Potentiometer on synth affecting things it shouldn't

Discussion in 'The Projects Forum' started by sharevari, Aug 28, 2011.

  1. sharevari

    Thread Starter New Member

    Aug 18, 2011
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    I have a relatively modern (around 10 years old) virtual analogue hardware synth, i.e. digital but emulating analogue sound in software, and it has developed a problem.

    There's a knob for changing levels of individual sounds, and next to it a row of 4 push buttons for selecting which of the 4 sounds the knob controls. Now, when selecting one of the 4, it often happens that turning the knob also affects one or more of the sounds that are not selected. Like some sort of spillover effect.

    This only just started happening recently.

    I opened it up, and everything looks very clean and tidy inside, nothing visibly wrong around the area of the knob.

    With my so far very limited knowledge I'm trying to get an idea of what could be causing this. Is it likely to be fixable by just replacing the knob, or does it sound like something more tricky?

    Thanks
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
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    I would check for oxides and dirt inside the pot and switches.
    But I need you to tell us more of the sort of pots and switches that is used.
     
  3. sharevari

    Thread Starter New Member

    Aug 18, 2011
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    As I'm still very much a beginner at this, I thought I'd post a pic.

    The potentiometer in question is the one on the top left, and the switches that control sound selection are SW21-SW24. The pot has a label B5K but no other markings.

    [​IMG]
     
  4. JingleJoe

    Member

    Jul 23, 2011
    185
    10
    The synth elitist in me says, you should have got a nice old analog synth!
    However this seems like your selection buttons are operating more than one selection, does it occur after the button is pushed or just when turning the pot? If the buttons are connected together (kind of like in a remote or video game controller D-pad) then maybe they have become sensitive to neighbouring buttons being pressed?
     
  5. sharevari

    Thread Starter New Member

    Aug 18, 2011
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    Hehe, the synth elitist in you may be right, but these things are damn good value for money and sound really good.

    Yeah, that's a good theory about the switches, I'm not sure though, as never more than one of the LED indicators light up. The behaviour I'm seeing is that when selecting patch 2, the knob operates only the patch 2 level, but when selecting patch 1, it operates both 1 and 2.

    Actually, I should do a few more tests to see whether it could be simply switch 1 accidentally enabling switch 2.

    (Another thing, revealing my noobness here, when checking the resistance across the pot, the two outer connections always have a resistance of 200 Ohm, but the left-middle and middle-right vary along a much wider range, something like 0-2 kOhm, with the highest resistance when the pot is in the middle. I thought a potentiometer always divided up the resistance between 0 and full, and that it increased linearly from left to right? This one follows some sort of parabolic curve.)
     
  6. JingleJoe

    Member

    Jul 23, 2011
    185
    10
    Potentiometers usually only have 3 connections (I have one with 4, the fourth was a center tap to the middle of the resistive trace)
    to test the resistance properly one would have to remove the pot from the board entirely or you're testing it in paralel with the circuitry.
    If the switches are connected together perhaps separating them may work? However one may not want to chop up bits of one's synth lest you risk permanent discombobulation.

    Additional info about potentiometers;
    They are not allways linear, some have logarithmic resistance response and commercial logarithmic pots aren't even logarithmic! They have some weird sectioned trace with a high resistance bit and low resistance bit.
     
  7. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    I think you're barking at the wrong part. Pots aren't that smart. My suspicion is that the brain chip has gone bad or you've read the instructions wrong and left channel 2 slaved to channel one.

    Be sure to read my signature line just below this.
     
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