Marshall JCM 900 Fuse Problem?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by thegearguy, Oct 22, 2008.

  1. thegearguy

    thegearguy Member

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    Hello, I'm sure this is an easy fix and I'm too dumb to figure it out. However, I have a Marshall JCM 900 2 ch 50W Hi Gain Dual Reverb Head. I plugged it in the other day and the 3amp fuse blew. I bought some more three amp fuses and replaced it. The Head powered up in standbye and then when you turn off the standbye the fuse blows. Any suggestions would be great.

    Thanks
     
  2. GearHeadFred

    GearHeadFred Member

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    Errrrr... Ahhhhhh... Hate to tell you, but I'm not so sure that it's an easy fix. You might have a very serious problem on your hands... I had a JCM900 a few years ago.. and without going into the gory details, it blew the power transformer and the output tubes.

    If you're not technically inclined, you should take it to a tech.

    If you are, you could pull the power tubes and see if the fuse still blows when coming out of standby.
     
  3. FortinAmps

    FortinAmps Member

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    +1 to the above. Remove the output tubes, replace the fuse. Then power up again. If the fuse doesn't blow then it's your output tubes.

    Good Luck,
    Mike
     
  4. skipm45

    skipm45 Member

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    :agree

    Unfortunately, if it's not an easy fix (bad power tubes) it can be anywhere in the power supply string. If you are not confident in your ability troubleshoot this , it's time to see your tech.


    Skip
    www.skipzcircuits.com
     
  5. SatelliteAmps

    SatelliteAmps Member

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    HT (500mA) is usually the one that blows if it is tubes.

    If one of the 3A fuse(s) is blowing, it is almost always in the power supply. Hopefully a bad filter cap. I think those also had a 3A fuse on the filaments. Could be a short there.
     
  6. GearHeadFred

    GearHeadFred Member

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    My JCM900 didn't have the HT fuses.. It only had the 3 amp main fuse.. I'm not a Marshall expert.. perhaps these were added on later models? And probably for a damn good reason!
     
  7. SatelliteAmps

    SatelliteAmps Member

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    Schematic calls for an internally mounted HT fuse, rated at 500mA.
     
  8. GearPaul

    GearPaul Member

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    A bit late on my reply, but....
    I have a JCM900 4100 (head), same thing happened 2x to me (over a couple years). Both times, the problem was output tubes.
    As previous poster mentioned: see if you blow fuses w/o tubes. I was able to find the faulty tube by going down to 50w mode and pulling tube pairs.
    Let us know if you resolved your problem.
     

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