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Peavey 400 70s

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  • Peavey 400 70s

    Have an older 70s 400 model here. This is the one with reverb/trem/distortion ...and the 8 RCA output transistors , i will post a pic.

    Everything works but , with the Master volume on 0 , i have a pretty loud gain hiss. Sounds like a high gain tube amp with the gain on 10 , but all volumes are off. The tone isnt distorted , but man its a bad gain hiss .

    Ive chopsticked it, looked at everything with a laser temp gun ...nothing is hot . I also shorted input jacks to ground, and the Master vol to ground. No change . Im assuming its in the output section .

    4 of the RCA 60142 measure =
    E -41vdc
    B -41v
    C .5v

    other 4 are
    E .5v
    B .5v
    C +41v

    Anyone came across this or worked on these 400s ?

  • #2
    scem

    peavey-musician-400-amplifier-schematic (1).pdf


    Click image for larger version

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    • #3
      It works but it is noisy. SO all the parts will test OK< because they work. Most likely a semiconductor is noisy. You need to track it down. Noisy on zero. Is it also noisy turned up? Does the master affect the level of the noise? Do you have the footswitch?

      Your amp model is Peavey Musician. Helps to identify it as that. Yours is the B series schematic below.

      There are a bunch of little 2uf/35v electrolytics/ PV used a ton of them, and they have not aged well. I generally find I need to replace all of them. You will note one on the master stage.

      Find the clockwise end of the maser pot and ground it. Noise stop?'
      '
      Attached Files
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Originally posted by Enzo View Post
        It works but it is noisy. SO all the parts will test OK< because they work. Most likely a semiconductor is noisy. You need to track it down. Noisy on zero. Is it also noisy turned up?

        Yes


        Does the master affect the level of the noise? no does the same on 10 or 0

        Do you have the footswitch? no

        Your amp model is Peavey Musician. Helps to identify it as that. Yours is the B series schematic below.

        There are a bunch of little 2uf/35v electrolytics/ PV used a ton of them, and they have not aged well. I generally find I need to replace all of them. You will note one on the master stage.

        I will try to find it


        Find the clockwise end of the maser pot and ground it. Noise stop?' noise does not stop


        i think i have some freeze spray...i can try that too

        '
        ....

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        • #5
          OK, if grounding the base or the master transistor (that is what the pot end connects to) does not kill the noise then I suspect the transistor itself is noisy.

          Schematic calls all transistors SPS953 unless noted otherwise. Any nice NPN signal transistor will work fine here MY factory sub list is for older types, they used: MPS8097 or 2N6520 in production, but use as subs also: 2N6539, MPSA18, 2N3391. But there are plenty of other good candidates.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            Thanks enzo. Im on hold . Told the owner the estimate , waiting for a response before i completely tear this thing apart

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