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JCM Blowing fuses after a strange hum. What is causing this to happen?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Yossi, Oct 10, 2005.

  1. Yossi

    Yossi Member

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    Marshall 100 Watt head. JCM 2000 blows the main fuse after humming for 5-10 seconds. Power on. Stand by was off.
    The hum is like a tranformer type of hum and it's not coming from the speakers. It did this twice. Prior to this the amp sat for about 6 months.
    Amp is not wet, not in a hot spot.

    What could be causing this to happen?

    Thanks for your expertise.
    Yossi
     
  2. Reeek

    Reeek Member

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    Although it may be the PT, it very well could be a shorted power tube as well. Remove all the power tubes and power it up and see what happens. If the fuse doesn't blow, replace your power tubes.

    Good luck!
     
  3. Erik

    Erik Member

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    Check the preamp tubes, too. This happened to me once and I kept blowing the HT fuse...everything "looked" fine, power tubes were glowing, mains switch lit up, etc. I had accidently put one of the preamp tubes in a little "off" center and bent the little metal contact points so it wouldn't sit right in the socket.
     
  4. Enzo

    Enzo Member

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    Happy New Year.

    SOmething is loading the power transformer down, possibly itself, or a short downstream. Do as the man says and remove the power tubes, but before risking another fuse, check the rectifiers for shorts. Also check the B+ rails for shorts to ground, possibly due to shorted filter caps.

    You can check the tranny by disconnecting the secondary wires from everything and applying power. If the fuses blow with just the tranny and nothing connected to its secondaries, then the PT is shot.

    JCM2000 is not very specific, which amp model do you have?
     
  5. Reeek

    Reeek Member

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    I just found this while looking for troubleshooting tips for my hum:

    If the fuse blows after rectifier warm up, and conduction, suspect the filter cap(s). While electrolytics may not fail in a catastrophic manner, they may develop leakage that will cause a high power drain. Get a VOM and take an ohm reading. While it has been suggested 1meg+ reading is ok, I suggest 2meg or greater resistance, or junk the cap.
    Avoid old reformed caps, as they are just that, old. The electrolyte dries out over time, and that's where your heartache begins. Considering you may not be able to get a replacement power transformer cheaply, is it worth it? If you want to retain the original external appearance, bury new caps in the chassis, and leave the old ones in place, disconnected.
     
  6. Wakarusa

    Wakarusa Member

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    +1

    Power tubes and main filter caps would be the very first place I'd look
     

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