Noisy Marshall JMC 800 Lead (2203)

Discussion in 'Marshall Amps' started by rbotn, Oct 9, 2020.

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  1. rbotn

    rbotn New Member

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    Hi.
    I am repairng guitar amplifiers and I have a problem with a JMC 800 (2203). The owner of the amplifier has short-circuited two pins on an output tube. There is sound at the amplifier, but there is a noise in the pre-amp, mostly at high input. There is no noise when the master volume is turned down. Attach a sound-file with the noise.
     
  2. Chris-in-LA

    Chris-in-LA Well-Known Member

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    So, what have you checked or replaced up to this point?
     
  3. Drinkingdeath01

    Drinkingdeath01 Well-Known Member Gold Supporting Member

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    Sounds like every other JCM800 that I've ever owned.
     
  4. FogDweller

    FogDweller Well-Known Member

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    Doesn’t sound like any I own.
     
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  5. rbotn

    rbotn New Member

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    I have change both preamp tubes with new one, no result. I sounds like the noise is comming for the high input. When I pull out V1, the first tube, the amp is silence.
     
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  6. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Both inputs run through v1. Do you have any experience working on amps? Do you have a voltmeter? Have you opened up the amp for a visual inspection?
     
  7. marshallmellowed

    marshallmellowed Well-Known Member

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    Try swapping components "C" and "M", should sound perfect!
     
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  8. StingRay85

    StingRay85 Well-Known Member

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    V1 is shared by both inputs. High input uses V1A while low bypasses it. So there's a massive difference in volume right there. Based on the very limited info you gave, it can be a lot of things. Maybe this should be moved to the workbench subforum
     
  9. JP2036

    JP2036 Well-Known Member

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    Thats nothing.
     
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  10. rbotn

    rbotn New Member

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    Yes I have experience working on amps, but this is a tricky one. There is nothing to see trough visual inscctions. The voltage around V1 and V2 is whitin 10%. The noise is the same with or without cable in the inputs. Are there anybody who have bad experiences with the jack inputs?
    Thanx, I will also post on that forum too.
     
  11. StingRay85

    StingRay85 Well-Known Member

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    Clean the inputs is for sure where you should start
     
  12. dro

    dro Well-Known Member

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    Are you using a Craftsman model 1019 Laboratory edition signature series torque wrench?
     
  13. Ufoscorpion

    Ufoscorpion Well-Known Member

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    No point in even tackling the job without one in my opinion.
     
  14. Chris-in-LA

    Chris-in-LA Well-Known Member

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    If the voltages look ok and you see no damage, you might need a scope to track down that noise.
     
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