JCM800 Output Fault

Discussion in 'The Workbench' started by Ken Stewart, Dec 30, 2020.

  1. Ken Stewart

    Ken Stewart New Member

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    Hi there,

    I have a problem with my 1994 JCM 800 2205 50w split channel head, which I am hoping someone may be able to offer advice on. I bought the amp brand new in ’94 and it performed outstandingly for the first few years. It developed a fault about 20 years ago, where it sounds as if the volume and tone is being rolled back to near zero gradually and then rolled back up again every few seconds or so. This becomes more apparent and frequent as the night rolls on. I initially thought it had to be valve related, but the amp has had more valves than the toilets at Grand Central Station. I thought maybe transformer, but it has been apparently tested by several service agents. I have spent mega bucks on the head, and where I live in West Australia repair shops are few. It has been to all of them, some many times, and I always get some horseshit story about dry solder joints and the likes. I love the amp and have a couple of other Marshalls which are also very nice, so I don’t want to throw it off a cliff just yet. If you have experienced this or have a theory I would appreciate any ideas.
     
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  2. neikeel

    neikeel Well-Known Member

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    The slow die and slow return sound like a power supply issue up stream of the filter caps (the charge in the caps softens the drop out) and as the PS comes back the caps have to recharge.
    Is the time it happens predictable (maybe as a component gets hot it goes open circuit? Maybe a rectifier diode for example?)
    Just musing.....
     
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  3. william vogel

    william vogel Well-Known Member

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    The only way to diagnose this is to bench test it with meters attached to find the sag. I won’t say it’s endless possibilities but guessing would be a bad idea.
     
  4. South Park

    South Park Well-Known Member

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    The amp is working you have a bad connection change out the out put jacks
     
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  5. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    First off, Don't randomly change parts.

    My 2205, a 1987 later preamp version, had issues with the impedence selector switch. It was cutting out in a similar manner as yours I got enough deoxit in there to fix the issue, and the amp has worked flawlessly ever since. That would be my first source of the problem. Clean that switch, and all the jacks.

    This isn't a horseshit cold solder joint theory, this is a fact I can back up by having been there. The 2205/2210's also have a tendency to have broken solder joints at the pots to pcb connections. Mine had this issue when I bought it in need of repair ($500) mine had almost no volume or distortion on the boost channel, and all the controls effected the boost channels tone. Reflowed all the pot to pcb connections and the thing was a monster.
    The 2205/10s have the pcbs half suspended by the pots which are mounted directly to the pcb. That stresses the connection, and they crack.

    Also my tube sockets were very dirty, and it took multiple times with the deoxit to remedy. Mostly the p.i. and the reverb tube..the latter had a swell up and down thing, as well as a crackle using the reverb before I fixed it.

    A wooden chopstick can be a great diagnostic tool. By pushing a bit near connections a lot of the time a broken solder joint will be exposed. Also a gentle nudge to a tube will reveal a dirty or otherwise compromised socket.

    Just keep your fingers out of things as I'm sure you know there are dangerous voltages in there.

    And don't use it until you figure it out! if it's a problem with the impedence selector or output jacks, smoking the output transformer is a definite possibility.

    That amp should be a monster.
     
    Last edited: Dec 30, 2020
  6. mickeydg5

    mickeydg5 Well-Known Member

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    Use a pedal or something if you have it and plug directly into the effects return jack.
    Turn up and play.
    Does it still do the same?
    This can help indicate where to start diagnosing.
     
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  7. johan.b

    johan.b Well-Known Member

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    When it fades in and out, look at the heaters. Sounds like intermittent heaters on one of the preamp tubes..... or it could just be a bad return jack in the loop
    J
     
  8. neikeel

    neikeel Well-Known Member

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    It is nigh on impossible to be definitive over the net.
    We are all stabbing in the dark.
    Need the amp on the bench and reproduce the faults and measure all parameters.
    Oz is a long way away!
     
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  9. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    These stupid intermittent issues are a bitch to diagnose. I just threw out my experiences with the 2205 I had, the biggest of which is that my impedence selector did cause a similar drop out/ return issue.
     
    Last edited: Dec 31, 2020
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  10. Ken Stewart

    Ken Stewart New Member

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    Awesome guys, I really can't thank you enough for your comments and suggestions. There are some great ideas in the mix there, some of which may be quite simple to check out. I will follow up on these for sure. After years of compounded bitterness, I am also going to forgive the repair guy with the horseshit solder joint theory Gunner64. Thanks again, you guys have a great forum going here, can't wait to engage with it some more. All the best for the new year to you all. Stay safe.
     
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  11. mickeydg5

    mickeydg5 Well-Known Member

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    We can do it!

    [​IMG]
     
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  12. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    And do this. If the condition isn't present plugging in the return this tells you the problem isn't in the power amp section of the amp, output jacks, imp select ect..and is pre amplifier section related.
     
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  13. Ken Stewart

    Ken Stewart New Member

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    Thanks Gunner64
     
  14. Gibson_83

    Gibson_83 Active Member

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    man my JMP is doing something similar even after 2 years and replacing all tubes and the filter cans (they were way overdue). Last time I jammed with it actually cut right down in volume to like just below one on the master volume, and stayed there for a bit. Turing the master volume up to full still made no difference its like there was no juice getting to it.

    Then after I turned it off and came back to it later, it was fine! Let me know what ends up happening with yours.
     
  15. cruisemates

    cruisemates Active Member

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    I am having similar issues and my 2204 and it seems to be related to heater solder joints at the valve sockets going cold on me after 40 years of building up oxide. One thing to check is to see if all of your tubes are not lighting up properly - especially when the sound goes down as you describe. Then when the sound comes back check to see if they are all lit up. If you see changes then some solder probably needs to be reflowed.

    If you do not see changes in your tubes - old filter caps sounds logical - inspect them for tiny bumps top & bottom. Otherwise, some other short as described above could be the problem.

    FWIW, those amps are designed with tone problems between channels. I had one 2205 as my first JCM800 and I naturally thought it made most sense to boost the channel volumes to full (like preamp gain volumes) and then turn down the final master volume (to get the most gain without too much output volume). Turns out that is wrong and will give you a TON of channel crosstalk between the tone controls on each channel (one TMB set will affect the other channel's sound badly).

    You want to turn the channel volumes up and keep the final master down. Then the tone controls can be turned up much higher without affecting the other channel.

    This is all IME - not a pro repair person - just things I have been learning here in these boards lately.
     

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