helpful. I'd replace it.

Caps can be expressed in pf or with decimals as mfds. No firm rule here expecially in the old days.

Re: Capacitor question - molded paper vs Molded Mica - exray - 11-29-2011

There's a general rule-of-thumb that under 1000/.001 is more likely to be mica. No guarantees, though, because there are plenty of exceptions. One thing you can count on is that line bypass caps like .01, .047, etc will be paper. If its in an RF circuit then the likelihood is that its mica.

Would I replace this one? Maybe not on the first round unless I had reason to suspect it was bad.

Re: Capacitor question - molded paper vs Molded Mica - morzh - 11-29-2011

http://www.antiqueradios.com/philcorestorer/page6.html

This discusses mica caps. The bottom line - usually no replacement is needed, unless there's a physical damage.

Mica caps of old type were "clamped type", where copper foil was clamped together with mica sheets. They were very unstable in tolerances, as there were airgaps, and the clamping mechanism provided the pressing of mica sheets against copper sheets.

New caps are "silver mica" ones, very stable and non-degradable.

I am not sure whether pre-war Philcos used clamp type or not; post-war used Silver micas.

In any case, if no physical damage is present, and the value is where it should be, those caps could stay, unlike paper caps, that do degrade over time..

Re: Capacitor question - molded paper vs Molded Mica - Arran - 11-30-2011

There is a code that most companies used to denote a molded mica from a molded paper cap, I think maybe Aerovox and Micamold were the two main manufacturers of molded paper caps. If I am not mistaken Philco used either their own in house capacitors or Sprague brand so the chances of a molded paper cap in there are pretty remote unless someone added it, Philco models, especially post war sets, had very detailed parts lists describing the sizes and types of parts used and I can't recall seeing a molded paper type used in any of them. Molded paper caps are usaully quite large and I don't think that they made them in sizes smaller then a .001. Leave it alone unless it's obvious that you have a problem.
Regards
Arran

Re: Capacitor question - molded paper vs Molded Mica - wakani - 11-30-2011

Thanks to all, you have been very helpful.

So helpful is this site, I actually have a new addition, a Philco 50-1723 (think). Have not yet seen it but looking forward to that taking up all the free time that I do not have.

Sincerely, thanks.

Re: Capacitor question - molded paper vs Molded Mica - morzh - 12-01-2011

Is this my imagination, or is the cap in the picture actually plugged between the 120V and possibly Chassis or another 120V input wire? As the input filter?

If yes - time to change it to Y-caps or X-caps. If not - disregard.

Mike.