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Problem testing very low value capacitors

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  • Problem testing very low value capacitors

    I'm trying to solve a problem with the Fun Master board in my Conn 650. On this board is a 0.0047 uF capacitor and the only success I have had with testing this is with the capacitance test. My ESR meter will not return any information and neither will my very expensive Gossen Metrawatt Metraport 3a analogue multimeter. Having at first thought that this capacitor must be faulty, I took two more with the same specs from different boards, but got the same lack of results. All three can't all be faulty, so it appears that none of my testing instruments are able to test capacitors with such a low value. I could go and buy some new ones, but if they turn out to be the same, I won't know weather the're OK or not. I am waiting for an expensive capacitor tester to arrive from Hong Kong, but as there are no flights from there to the UK at the moment, it has been sent by sea!! So I might have to wait until the end of December to get it and even then, there's no guarantee that it will solve this problem. In the meantime, have any of you techie guys got any ideas on how to test these low value capacitors? Any help would be greatly appreciated.

  • #2
    Remember that your connecting wires can have a capacitance so keep them short.

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    • #3
      Rather than trying to measure, I'd try substitution. The exact value might not be critical to the circuit, but without a description of your problem and a schematic that's an unknown.
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      • #4
        Just having two test leads together there is capacitance between them.Have you tried just sticking the cap leads directly into the meter?
        Can't play an note but love all things "organ" Responsible for 2/10 Wurli pipe organ, Allen 3160(wife's), Allen LL324, Allen GW319EX, ADC4600, many others. E-organ shop to fund free organ lessons for kids.

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        • #5
          ESR meters are designed to test electrolytic capacitors, and they rely on the fact that the capacitive reactance of a large capacitor is very low compared to its ESR. Some state in the documentation that they are not for anything under 1uF. They are useless for testing a 0.0047uF capacitor because it's far outside their range. The Gossen unit, as far as I can tell, is not designed to measure capacitance at all.

          My Fluke multimeter has a capacitance test function that would check capacitors in this range. Quite a few digital multimeters do.
          I'm David. 'Dave' is someone else's name.

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          • #6
            I've been using a Philips ECG CX-920 capacitance meter. It has a 200pf range. It's an older meter, but works very well on small value pf and uf capacitors.

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            • #7
              Thanks for all your comments. You've confirmed what I was beginning to think, that such a small value capacitor would not give satisfactory results. But, you know, I had no trouble checking the capacitance, it was the charging and leakage I wasn't able to check on just this one capacitor, all the rest produced results. One thing I would like to know though, is how long should it take using x10K ohms on an analogue multi meter and a good 250 uF 35 volt capacitor, for the needle to show from a full scale low resistance to a full scale high resistance? With the ones I've tested, it takes up to 8 or 10 minutes. Does this sound about right? What about these results, do they seem OK? uF 312, ESR 0,175.

              I have a 628 manual and a 650 manual, and what I've noticed reading these two manuals and studying the circuit diagrams of the Fun-Master, is the number of inconsistencies there are, between the theory of operation and the circuit diagrams, even between the two manuals. How anyone can work out how the system is supposed work with these inconsistencies is a real problem.
              Last edited by ukmusicman; 10-31-2020, 06:51 PM.

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              • #8
                Originally posted by ukmusicman View Post
                One thing I would like to know though, is how long should it take using x10K ohms on an analogue multi meter and a good 250 uF 35 volt capacitor, for the needle to show from a full scale low resistance to a full scale high resistance?
                My question would be: Why does this matter? Most analog multimeters don't apply sufficient test voltage to a capacitor to reveal leakage that might occur with full working voltage. Capacitor leakage is not necessarily linear with voltage.

                One thing that's starting to frustrate me more and more on these forums -- because I see it more and more -- is people using electronic test equipment incorrectly. People seem to burn up hours and hours of time worrying about readings that aren't particularly diagnostically useful. A multimeter, digital or analog, will only reveal DC leakage on a capacitor that's failed pretty catastrophically.

                Past a certain point, if you're that worried about a capacitor and you don't have proper test equipment, just replace it with a new one.

                With regard to schematics, some companies documented their products much better than others. With some products, no schematics exist. At that point, you simply have to rely on an underlying understanding of circuits to work out the problem. For example, I had to fix an Ace Tone amp with no schematic, but it's similar to the Leslie and early Rhodes SS amp circuits.

                I'm David. 'Dave' is someone else's name.

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                • #9
                  Capacitors are mere pennies, well maybe a bit more, but it's easier to just replace the suspect. *Bay is great for buying onesies and twosies of small parts. Same with IC's. If I suspect one - I just buy another, it may take a few days to get it, but I 'll have an answer to my question.
                  Can't play an note but love all things "organ" Responsible for 2/10 Wurli pipe organ, Allen 3160(wife's), Allen LL324, Allen GW319EX, ADC4600, many others. E-organ shop to fund free organ lessons for kids.

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