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Peavey M-3000 Compressor Help Please

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  • Peavey M-3000 Compressor Help Please

    I have a Peavey M-3000 that was working and then stopped working and the Clip/Compression light came on and remained on.

    I pulled the 87478 chip and did some checking:
    I have less than 5mV on the output without speaker load. As the amp is very similar to the 400 B/H board, I checked the voltages listed on the BH400 schematic against the M3000 board
    and they they are very close (within 2 tenths of a volt). When I apply guitar input without a speaker load and turn up the volume, the LED meter shows signal at the output.
    I also tried another TL074 and the clip light remains on.

    I am somewhat stumped as to how to proceed with trouble shooting. The Clip light comes on without signal and very little DC on the outputs.
    I'm kind of down to Q1, Q2 and Q4 as the culprits.

    Can someone please provide some guidance how to proceed with this problem.

    Any ideas and advice is greatly appreciated.
    Attached Files

  • #2
    So the problem is low output?

    You pulled the compressor IC and no help, so leave it out for now. Did you remove Q1 to see? Q4 won't be it because it controls the compressor IC. Got voltage to the gate of Q1? Look up by Q2, that is all the more circuit there is to control the two JFETs.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      I plugged in a guitar as the input and turned the volume all the way up and the amp does work (outputs signal) to the speakers.
      The volume was low but appropriate for an instrument level input and the sound was clean (no distortion detected by ear at least).

      As for the voltages:

      the gates of Q1 and Q2 are at +14.5V
      The - side of C2 is at -0.87 V
      The + side of C6 is 0.0V
      The + side of C9 is +0.006 V

      Does that make sense?

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