** The Marshall TSL122 JCM2000 Repair/Mods Page **

[Steve A.]

Richie Hall just posted a link to this article over at the Tube Guitar Amp Builders group at Facebook. It seems that negative temp-coefficient resistors were used in the bias circuits for some of these amps causing a runaway bias problem after maybe an hour.

The Marshall TSL122 JCM2000 Repair/Mods Page

Steve Ahola

[Jazz P Bass]

That and/ or a crappy pcb board.

[Steve A.]

That and/ or a crappy pcb board.

True enough but I don't think a crappy board would be a real headscratcher like the negative temp-compensated resistors. What the heck- they tested out okay! Speaking of mysterious board problems I had heard that some of those old Fender amps with the eyelet boards would soak up moisture in high humidity areas and have some really strange symptoms.

Steve

EDIT I see that the crappy boards can be a real headscratcher, too. My bad!

[drewl]

I've referenced that site here several times over the past few years as it's been there for a little while.

I also contributed some of my findings on the many problems with these boards and bias circuit.

[Leo_Gnardo]

I don't think a crappy board would be a real headscratcher like the negative temp-compensated resistors. What the heck- they tested out okay! Speaking of mysterious board problems I had heard that some of those old Fender amps with the eyelet boards would soak up moisture in high humidity areas and have some really strange symptoms.

Steve, and all others, I got a big clue on what to do with the 2000 marshalls (besides heave 'em in the dumpster) when I found this,

The Marshall TSL122 TSL100 thermal bias drift repair page

2000's sure are "buggy" amps. OTOH they are capable of excellent tones!

Too bad I found the article from hullerum too late to help my first customer who showed up in 2006 with a 100W JCM 2000. His output tubes got so hot they melted the glass! With new tubes installed I watched the bias current drift up and up over the course of 2 hours warmup. All I could do at the time was set bias super cold & recommended he use the amp for no more than one hour at a time. Now I know there's a better solution.

And those Fender eyelet boards - I've found leaky ones whether damp or dry. But every year @ July/August I get a stack of Fenders to fix - summer humidity increases their leakage problems. And that's not all - I've had customers bring in BRAND NEW kits using the same type board, and they have leakage right from day one. How frustrating. So if you're tempted to build on that stuff, do yourself a favor and DON'T! Build on turret board or even tie strips.

[Jazz P Bass]

I had one that responded very well (bias drift wise) to the 'resistor in the air' fix.
But it still had a persistant, progessively louder hum. (after an hour it was above 10mv's on the output)
I recommended a new board but the customer was okay with it.

[Ronsonic]

Darwinian processes seem to have weeded out any with the NTC resistor issues by now. But .... The buggy boards remain both among TSLs and DSLs.

I have one very large problem with that page and much of the internet discussion about those amps. You can just buy the damn board and replace it for much less cost and labor than trying to fix the thing. The boards are relatively inexpensive and although a pain to swap provide a sure fix complete with a fresh complement of filter caps and all new other parts.

If you are not a Marshall service center you can still get them pretty reasonably from CE and others.

Lift off, nuke from orbit. It's the only way to be sure.

[Steve A.]

Richie Hall just posted this link over at Fakebook- these are supposed to be redesigned boards for the amps (perhaps someone can confirm that):

Marshall Amp Parts Hot Rox UK

Here is the link to the TSL 100 main board:

Marshall TSL100 Main Board - Amp Parts at Hot Rox UK

Steve

P.S. Most parts are pretty cheap today so I think it makes more sense shipping stuffed boards rather than having to transfer all of the parts over.

[drewl]

Darwinian processes seem to have weeded out any with the NTC resistor issues by now. But .... The buggy boards remain both among TSLs and DSLs.

I have one very large problem with that page and much of the internet discussion about those amps. You can just buy the damn board and replace it for much less cost and labor than trying to fix the thing. The boards are relatively inexpensive and although a pain to swap provide a sure fix complete with a fresh complement of filter caps and all new other parts.

If you are not a Marshall service center you can still get them pretty reasonably from CE and others.

Lift off, nuke from orbit. It's the only way to be sure.

No, I can fix the board for cheaper than what they cost.

[Leo_Gnardo]

Richie Hall just posted this link over at Fakebook- these are supposed to be redesigned boards for the amps

P.S. Most parts are pretty cheap today so I think it makes more sense shipping stuffed boards rather than having to transfer all of the parts over.

Thanks Steve & Richie Hall! If any of my customers want to spend extra for a better board I'll consider this. I wonder where Hot Rox gets their 11mm mint green pots that are "nonobtanium" from Marshall, CE and other sources? Those are another bugaboo with this series of amps.

Presumably USA customers would get the 75 quid "no VAT" price plus shipping = @ $125 as one happy recipient reports. Sure beats the $300 that other techs have mentioned.

[Gtr_tech]

Mar$hall has copped to the crappy board material problem and had replacements made from FR4 board. Thats thew way it shoulda been done to begin with. Corner cutting plain an' simple.

[g1]

Presumably USA customers would get the 75 quid "no VAT" price plus shipping = @ $125 as one happy recipient reports. Sure beats the $300 that other techs have mentioned.

At current exchange rates, basket shows approx. $115 plus $25 shipping to US, no VAT.
Much cheaper than the $225 plus shipping from AES Marshall, Valve PCB TSL100/TSL122 | Antique Electronic Supply
Thanks for posting the link Steve!

Mar$hall has copped to the crappy board material problem and had replacements made from FR4 board. Thats thew way it shoulda been done to begin with. Corner cutting plain an' simple.

They should have really copped to the problem and replaced them all under warranty, right from the beginning. I think some of the other major manufacturers would have. I guess they don't figure it cost them much in sales, so if such a problem occurs again, it will probably be the same scenario.

[Alex R]

Well I too just put the new boards in when I meet this quite common problem, but I appreciate they are more expensive if you're not Marshall approved and in the UK. I must have a dozen faulty boards hanging around. The fault never occurs with the FR4 boards, but often does so with the old plain pale green ones. Usually it's the 100 watt TSL and DSL amps that do it, I guess because of the extra heat they make.

I don't know about this business with heat sensitive resistors, but I guess it is open to me to clip a few off the faulty boards I have and test them. The boards are heat sensitive for sure, whenever I get one of these in I test for it by clipping a vm to pin 5 with the amp running and blowing hot air at them. If the fault is present the bias voltage drops away very quickly and noticeably, often losing 20v or more. It would take a very big change in a resistor value to achieve that. HT leaking across to a trace with bias on it seems the likely explanation given the fact that FR4 boards never show the problem.

There is a danger of jumping to the conclusion that you've fixed it because the amp stays stable for an hour or so. Some of them take a few hours to get into the vicious circle of overheating and bias loss, some will only do it if you play them hard. I suspect some of the internet 'cures' are subject to this error.

If anyone who thinks they have isolated the resistors that they believe cause the problem I will clip them off some of my faulty boards and test them for this alleged value change when heated. For some reason I hang on to the old boards I replace, probably so I can steal caps off them but I never do . So that puts me in a good position to shed some objective light here... though I admit that I am very sceptical about the heat sensitive resistor theory!

One last eta: Hot Rox are a good firm in my experience, quick friendly efficient service with reasonable prices. Their competition forced some other UK suppliers to drop their prices on tubes to a sensible level.

[Ronsonic]

Mar$hall has copped to the crappy board material problem and had replacements made from FR4 board. Thats thew way it shoulda been done to begin with. Corner cutting plain an' simple.

I'm gonna blame korg*. Anybody can get a bad batch of anything, but to not tell your service centers or suggest it when they call for assistance, that's special.


* just because they're out of the loop now and are easy to blame.

[Mick Bailey]

The new boards have quite a few differences beside the board material and bias circuit fix. There are numerous other changes to the boards and many more resistor changes (a lot more metal film types). It becomes a spot-the-difference competition when comparing the latest boards to the original ones. There are also track changes and some capacitor rating changes as well as other minor component and circuit changes.

For me to re-work an original board to the latest spec would not be economical. Plus it would still be on the inferior substrate. I swap the boards out because here in England they're not too expensive, but I save any good originals for the time when new ones are discontinued and then we'll have no chioce but to upgrade/fix them. Bear in mind that some (if not all) of the 24 electrolytics on a 1998 DSL amp probably need changing by now.

It's important when working on these amps to re-solder the bias pot board. The solder joints crack on the solder pads, but for good measure I replace the pots with new ones. If you don't attend to this little board the amp loses its bias completely on at least one side when the joints eventually fail. A recent amp I worked on had melted the EL34 envelopes and the vacuum had sucked them so that they looked like fun-fair prizes. I've identified this board as another culprit for bias drift; when the joints begin to break down they become thermally sensitive and increase in resistance, throwing the bias off.

My feeling is that there's a lot of emphasis on the bias netwok resistors and not enough on the bias board.

Interestingly, not all of these early amps suffer problems as a result of the original resistors being thermally sensitive. Yes, they'll shift if you heat them with a hot-air gun or soldering iron, but in use the bias remains stable once the amp is up to temperature and the bias set when the amp is warm.