Marshall TSL 1000 high bias voltage - help

[sonic_tooth]

Hello,

I am trying to fix a Marshall JCM 2000 TSL. When I got the amp one of the power tubes was arcing. I replaced the tubes. Now I can only bias half of the tubes to the suggested 90mv. The other 2 tubes come in at about 120 mv with the bias control all the way down. The voltage increases slowly as the amp sits powered up. It seems like this happens to other users..... Has anyone ran into this or know where to start troubleshooting? A bad cap perhaps??

[tim]

Some of the earlier tsl's had a 22pf cap ( I think it was 22pf) tacked onto the end power tube socket on the solder side of the pcb. that cap was grossly under rated voltage wise. replace it with a 1kv cap. I've actually seen that cap catch fire....for the longest time, Korg didn't even know it was there (it didn't show up in their schematics) so I'd just cut it out and leave it out...now I just replace it when I have one on my bench...it probably helps keep down parasitics...I'm not saying that is wrong with your amp but check and make sure that it's not cooked

[Enzo]

The little cap is between pins 3 and 4 of the end power tube next to the transformer. It is only wired to the one tube. If it looks OK, see if pins 3 and 4 are shorted there.

[sonic_tooth]

Thanks guys, I will check that cap tonight.

[sonic_tooth]

OK, I checked for that cap... I don't think that one is there, but I am not sure. I did not see one across the sockets where they are soldered to the board. When you say to check the solder side of the board do you mean where the sockets are soldered, or where the components are soldered? I just wanted to double check before I remove the sockets and the board from that plate on the chassis.

[Enzo]

With the amp upside down on the bench like we would usually have it, the rear panel of the amp away from us. the little cap - which your amp may not even have - would be on the right end of the main tube board. it is not soldered right to the pins of the end socket, it is right next to them.

You can check this from the top even. With the amp chassis sitting tube side up on the bench, remove the power tube next to the transformer. Stick the ohm meter leads into socket holes 3 and 4 and see if they are shorted together. Power off of course. If they are shorted, then out cap is suspect. If they are not, then cap or no, that ain't the problem.

In the tubes that are conducting too hard, check the bias voltage at pin 5 of each tube. If it is collapsing, then by all means explore the little caps in teh bias circuit.

[sonic_tooth]

Thanks everybody, I clipped out that cap and was able to bias the amp properly. I am not sure if the cap was the problem or if I just had not played around with the bias controls enough. I had no idea they interacted with each other so much.... Anyway now I am working on the footswitch. The cord is shot on mine. Does anybody know where I can get a replacement cord? I was thinking of just using a long midi cable and replacing the end with the 5 pin connector.

[tim]

I'm glad it worked for you....I think that cap is there 'cause they may have problems with parasitics sometimes. I've done a lot of those Marshalls with that cap in there and there are alot of those being played often that don't have that cap in there any longer and no complaints from their owners. I just replace them with higher voltage ones now...they can look good on the outside (the lower voltage ones) but still cause the symptoms you were seeing. The bias pots are interactive to a degree but not that interactive.