Problem with 2003 Marshall JCM 2000 TSL100

[Luco]

Hi all, I am new to this forum and have found much useful information. Alas I have not completely solved my problems and I require some help.

History of Amp
I got my JCM 2000 TSL 100 new in 2003, about 10 months on it developed a crackle. I had it fixed under warranty. I don't know what exactly was wrong, the techs told me it was a bias problem and because I didn't know what they were talking about I asked no further questions. The Active valves were replaced with Mesa EL 34s at the same time.

I replaced pre-amp valves 1 and 2 approx 3 months ago. Pre-amp 3 and 4 are the original valves. On average the amp would get 5 - 10 hours a week usage generally at low volumes. I rarely touch the Mute or VPR buttons because I am aware of problems with this model amp.

Current Problem
The amp has been brilliant until last night, when it suddenly dropped to a very low volume. I turned it off and let it cool down for about an hour, turned it back on, it went fine for about 5 minutes then dropped volume again as soon as it warmed up.

This morning I tested again and with more sober ears and less bad playing in the background noticed the amp was also crackling when I played, after approx 10 minutes I lost volume again.

I then adjusted the bias (I found left side at approx 60ma and right side approx 90ma). Adjusted to 87ma (backed off a bit from Marshall's recommended 90ma) each side and allowed to cool down.

Turned back on, crackling was still present (again, only while playing) but the volume has remained constant (by the way, no valves are red plating). Tested it over about 1.5 hours, volume did not drop out, crackling remains.

I ran a line out from the effects loop to another amp - no crackling. I then plugged another amp into effects return - crackling was present while playing. Based on what I've read on this forum this suggests that pre-amp is fine and my problem is something to do with the active.

I have just moved approx 4 hours drive from the nearest tech I would trust to fix the prob and no rip me off. Any suggestions would be greatly appreciated and save me a long drive, but please explain as much as possible as I am very much a novice when it comes to electronics - but I have a multimeter and a willingness to learn.

Az

[Luco]

Update - the volume problem is now back. I have also been monitoring the bias readings which keep changing, twice now it has changed so signifficantly that it exceeding 150ma and began red plating. It is always the right side (left side adjustment pot) which differs signifficantly

Could this be as simple as a valve? They are close to 6 years old.

[bluesfreak]

If you would have used the search function you probably would know already that there has been an issue with runaway bias in TSLs. The problems are mainly resistors having a negative temp coefficient which causes the bias to drift away when the amp warms up, Marshall back then fixed this by replacing the whole board so either you order a new board or if you have the knowledge replace these resistors with metal film ones.
About the crackling: due to the fact that your tubes already redplated (how often did they do that before?) it could be that they got a hit already and need replacement. Check out this link for more information: The Marshall TSL122 JCM2000 Repair/Mods Page but be aware, working on these amps guts can be dangerous a potentially lethal voltages are present inside the amp. Just do it if you are really confident about your knowledge and skills...

[Enzo]

Tubes are certainly the first suspects. if you suspect a tube, take the existing output tubes and swap positions. Now the red plating either moves with the tubes or it stays with the socket.

[Luco]

Thank you both,

The tubes only red plated twice, I was watching closely both times and switched the amp off immediately both times. This happened after I had switched positions of the tubes. Unfortunately, I didn't write down which tubes I had switched around - bit silly of me. I am fairly certain the tubes had not red plated prior to this point.

I have run the test suggested by Enzo, and wrote down the results. In original position I read 60mv right pair, 90mn left pair. When I switched positions the results were reversed. (I re-adjusted the bias yesterday to 70 either side, but switched some tubes).

I ran this experiment a few times with fairly consistent results. However, on 2 occasions I got spikes in the right side pair, at one stage up to 168mv. I turned the standby switch to off before tubes could red plate. This is exactly what happened yesterday when I got the red plating.

I have opened it up and there are no obvious signs that the mother board has been heat affected or that any fuses had blown. However, I do have quite the untrained eye.

So, I seem to be getting 2 conclusions out of this. Firstly it appears that the valves have reached the end of their useful life. Secondly I am getting some bias drift.

Unless there are any better suggestions, I am going to try new valves. If the bias problem persists I will replace the motherboard also.

Az

[Enzo]

Of course new tubes are a good idea. Install new tubes and bias it up. See if any issues arise. if it turns out the old tubes were still working, than at worst you now have spare tubes.