Jcm900 problems... Help needed!

[zw44]

Hi
The amp in question is a marshall jcm 900 dual reverb 4502.

The problem is the volume on channel A goes up and down by itself, and doesnt seem that loud when its on 10 on the master. I have replaced the volume pot but the problem still remains. Infact when the volume is turned down to zero you still sound out of it. When the volume drops and gets louder on its own it sometimes makes a strange buzzy sound, kind of like a bee stuck in a smarties tube.

It is only Channel A which is effected. I have put a patch cable between the send and return on the effects loop and the problem still remains. I have a multimeter so can check things if needs be.

Please help, this is driving me nuts!

[anthillrich]

which vol pot did you change? the master vol, or pre amp(gain)vol pot?
either one could cause the problem you mention.

[zw44]

which vol pot did you change? the master vol, or pre amp(gain)vol pot?
either one could cause the problem you mention.

Today I changed the master volume pot for Channel A, but on a previous occassion (not that long ago) I have replaced the preamp gain pot for channel A too.

[zw44]

Right, I replaced the preamp tubes and it didnt solve the problem. BUT... I was messing around with the volume knobs and it appears there is some cross over between channels. When Channel A is selected and volume is on 0, there is still sound produced except when the Channel B volume knob is set to 0 as well. When channel A is selected and the volume knob for channel B is set to max, channel A works as normal.

I tried channel switching with a footswitch and the problem still occurs. Therefore it must be something to do with the channel switching on the circuit board. I read somewhere that IC8 on the circuit board can go bad - anyone had any experience with that? It is some sort of op-amp.

Any help is greatly appreciated!

[Robert M. Martinelli]

Hi,
both IC8 and IC4 are controlled by the boost switching signal ( "C" on the schematic ).
M5201s are switching dual op-amps, they're otherwise similar to 4558s, the additional switching circuitry is controlled by the status of pin 1, if you' re experiencing some crosstalk between the channels, chances are either IC 4 or IC 8 could be defective.

Hope this helps

Best regards

Bob

[zw44]

Thanks for the reply Bob! Is there anyway of easily testing to see which IC is defective? I'm trying to find a place to buy the IC in the UK, then I was thinking of perhaps buying a couple of them incase more than one is defective.

Thanks for the help so far!

[Robert M. Martinelli]

Hi zw,
If you have an oscilloscope and a signal generator it would be easy to check for crosstalk by injecting a sine wave at each op-amp input, operating the switching circuitry and checking the output with the scope.

These ICs are not as cheap as standard 4558s and the like, but you could probably get two for some 15-20 bucks.

M5201 are manufactured by Mitsubishi, equivalent types are the Japan Radio
JRC2120D and NJM2120D ( both are pin-to-pin compatible ).

You could try to contact Marshall to get 'em, I have also seen the tube amp doctor has them listed on his site, and I bet the biggest EC suppliers ( Mouser, Small Bear etc. ) have them too.

If the originals are not socket-mounted, I would advise you to add IC sockets for future easier replacement....you never know....

Hope this helps

Best regards

Bob

[zw44]

Thanks for all the help! I have now cracked it!
I emailed Marshall and they sent me a JRC2120 chip to replace the M5201 in IC8 position and it has fixed the channel cross over problems! I soldered in a DIL 8 pin chip socket to make the job easier in case it happens again in the future!

Thanks again for the help!

[Robert M. Martinelli]

Hi zw, and congrats!

I'm truly glad you did it and I'm glad I've been able to offer some help.

Cheers

Bob