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Peavey MiniMax DC on output

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  • Peavey MiniMax DC on output

    I have a Peavey MiniMax bass amp that has DC on the output. With no speaker attached, the + and - rails are reading solid +/-75VDC, but P6 (red positive speaker connection has -3VDC on it). If I connect a dummy load (8ohm power resistor to it), the positive rail will oscillate from +75 to some much higher DC voltage, while the negative rail oscillates from -76VDC to something near 0V. The positive speaker terminal also goes from -3VDC to 0VDC during this oscillation. This oscillation continues about every 1-2 seconds.

    I've pulled the board and measured the diodes and found no shorts. I also didn't find any shorted caps, but that's not to say I didn't overlook something in my testing. Maybe U6? Any suggestions on what possible causes are or measurements to take? Thanks for your help/guidance!

    See sheet 2 of attached for the power amp schematic.

    MINI MAX PWR.pdf

  • #2
    U6 is an all in one power amplifier ic.
    At idle (no signal) it should not do anything as far as driving the output mosfets is concerned.
    If there is not a spurious input signal observed then it comes down to ic 6.
    Look at Q7/ Q8 gate pins to see if they are being driven at no signal.

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    • #3
      There are a number of possible reasons to have DC on the output;
      The saw tooth wave is not symmetrical causing an offset,
      There is a leak in an output transistor etc.
      Mos Fets usually destroy themselves when they fail under excessive current/overload conditions.
      Q6 is unlikely the culprit.
      With your scope, is the output waveform symmetrical?
      Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
      If you can't fix it, I probably can.

      Comment


      • #4
        Just put scope on gate for Q7 and Q8. Now my output is measuring a solid 75VDC. Also have 75VDC on Q7 gate and source. Looks ike Q7 is now perhaps shorting out.

        I did notice prior to this test, that sometimes the DC offset would dissipate towards 0V after about 10 seconds, and wasn't always a consistent DC offset value, but now it's a solid 75VDC. I read 185 ohms from Q7G to speaker positive. Q7D is 1ohm to speaker red, and Q7S is 0 to speaker red.

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        • #5
          Maybe Q7 has failed ... change the pair.
          Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
          If you can't fix it, I probably can.

          Comment

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