jvm410c down again 2nd time in 6 months

Discussion in 'The Workbench' started by merrill, Oct 26, 2020.

  1. merrill

    merrill New Member Silver Supporting Member

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    i cant believe it, the amp is down again.

    6 months ago a tube failed so i took it in and got all new tubes and a general service.
    sounded nice seemed to work great.

    i did notice some noise that i didnt think was there before..

    anyway, playing a little tonight, still hear that noise lets call if background noise, after a while some low volume popping but nothing big

    then the sound decreased in volume , i looked at the amp and all the led's on the front panel blinked once and no more sound and i smelled something hot, not necessarily burning electrical smell. sigh.

    oh well back to the shop,

    i need to see if there is an authorized marshall amp repair shop in austin. i am getting skeptical with the well known and recommended shop here in austin
     
  2. South Park

    South Park Well-Known Member

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    Get some gut shots
     
  3. avspecialist

    avspecialist Active Member

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    The only time I had a problem with my 410H was using a Russian tube in V3 I believe. It has failed 2 times. I put in a Preferred series 725 from the Tube Store and no more issues.
     
  4. merrill

    merrill New Member Silver Supporting Member

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    well heard back from the authorized marshal amp repair shop,

    here is what they found
    power tubes were running too hot and HT fuse blew ( tubes got hot but are still functioning) - thermal bias drift issue detected - circuit board getting hot and causing bias voltage to drift / tubes to draw too much current

    recommended repair
    Repair circut board to prevent thermal bias issues. - clean up and secure all connections - run unit on bench - and monitor voltages

    $132 repair, which is nice, my question back to them is was this something that the other non marshal amp shop that last repaired the amp missed or caused?

    interesting the shop said this issue is "common" for this amp.
     
    Last edited: Nov 16, 2020
  5. merrill

    merrill New Member Silver Supporting Member

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    The tech has finished repairing the amp, he was open to reading all of the various suggested mods for the jvm410.

    the latest is bias issue was resolved and amplifier is stable under full load
    - additionally_
    I replaced six resisotrs with metal film type to lower hiss - worked good. I also installed a Sovtek 12ax7WA low noise type preamp tube into OD1 / OD2 posistion.

    excited to see how it sounds when i get it back
     
  6. merrill

    merrill New Member Silver Supporting Member

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    i finally picked up the amp and had some time to use it.
    the noise levels are much lower, especially the OD2 channel, OD1 is still hissy but much less annoying.

    now i have to turn up the volume on the OD2
     

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