Marshall JCM 900 Model 4500

admin · Post by **admin** » Sun Jan 21, 2007 5:08 am

I have a 1993 Marshall JCM 900 which has recently taken to cutting out after about 10-15 minutes.

The problem is with the A Channel only and the B Channel still works fine. I replaced all the tubes and a fuse has never blown. The problem appears to be worsening, but all is well following a cool down only to begin this cycle once again. No smoke yet, but there is considerable hum in the A Channel when the problem is present. Whisper quite in this channel after cooling.

It certainly sounds like overheating to me, but I am all eyes and ears if anyone else has any ideas.

soundmasterg · Post by **soundmasterg** » Mon Jan 22, 2007 10:34 am

It could be a cap or resistor that is bad, but only when it gets hot. A tech would fire the amp up, put a signal into it, hook up a speaker, and poke and prod parts to see if he/she could narrow down where problem is. You can also pull tubes to try to isolate the circuit area where the problem is. You would also check voltages and compare them with published schematics to make sure nothing is amiss. If a heat issue is suspected, then you apply heat to various parts to see if they have the problem, and you also use the cold spray on parts to try to narrow things down. This is the type of thing that can be very frustrating to figure out and can cost lost of moeny sometimes to fix.

If the amp is original, you can do the shotgun approach and replace all of the electrolytic caps, and see if it still has the problem. The e-caps are now going on 14 years old and are probably due for replacement. If it still has the problem after that, then you can replace coupling caps one by one until problem goes away.

admin · Post by **admin** » Mon Jan 22, 2007 10:52 am

Thanks Greg. I really enjoy the tone of this amplifier and will be glad to have it back in good working order.

admin · Post by **admin** » Tue Jan 23, 2007 12:32 pm

Greg: If I put a patch cord from send to return the problem goes away. Any ideas?

soundmasterg · Post by **soundmasterg** » Tue Jan 23, 2007 2:03 pm

Is that a reverb send and return or an effects loop send and return?

admin · Post by **admin** » Tue Jan 23, 2007 2:20 pm

It is an effects loop send and return, Greg.

admin · Post by **admin** » Wed Jan 24, 2007 7:00 am

Greg: I put some contact cleaner on a guitar plug and inserted it in "send" and "receive" a few times and the problem has vanished with a much louder amplifier than before. Could the solution be this simple?

bitzerguy · Post by **bitzerguy** » Wed Jan 24, 2007 8:31 am

I don't know about the Marshall, Peter, but this is must do maintenance with the Vox blue Valvetronix series (AD60/AD120) amps. Not only the sends and returns, but also the line outs, headphones, speaker outs and input jacks. In the Vox case the problem is mainly due to cheap jacks. I have to do this at least once a month.

...Dean

soundmasterg · Post by **soundmasterg** » Wed Jan 24, 2007 10:07 am

It could very well be Peter. It sounds like you have cheap or dirty jacks. I'd suggest to make sure all the jack and pot nuts are tight and clean them all as you did with those two.