Determining Cap Value

Discussion in 'Tele-Technical' started by youdevil, Jun 24, 2019.

  1. youdevil

    youdevil TDPRI Member

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    i'm hoping you can help me solve a mystery. there's a capacitor in a guitar tone circuit that i built years ago and embedded in epoxy....this is now a problem because i have misplaced the wiring diagram and don't remember what the cap value is! argh.

    so here's my question.

    is it possible to tell from my ohmmeter what value cap is in there by comparing the pickup output with the pot turned off and comparing it with the pot cranked to 10?

    i'm hoping that there's some simple math here...that is, if the pickup reads 4.50 with the pot turned off, and reads X.XX when the pot is cranked up, you simply subtract X.XX from 4.50 and that somehow reveals what the cap value is....etc.

    thanks for whatever you can pass along apart from “don’t lose ur diagrams” which i hav already learned...!

    chris
     
  2. AAT65

    AAT65 Friend of Leo's Silver Supporter

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    No, not with an ohmmeter. Not reliably anyway. A capacitor looks like an open circuit to the dc voltage which the ohmmeter uses to measure resistance — once it’s charged up. In principle the time it takes to settle should give you an indication of the capacitor value... if it seems to settle in a predictable way you could compare to the settling time with other resistor / capacitor combos to see if you can estimate the capacitance.
     
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  3. BryMelvin

    BryMelvin Tele-Afflicted

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    There are many quality multimeters that measure capacitance. Borrow one from someone or get them to measure the capacitor.

    Harbor Freight has one that costs about 40 USD

    You will need to somehow isolate the capacitor out of the circuit.
     
  4. sjtalon

    sjtalon Doctor of Teleocity

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  5. kbold

    kbold Tele-Holic Silver Supporter

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    Embedded inpoxy - you won't be able to measure the cap, even with a meter that measures capacitance.

    Why do you want to know this cap value?
     
  6. youdevil

    youdevil TDPRI Member

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  7. youdevil

    youdevil TDPRI Member

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    IMG_0154.jpg Thanks for these helpful answers. Now I have another question as a result! I ended up scraping away the epoxy and successfully identified the cap: it's a yellow one stamped 393k.....which is what I thought it would be,,,,,HOWEVER....it sounds much different from, and better than, a green cap I recently purchased online, stamped '2A393J', which I'd bought because I thought it was going to be the same as my old 393k supply. (The '2A...' one sounds much darker and woolier.) So what's the difference between these two, and what technical spec should I add to my search terms in order to ensure I get more of the yellow, '393k' ones? When I search for .039 or 393k caps online, the '2A...' ones keep coming up and I know I don't want any more of those....
     
  8. youdevil

    youdevil TDPRI Member

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    Please see my answer in my general Reply to the thread. Thank you!
     
  9. preactor

    preactor Tele-Holic

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    Either one or both could be off their stated values.
     
  10. kbold

    kbold Tele-Holic Silver Supporter

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    Both are 39pF. The J and K are tolerance codes.
    J is +/-5%, K is +/-10%
    The darkness you hear is possibly that cap being at the upper end of its tolerance (i.e. slightly greater capacitance)

    The greencap is common garden variety polyester cap. Reliable, but not very precise. I'm not familiar with the yellow cap (i.e. what its made of). Looks like a metal foil cap.

    I doubt, at these values, you would hear the difference between capacitor types.
    (Making such a claim, however, can start a lengthy debate on differing opinions.)

    I use PIO caps in my guitars, because they're expensive, because I have them, and because some claim they have a superior sound to other types.
     
  11. bblumentritt

    bblumentritt Tele-Afflicted Platinum Supporter

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    There are two immediate differences.
    J = ±5% tolerance
    K = ±10% tolerance

    The green 5% cap appears to be a 100V polyester film dielectric, with aluminum foil electrode, copper ply leads and epoxy resin coating in inductive type.

    The yellow 10% cap appears to be a 50V polyester film cap, with inductive construction, using a dielectric of polyester film together with aluminum foil.

    IOW, they're similar .039µF caps except for voltage rating and tolerance. The "K" cap could be as high as .043µF and the "J" cap could be as high as .041. Not much difference... if they're both in spec. If the 5% 100V cap is "darker" then maybe one of the caps is out of spec.
     
  12. Matt_Black

    Matt_Black TDPRI Member

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    I'm intrigued as to why you embedded the caps in epoxy. This is a new one for me.
    Thx
    Matt
     
  13. youdevil

    youdevil TDPRI Member

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    No technical reason for the epoxy. It was really more of a poor man's circuit board: I was experimenting w combinations of several components and wanted to be able to move them from instrument to instrument as a single unit without worrying about wobbling the connections etc.
     
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