JMP1 defect OD2 Channel. Can someone help me?

Discussion in 'Marshall Amps' started by Carlos_ring, May 20, 2019.

  1. Carlos_ring

    Carlos_ring New Member

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    Hi

    i registered here, i thought you could help me out. I am from germany, so my english could be a little bit weird.

    MY JMP1 OD2 Channel has issues since today. If i turn up the gain theres a loud pheep, screaming noise to hear, but on gain stage 20, 15, 10, 5, 0 there is no pheeping noise. In Area 10-15 the noise changes little bit in the tone height..

    There must be a logical issue, but i cant detect it on my own.

    Whats wrong here? That amp is modded after the french jmp1 article.

    Thank you .
     
    Mitchell Pearrow likes this.
  2. Fender

    Fender Active Member

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    "french jmp1 article" meaning the voodoo amps mod ? did the problem occured just after the mod or a long time after ?

    usual suspect : did you try fitting a new preamp valve ?
     
  3. Carlos_ring

    Carlos_ring New Member

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    Hi it accured today, the amp was on about 6 hours yesterday and switching it on today, it occured. I tried another valve, same thing.

    to get to the mod, try google " french article jmp1 mod", i cant post links here.

    Found out something special, if i breathe or blow warm wind on the board especially on the drive tube, the tone changes, its getting much higher.

    Ok another thing, i can hear it through the valve. The valve itself makes noise on the board. Can a valve "ring"?
     
    Last edited: May 20, 2019
  4. Fender

    Fender Active Member

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    microphonic valve, symptom of a faulty valve, or there's something wrong around it which kills it anytime you change it… you sure this other valve you tried was ok ?
    could be an unreliable solder joint from the mod, too, did you do it yourself ? this is a fairly complicated mod which should be done if you know how to troubleshoot the modded piece of equipment.
     
    JAC likes this.
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Can a valve "ring"?

    Yes.
    I think this is oscillation and not a microphonic tube.
     
  6. Fender

    Fender Active Member

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    I thought he meant the blow itself coule be heard, but you're right that's not it
     
  7. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

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    :cheers::welcome: To the forum
     
  8. Carlos_ring

    Carlos_ring New Member

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    From the view of an electrician, it sounds like clearly oscillation ringing, because it begins quiet and builds up. But what is it? And how can i measure it?
     
  9. Fender

    Fender Active Member

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    It can have multiple causes. One I experienced is the presence of many buffers in the chain with an active fx loop and the first gain stage set too high...

    could be a faulty active solid state component in the chain before the tube, but on this beast it's very hard to find it if you don't know anything about troubleshooting circuits.

    did you check if the oscillation is also present in the send of the fx loop ? have you tried clean presets ?
     
  10. Carlos_ring

    Carlos_ring New Member

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    Its only on od2. and even in the send of the fx loop.
     
  11. Carlos_ring

    Carlos_ring New Member

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    Ok, i think i have it. It has to do with the input Jack. The mainboard near the input jack squeals if i push it towards the housing at the bottom, if it gets mm closer the squaling gets louder and louder. I think i must put something there to get more space to the ground.
    The Capacitors near the input Jack are also microphonic , drumstick tipping is loud to hear and the metal shielding plate between the two Jacks was loose. I removed it and now it squeals like hell.
    And the input jack is defect, when unplugged, the input is not shorted, but must be i think. If no plug is connected it squeals like hell on OD2-.

    I will solder a new input and phone jack and install the shielding and then the search goes on.
     
    Mitchell Pearrow likes this.

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