JCM2000 DSL100 bias drift issue

[atsares]

Hello,

I can't seem to solve the bias drift issue on my JCM 2000 DSL100.
The amp is a 1998 model. Main board JCM2-60-02 Issue 5. Schematic: https://www.drtube.com/schematics/marsh ... 20iss5.pdf
The things already done to remedy the problem:
Everything in this post : http://www.hullerum.de/Marshall/TSL122repair.html
Drilled pin5 on the power tubes, replaced the resistors.
Changed the PI coupling capacitors.
Made a direct line from the PI coupling caps to power tube resistors, destroyed the old traces.
New power tubes.

The standby ON/OFF test without the tubes installed is ok, voltage readings remain stable ( -45V / -44.8V ).

The odd thing is that the bias rermains rock solid with no PI tube installed ( changed the PI tube also ).
After i put the PI tube in the bias starts to drift. Right side way more than the left.

Any suggestions at where to look next would be greatly apreciated.

Ty.

[John_G]

Hi,
Try lifting the newly fitted PI caps from the bias side up in the air.
Run wires directly over to the O/P tube grid resistors.
Not pretty but it will ensure that there is no longer any contact with the PC board on the bias side. I found that there mustn't be any contact with the PCB on the bias/grid side
Just my 0.02cents worth.
John

[bepone]

amp is disaster. it must be thrown in the trash. or remove all motherboard and rebuild one good JCM800 from it, using only transformers. i quit from repairing this

[martin manning]

That's a real mess. Apparently it can be made to work by hacking up the original board, but life is too short. Marshall dealers can supply a replacement board with improved material and components, which is a better fix in the long run, IMO: https://www.tubesandmore.com/products/b ... 00-dsl-100