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Yamaha B2 Swell - Volume Pedal Fault

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  • Yamaha B2 Swell - Volume Pedal Fault

    Hi everyone,

    I have a Yamaha organ on which the volume pedal has suddenly stopped working and its stuck at full volume; my knowledge has run dry in trying to sort it so i hope someone here can point me in the next step.

    It appears the pedal is stuck at full volume; though i can adjust volume with the stops, but you can hear the amp driving flat out all the time, this worries me!

    I have dismantled the pedal - it is a photoelectric type with a bulb and LDR. Ive tried cleaning it and putting a torch directly to the LDR and blocking light from it which makes no difference. I have attached a DMM though and can see resistance change from about 200 ohm to about 100 ohm when i put the torch on it.

    Ive traced the wiring, it looks as though the sound signal goes from a pre amp section, through the LDR to the Power amp section; is that about right? I seem to have continuity in the wiring and i can watch the resistance change between the 2 points..... so i cant understand why the volume remains constant? Is the LDR badly out of spec?

    The volume behaves the same no matter what register/source/stops are used.

    Any help would be greatly appreciated!

    Cheers,

    Jonny

  • #2
    OK, So faulty LDR it is. I disconnected it and all went quiet, then i temp fitted a couple of different value resistors and achieved different volumes. Anyone have any idea of the proper value LDR? I tried a 460 ohm fixed which seemed full volume, and a 270kohm, which sounded a little louder than with the resistor disconnected. Thats a massive range! Of course... something else in the circuit could be out of spec.......

    Also I've still got a hum. Volume has no effect on it. Ive read a lot about electrolytics... i assume maybe a cap in the power amp section? is the power amp section likely to be the board next to the power supply?

    Cheers,

    jonny

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