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Strange fluttering hum out of an A100

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  • Strange fluttering hum out of an A100

    Hi there!

    Does anybody knows this problem?

    A strange noise, like fluttering hum, comes out of an A100.
    With or without Leslie. Only on Volume normal, volume soft there is no problem at all.
    The noise occurs at the last quarter of the swell pedal​.
    I think, the problem is in the AO28.

    Thanks for any input.

    Attached Files

  • #2
    I would not call this fluttering, or a hum. I would call it a periodic thump, and it definitely sounds like it matches the rate of the vibrato scanner.

    If you disconnect the D terminal does it still happen? Is the swell capacitor touching the metal chassis on the inside the dog house?


    • #3
      Thanks! I will disconnect the D terminal. Also look inside the dog house.
      The periodic thumb is also gone, when the swell pedal is complete full opened. This seems It looks like the swell capacitor inside the dog house is not adjusted​.
      The problem is only on the last quarter and gone, when the swell pedal is full opened.


      • #4
        I only brought up the swell pedal because if the swell capacitor touches the dog house (a common problem when mis-adjusted) you'll get a single thump and a significant loss in volume. That alone won't necessarily cause a periodic thumping, but if there's a problem with the vibrato circuit of the preamp or the scanner itself, it might amplify those problem if they are ground-related.