TSL100 bias again

[Alex R]

I have two TSLs in just now with overheating problems. Sigh. A head and a combo.

I've been working on the TSL122 combo amp. Having replaced everything in sight in the bias circuit (details below) and installed two different sets of new EL34s I find it still overheats. I can settle the bias down after half an hour, then I leave the amp on and after about an hour the plates of all four tubes start to glow, and then no adjustment on the (new) trimmers will bring them back.

By that time the whole amp is very hot. Heat-related problem I thought. Took the chassis out and found that hot air applied to the general area of C36, C37 etc (bias smoothing caps and associated resistors) produced a drop in bias voltage, quite noticeable.

With the board good and hot the bias voltage was down more than 10 volts. Looks like a vicious circle of overheating going on. Tried the freeze spray on everything I could see, but that generated nothing indicative.

OK tried then to isolate the problem. Lifted the PI coupling caps, removed the link to the mute/VPL and the link to the opamp supply on the back board. Now it was a bit harder to produce the voltage drop by heating the board, but it was still there. Not so much current draw on the circuit I guess.

Anyhow I don't seem to be able to trace this down. So maybe I'm on the wrong track altogether. Any ideas? Or shall I just fit a fan?

Parts replaced:

output tubes
C6, C7 (post-PI coupling)
R67, R69 (220k bias feed)
C36, C37 (bias smoothing)
R68 (bias supply to ground)
C42 lifted - symptoms unchanged
bias trim pots
C43, C44 (bias feed caps)
C36, C37 (VPL/mute coupling)
C46 (plate-to-screen)
D7-D10 all check ok (bias supply bridge)

[guitician]

Have you tryed different makes of tubes? Some are better than others in any particular application.

[tim]

I thought I remembered reading somewhere about these series amps that Marshall used negative (or positive, I can't remember which) coefficient resistors in the bias circuit on these which were causing all sorts of bias drift problems...I'll see if I can dredge up that site.


here ya go...

http://www.lynx.bc.ca/~jc/TSL122.html

[Alex R]

Yes, tried a set of ok Chinese ones, and a set of EH.

[Alex R]

I thought I remembered reading somewhere about these series amps that Marshall used negative (or positive, I can't remember which) coefficient resistors in the bias circuit on these which were causing all sorts of bias drift problems...I'll see if I can dredge up that site.

That sounds interesting - thanks.

[Alex R]

Is this the page you were thinking about, Tim?

http://www.lynx.bc.ca/~jc/TSL122.html

Scrolling down to the comments was interesting.

Looks like a new board or a fan is the answer.

Any views on all this?

[tim]

yup...I hate working on these amps because for one thing, they are not "tech friendly" to work on, plus, as you can see from this web page, they are fraught with issues and sometimes you just have to pack 'em up, give 'em back to the owner and wash your hands of it. I try not to work on this series amp....

[Mars Amp Repair]

Hello,
On the TSL100 or 122 schemo there are actually 2 sets of capacitors with the references C36 & 37. One set are on the power amp schemo & as you mentioned are bias filters.

The ones that really cause havoc with the bias are on the 'OD channel' schemo in the upper right hand corner and are coupling caps to the DI buffer opamp IC5a (C36 & 37). They are 100n 63V rectangular block types on the front pcb near the VPR & muting switches.

When the power tubes short like grid to screen or plate, these little 63v rated caps get hit with a jolt & become resistive or just plain flakey.

So try replacing these little buggers. I use a much higher voltage rating, at least whatever I can fit in that location.

I've seen this so many times that when I have any weird bias issues on the TSL, I just automatically replace these guys.

Also as I have mentioned before, the 1kv 22pf replacement for that little anti-oscillation cap on the right most power tube is beginning to come back leaky or shorted. I am now replacing it with a 2KV cap.

hope that helps, glen

[Alex R]

- thanks glen!

yep changed the VPL/mute coupling caps too, and the plate/screen cap with 2 x 66pf 1kV in series!

Called Marshall this morning and ordered two new boards, trade price not bad and they said, we always recommend a new board for bias problems on these and keep the price low for that reason. The fact that those VPL/mute caps aren't actually on the main board is well worth remembering in this context!

Nest time I see one of these it's going to be new board straight away, no messing. I put two whole days into that bloody combo, can't charge for that...

[Amp Kat]

If you think your getting bias drift simply measure it and monitor for awhile. If it has a problem it will fluctuate at idle under load. The bridge rectifier in these coming off the heaters from the power tubes to DC the preamp tubes are a problem also so look for burn marks or dark spots around that rectifier. If you replace it put a higher current type than the one in it.

[oskar]

Yes, tried a set of ok Chinese ones, and a set of EH.

Try the Svetlana SED -C- theese are the most stable on G1 current caracteristics tube on current production , they are dead stable , they are my recomendation on EL34 upside down mounting amps , the worst are the JJ , never use them upside down

And yes the JCM2000 bias circuit is a crap.
If it is old ones , from 90ś , it will have the 220k grid stopers , reemplace them by 22k.
Always put a 100k resistor from pin 1 and 2 of the bias trimers (from B- to wiper) in the event of the wiper of the trimer made bad contact (by hot or age) the tubes get without bias , putting this resistor the tubes will receive the max B-.-

Sorry for my poor english , I hope you understand it.
Regrads
Oskar.

[Alex R]

Thanks for all the suggestions!

I got the new board(s) from Marshall. Once fitted, the heat problem went away. Hot air gun on the board, no drop in bias voltage. In future when i see that voltage drop with the heat gun on the board i will go for a new board straight away.

[Mars Amp Repair]

I'm curious,
what is it that is different about the new board that would keep the bias from drifting? has Korg indicated any improvements, or is it just a matter of components getting older or drifting/drying up from the heat.

Persoally I'd have to have a really good reason to replace an entire board for one issue. PLEASE...I'm not passing judgment...I just wonder what they think is the route cause.

I have only run into the bias drift issue when the DI coupling caps have been leaky...has anyone ever found anything else actually verifyably causing the bias drift?

thanx, glen

[Alex R]

I'm curious,
what is it that is different about the new board that would keep the bias from drifting? has Korg indicated any improvements, or is it just a matter of components getting older or drifting/drying up from the heat.

Persoally I'd have to have a really good reason to replace an entire board for one issue. PLEASE...I'm not passing judgment...I just wonder what they think is the route cause.

I have only run into the bias drift issue when the DI coupling caps have been leaky...has anyone ever found anything else actually verifyably causing the bias drift?

thanx, glen

Fair enough - I feel the same, and I put over two days work into this amp before giving up and calling Marshall (we can call Marshall direct in the UK). Earlier in this thread are listed some of the things I changed - but basically I changed everything. The condition was, that with the power tubes out, a hot air gun gently applied to the board in the area of the bias filter caps and resistors would cause a distinct drop in bias voltage - up to 10 volts before i started to worry about getting the board too hot. However the freeze spray did not isolate a component for me, though applied generally to that area of board it brought the bias voltage back up.

This mirrored the situation that would occur inside the amp with the power tubes IN, after an hour or so, as a vicious circle of heat and current developed and replating began.

None of the component changes I made stopped the bias voltage drop under the test conditions mentioned. The new main board stopped it dead. No bias drop under heat conditions at all now.

Obviously the upside-down combo amp is going to be more prone to this heat-based problem, but the TSL100 head I have in shows the same symptom. Not going to start changing components on that one, life is too short, new board straight away! - I ordered two. The owner of this one has a Greek island gig for six months coming up....

The advice from Marshall was, just fit the new board if there is a problem with heat on these amps. The trade price of the board was reasonable.

So there you go. It might be a component, but who knows which one. I don't like 'electrically porous boards' type explanations (specially not with Mesa Boogies!!) - but it looks that way.

Marshall have been very supportive on this issue. In the UK their service and support for the small repairer - and thus for the musician - is second to none.

[Mars Amp Repair]

Fair enough as you said.

Yeah in that scenario I would do the same. Sure don't want a failure when the guy is off in a far-away land trying to make a living gigging...I think he'd be back in your face for sure.

I also certainly understand how we feel after about 4 hours plus beating our heads against the wall. Been there, done that...ya gotta make a living.

I'm with ya now...I'll have to try ferreting out this heating issue the next time I have one of these & I have the time (like that will ever happen ;-] ). Thanx, glen