Marshall JCM2000 TSL122 bias headache....help?

mr crunchyfrog · Not So Newbie Joined: 28 May 2008 Posts: 5 Location: uk

I have a TSL122 100W combo that's driving me nuts!
Something somewhere is making the bias unstable, and the tube current draw just keeps slowly rising. With both bias pots fully cold it starts off ok with about 32mA through each side, then slowly rises, a bit faster on one side than the other.
I have replaced all the components in the bias circuit, resoldered most of the main board, cleaned all the tube sockets thoroughly, all with no effect.
All the voltages check out: 490V plates, -47V bias (at max), 480 on the screen grids. The plate and screen voltages fall slowly when the current draw starts increasing, falling to about 480 and 465 when the tubes are drawing 90mA (one side).
I would have thought that if the B+ was falling then the tube current would decrease, so it must be the tube current making the B+ fall???
All the tubes are fine (tested on a Hickok tester), so what is going on? I guess it must be something heating up but what? Nothing seems to be getting very hot (apart from the tubes obviously and they're no hotter than they should be).
I am stumped! Any ideas please...

russ@savage · Posted: Wed May 28, 2008 5:34 pm Post subject:

your tubes may test fine on the ol' hickok, but try another set any way. The tube tester will tell you if you tubes are good, but they still can have issues it can't sniff out. If another set does the same thing, then check your OP XFMR.
I have seen a lot of bad ones in those amps. Remember, if everything makes sense and the amp is still messed up, it's probably the transformer.

mr crunchyfrog · Not So Newbie Joined: 28 May 2008 Posts: 5 Location: uk

Thanks for the input, but I should have mentioned I've tried different tubes with the same result...
I've also tried wiring in another OPTX, and again that's not the problem...
I just read a depressing post on the Marshall forums with some poor guy with what sounds like the same problem on the same amp - he's had it back to Marshall 8 times and they can't fix it! Just goes into runaway and burns out tubes...
They might sound good but they ain't well built - just look at the crappy pots and the crappy fotswitch socket that just can't wait to break...
Still, I haven't given up yet...

russ@savage · Posted: Thu May 29, 2008 3:58 pm Post subject:

[They might sound good but they ain't well built - just look at the crappy pots and the crappy fotswitch socket that just can't wait to break...
Still, I haven't given up yet...[/quote]

Yeah I know. It's hard even consider them Marshalls, but that's a subject for another post.

Have you pulled the main tube ckt board to check for soldering? Checked the little jumper to the bias board? Screen grid resistors OK? If so, I have seen the little cap (20pf @ 1kv?) on the screens (I think, it's been a while since I've been in one) of the power tubes cause all kinds of weird oscillations if it shorts. That could cause your bias to run away. Just pull it out to see if that is the deal.

mr crunchyfrog · Not So Newbie Joined: 28 May 2008 Posts: 5 Location: uk

Yep, resoldered all of the power tube section of the main board, replaced all the grid resitors, checked the little bias board - swapped it for one from a DSL50 which has the same board...
This appears to be an old one so it doesn't have the little 500V cap on the last power tube...
I guess 2 unpleasant solutions would be to try and get a new main board (how much would Marshall want for that, and what are the chances of it doing the same thing later on? The more I look, the more stories like mine I find, unfortunately not with solutions..) - or to just rebuild the whole main board properly on tag boards...not exactly quick...
Hmmm....

russ@savage · Posted: Fri May 30, 2008 4:34 am Post subject:

Are you an authorized Korg/Marshall service center? If not, you should send the amp chassis to some one who is. Parts at cost and direct support to the Marshall Techs. The head guy in New York is pretty sharp. I see you are in the UK, shipping to the US would be pricey, but so is a new board. If you can even get one, they are about $250. They are not listed on the service center parts list, but I'm pretty sure that's about the going rate.

mr crunchyfrog · Not So Newbie Joined: 28 May 2008 Posts: 5 Location: uk

I'm not a service center, just a simple tech guy. The reason I posted this stuff in the first place was 'cos I've never failed to fix a valve amp before, and I don't intend to start now if I can help it...

Ended up ordering a new board - talked to Marshall, and while their tech people were pretty unhelpful, they appear more than happy to sell you things...
New board only £70 - it's nice for once to pay less for something than you folks in the US! I guess it's because the factory is only about 80 miles away from me.
I'll be pleased when the board's in and it goes home to it's poor owner.
Cheers.

russ@savage · Posted: Mon Jun 09, 2008 6:25 pm Post subject:

Hey man, did you fix it?

mr crunchyfrog · Not So Newbie Joined: 28 May 2008 Posts: 5 Location: uk

Yep, all ok now with the new board in, although the mains tx was putting out 7.2V for the heaters so some serious resistors were necessary to get it back down to 6.3

The new board is "issue 22" which I guess means they've had to fix something on it 21 times! I notice they've uprated the little 500V cap to 2kV too...

I hope I never have to work on one again...

nigelwright7557 · Posted: Wed Jun 11, 2008 2:03 pm Post subject:

russ@savage · Posted: Wed Jun 11, 2008 4:12 pm Post subject: