[Boatanchors] Capacitor question

[J. Forster]

Hi,

The outside foil marking is important for stray capacitance and shielding
issues.  Think of it as a capacitor in a metal box, with the box connected
to the outside foil terminal.

The capacitor is quite probably OK, but if you want to recap it a film cap
with the same specs, connected the same way, should work just fine.

Because the capacitance may not be identical to the existing cap, you may
have to recal, especially on the AC ranges.

Best,

-John

==================






> Hi all, I hope everyone is having a nice Sunday (or whatever day it is at
> your locale).
> I have a question about an old capacitor. I'm currently freshening up an
> Eico 249 VTVM. It was my Dad's, was working when last used about 10 years
> ago, and still does although it's way out of calibration now. I'm
> attempting to restore it electrically for daily bench use. Of course, one
> of the repairs I'm making is to replace most all of the caps. Here's where
> the question comes in.
>
> There is a .1 mfd, 1000 volt capacitor that has no brand name on it, but
> is
> marked "Type 85TOC". It is rather large and cylinder shaped, heavily
> coated
> in wax, and appears to be made of yellow-colored paper or cardboard. In
> the
> schematic, it is connected directly to the test probe in certain function
> switch positions.
>
> Here is my question: one end of this capacitor is marked "Outside Foil". I
> can't say I've ever seen this before. Could someone enlighten me about
> this
> marking? I understand how this capacitor is built, likely two long foil
> strips seperated by a dielectric strip and all rolled together. But why
> would one need to know which foil strip was the outside one? It doesn't
> appear to be an electrolytic capacitor as there are no polarity markings.
>
> I can get a very reasonably priced replacement film capacitor from Mouser
> with exactly the same specs; will that work as a replacement?
>
> Thanks much in advanced! I should mention I'm not new to electronics but I
> am fairly new to Boatanchors. The bug bit me about a year ago...quite hard
> ;-)
>
> Chris Bower
> N8NW
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