theman1001

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!

Jesstaa

Have you tried changing the tubes? It may only be on 1 channel, but the tubes could be the problem.

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AussiRock

How old is the amp? How long Have you had it? Were the Valves (Tubes to you yanks) replaced in the last 5 years? Were the filter caps changed in the last 5 years? Do you know a good tech? If no to the 3rd and 4th Q, change them now!!
Thats a new set of MATCHED valves and new high voltage filter caps....as matter of fact ...change all the electrolytic caps while you are in there. BE CAREFUL many good techs have died working on high voltage valve amps.

Good luck!!!

theman1001

Hi, thanks for the replies.
The tubes are fairly new (18months old) and havent had a great deal of use. I think the amp is a 1992 or 1993 model and I've had it for the past 3-4years. The filter caps look like the original ones to me, but I would have thought if they were the problem that both channels would be effected?

Jesstaa

AussiRock is kinda right. Although filter caps only have to be replaced every 10 years, and it sounds like they've had their time, I'd say have them replaced.

Also 18 months can be new for tubes, or very old, I personally try to cut it down to about 6 months at most. And only 1 channel going can be one of the first signs I guess.

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theman1001

ok, Im going to swap out the preamp tubes to see if that solves the problem. I would be surprised if they are dud as they havent had much use - but they are crappy marshall ones which I took out of my 2203kk. I will let you know the outcome... fingers crossed!

theman1001

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!

Jesstaa

Now I'm kinda paranoid mine will screw up if it's rumoured it can go bad. Although there might be a difference with heads and combos.

If you have a good local tech, I'd say take it in, explain the problem in detail to him and get him to replace the caps. Hopefully they can fix it and have it all good as new again, which is what I love about valve amps, if something goes wrong, 99% of the time you can replace a bad part and off you go.

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stargazer76

The unfortunate thing about these 900's is the use of these dang op-amps! When all is working well, everyone is happy. But when something fails, and it's determined that it's NOT something routine, like tubes, fuses, or maybe a resistor or cap, then you must start considering the possibility of a faulty op-amp. These modern marvels are pretty reliable devices, but when bad can be tough to track down in these amps. The 900-4100 DR has 8 of these buggers, most of which are dual-sided. Channel A using one side and channel B the other through the switching circuit. Input voltage is both +15 and -15vdc supplied from BR1, D3 and D4(zeners). Inputs and outputs are critical to normal operation, and they are very susceptible to static electricity. Not to mention they are troublesome to remove and replace. Take this amp to a Tech who is competent with hybrid and SS amps.

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theman1001

Thanks for the reply stargazer76. I read a post on another forum by FJAMods (a guy which mods quite a lot of marshall amps) and he said they he has seen a fair few jcm900's with a faulty IC8. On another forum I was advised that the problem I described may be due to either a faulty IC4 or IC8. Therefore I'm tempted to replace IC8 and see if that resolves the problem.

Can I measure the voltage across the IC to see whether or not it is faulty? The next problem will be tracking a replacement down! The chip is an M5201.

HAMPAMP TUBE AMP SERVICES

Measuring voltage will only tell you if you are getting the correct B+ and B- to operate the IC, you need to scope the input and output to see if the chip is failing with regards to the signal flow. You can buy the M5201 from Antique Electronic Supply for $5.95, p/n is P-QM5201

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AussiRock

Jeestaa,
Kinda right, depends on the work load. Under normal conditions 10 years for caps is about right.
My amps would need complete overhauls every five years during the 70s, 80s and 90s as I played large gigs 4 to 5 nights a week, usually with 2 Marshall 100w SLs with every thing on 10. The caps would die after 4 to 5 years and the valves every 15 months if they were lucky.
Call me old fashioned but I'm a bit dubious about PCBs in valve amps and am considering going back to plexis from my 900s.

Cheers

theman1001

Thanks for all the help guys! 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!

I've now got another problem with a rattle from the speaker... I guess that warrants a new thread!