# JCM 900 Nightmare



## nonreverb (Sep 19, 2006)

Gentlemen! I come to you all hat in hand. I seem to have hit a ceiling regarding the repair of this amp and humbly ask for opinions from those here who have extensive experience. Usually, foolish pride would have me fighting with this for hours but I seem to have run out of ideas.
The problem: Clean channel A( I use the term loosely) has a nasty buzz throughout the preamp volume's full sweep. The scope output waveform shows a distinctive shelf on one side of the output. The overdrive channel B operates normally. Interestingly, the overdrive channel's master affects the channel A's signal as well. I've already replaced IC's 1, 4 and 6(for good measure) as well as the LED's used for clipping. I took the liberty of taking a pic of said ugly waveform...Let the fun begin...


----------



## jb welder (Sep 14, 2010)

Which 900, dual reverb or dual master? Is IC6 the switcher op amp? That would be my first look, the 5201 that's last before the power amp.
They're extinct, the 2120 sub is now extinct, I think 2121 sub is still available via mouser digikey etc.


----------



## nonreverb (Sep 19, 2006)

Switched out IC 1, 4 and 6 with M5201's I had. IC 4 seems to be the one that controls the clean channel. replacing both and IC 1 had no effect.
Marshall 4100


----------



## jb welder (Sep 14, 2010)

Ok, for the 4100 the one I meant would be IC8. If you are out of those IC's you can try moving IC7 in there (and kill the reverb circuit via footswitch).
But the IC's you removed are probably ok, so you could try one of those too.


----------



## nonreverb (Sep 19, 2006)

Pull V2, the issue clears up...I'm usually pretty good at figuring things out however, once and a while I get a dog.


----------



## jb welder (Sep 14, 2010)

nonreverb said:


> Pull V2, the issue clears up...


Not sure what you mean, pulling V2 should kill both channels? Did you try changing IC8?


----------



## dtsaudio (Apr 15, 2009)

I was wondering how you made out with this. 
About what frequency is the buzz, and for that matter, the frequency of the waveform you posted? It looks like one or more of the M5201's is having trouble switching.
Are all the other functions working. Reverb switching etc. 
Check the +15V supply. If ripple is too high it will affect the switching.


----------



## Milkman (Feb 2, 2006)

Good luck NR.

I’d need a translator to know wtf you guys are on about.


----------



## nonreverb (Sep 19, 2006)

*jb welder*. I should have been more clear. If I remove V2, while scoping the signal at IC4, the distortion disappears. I thought there might be a loading issue somewhere but I don't think that's the case. It's somewhere else. As for IC8, the master's both operate normally except for the bleed through. The gain control is where the issue seems to be. Having said that, I may just replace it anyway. At this point, I'm running out of options. Also, if it were to solve the problem, I'd want to understand WHY it solved it.
*dtsaudio: *It's not frequency dependent at all. It occurs at all frequencies and the waveform distortion stays the same regardless of what frequency it's at. One of the notable things is it starts to occur as the gain control increases. I tried removing the led clipping diodes as well as replacing them...no difference. I was thinking the same thing you are about the supply though. It's my next place to look. There is a 2 stage filter on this supply. 470uf(s) to a 270ohm resistor(s) and then to 1000uF(s). I had replaced one 1000uF on a previous service as it was bad. Of course, I've checked for ripple and both zeners are producing +/- 15V with no ripple but as we all know that doesn't mean there isn't a problem....especially since one of the 1000uF caps already went down previously. Also, it came in with non-functioning reverb as well. I replaced clamping FET (J111) and it's back to normal now.

I tried a guitar through it and as the gain increases, the distortion sounds like a blown speaker...it's awful.


----------



## greco (Jul 15, 2007)

This is more interesting than a lot of mystery novels I've read. (seriously)


----------



## jb welder (Sep 14, 2010)

nonreverb said:


> *jb welder*. I should have been more clear. If I remove V2, while scoping the signal at IC4, the distortion disappears.


Ah, got it. Maybe 2 separate faults, or the glitch is getting into control voltage 'C'? The bleed at the masters tells me that chip (IC8) is not working right so that's why I went straight to that. I had to replace quite a few of those when these things were current. The scope glitch looks real familiar also.
Maybe scope control voltage 'C' and check it's DC levels. Could be glitch getting back to IC1 & 6 from 8 ?

@greco , schematic attached. pg.2 most legible


----------



## greco (Jul 15, 2007)

jb welder said:


> @greco , schematic attached. pg.2 most legible


Many thanks! Now a mystery novel with pictures.


----------



## nonreverb (Sep 19, 2006)

Gentlemen! I've finally found the problem. I had suspected a potential problem in the supply and as dtsaudio mentioned, a bad filter cap could cause all kinds of problems and I was thinking the same thing. Although I had replaced one of the caps previously, I was not convinced that the supply was behaving even though the meter said it was.
Here's what I found and think was happening. One of the primary 470uF caps in the supply was measuring close...but not close enough. I suspect that there was a small amount of DC being conducted across it causing a voltage divider between it and the 270 ohm resistor. This seems to have occurred only when the clean channel was activated which coincidentally is when TR1 is turned on in the switching circuit. Now if I'd only looked at the power supply first instead of over thinking it.....thanks for your time gents.


----------



## dtsaudio (Apr 15, 2009)

Glad you found the problem.


jb welder said:


> The scope glitch looks real familiar also.
> Maybe scope control voltage 'C' and check it's DC levels. Could be glitch getting back to IC1 & 6 from 8 ?


That glitch looked familiar to me as well, although not with the same IC's. I'd seen that in a muting circuit from a CD player. Completely undetectable until a signal was passed. The mute was turning on and off with the ripple from a defective power supply. In this case when TR1 is turned on there is enough of a voltage drop to prevent the M5201 from switching properly.
You see this with CMOS switchers, and sometimes sold state relays as well.


----------



## nonreverb (Sep 19, 2006)

dtsaudio said:


> Glad you found the problem.
> 
> That glitch looked familiar to me as well, although not with the same IC's. I'd seen that in a muting circuit from a CD player. Completely undetectable until a signal was passed. The mute was turning on and off with the ripple from a defective power supply. In this case when TR1 is turned on there is enough of a voltage drop to prevent the M5201 from switching properly.
> You see this with CMOS switchers, and sometimes sold state relays as well.


Indeed.....That was throwing me off as the rest of the switching circuit(s) were operating normally. Also because there was no ripple present. It looks like the cap was dragging the + rail just enough to mess up the TR1 turn-on circuit to the IC. I've seen similar problems before with slow failing filter caps. Too bad I decided to try everything else first.
Thanks again for your help gents.


----------



## bigboki (Apr 16, 2015)

I would strongly suggest to post issues with Marshall amps in one of the corresponding forums at:
Marshall Amplifiers

Steve Dawson one of the main former Marshall designers is frequenting the forums there and is willing to help Marshall brothers in need with a lot of advices. 

Happy that you resolved your issue!


----------



## nonreverb (Sep 19, 2006)

bigboki said:


> I would strongly suggest to post issues with Marshall amps in one of the corresponding forums at:
> Marshall Amplifiers
> 
> Steve Dawson one of the main former Marshall designers is frequenting the forums there and is willing to help Marshall brothers in need with a lot of advices.
> ...


Thanks! I think this is the first "ask a question" post I've done in years. I usually figure them out in a timely fashion. If by some chance I hit a wall with one of their solid state abominations, I'll drop over there.


----------

