JCM 900 4100model - caps and rebuild advice

Discussion in 'The Workbench' started by mikroman, Jan 15, 2022.

  1. mikroman

    mikroman New Member

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    1997 amp. There are no hotspots anywhere on any pcbs. All of the caps look original and new. No swelling or leaking. I tested all of the tubes with a tube tester and are in great shape. The mains fuse will blow after 5-6 seconds of power on. I think I should recap it. Check resistors. What would any of you suggest. Thanks
     
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  2. Pete Farrington

    Pete Farrington Well-Known Member

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    Use a light bulb limiter rather than repeatedly subjecting an amp to fault current.
    Use a low power bulb, eg 40W, 60W.
    Take the valves out.
    Power up in standby mode and note how the light bulb brightness responds over time, eg 30 seconds.
    Switch standby into play mode and note how the light bulb brightness responds over time.
    https://www.drtube.nl/schematics/marshall/4100-60-02-1-iss11.gif
    https://www.drtube.nl/schematics/marshall/4100-60-02-2-iss11.gif
    All but the most expensive valve testers don’t tend to subject the valve under test to anywhere near the voltage level found in a guitar amp. They are fine for weeding out badly failed valves, but no good for confirming the remainder really are fit for service.
     
    Last edited: Jan 15, 2022
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  3. tschrama

    tschrama Well-Known Member

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    No indication yet to replace capacitors.
    Try to isolate the problem
    Pull output tubes, still blowing fuse? If not, output stage is the problem.

    Put the output tubes back one by one and test in between. Etc etc etc
     
  4. DreamerDeceiver

    DreamerDeceiver Well-Known Member

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    Thanks for having the same year and model.

    This info is priceless.
     
  5. mikroman

    mikroman New Member

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    I have seen the light bulb used in radio circuits. Great idea.
     
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  6. Tatzmann

    Tatzmann Well-Known Member

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    Mains Fuse, how hard can it be.
    You are not fit to work on an amplifier.
    Turn it in.
     
    Last edited: Jan 15, 2022
  7. mikroman

    mikroman New Member

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    Thanks for the advice. There's always "one of you" around forums
     
  8. mikroman

    mikroman New Member

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    I built light bulb limiter today and tested the amp without any output tubes. Fuse OK. Now to the nervous part; move on and put back one tube at a time. I'll post news after I dig up some bravery.
     
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  9. mikroman

    mikroman New Member

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    Fails after first tube goes in, within 8 seconds. Light bulb starts to glow brighter, then....loud buzz as before and fuse blows.
     
  10. Pete Farrington

    Pete Farrington Well-Known Member

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    What bulb wattage?
    With no valves in, note the VDC at pins 3,4,5 of the output valve sockets.
     
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  11. mikroman

    mikroman New Member

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    Thank you sir. 40 Watt bulb. I will check the voltages.
     
  12. Pete Farrington

    Pete Farrington Well-Known Member

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    I wonder how a 3A mains fuse could blow, with the current through it limited by a 40W bulb?
     
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  13. neikeel

    neikeel Well-Known Member

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    You’ve had good advice thus far.
    Let’s break it down.
    3A mains fuse is there to protect/detect mains transformer, rectifier and first filter cap problems. So if it fails with mains on and HT off (ie in standby) the issue sits there somewhere.
    It is more common for a shorted out tube (or screen resistors) to take out the 1A HT fuse.
    I presume that your are using T rated (ie SLO blo) fuses?
    A 40w bulb at normal brightness will be 40/240 ie 1/6 of an amp. When it goes brighter- switch it off.
     
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  14. Spanngitter

    Spanngitter Active Member

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    On a side note:
    For 230/240Vac a T1.5A or max T2A is sufficient, for 110/120Vac Input you should be able to run on a T2A or max T3.15A
     
  15. mikroman

    mikroman New Member

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    This is a NA model with a 4A slo blo fuse. It won't blow in stand-by; but, about 8 seconds after switch on it fails. My thinking was the filter caps. And yes, excellent advice and thanks for steering me there without solving the problem for me.
     
  16. Pete Farrington

    Pete Farrington Well-Known Member

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    I’m struggling to envisage how a 40W bulb could pass enough current to blow a 4A fuse
    Was the light bulb limiter in circuit?
     

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