Marshall TSL 122 NOT bias drift.

[The Dude]

Ok, got the bias sorted.... apparently the left pot adjusts the tubes on the right??? Weird... Also, when the Standby switch is engaged (sound off), I'm reading ~259 volts DC at pins 4 and 5. When I disengage the Standby (sound on) I'm reading ~ 530 Volts DC.... Do these voltages seem a bit high?

Is that with tubes in or out?

[Danglin' Fury]

It was with the tubes out...

[g1]

It was with the tubes out...

That sounds about right for this model. Seems they run higher B+ in these than a lot of other Marshall models. Ballpark 500V with tubes in seems common from other posts.

[Danglin' Fury]

Gotchya.... Well she is fixed... Had an oops while I was testing it... R63 smoked. I changed it and beefed it up to a 2 watt 4.7k. I'm gonna recommend that the customer run a fan on the power tubes... I really don't know why Marshall didn't put some sort of vent on the top of the amp so heat could leave the damn chassis..I may make one... The rear panel gets so hot I can almost not touch it. But I just wanted to say thank all of you for your help. You guys are the best forum on the net. Hands down. May God bless you all...

[Greg Robinson]

Not sure this has been addressed yet - but does this amp have the rev20 main board? If not, it's time to give up. There was some kind of manufacturing problem with the FR4 used in earlier revisions that causes the circuit board itself to become conductive, and while cutting slots in the pcb to isolate pins may slow down failure, ultimately the only proper fix is to replace the board itself.

[Danglin' Fury]

This amp is a 2006. I thought they fixed the bias drift after 2004? When this amp first came in, it had a rock solid bias. It would not drift. The main problem was a tube shorted, causing C46 to go and the HT was half of what it was supposed to be. Also R76 and R6 and R9 were toast. But the bias would stay solid for over an hour. The switch was the culprit. The person previous to the current owner crossed the standby wires one on the left, one on the right and when the tube blew, the surge smoked the switch. Jumpered the HT wires and full voltage came back.

[Greg Robinson]

The board issue did not show itself until after a number of years and use. I still occasionally get amps across my bench that are only now beginning to show the symptoms.
I'm not sure what year of production they fixed the issue - what I do know is that anything prior to revision 20 is suspect.
So which revision is it?

[Danglin' Fury]

Well shoot... Apparently my board is issue 6.. I guess this means I'm screwed, huh. All I saw was amps after 2004 were "good"...

[g1]

For the bad board problem, the replacements are reported to be discontinued. But the repair kits are still an option: https://www.drtube.com/product/dr-tu...-kit-of-parts/