JCM2000 DSL 100W Head issue resolved!

[sweatyk]

This issue started much worst and was part of another thread here that became very long so I decided to stop that thread and dig into this thing for a while without assistance. As of today the amp has been repaired. But not because I found a bad component or cold solder joint or broken trace. This amp came to me with what turned out to be multiple issues. The first issue was that not all of the tubes were lighting up and yes bad connects on the pcb along with a few bad sockets were to blame. After that it was only getting a very weak output signal losing it somewhere between V2 and V3. That’s where I ended the very long thread on here and decided to take a break and then come back to it before posting a new thread. I traced the lost signal back to the reverb pcb. Everything there seemed ok and then I had a hunch concerning the 2 jumper connections on it. When I would unplug connection 2 on the reverb board the amp came to life! Not being able to trust the schematics for this particular model I was at a loss. Until today when a technician friend said that he had the exact same model come into his shop for repairs and it was on his bench. A rare opportunity to visually look at another one to ensure that all of the connectors were in their proper place. As it turns out conn1 and conn2 needed to be swapped. Whomever worked on this amp before it came to me put them in the wrong place. I swapped them and now the amp plays like a champ. Before I disconnect any wiring harnesses I always make a diagram and list where all connections go and take pictures.

Now one thing that I have noticed is that the schematic for JCM2-60-00 PCB clearly does not match up with the labeling and some values of the components on it. The best match that I have been going off of was for JCM2-60-02. The schematic for the reverb board looks correct even though it has no stamping on it which should be JCM2-63-00. I’m posting all of this in case someone else runs into the schematic issue. From what I have seen Marshall goofed on the schematic when it comes to the JCM2000 DSL 100W head. Thanks to all that chimed in to offer assistance in the previous thread on this amp. The combine knowledge of all that are part of this forum is more valuable than anyone can realize…………….Thank you!


P.S. ......I have learned quite a bit from this experience

[Mark Black]

Wow - that was a tough one. Glad you got it fixed!

[sweatyk]

I guess celebrating was short lived. Trying to set the bias it is apparent that this is one of those models that suffers from that dreaded bias drift that plagues the TSL and DSL's from the early 2000's..........sigh.

Doing a lot of reading but the more I read the more I'm gonna advise this guy to get rid of this thing before it becomes a serious problem.

[Leo_Gnardo]

I guess celebrating was short lived. Trying to set the bias it is apparent that this is one of those models that suffers from that dreaded bias drift that plagues the TSL and DSL's from the early 2000's..........sigh.

Doing a lot of reading but the more I read the more I'm gonna advise this guy to get rid of this thing before it becomes a serious problem.

Some German guys put up a terrific writeup w/photos on how they did it, and I've done it now about 5 or 6 times. Takes a while but the amp gets fixed, no more drifty after you remove the offending bits of circus board, also good to change a couple resistors (put in 5K6 grid stoppers instead of Marshall's 220K - ?!? - what were they thinkin? I use carbon comp, and you may want to put in fresh bias adjust pots too) IMHO it's worth it. Though I'm not crazy about the way they're built, these amps do sound good and are versatile.

I use a Dremel with appropriate bit and tried it with the sockets in. Not a comfortable thing to do, the bit hits the socket pin & becomes unstable. Although it will take a good while longer, remove the output tube sockets then the task can be done a whole lot more neatly & safely.

Some people have replaced the entire board. Last I heard Marshall wants about $300 plus shipping.

here's the German website: The Marshall TSL122 TSL100 thermal bias drift repair page

[DrGonz78]

I guess celebrating was short lived. Trying to set the bias it is apparent that this is one of those models that suffers from that dreaded bias drift that plagues the TSL and DSL's from the early 2000's..........sigh.

Doing a lot of reading but the more I read the more I'm gonna advise this guy to get rid of this thing before it becomes a serious problem.

On the bright side... This one amp forced you to have to learn many different repair lessons. After I had an interaction with a beast like that the next three amps were like a walk in the park!!

BTW: Sweatyk the reason I clicked the like button on your last post was because you said "I'm gonna advise this guy to get rid of this thing." The truth is those amps are great sounding when they are working, but once they go bad it is all downhill.

[sweatyk]

The customer expressed interest in replacing the main board but sometime down the road. Knowing this individual as I do it will end up being a 90 pound paper weight before that happens. I warned him about the possible melt down if he continues to play it. When he picks it up I will advise him of the tail light warranty under these circumstances. I will look into the mod some more and if I feel up to the challenge I might offer that option to him.

[Leo_Gnardo]

The customer expressed interest in replacing the main board but sometime down the road. Knowing this individual as I do it will end up being a 90 pound paper weight before that happens. I warned him about the possible melt down if he continues to play it. When he picks it up I will advise him of the tail light warranty under these circumstances. I will look into the mod some more and if I feel up to the challenge I might offer that option to him.

Tail light warranty, that's what it should get unless that board is modified or replaced. I'd strongly recommend he carries a spare amp to every gig too. And works up his schpiel to the audience "talk amongst yourselves awhile, we have a little technical problem, be back in just 5 or 10 minutes..."

Yes it's like trying to drive around with bashed up tie rod ends, just a matter of time before one breaks and - no more steering - ka-rasssshhhh.

Forgot to mention, besides the leaky PC board, Marshall also has a crappy little disc cap between plate & grid on one of the output tubes. Never seen anything like it in any other amp. On occasion I've found this cap sizzled black and shorted. I just take 'em out when I see them. Maybe they were trying for a sort of hi frequency feedback stabilizer? Seems more like a disaster waiting to happen. Anybody thinks it's a good idea to have that cap there, replace it with one rated for a couple thousand volts. FWIW I do watch the amp's output on a scope while driving speaker loads, never saw any instability - ultrasonic bursts or anything of that nature - so the amp seems to work just fine without that disc cap.

[sweatyk]

Well I'll be a son of a.........I see Goldtop5 posted on the drift and darn if this amp didn't start to cook the tubes. It was idle and started at 81mv. After about 5 minutes it had drifted to 88mv and the tubes could be smelt from a good distance so I shut it down. And probably unrelated but the PT has developed a rather noticeable hum to it. I'm getting the feeling that this amp doesn't want to leave............................

[Leo_Gnardo]

Crank down the mounting bolts, the transformer cover on these does start to buzz. And the more current, the more buzz so when those tubes go to red plate the tranny sings along, what a circus.

When they really get going the plates glow yellow-white and sides of the output tubes cave in. Seen it. Hell of an amp, Marshall!

[TimmyP1955]

Only 7mv drift in 5 minutes? And only 88mV? No problem there unless it continues to drift. The problem is the interactivity between the two pots. Tweak one pot, then the other. The measurements match so all is well, yes? Nope! Put a bias meter in there and you may find that there's a huge imbalance in the actual cathode current.

[Drewline]

I generally set the bias to 70mv & monitor it for an hour & reset it when it is good & warm.