JCM 2000 DSL 100 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.
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.

 

[Gunner64]

Did you change the bias pots?. They are less than robust, and do go bad resulting in unstable bias.

 

[Dogs of Doom]

one thing to check, is R6 & R9: (for stability)



also, what speaker cab are you using, more importantly, which speaker out jack(s)?

If using the 4/8 ohm outs, there could be an issue w/ the ground on the 16 ohm out jack, causing a break in the ground. That'll heat up an amp's circuit in no time, once pushed...

Have you done the ground mod yet?

schem here:

https://www.drtube.com/schematics/marshall/JCM2-60-02%20(1998)%20iss5.pdf

 

[Chris-in-LA]

Make sure that the coupling caps after the PI are not leaking DCV. Than can also cause drifting bias.

 

[Atsares]

one thing to check, is R6 & R9: (for stability)

View attachment 117992

also, what speaker cab are you using, more importantly, which speaker out jack(s)?

If using the 4/8 ohm outs, there could be an issue w/ the ground on the 16 ohm out jack, causing a break in the ground. That'll heat up an amp's circuit in no time, once pushed...

Have you done the ground mod yet?

schem here:
[URL

 

[Atsares]

R6 and R9 were already replaced. Also the PI coupling caps.
Will try the 16 ohm tap ground mod.
Tnx for suggestion.

 

[Atsares]

The bias pots are also new.

 

[Atsares]

Update.

The bias drift issue was fixed, by moving all the bias circuit parts to a dedicated PCB.
The pin 5 holes were drilled for the powertubes of course.

Tnx to all for the input.

 

[Adrian R]

REPLACE THE MAIN BOARD...period. All problems solved. It's a one hour job you can do yourself for about $285.