Marshall JCM 2000 DSL

[andyjazz]

I just finished fixing this thing. It started on fire. Near as I can tell one of the unused lugs of the PT shorted against its mounting stud causing all that grey-foam insulation stuff to burn and melt all over the place. Anyway, took it apart replaced a few (Literally) burnt components and it seems to be running fine. My question: What is the conventional wisdom about removing the two outside power tubes and turning it into a 50 watter? Can this cause some kind of power feedback that will damage the amp? Your thoughts would be most welcome.

[Alex R]

I don't think so, no.

These amps are a bit prone to overheat. Look at the main board that has the power valves on it. Can you see woven material just showing under the green of the board? If not, take out the power tubes, put a voltmeter between chassis ground and pin 5 of a power tube and blow hot air at the board with a hairdryer or something, mostly in the area around the two small bias caps near the power tubes but just blow it around and get the board hot. If the voltage starts to drop more than a volt or so as the board heats up, and keeps dropping as it gets hotter, you will need to replace the board - the new boards with the woven material don't have this problem.

[andyjazz]

Thanks Alex,

I'm not going to replace the board. The amp belongs to someone else. I just fixed it for him. It would be useful for me to know where one could order a board in case I get something similar in the future. I've discussed the heat issue the him. I made an isolation barrier inside of the chassis so the major short that caused the fire won't happen again. I guess things could get hot enough for other things to happen - not my problem. Customer is probably going to trade the amp for a smaller combo or pull two of the power tubes and use it as a 50 watter. He didn't want to put up the $ for a replacement board anyway. If you could post a link to where I could order Marshall boards in the future I'd really appreciate it.

[Alex R]

They come from marshall, direct in the UK, I guess via Korg in the US.

Symptom of this problem is, you turn the amp on and leave it and it is fine, come back in an hour or three hours and the plates are all aglow, or it has already fused.