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Testing Caps

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  • Testing Caps

    I'm just curious if anybody has any interesting ways of testing caps to see if they're leaky? Obviously you could hook up a DC supply and take a look at it that way, but does anybody have any other clever and perhaps less invasive ways of doing it? Thanks!

  • #2


    • #3
      ESR meters ie equivalent series resistance meters for electrolytics.
      Do a search for them... loads of info about them
      Here's a good 'ol Aussie page for starters...
      Welcome to Bob's ESR Meter Page!

      and quote from

      "The ESR of an electrolytic capacitor is normally just a small fraction of an Ohm for a high capacitance, low voltage capacitor (such as a 1000?F, 16V cap), and can be as high as two or three Ohm for a low capacitance, high voltage cap (1uF, 450V). When the capacitor ages, this resistance increases, and it often does so in such a dramatic way that the equipment completely ceases to function or even blows up semiconductors. It's very common to find capacitors that have degraded to 100 times their normal resistance, while their capacitance remains fine! On a typical capacitance meter they will measure close to their correct values, but they are completely bad! This is where the ESR meter comes in: It measures the equivalent series resistance of the capacitor, almost independently of its capacitance. "

      Apart from that you get a feel for it after a while..just observing charge discharge using a multimeter and measurements in the circuit can show dc getting through.
      Anyway they are so cheap (unless your talking black gates) I often replace while I'm there particuarly if they are the older 85 deg type.
      When in doubt replace.... saves time and only a few bucks


      • #4
        I run a commercial repair bench, so keep this in perspective. I find parts are a lot cheaper than labor. At a dollar a minute to work on something, how much time should I invest in testing a $1 cap? Obviously swapping out a three section can cap is not the point, but if I think a cap might be bad, I just replace it and find out. Usually a voltmeter has already told me the tale.
        Education is what you're left with after you have forgotten what you have learned.


        • #5
          Thanks guys! I might have to get an ESR meter - I knew about the problem, though I've sort of ignored it for a while for lack of a test equipment budget. I didn't know that it's more common than a leaky cap, but that isn't hard to believe. After all, I'd rather test parts before replacing them, so I don't go pulling all 45 years of good sound out of my amp. :-)


          • #6
            Electrolytic capacitors don't mature like scotch whisky. More like fish. You wouldn't eat 45 year old sushi, and 45 year old electrolytics will need replaced.

            Of course, maybe you've got used to the tone of an amp with old, dried-out capacitors, and won't like the effect of replacing them. But that's a different issue.
            "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"


            • #7
              I'm with Enzo here & have expressed this before...I actually inherited one of those $200 cap checkers & I have still not found a situation beyond my usual substitution & logic method where this has really told me anything I didn't already know.

              In most guitar/bass amps if it's electrolytic caps & they're in a 20-30yr old amp...well, I know what I do. If you have a cap in an oscillator circuit or even a small electrolytic as a cathode bypass that you suspect...bridge it or lift one end & substitute it. The quick & dirty methods seem to work just fine & for much cheaper, too ;-]

              I just don't see the $200 cap-checker as the Holy Grail for servicers. But nice for the cap-checker manufacturers ;-]

              Mi dos centavos...glen