Marshall TSL 100 Bias Drift

[gbono]

I have a TSL 100 that appears to have an issue with bias drift. Initially, I can set the bias on both pairs of EL-34s. After about 10-15 min one side keeps drifting even with the fine tune bias pot maxed out. The date code (1998) and board serial # put the amp in the time zone for the issue where the PA PCB epoxy failed overtime. Was it common for only one side to fail?

I'm thinking I should:
*Switch the tubes and see what happens?
*Monitor the grid pins to see what the voltage is doing (worst case going positive)?

The plate voltages on this amp are above 500V - I see that 90mA is "generally" recommended for "correct" bias but I'm think 70mA makes mores sense.

Other issues that were uncovered:
*I replaced the grid stopper resistors since they were 200K instead of 5.6K.
*Output jacks are connected such that the jack for the 16 ohm output also supplies ground. The 16 ohm jack had a burned contact causing an open.
*There was a burned ceramic cap in power section (C46) on V8.

THX for the help.

[gbono]

TL10-62-02 (1998) iss6.pdf TL10-60-02 (2001) iss4.pdf

[Leo_Gnardo]

The plate voltages on this amp are above 500V - I see that 90mA is "generally" recommended for "correct" bias but I'm think 70mA makes mores sense.

*Output jacks are connected such that the jack for the 16 ohm output also supplies ground. The 16 ohm jack had a burned contact causing an open.

I jumper over the dodgy contact on 16 ohm jack.

Yes 70 mA is plenty enough.

Recent thread details*, there's no point in fiddle faddling with the board any more. One pair drifts more than the other? In time both will just get worse. I've worked on 8 or 10 or a dozen of these maybe a couple more. (First one I saw 2006, was so bad glass melted and imploded the output tubes, OW!) Antique/Tubesandmore.com has the whole board, I think it was $210. Dig deep & do it right. Imma no more feex these dam' boards! Replacement is the answer, don't mess around.

When working right, imho these are actually quite good sounding amps, also versatile with 3 discrete channels. Worthy of restoration/repair.

* here 'tis so you don't have to search:

https://music-electronics-forum.com/...-drift-problem

[vintagekiki]

http://www.hullerum.de/Marshall/TSL122repair.html
The Marshall TSL122 / TSL 100 thermal bias drift repair page

https://www.squier-talk.com/threads/i-just-couldnt-resist-jcm2000-tsl122-needing-love.46186/
I just couldn't resist... JCM2000 TSL122 needing love...

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/41741-marshall-tsl-100-bias-drift-problem
Marshall TSL:100 bias drift problem

https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-JCM2000-TSL100-100W-Schematic.pdf

1)
http://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-JCM2000-100W-DSL100-Schematic.pdf

On Marshall JCM2000 100W DSL100 schematic see page 5 for bias board where are separate bias pots for V5, V6 and V7, V8.
Check CON1 and CON11 (bias connectors) for secure contact.
Bias is measured at CON2 separately for each pair tubes.

https://www.youtube.com/watch?v=VrUNJhD0yv4
How to bias tubes in Marshall JCM 2000 DSL

[g1]

Dr.Tube is still selling the bias mod board. I'm not sure how much cheaper it is than the full board replacement.
Also, if the main board is conductive, eventually there will be other faults aside from bias issues.

[gbono]

Thanks for the info/links - what I suspected but thought would be repairable. The board is on backorder so this repair goes on the back burner for now.

[GainFreak]

I can set the bias on both pairs of EL-34s. After about 10-15 min one side keeps drifting even with the fine tune bias pot maxed out.

I had recently the same problem with a friend's amp. Here's how I solved the problem:

https://music-electronics-forum.com/...-drift-problem

However note that this is only a temporary solution as with the time the board ages and generates more problems.