Jcm 900 50w 4500 Volume Dropping

Discussion in 'Marshall Amps' started by Jimmy3000, May 11, 2017.

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  1. Jimmy3000

    Jimmy3000 New Member

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    I have a JCM 90 50W Hi-Gain Dual Reverb with GrooveTubes biased at by a Marshal qualified tech years ago. This Amp periodically drops volume and I have never gotten it to stop for more than a few weeks. It appears once the AMP is used quite a bit it wont do this but I use it less and less now that I have less time to play :(
    Nothing is in the effects loop and I just had an SG plugged into the front end of the Amp with the volume on about 5 gain cranked but it will do this on the clean channel as well. The power mode is set to high
    The foot switch wont work and it's not plugged in so the only cables are power, speaker and guitar.
    the speaker cab is a 1968 Marshall 4x12 set to 8 Ohm, amp is set to 8 as well. The power in the house is clean, the cables are good and working, Ive had this happen with more than one speaker cable. I've had the Amp for about 20 years and I'm wondering if I should just cut my losses and get something smaller.

    Also about how long should I warm this up before I play?

    Apologies if this is the wrong forum, this is my first post.
     
  2. Gunner64

    Gunner64 Well-Known Member Gold Supporting Member

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    Put a cable across the send and return on the effects loop..see if it makes a difference. The Effects loop jacks get dirty. Get some cleaner and clean the crap out of them. That is usually the problem.
     
  3. Buzzard

    Buzzard Well-Known Member

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    Sorry your amp has issues.but if you like the amp dont give up on it.I got a jcm 900 hi gain mast vol mkiii 2500 used for cheap.I have a great tech who is a marsh authorized service tech.He works on ALL marshalls.He showed me video of my amp having a loose wire (sorry for my lack of technical specifics)somewhere in a not so common and hard to see area.and remarked that this was probably driving someone crazy for a long time causing intermittent probs.If you can afford it you might want to have a pro look at it if you dont know how yourself. good luck.
     
  4. Gunner64

    Gunner64 Well-Known Member Gold Supporting Member

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    Try jumping the effects loop before you go looking for anything else. I have had this issue with my 900..dirty contacts in the jacks.
     
  5. Jimmy3000

    Jimmy3000 New Member

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    @Buzzard, thanks. Do you know the link to the video?

    @Gunner64, I will try this weekend. When you say cleaner do you mean compressed air or something else?
     
  6. Gunner64

    Gunner64 Well-Known Member Gold Supporting Member

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    I use a lubricating cleaner like deoxit. Best to remove the amp from the headshell so you can put some paper towel under the jack so as not to saturate the whole pcb with cleaner. Work a plug in and out several times as well.
     
  7. JCMDOUG

    JCMDOUG Well-Known Member Gold Supporting Member

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    If its not the effect loop it could be the pots. I had the same problem in my 900 MK iii the volume would fade away. I replaced all the pots and so far things are good. one master volume was tight and the other was loose and the mid control would make all kinds of noise, I would clean them but after a month the volume just start fading again. They were cheap to and easy to replace.
     

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