Time to fix my JCM 2000... help?

[GuitarIV]

Okay folks.

My JCM 2000 DSL 100 from 2004 died from the infamous bias drift issue about a year ago. I haven't had the funds nor the time to fix it so far, now I do.

My parents bought it for me as a birthday present so I'll never sell it, I'm handy with a soldering iron and I know how to discharge caps and work around electronics without killing myself.

I decided to go for a replacement board and found this:

Marshall JCM2000 DSL100 Power Amp PCB #M-JCM2-60-02-01

Marshall JCM 2000, DSL100 Power Amp PCB Marshall Amp Power Amp PCB for JCM 2000 DSL 100 Watt Amplifiers. This is an Issue (Revision) 20. This board fixes all issues with all older version boards. Part# M-JCM2-60-02-01

britishaudio.com

Unplugging and plugging in some connectors seems easy enough but I do remember that some guys here have recommended replacing the external bias pots as well. Found this site:

Marshall Amp Parts - DSL100

Marshall Amp Parts - Marshall Jacks

www.amprepairparts.com

Now my question is: how hard is it to replace the little buggers? Should I look for a new bias circuit PCB instead? If so, where can I find one?

Cheers and thank you!

 

[Purgasound]

If you're replacing the board that will solve the bias issue. There are ways around that but they're fairly involved and require things like using a hollow drill and building a satellite board. I don't recommend it for anyone that doesn't work on amps often.

As far as the trim pots go you should be fine to leave the existing ones. Not to say that they're trouble free, but I haven't had any issues with them in my experience. That PCB really only has the trim pots on it and the test points so it's not a complete bias circuit.

The solution to bias drift is no where to be found on the internet so you can't really use the two main tutorials out there as reliable sources. They don't address the problem, they're both pointed in the right direction but shots in the dark nonetheless. I'm speaking specifically about lynx.net and hullerhum.de. In those fixes they missed the problem entirely. Lynx.net goes on to move the bias trim pots but they were never the issue with the bias drift problem. They go a little further in depth later in the page but they're still missing the problem, all the work was a waste of time. There's no way around it and there's no shortcut except for replacing the main board. You have to physically remove every component from the bias circuit from the PCB entirely and this goes all the way back to the PI coupling caps and grid leak resistors and even further to the point where the bias supply is tapped way off to the opposite side of the board. A secondary board must be made with a complete bias circuit and mounted elsewhere in the amp. A JCM2000 just came into the shop last night so I can do a detailed video on this finally. But hey, you really don't have to worry about any of that if you're replacing the main board. I just mention it so you can take what those sites have to offer with a grain of salt. I'm sure both sites do contain very insightful and helpful information, just not about the bias drift issue.

 

[Tatzmann]

If youre dead set on replacing the board then go ahead, its all plug and play bassically.

Be careful with the connectors, use pliers if necessary and as little force as necessary.

I repaired mine with the Dr.Tube SBK, works too
and way less money involved if thats a concern.

I havent replaced the biaspots because they work fine in my amp, but some had problems with them or had bad solderjoints on them, its an option that needs case by case evaluation.

 

[Tatzmann]

@Purgasound

Why didn't you blow yours up?

(I know you didn't because i've seen it sitting on a shelf in one of your vids)

 

[Purgasound]

@Purgasound

Why didn't you blow yours up?

(I know you didn't because i've seen it sitting on a shelf in one of your vids)

lol, I used to be a DSL hater for sure. I think I've found peace with them at this point. One of them was given to me by another MarshallForum member in a trade and working on that amp was an epic saga. I would work on it a bit, get frustrated and shelf it for another year and try again later. It ended up being a series of events that led a bunch of techs including myself down the wrong path. The bias drift issue I think started the whole thing, ended up arcing a tube socket which blew some fuses, the OT and PT get replaced but no one could address any other issues in the amp because it wouldn't cut on without blowing the mains fuse. Ended up being a Classic Tone PT with mislabeled primary wires. I ended up disassembling the amp and was going to use it for pieces and it wasn't until I had the PT out for months I had to go back and test some parts to clear up space and low and behold the PT tested good but the voltages didn't match up to the spec sheet. Once I got that down I could move on to replacing the socket and solving the bias drift issue. Such a nightmare. Never give up, never surrender, unless it's an SVT reissue.

 

[jaallen]

For peace of mind, do change those bias pots while you are in there. Once that bias drift happens, they become very spotty, and bias setting on these is touchy at best. Besides, you don't want to have to bust back into that amp again, for something that is fairly easy to do now.. Its a small seperate board you can remove from the chassis.
Go slow working, but be quick on the soldering iron on the new ones, to keep heat to a minimum, and get the pads clean with a solder sucker, and don't linger with the iron on the pads too long. They can lift easily if overheated. You'll be fine and wont have to have it on your mind. And you will be able to bias with no jumping around.
I did it on my two TSL 100 heads during bulletproofing mine, and I am not as good as our great experts here. Never a problem afterwords. Just go slow and with purpose.

 

[GuitarIV]

If you're replacing the board that will solve the bias issue. There are ways around that but they're fairly involved and require things like using a hollow drill and building a satellite board. I don't recommend it for anyone that doesn't work on amps often.

As far as the trim pots go you should be fine to leave the existing ones. Not to say that they're trouble free, but I haven't had any issues with them in my experience. That PCB really only has the trim pots on it and the test points so it's not a complete bias circuit.

The solution to bias drift is no where to be found on the internet so you can't really use the two main tutorials out there as reliable sources. They don't address the problem, they're both pointed in the right direction but shots in the dark nonetheless. I'm speaking specifically about lynx.net and hullerhum.de. In those fixes they missed the problem entirely. Lynx.net goes on to move the bias trim pots but they were never the issue with the bias drift problem. They go a little further in depth later in the page but they're still missing the problem, all the work was a waste of time. There's no way around it and there's no shortcut except for replacing the main board. You have to physically remove every component from the bias circuit from the PCB entirely and this goes all the way back to the PI coupling caps and grid leak resistors and even further to the point where the bias supply is tapped way off to the opposite side of the board. A secondary board must be made with a complete bias circuit and mounted elsewhere in the amp. A JCM2000 just came into the shop last night so I can do a detailed video on this finally. But hey, you really don't have to worry about any of that if you're replacing the main board. I just mention it so you can take what those sites have to offer with a grain of salt. I'm sure both sites do contain very insightful and helpful information, just not about the bias drift issue.

If youre dead set on replacing the board then go ahead, its all plug and play bassically.

Be careful with the connectors, use pliers if necessary and as little force as necessary.

I repaired mine with the Dr.Tube SBK, works too
and way less money involved if thats a concern.

I havent replaced the biaspots because they work fine in my amp, but some had problems with them or had bad solderjoints on them, its an option that needs case by case evaluation.

For peace of mind, do change those bias pots while you are in there. Once that bias drift happens, they become very spotty, and bias setting on these is touchy at best. Besides, you don't want to have to bust back into that amp again, for something that is fairly easy to do now.. Its a small seperate board you can remove from the chassis.
Go slow working, but be quick on the soldering iron on the new ones, to keep heat to a minimum, and get the pads clean with a solder sucker, and don't linger with the iron on the pads too long. They can lift easily if overheated. You'll be fine and wont have to have it on your mind. And you will be able to bias with no jumping around.
I did it on my two TSL 100 heads during bulletproofing mine, and I am not as good as our great experts here. Never a problem afterwords. Just go slow and with purpose.

Thanks folks. I'm dead set on the new board and will order the bias pots too, just in case. If this was a regular DSL I probably would have scrapped it, but since the sentimental value is there I'm willing to save it. I'll be posting more once I have the parts and start working on it, that's for sure.

Cheers!

 

[PelliX]

Whatever you do, one of the key contributors to the failure of the boards and drift issues is heat. I'd wager that at least half the faults that occurred of that nature could have been prevented by means of a little ventilation. A small fan on the back gently extracting warm air will do a lot to counter this. Of course crap quality boards are more susceptable to heat damage, but a cool circuit is almost always a happier circuit.

 

[GuitarIV]

Whatever you do, one of the key contributors to the failure of the boards and drift issues is heat. I'd wager that at least half the faults that occurred of that nature could have been prevented by means of a little ventilation. A small fan on the back gently extracting warm air will do a lot to counter this. Of course crap quality boards are more susceptable to heat damage, but a cool circuit is almost always a happier circuit.

Thanks for the tip mate!

 

[Tatzmann]

Of course crap quality boards are more susceptable to heat damage, but a cool circuit is almost always a happier circuit.

Thats what they figured.

"How can we pull our asses out of this stupid circuitboard situation? Any suggestion with the least amount of work needed earns the extra biscuit tomorrow!"

"I got it! We have to put the heads in a secluded place where noone can see that it's actually onfire! A cool place!"

"This is how you sell merchandise! Well done!"