JCM900 Missing Reverb.

Discussion in 'The Workbench' started by Brian Jones, Nov 21, 2019.

  1. Brian Jones

    Brian Jones New Member

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    Hi folks,
    I have a JCM900 that the reverb has quit working on. I am looking for suggestions of any common issues before I commence surgery.

    Brian.
     
  2. South Park

    South Park Well-Known Member

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    Thar is a digital chip related problem.
     
  3. Jon C

    Jon C Well-Known Member

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    Solid State ……Yes
    Digital … No
    It’s a ‘90s Amp
    900s have a Reverb Tank
     
    Last edited: Nov 21, 2019
  4. Lance Chambers

    Lance Chambers Well-Known Member

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    What model?:hmm:

    My JCM 900 4100R has a tank.
     
  5. Gunner64

    Gunner64 Well-Known Member Premium Member

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    The in and out rca jacks and plugs at the tank can be the source of the problem. That's where I'd look first.
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Brian Jones, of Rolling Stones? Are you the Ghost of rockstars past?

    Turn up the reverb control, turn up the master volume shake the amp around and bump it against the cabinet.
    Do you hear the springs rattling?
    (should hear this noise from the springs if the reverb return is working)
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Turn on the amp.
    Turn up the reverb and master volume controls.
    Un-plug the reverb return from the reverb tank.
    Touch just the tip of the reverb return cable RCA plug with your finger.
    When you touch just the tip of the reverb return cable, you should hear a buzzing noise from the amp.
    Does this happen as above?
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Take the reverb tank out of the amp.
    Look inside the reverb tank where the wire are soldered to the RCA jacks.
    Is there a broken wire right at one of the RCA jacks inside the reverb tank?
    This is where the wires break sometimes inside the reverb tank.
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    There is actually dual analog op amps that have an A/B switch built into the op amp.
    The A/B switching is controlled by a voltage from the switching circuit.
    Sometimes the op amps with A/B switching do go bad.
    But look inside the reverb tank first for a broken wire at the RCA jack.
     
  10. Gutch220

    Gutch220 Well-Known Member

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    let us know if you can isolate the issue and fix it.
    If not, i have an old accutronics tank from 1990 that came from a 4100 that works that you can have.
    ....but new replacement tanks are easy to find.
    get a multi-meter and check continuity, that's a good place to start
     

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