weird Mallory FP filter cap readings, need some input

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by SeanF, Nov 25, 2014.

  1. SeanF

    SeanF Member

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    so I'm looking at this McIntosh C28 pre-amp for a buddy, I check the filter caps, and some of them are reading way high. For instance, one's got 3 sections: 50/200, 2,000/20 and 2,000/25. The 50 is completely dead, but each of the 2,000's are reading nearly 3,000 uF. Their ESR is really good, so I'm inclined to leave them there, but I can't recall ever seeing electrolytic filter caps drift up that much from nominal value.

    I checked another one (this one's a GE), and they're high as well, though not by as much; the worst section on that one is a 50 uF that reads at 120 uF.

    so, do I leave 'em in? with the first one, I could scab in a 50 uF on the existing terminals and call it a day (I think I've got room for that), but I figured I'd run it by y'all and see if there's any good reason not to. ESR on all of them (except the dead one) is really good, so I'm thinking they're ok. These things are a real pain to find, especially in those values, I don't wanna try to replace them if I don't really have to.

    The pre-amp works fine, for what it's worth, the guy just said it sounded like it had some bad connections, so I'll definitely clean the pots/switches/etc., but while it's here, I figured I'd go the extra mile.
     
  2. mark norwine

    mark norwine Member

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    what sort of meter are you using to check them?
     
  3. SeanF

    SeanF Member

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    a Fluke 112 and a Capacitor Wizard
     
  4. Davidg1974

    Davidg1974 Member

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    There is a lot more to a cap than esr. They are shot so replace them all. From. What uu describe the "good" ones are 50%+ out of spec, would u keep a pt in there that was putting out 50-100% higher voltages? Cause I gurantee they are drifting even higher with heat while under a load. Change the caps.
     
  5. mark norwine

    mark norwine Member

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    If the wizard says it's good, then I believe it to be so....

    (one of the best tools I ever bought!)
     
  6. SeanF

    SeanF Member

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    good to hear, I like mine. I just don't understand how the capacitance can drift up that high; pretty sure they weren't made that way.
     
  7. mark norwine

    mark norwine Member

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    I think the issue is likely the Fluke. I've not found the cap measurements to be very accurate or useful.

    If you're an electrical verifying capacitors on HID metal halide light fixtures, it's fine. But for what we do, they leave me flat.

    These are odd-value FP caps, so "just getting new ones" isn't likely (unless / until CE perceives a need...), so if they're working, and if the ESR is reasonable, I'd say "just leave them alone for now".

    As for the 50 that's confirmed dead.....remember: never parallel a new cap with a dead one. Disconnect the old node & tie in the replacement elsewhere.
     
  8. pdf64

    pdf64 Member

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    I don't perceive that measurements made on such caps, using a polarizing voltage that's tiny in comparison to their normal working voltage, are reliable.

    Are you sure, ie what is their tolerance specification?
    I recall +50% to -20% as being a standard for electrolytic caps.
    So I don't see the discrepancy between their actual measured value and nominal value as evidence that they're unstable.
     
  9. SeanF

    SeanF Member

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    this is one of the few times that I've had to measure values that high; it seems to be ok for 100 uF and down. I've never measured a new cap that was high by 50%, so that's why I was thinking it was the cap, not the meter.

    cool, that's what I'm thinking.

    really? just to make sure the dead one doesn't short and take the circuit out with it in the future? I was thinking of using the old one as a terminal block, but I guess I can do without it.
     
  10. SeanF

    SeanF Member

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    yeah, the discrepancy leaves room for error, I guess, but how else do you measure capacitance? some dedicated high-voltage meter or something?

    not sure I've ever seen a rated tolerance that high, I was thinking it was 10%; I think that's what I found on a Mallory FP data sheet the other day.
     
  11. pdf64

    pdf64 Member

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  12. SeanF

    SeanF Member

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