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  • Peavey MP-400

    Greetings,

    An MP-400 that I'm checking has distorted audio which increases as the volume is increased anything above a low setting. It appears to be happening in the power section, although there seems to be plenty of power. No indication of shorted components; voltages that I have checked have been normal; DC voltage on the output is low (0.6 mV). I have injected the signal directly to the board so as to eliminate the pre as a source. I don't have a scope to see what is happening here. On the power board I found the 5W resistors needed the solder joints touched up, and I tested the caps at both ends of the signal path. Where do you suppose I should look for the problem?


    Thank you for taking the time,

    R.
    Attached Files

  • #2
    What are the voltages on your output transistors Q3 & Q4?
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #3
      Originally posted by The Dude View Post
      What are the voltages on your output transistors Q3 & Q4?
      Sorry it took so long...

      ~+/-35VDC

      Comment


      • #4
        Originally posted by soundinvst View Post
        Sorry it took so long...

        ~+/-35VDC
        Please supply the voltages on the base, emitter & collector pins.

        Comment


        • #5
          Agree and add: we need 4 voltage measurements for each transistor:
          * From ground (black test probe) to each emitter/base/collector , that makes 3 per transistor.
          * from emitter (black probe) to base (red probe) , thatīs the fourth.
          Thanks.
          Always amplifier on, no signal applied, all controls on 0.
          Juan Manuel Fahey

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          • #6
            Originally posted by J M Fahey View Post
            Agree and add: we need 4 voltage measurements for each transistor:
            * From ground (black test probe) to each emitter/base/collector , that makes 3 per transistor.
            * from emitter (black probe) to base (red probe) , thatīs the fourth.
            Thanks.
            Always amplifier on, no signal applied, all controls on 0.
            Q3 (A1941)

            1. base 34.9
            2. collector -0.1
            3. emitter 34.9
            base to emitter 0.0

            Q4 (C5198)

            1. base -34.9
            2. collector -0.1
            3. emitter -34.9
            base to emitter 0.0

            Both collectors have neg. voltage. Measurements taken as instructed with no load.

            Thanks!

            RM

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