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Thread: Need schematic for Yamaha PS 55 keyboard

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    Senior Member fredcapo's Avatar
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    Need schematic for Yamaha PS 55 keyboard

    Most functions seem to work, individual volume controls function except for the master volume control which has no effect. The amplifier sounds thru the speakers, but it is quite low level. Schematic section covering the audio stages pribably all that is needed. Thanks

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    Lifetime Member Enzo's Avatar
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    If that. The MV is a slider. It controls DC, does the wiper go up and down DC?

    The actual control of volume is through optocouplers.
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    Senior Member fredcapo's Avatar
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    Enzo thanks.I dont see any DC on what I think are the pins on the circuit board coming from the master volume slider But if i touch DC to one of those pins the volume comes up full on both channels. so it looks like no DC going and/or coming to the slider. I looked again and see the area of interest on the schematic and will try to localize the problem. Thanks

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    Last edited by fredcapo; 03-29-2018 at 07:32 PM.

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    Lifetime Member Enzo's Avatar
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    I can't call it up at this moment, but seems to me the slider has DC across it, and the wiper makes a control voltage for the volume circuit. So look for the source of (probably) +5v on top of the slider.

    OK, I went and got it. +2.5vDC. The other sliders work the same, so look at other volumes or tempo control, is there 2.5v on the tops? If not, then they all should be dead. If they work, and have the voltage, then look for the interruption between that 2.5v source and the Master control.

    The 2.5v supply is a simple transistor regulator off the +5 supply. Lower right corner of schematic.

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    Senior Member fredcapo's Avatar
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    Thanks. Yes its pretty clear on the schematic you provided...just needed to expand the print

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