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audio output distortion that corrects after 20 minutes

Discussion in 'Electronic Repair' started by mikeh, Feb 14, 2007.

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  1. mikeh

    mikeh Guest

    This is a piece of music gear, a Roland MKS-20 piano module, which has
    an audio output problem where the volume of both channels is low and
    very distorted when the unit is first turned on.

    Gradually, over the course of 15-20 minutes, the volume surges up and
    down, and the distortion becomes less and less. After the 15-20
    minutes the distortion is gone, and the volume stabilizes too.

    Both the left and right 1/4" jacks and the stereo headphone jack
    experience the same problem, but the problem corrects usually quicker
    at the headphone output.

    Can anyone suggest where I might start to look for the cause. I've
    thoroughly cleaned all the outputs already, even though that doesn't
    seem likely to be the cause.

    I'd appreciate any suggestions.

  2. Push-pull amplifiers can suffer from crossover distortion. It is a bias
    issue ... some modest forward bias is required to reduce this distortion. A
    simple check: is the distortion much worse at low volume levels? If yes,
    then it is probably crossover distortion and you will have to investigate
    the forward bias circuit for the output transistors. If no, then it could
    be any number of things such as a bad electrolytic capacitor, a transistor,
    and so on. Not an easy one to fix, by the way.
  3. Arfa Daily

    Arfa Daily Guest

    I wouldn't have expected an electric piano to have contained a power amp. Is
    this a power issue we are talking here, or low level audio output to an
    external PA amp ? Either way, it should be easy enough to track the problem
    down with a hair dryer and a can of freezer.

  4. Meat Plow

    Meat Plow Guest

    It's a rack mount piano MIDI module.
  5. Since it's common to both channels I'd first look at the power supply
    rails feeding the audio amps - sounds like one leg starts off low then
    gradually comes good, so likely a smoothing or decoupling capacitor.
  6. Eeyore

    Eeyore Guest

    Sounds like the power supply to the audio output to me.

  7. mikeh

    mikeh Guest

    Thanks, distortion is just as bad at all volume levels. And at all
    outputs too: there's a headphone, L/R Hi-Z and L/R XLR. Sounds like
    this one might be a bear.
  8. Eeyore

    Eeyore Guest

    No, it's simple. Find where the volts are dropping.

  9. mikeh

    mikeh Guest

    Thanks, it does sound like a good place to look first. Lucky me, I
    found a source to get the service manual.
  10. mikeh

    mikeh Guest

    Thanks Graham
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