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MESA Express 5:50 crackling noise

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  • MESA Express 5:50 crackling noise

    Hi,

    So I have here a Mesa Express 5:50 which has that annoying crackling noise. Not loud but even with all controls at zero you can still hear it at the background.
    I narrowed down the problem to V3B at the FX loop. When C31 is shorted to ground the noise is gone. With pin 2 shorted the noise is still there. What I did so far:

    1/ tried different tubes - NO change
    2/ suspected leaky cap C31 - tried a new one - NO change
    3/ lifted one end of C30 - NO change
    4/ resoldered all components around V3B - NO change

    By this time I ran out of ideas. Any suggestions would be appreciated.

    Another thing is switching Standby OFF produces a loud pop. I fixed that by installing a 100nf/630V cap across the Standby toggle switch but I have the feeling the actual problem was masked and not solved but this can wait for now.

    Express 525-550 schem.pdf

  • #2
    Pin 2 doesn't belong to V3B. Do you mean pin 7?
    R59 might be bad.
    - Own Opinions Only -

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    • #3
      Thanks for the quick reply.

      Pin 2 doesn't belong to V3B. Do you mean pin 7?
      Yes, I meant pin7.

      R59 might be bad.
      I'll replace it and report back.

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      • #4
        My thought process with rustling noise is pre amp valves, then power valves
        But if it is more of a crackle, sizzling bacon then I usually find that it is plate or cathode resistors that have had a hard life (high volt drops), couple caps also but less common and easy to diagnose with a voltmeter, resistor body contact with other traces / PCB (esp . boogie) and coming off another thread recently corrosive glue / silicone goopy.

        HTH

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        • #5
          Problem was solved by replacing the R59 as Helmholtz suggested. Thanks for your help. That saved me time replacing all other components around V3B.

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