hifi amp, tremolo-like distortion @ volume

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by wall_of_sleep, Feb 12, 2012.

  1. Feb 12, 2012 #1
    wall_of_sleep

    wall_of_sleep Member

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    I have a small vintage hifi amlifier that is breaking into tremolo-like distortion at volume. Dialing up either the volume or the bass affects the rate and severity of the distortion.

    The amp works great to about 75%, so I don't want to tear into it without a better idea of what the cause may be. Thanks.
     
    wall_of_sleep, Feb 12, 2012
    wall_of_sleep, Feb 12, 2012
    #1
  2. Feb 12, 2012 #2
    diagrammatiks

    diagrammatiks Member

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    it's motorboating. inadequate power supply decoupling.
     
    diagrammatiks, Feb 12, 2012
    diagrammatiks, Feb 12, 2012
    #2
  3. Feb 12, 2012 #3
    pdf64

    pdf64 Member

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    Yes, the vintage electrolytic caps are likely to be unfit for purpose.
    Time to replace all electrolytics.
    Pete.
     
    pdf64, Feb 12, 2012
    pdf64, Feb 12, 2012
    #3
  4. Feb 12, 2012 #4
    SatelliteAmps

    SatelliteAmps Member

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    Most often the electrolytics in the preamp section.
     
    SatelliteAmps, Feb 12, 2012
    SatelliteAmps, Feb 12, 2012
    #4
  5. Feb 12, 2012 #5
    wall_of_sleep

    wall_of_sleep Member

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    Thanks all. I had already swapped the filters and a couple others. Will look into completing the cap job.
     
    wall_of_sleep, Feb 12, 2012
    wall_of_sleep, Feb 12, 2012
    #5
  6. Feb 25, 2012 #6
    wall_of_sleep

    wall_of_sleep Member

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    I completed a full cap job on this amplifier and am still getting the motor boating on the magnetic jack channel at around 50% volume. There is also some hum that goes off and on with pressure on the 6sc7.

    Will continue troubleshooting, but am welcome to any suggestions.

    [​IMG]
     
    wall_of_sleep, Feb 25, 2012
    wall_of_sleep, Feb 25, 2012
    #6

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