My Marshall is in trouble.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by rjpilot, Sep 11, 2008.

  1. rjpilot

    rjpilot Member

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    I got a Bray mod 1987x.

    Here is the issue.

    I play the amp for a little while and everything is fine. After about 15 mins I get some volume drop accompanied with popping that sounds like a loose Guitar jack input (which of course its not). Then I get a screeching sound, and by this time I turn the standby switch on and all goes quiet (thankfully). I flip the standby switch off again and keep playing but the problem reoccurs after a brief reprieve.
     
  2. rjpilot

    rjpilot Member

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  3. Hacksaw

    Hacksaw Time Warped Gold Supporting Member

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    Usually, this is tubes failing usually. Have any replacements on hand to test with?
     
  4. SatelliteAmps

    SatelliteAmps Member

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    Tubes are always the first to check. If it's not that, then you will probably need to see a tech, as it might be something in the B+ or filtering.
     
  5. rjpilot

    rjpilot Member

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    Thanks guys I suspected the tubes too.
    I put in new power tubes a week ago. I got some preamp tubes on order. What was that about the B+ and filtering?:bow
     
  6. SatelliteAmps

    SatelliteAmps Member

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    To have something fail like this, and not be a tube, would indicate some sort of high voltage supply problem (B+). There are usually tell tale signs of amplifiers doing things when they go bad. When something takes 15 minutes to fail, it is because something is slowly heating up to the point where the component is failing. Filaments don't do that. Actual signal doesn't, so that only leaves B+. Most B+ issues come from either the filter caps, a resistor, or the power transformer.

    I still think it is tubes.
     
  7. rjpilot

    rjpilot Member

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    thanks for the replies

    I hope the new tubes work out.:bkw
     
  8. johnny q

    johnny q Member

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    rjpilot - Did you ever resolve this? I am curious because my JCM800 is doing the exact same thing.
     
  9. TED STEVENSON

    TED STEVENSON Member

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    I had a friend's JCM 800 on the bench recently with the same symptoms. Turns out the power tubes are grounded with a small screw to the chassis and these were loose. Tightening them up did the trick.

    ted
     

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