Marshall TSL 122 positive bias voltage

[Jon Snell]

I don't think that's it.. I've done measurements on V7 and V8 and they measure the same as V5 and V6 and there is no visible charring. I'll look again, but... What gets me is R67. How in the hell does one side of it read a good negative bias voltage, the other side of it read +120 volts, I change it and it stays the same?

Are you saying R63 has nothing to do with your bias voltage?What is going on at CON12? Disconnect it and what happens?
Please measure the exact voltages, with respect to point E and post them. That will show you where the annomally is coming from.
Don't get side tracked with spurious things like hot resistors etc.
Use a Fluke DVM, not a cheap meter (for accuracy).

[g1]

Your other thread had 'not the bias drift issue' in the title. Yet this kind of problem is exactly what the bias drift problem can cause. How did you determine it's not the infamous 'bad board' issue?

edit: just caught this from the other thread:

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]

Amp is a 2006. They fixed the problem in 2004. When the amp first came in, bias was rock solid. The switch was taken out by a shorted power tube so was C46 and a host of other components. I played this amp for an hour solid before it left the shop and my bias did not drift. He was playing it in a hot garage for 3 hours when the amp started smoking and the back plate was too hot to touch. This amp needs fans on it. The plate voltage is < 530 volts with the tubes out and in the "play" position. When he brought it back to me, I noticed the burnt smell and saw the only visible burnt resistor was R63 and the HT fuse was blown. 4.7k, 1 watt. I replaced it and took a couple resistance measurements and turned it on with the tubes out and that's when I noticed that V5 and V6 read positive 130 volts DC on pin 5. V7 and V8 both read -40 volts DC on pin 5. And here we are.

[Danglin' Fury]

Hey Jon, here are those voltages with respect to (E):
D. -40.6
C. 133.5
B. 134.8
A. 145.3


I noticed the voltage at R67... On one side it is -40.6, the other it is +133.5.... I thought "R67 is bad"... So I changed it. No change in the voltage. So I changed it again, after measuring the replacement. Same thing. No change.

[Helmholtz]

Hey Jon, here are those voltages with respect to (E):
D. -40.6
C. 133.5
B. 134.8
A. 145.3

Is this with CON12 disconnected?

[Danglin' Fury]

It was not disconnected. I will do the same measurements with it disconnected.

[g1]

If it's the bad board problem, the replacement boards are supposedly discontinued. But the repair kits are still an option: https://www.drtube.com/product/dr-tu...-kit-of-parts/

[Helmholtz]

If it's the bad board problem, the replacement boards are supposedly discontinued. But the repair kits are still an option: https://www.drtube.com/product/dr-tu...-kit-of-parts/

Did you ever use this repair kit?
I'd be interested to see the instructions.

Isn't there a way to merge the two threads?

[g1]

Did you ever use this repair kit?
I'd be interested to see the instructions.

I haven't used the kit. There is more info about it (and the cause of the issue) here: https://www.drtube.com/modification/...able-bias-mod/
From the sounds of it, there is no real year when the issue was finally cured, and even the rev.20 boards could end up failing.

[DrGonz78]

You know the whole thing is predictable but not predictable. I ran a jcm2000 tsl100 from 2003-2009 nearly daily. I went through a set of tubes every 9-12 months. I had a really good company Field Services always do our bands repairs. Just mentioning them because they were always an honest reliable business. In all that time the amp worked beautifully. Very dependable and predictable. I’m pretty sure my amp was late 90’s or early 2000’s. I’m betting that most of the amps that were not well maintained could have overheated boards. My other guitarist in the band had the same amp.

Later on around 2012 I fitted his amp up with new tubes and sold it off for him. It did not have overheating or any symptoms of runaway bias. It sounded great and had been well maintained.

Around 2014 I picked up a jcm2000 for $225. For that price I was willing to just had over the money. It actually worked pretty good but within a few minutes there were pops and issues. Nothing actually that bad but opening the amp revealed my suspicions of a bad board. I waited till 2020 as I kept up with researching the board issues. By the time I had to choose between doing some weird things to the circuit board or ordering a replacement. I chose a replacement and never looked back. So I think some of the amps hold up better than some other ones.

[Danglin' Fury]

Ok, I finally got the A,B,C,D voltages with CON12 disconnected
Voltages are:
A). 164
B). 134.8
C). 144
D). -43.4

[Helmholtz]

Ok, then the positive voltage must be coming from the track between point C and the grid stoppers..
A conductive board is very likely.
I'd consider getting the drtube repair kit which looks like a good solution.

[Leo_Gnardo]

For what little it may be worth, I discovered this article and used these clever German fellows' method @ 15ish years ago and been doing that since. http://www.hullerum.de/Marshall/TSL122repair.html Replacement boards apparently were theoretically available, expensive of course. Not always in stock, naturally.

I must admit, the dr tube repair kit plus hollow drill certainly can ease the difficulty of isolating pins 5, PLUS bolster the reliability of this repair because Marshall's bias adjust board is another poorly thought out disaster with trimpots often defective as well. Next time. And there will almost certainly be a next time. Some of these amps have arrived with output tubes MELTED. Geeze!

Someone wrote on an earlier discussion about the JCM2000 meltdown problem "If it was an automobile, there would have been a recall and the manufacturer would have to fix them properly at no cost to the purchaser." Since it's Marshall, it's ha ha we got your money and too bad for you. Throw it in the dump and buy another one from us.

Back then, 2010 or so, word was out the problem had been avoided starting around 2006. Marshall may have changed the material the board was made from to one that didn't exhibit the problem of becoming partially conductive when subjected to heat over the course of time. Indeed I haven't seen this problems in amps dating from 2007 on. So, up to 2006, consider them suspicious.

[Helmholtz]

Also interesting the point about the "wrong" 220k grid stoppers.
Seems the typical 5.6k value is preferred soundwise.
That's interesting because the main difference should be much less blocking distortion with the 220ks.
So maybe some blocking distortion is desirable for a Marshall sound?.

[Danglin' Fury]

Damn. And this is a 2006. Good to know... Well, I guess I'm ordering the Dr. Tube bias board. I was REEEEEAAAALLY hoping it wasn't that. Oh well. Lol! Thank you all for your time... I really do appreciate it. I will report back on this thread when it is truly fixed. And man, I really wish they had cut a vent in the top of these damn combos. They get so flippin hot.