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Marshall 2100 JCM 900 head oscillating

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  • #1

    Marshall 2100 JCM 900 head oscillating

    A friend picked up this beast for $50 at a swap meet. It had a PCB short between the OT feeds at the LO/HI power switch which I removed. But now it goes into ultra-sonic oscillation near clipping in the Hi power mode only. If I feed the FX Return, then it only oscillates a little bit on the top half of the waveform before it sort of punches through the oscillation at clipping. I've swapped tubes and even tried another OT, but it still acts the same. It does not oscillate with only 2 power tubes in place. With the power tubes out, the output of the PI looks good on a scope.

    Any ideas?
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  • #2
    If the oscillation is there when the preamp is bypassed, and the power mode and number of tubes affects the problem, I might suspect the feedback loop first.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #3
      Are you sure this issue has nothing to do with the other issue you fixed? What kind of pcb short was it, something you had to cut away or a solder bridge? Could the short have damaged something else, like the Hi/Lo switch?
      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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      • #4
        I have disconnected the feedback loop and it still oscillates. The PCB short was an arc between the OT feeds at the Hi/Lo switch. I had to drill out the carbon on the PCB. I think I will pull the Hi/Lo switch and hard wire it and see what happens. The switch measures OK in circuit, but I'll pull it anyways. Thanks for the suggestions, guys!

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        • #5
          It's still oscillating with the switch removed and hard wired for Pentode mode.

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          • #6
            Originally posted by rf7 View Post
            It's still oscillating with the switch removed and hard wired for Pentode mode.
            What sort of grid stoppers are there at the power tube inputs? I seem to recall 2 or 3 different revisions to this circuit.

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            • #7
              Originally posted by 52 Bill View Post
              What sort of grid stoppers are there at the power tube inputs? I seem to recall 2 or 3 different revisions to this circuit.
              Good call about the grid stoppers. I moved the 1.5Kohm grid stoppers off of the rear circuit board and moved them to the tube sockets. I ran the wires above the sockets straight to the resistors. That took care of the problem. I did some heavy duty web searching and found that this was a mod from Marshall apparently. Thanks for the help!

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              • #8
                Just to chime in +1 on moving the 1k5's to reduce the terrible oscillation. Was there a factory update on this? Seems pretty obvious/bad.

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