Marshall JCM2000 DSL100 bias drift question.

[errrrrl]

A friend needs to unload a good working condition JCM2000 that has the bias drift issue. He offered it to me for $400 with all new tubes but let me know that the amp is still susceptible to bias drift and said I can get a new pcp board that fixes the issue....

Is this amp worth trying to repair? Anybody ever fix their bias drift issue? I need another amp like I need another hole in my head but $400 and new tubes has me curious. What do you guys think RT?

 

[SpiderWars]

I replaced one and it did not fix the issue. It did improve but would still drift after it got warm. Easy to replace tho, it's all plugs. The DSLs do sound pretty good but...is it worth the headache?

 

[Rick Lee]

If the board does not have charring on it, the bias drift issue can be repaired without replacing the board. I had it done for $50 on my DSL50, which DID have the bias drift issue. I have under $350 total into the amp and I run it full tilt all the time.

 

[Wizard of Ozz]

Known issue with the DSL amps. Not a deal breaker. Swapping the bias pots can help. Worst thing is you have to rebias more often every now and then.

 

[fusedbrain]

If the PCB has the bias drift issue, then the board material has become conductive and is Fuk'd.
Voltage from (I think the HT circuit, might be the heaters ) conducts into the traces for the bias voltage, making the bias voltage less negative and causing the runaway bias condition. Gets worse as the amp heats up whille you play.
You have to replace the main board.
A replacement board is about $225 and used to be available as a spare part from Marshall.
Amplified parts used to sell them, but are backordered on them.
I'd contact a Marshall distributor and confirm availability and price before commiting to buying the amp.
I've done the board swap on a DSL100 that I picked up cheap.
It's a pretty simple job, and it fixed the issue. No soldering involved, but you have to unplug a boatload of ribbon/connector cables between the main board and the other boards, so you better take lots of pictures and use some label tape to keep it all straight. Plan on about an hour - hour and a half of your time.

It's up to you whether you think its worth the time, effort and cost.
I'd pass.....

 

[errrrrl]

I replaced one and it did not fix the issue. It did improve but would still drift after it got warm. Easy to replace tho, it's all plugs. The DSLs do sound pretty good but...is it worth the headache?

I googled the issue today and saw a few post saying it didn't fix it entirely too... yeah honestly prob not worth the headache. Thanks!

If the board does not have charring on it, the bias drift issue can be repaired without replacing the board. I had it done for $50 on my DSL50, which DID have the bias drift issue. I have under $350 total into the amp and I run it full tilt all the time.

I think if I could be only $350 all in I'd do it but $400 + @245 plus shipping for shits and giggles probably isn't worth it.

Known issue with the DSL amps. Not a deal breaker. Swapping the bias pots can help. Worst thing is you have to rebias more often every now and then.

After looking at gut shots i'm not sure I want to that route now.

It's up to you whether you think its worth the time, effort and cost.
I'd pass.....

And that's pretty much where I am at. I mentioned to my wife about a cheap amp/good deal/helping a bud out and just got a semi shit look aka the are you serious look lol. I'm so maxed out on space I need to make new amps really count and probably not be fkn with pcp's n stuff. Thanks for chiming in dude

 

[SpiderWars]

IIRC, it is actually the screen traces (they are adjacent pins) that conduct and cause bias to drift less negative. As mentioned the board only gets conductive as it warms. The fix is to cut the traces and use jumper wires (I think it was the screen traces that were cut/rewired but could have been the grids).

 

[JSutter]

https://www.squier-talk.com/threads/i-j ... ove.46186/

Good read on the similar TSL.