jcm 900 4101

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ChrisP, Oct 2, 2006.

  1. ChrisP

    ChrisP Member

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    I own a JCM 900 4101 Dual Reverb Combo.

    never really liked it all that much in the first place, but I would like to have another amp available to me. I own several other amps but only 18 watt amps. I have some friends who like to rawk so having a 100 watter is cool incase I need to make them cry. There are some things I like about the amp and it’s actually sounded <very> nice at times. They go for about $500 used. I’ve been quoted roughly $300 + to get it fixed. I really don’t want to spend that much.

    Here’s the good part. There is a master electrician that works with me that would love to help me out. He doesn’t know anything about guitar amps but he is a master electrician after all. A circuit is a circuit. He is willing to trouble shoot it and replace whatever needs replacing. However, he has no idea how to trouble shoot it!

    Here’s what happened. Went to a jam, plugged in amp, put it on standby. Came back 20 minutes later, smelled a burning smell, turned off amp, realized speaker harness had come loose meaning that the transformers were running w/no load, cried as it was my only amp.

    Fuses are fine. Visual inspection doesn’t show anything unusual. No exploded caps or anything like that. The amp has been sitting for a couple years but when I tried it the night after the incident there would be a bit of sound (very low volume) and the burning smell. I tried it the other night and there was no sound at all and no burning smell!

    Can someone tell me what to tell the electrician to look for? How does he go about checking the transformers to see if they still work? I assume that is the problem.

    Thanks!
     
  2. Mark Robinson

    Mark Robinson Member

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    I had one of those amps, and went through a few repairs.

    The potentiometers can break off the circuitboard, that happened twice. I also had something called a coupling resitor burn, and that shut'er down too. It doesn't sound good, the circumstances of original departure, but best of luck to you.
     
  3. ChrisP

    ChrisP Member

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    Thank you...

    Forum, is this an unreasonable request?
     
  4. Dave C

    Dave C Gold Supporting Member

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    Download and printout the schematic for your electrician buddy .....see if it looks like something he'd be willing to try, make sure he is aware of the 400v+ inside the chassis . If you smelled something burning....there is at least a burned component or possibly tranny...hope not , but the possibility does exist. Find a Marshall voltage chart and check it against your amp. You/he might have to do a bit of research on how to check trannies and various circuits , but it's not rocket science.....you'll find a lot of info if you search this forum and other sources on the net as well. You can always post more questions here once you get into it...just be careful and good luck. Let us know what you find.
    Dave C
     
  5. ChrisP

    ChrisP Member

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    High voltage bad? gotcha. Thanks.

    the boards look fine. I'm pretty sure it's a txformer.
     
  6. JJman

    JJman Member

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    I had the head. I never liked the tone much and was overwhelmed by the extreme amount of components on the boards. I upgraded the screen resistors to accept EL34s but the tone was still disappointing. Was a little quirky if the loop was not in use. The schematic for the pre-amp is colossal. Reverb sucked. I sold it mostly because of the PCBs but also because my other amps sound better.
     
  7. ChrisP

    ChrisP Member

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    I hear ya...it's not a great amp. I'd grab my SFPR for cleans or my other EL84 amp for a certain type of lead sound but the amp *does* have it's merits. Plus it's my only amp that has more than 20 watts.

    If I had disposable income this amp would probably be in the trash but I don't...I'm just trying to fix it, cheaply. I ought to just toss the thing and be done with it. There's no way I'm going to fix it for less than 150-200. They sell on Ebay for about $400 or so I believe. Back to the basement it goes.
     
  8. Rich M

    Rich M Member

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