JCM900 Cutting in and out

au_rick

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Hi All,
I have a JCM900 2x12 50 watt combo and after a few hours of jamming it starts to cut in and out, sounds exectly like when you have a dodgy guitar cable / guitar jack but I'm certain it's not that (yes, I've swapped guitars and cables, etc.... to isolate it to the amp)
Does anyone have any clues please, maybe valves failing or dry joints on the input jack, any other things to check before I take it to the shop ????

Thanks in advance.
 

Matthews Guitars

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Swap in your spare set of tubes and see what happens. You DO have a spare set of tubes, rigiht? If not, why not? It's something that is really mandatory for a tube amp owner. Or it should be.
 

Spanngitter

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Swapping tubes right away is for me pure waste of money and there is too many tubes swapped out with the failure sitting somewhere else. Also they normally need to be biased which is also a task outside of the scope of a normal user. First thing to check anyway is always the FX Loop. Marshalls are known to have constant issues on the contacts of these (or better said on the return jack) when getting older and its a quick check: just put a patch cable in between Send and Return and recheck. However, the fact that you state that it only occurs after a while could indicate a cold solder joint or a failing bridge rectifiert (depending onw hat model of JCM900 you have exactly). Especially the Bridge Rectifier are some point of woe, no matter if it is the one for creating the +/-15Vdc Voltages or the one for heating V1 with DC. Also I have seen numerous cold solder joints around the PCB mounted jacks, no matter if Input, FX or Speaker so this should be checked as well, however, it requires pulling the PCB so not a easy job. In case you decide to hand the unit over to a tech I would recommend to have him also check the main and bias capacitors, considering that they are alreay at least +25yrs old and might require replacement.
 
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au_rick

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Swapping tubes right away is for me pure waste of money and there is too many tubes swapped out with the failure sitting somewhere else. Also they normally need to be biased which is also a task outside of the scope of a normal user. First thing to check anyway is always the FX Loop. Marshalls are known to have constant issues on the contacts of these (or better said on the return jack) when getting older and its a quick check: just put a patch cable in between Send and Return and recheck. However, the fact that you state that it only occurs after a while could indicate a cold solder joint or a failing bridge rectifiert (depending onw hat model of JCM900 you have exactly). Especially the Bridge Rectifier are some point of woe, no matter if it is the one for creating the +/-15Vdc Voltages or the one for heating V1 with DC. Also I have seen numerous cold solder joints around the PCB mounted jacks, no matter if Input, FX or Speaker so this should be checked as well, however, it requires puööing the PCB so not a easy job. In case you decide to hand the unit over to a tech I would recommend to have him also check the main and bias capacitors, considering that they are alreay at least +25yrs old might require a replacement.
Thanks for the advice.
If it cuts out next time I jam I'll take it to a pro for a "midlife" service:)
 

fat_lenny

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When my 4100 head was doing this, it was the capacitors that needed replacing. The tech also did something about "spurious oscillations", but thats way above my pay grade and wasn't in anyway connected to the fade in/out. Ive actually got the work sheet somewhere, if I can find it I can come back with what was done to it.

That was all done about 7 years ago and its been sweet since.

I already had a patch cord in the loop, so didn't need to go there.
 

mrjones2004x

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Hi All,
I have a JCM900 2x12 50 watt combo and after a few hours of jamming it starts to cut in and out, sounds exectly like when you have a dodgy guitar cable / guitar jack but I'm certain it's not that (yes, I've swapped guitars and cables, etc.... to isolate it to the amp)
Does anyone have any clues please, maybe valves failing or dry joints on the input jack, any other things to check before I take it to the shop ????

Thanks in advance.
Mine was doing the same and it was the volume pot apparently. It had a full service at Marshalls with new pots and jacks and power tubes and everything is perfect now. Well worth the money
 

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