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Power filter capacitors fail by shorting or opening.

Discussion in 'Electronic Repair' started by mm, Dec 26, 2006.

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  1. mm

    mm Guest

    I can't do subtle repairs, but I"m ok replacing power supply filter
    capacitors when I hear the typical 120 cps hum in the speakers.

    And I had always thought that the filter capacitors fail by opening,
    rather than shorting, because I figured there would be sparking and
    smoking if the cap was shorting.

    Is this so?

    In this case, I'm trying to fix up a high quality Panasonic table
    radio from the 70's I think. It used to give perfect sound, but the
    hum has appeared about 3 months ago.

    There are 3 16-volt 2200mF capacitors in the filter, and two of them
    are connected at the posive end to the +25 volt, and one is connected
    at the negative end to the -25 volt location. The other ends are
    connected to the chassis. Normally I cut out the old caps and replace
    them, but to learn something, I used some wires with alligator clips
    to connect a new 2200mf cap in parallel with the old one, or a new
    4700mf cap in parallel with the old two in the other case. This did
    nothing to make the hum go away. Should this have worked if one or
    more of those 3 caps were bad?

    !!I just found another fairly large cap, 1000mF, on the circuit board.
    I'll have to take the whole thing apart to get to it. Given the
    previous paragraph, is there any point to first changing any of the
    first 3 caps, which are very easy to get to?

    If you are inclined to email me
    for some reason, remove NOPSAM :)
  2. Guest

    lytic caps can go leaky, this can cause hum. But usually they go high
    ESR or sometimes the capacity falls, either of which causes hum, and in
    either case piggybacking them would make the hum go away. So its not
    certain but most likely the problem lies elsewhere than those caps,
    since piggybacking them did nothing.

    Hum is more likely to come from a bad signal ground connection than bad
    caps. You've some some real troubleshooting to do.

  3. They can short and they can also develop high series resistance.
  4. CJT

    CJT Guest

    They can even explode, spray goop everywhere, and disintegrate, at
    which point their resistance is a bit moot.
  5. DaveM

    DaveM Guest

    Electrolytic capacitors usually fail by
    (1) drying or chemical change of the electrolyte that gives the capacitor most
    of its characteristics, resulting in loss of capacitance and/or excessive series
    resistance. Sometimes, this is due to age or non-use (eqpt being stored for many
    years without being used). High leakage current through the capacitor is
    (2) exposure to excessive heat, whether external or internally generated.
    Internally generated heat is usually the result of the capacitor developing
    excessive series resistance. The current through the capacitor's series
    resistance generates heat, and will cause rapid deterioration of the electrolyte
    and failure of the capacitor.
    (3) Exposure to excessive voltage, beyond the rated voltage of the capacitor.
    The result of extended exposure to higher than rated voltage is usually
    punch-through of the electrolyte, resulting in catastrophic failure of the
    capacitor (a short).

    If the capacitor is used as a power supply filter, and the capacitor develops a
    short, then the most likely result will be overheating of the power transformer.
    If not caught in time, the transformer will be destroyed, usually accompanied by
    a large volume of pungent smoke. If the equipment has a power line fuse, the
    transformer may be saved.
    A failed rectifier can absolutely cause a capacitor to fail. Since the capacitor
    should be exposed only to DC voltage, if there is an AC source connected (caused
    by a failed junction in the rectifier) it will fail in short order. Violent
    explosion is possible in this case.
    If the capacitor fails with high series resistance, the bad effects will be more
    subtle. As indicated in your post, excessive 120 Hz hum might result.
    Sometimes the capacitor develops high leakage current (1). Bridging a good
    capacitor across the bad one can sometimes reduce the hum.
    There are other things that can cause hum in audio equipment. Open shields in
    the signal path can certainly allow induced hum to be amplified. Poor
    connections of circuit components can allow hum to pass. Poor grounding of
    components can cause hum. Missing cover plates or other chassis components can
    allow hum to be picked up.

    If this is a tube amplifier that you're asking about, then it's not uncommon for
    one of the tubes to develop a heater-to-cathode short. That will allow the AC
    filament voltage to be superimposed on the signal. A tube tester can easily
    identify defective tubes. Find a friend who has a tube tester and test all the
    tubes. Alternatively, substituting the tubes with known good tubes can quickly
    identify the bad tube.


    -- Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

    Some days you're the dog, some days the hydrant.
  6. mm

    mm Guest

    Thanks to all and thanks to you Dave for the detailed answer.

    This is a background project, and new things came to the fore today,
    so it's going to take me several days at least to do all the testing I
    see I need to do, and to read this thread thorougly as well.

    So I plan to get back to you all in a few days.

    Thank you.

    If you are inclined to email me
    for some reason, remove NOPSAM :)
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