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Marshall jcm900 oscillating

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  • #1

    Marshall jcm900 oscillating

    This amp came in with the volume going in and out. I found R30 on the high/low switch opened. REplaced and it's working fine except there's also an oscillation that can be heard when inserting half way into the input. IOW, when the tip lifts the ground of the input jack the oscillation can be heard. I replaced C5 because it sounded microphonic when tapped. But not the problem. When tapping on the ICs 1 and 4 the oscillation is affected, it sounds like it vibrates with the tapping.
    It seems like IC 1 is most affected by the tapping. But I'm not sure.
    How do I determine what's causing the oscillation?
    And if it is one of the ICs is a 4558 a good sub for the 5201?


    Strike the last question: I found jrfond answered way back, NJN2120 or 4560 work as subs. But which one if any to stop the oscillation?
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    Last edited by pontiacpete; 08-08-2012, 04:51 PM.
    Tags: None
  • #2
    What kind of gain and volume settings do you have when the squeal is caused by the input jack? The ground switch of the jack connects to point F of the V1 grid circuit. So if the gain and volumes are set high, I would expect some squeal when plugging halfway into the input.
    As far as microphonic IC's go, if they exist they are extremely rare. I would expect something else near they IC may be the problem.
    The 5201 is a switching op amp, looks like the NJM2120 is also, the 4560 is not.
    Last edited by g1; 08-08-2012, 05:00 PM.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


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    • #3
      Thanks g-one. I neglected mention that it only happens when the boost channel is engaged and the gain is set to '8' and the vol to '5' half way, the oscillation tone occurs.

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      • #4
        Is this really a problem? If you plug a cord into the front of the amp that is not plugged into a guitar the amp will make a lot of noise and will probably oscillate if the gain is high enough.

        What normal operation of the amp will require the plug to be half inserted into the input?

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        • #5
          What normal operation of the amp will require the plug to be half inserted into the input?
          I guess there's no problem then, I was just over worrying when I kept hear this somewhat loud tone as I plugged into the amp.

          Thanks for the help

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