Marshall TSL100 main board mod

[Jazz P Bass]

I have a TSL100 with the Rev 5 main board on the bench.
This is the "thermal runaway" board.
I found a link where the tech dremeled out the board around each EL34 Pin 5 & ran the 5.6K resistors directly to the pin.
Thereby isolating the bias pin from the crappy board.
It is claimed that this fixed the thermal issue.
Has anyone tried this?
Link:The Marshall TSL122 TSL100 thermal bias drift repair page

[Jazz P Bass]

Well, I went ahead & scooped out the PCB material around pin 5 of the four output tubes.
I can report that the bias wandering is gone.
What the mod did not do is eliminate the steadily increasing hum on the output.
As the amp temp increases, so does the hum.
(it may be leakage from the B+ to the heaters.)
I measured 3mv's at start & 20mv's after running the amp at half power for 30 minutes.
So, yeah, the board is crap.
The scooping got rid of the bias drift, but the hum remains.

[g1]

Thanks for the link, also followed the link to jc maillett's info. Did you have NTC resistor issues discussed? How about the phase splitter issues? Just wondering if you did all the mods or just the reroute of the grids.

[Jazz P Bass]

I did not want to do everything at once.
Next step is to change out the 5.6K's & the bias resistors 33K,10K & 220K.
By the way, this is for grins & giggles.
The customer already said o/k to replace the POS board.

[drewl]

Below is a link to another page about fixing the negative temperature coefficient resistors used in the bias supply.

So far just replacing these resistors has fixed the ones I've had in for repair.

The Marshall TSL122 JCM2000 Repair/Mods Page