#1
So I made this post quite a while back, but I've been busy and unable to work on the amp:

Quote by guitar42697
Hey all, I have a 2002 Marshall JCM2000 DSL401. I know the pre-2003 models are prone to overheating, however I doubt that's what happened here. I've installed a 12v computer fan in the chassis, blowing onto the tubes to help reduce heat, and it definitely helped as the amp is infamous for it's scalding front plate, which the fan got rid of.

Basically, it was working fine maybe 3 or 4 days ago. I moved it a few days ago, no bumps or anything, brought it into a different room, then back to it's regular spot. The amp powers on- power LED active, I can hear the transformer buzzing, but both power and pre amp tubes (yes, all 7 visible tubes; ignoring the covered pre amp tube) are not lighting, and I'm not getting any sound whatsoever. I checked the mains & HT fuses, both tested good with a multimeter.

I replaced the power tubes about a month and a half ago, maybe a little longer. I biased them to within Marshall's recommended voltages. Pre amp tubes are original as far as I know, I bought the amp used, but in excellent condition, guy said it had never been serviced, but he hardly used it as he didn't play much guitar or something.

If I remember correctly, there might be a fuse on the board itself, but I really am not in the mood to pull the amp apart at the moment. Anyone got a clue what it could be?

I only have spares for the mains fuse, and the original (but still functioning) EL84s, no spare HT fuse though, and if there is a fuse on the board, no idea what it is. I guess I'll be finding out soon though. Any ideas? Thanks.


I've finally gotten around to breaking it open, and I checked the underside of the board. I cant find ANY joints that look like they melted or anything like that, which makes sense because I don't crank this amp. Only signs of any real heat is around the tube sockets, which is to be expected.

(UG & IMGUR are giving me issues with posting the pic so here's the link: http://imgur.com/a/PKhYW)

Here's part of the PCB, There's no point in taking a photo of the whole PCB as the whole thing looks solid all the way through. The solder connections circled in red are for the bridge rectifier.
Last edited by guitar42697 at Jul 26, 2016,
#2
Turn your amp on and check if there is voltage on the heaters. If there's not, follow the traces to find the failure point. In the best case it's a fuse on the board, in the worst case it's the power transformer

edit: I forgot if it's AC or DC going to the heaters, you need to check the schematic for that
Last edited by ArturPr at Jul 26, 2016,
#3
ArturPr well, funnily enough, I had checked that fuse a few days after I made that post, and it was fine, that said, I didn't remove it from the board. I just pulled it, and well, not working, so I guess it's off to amazon, the few radioshacks in my area don't carry that fuse
Last edited by guitar42697 at Jul 26, 2016,
#4
Good to see you`ve found the issue. BUT buy 2 or 3 of those fuses because if the new one fails in a short time, it probably means the filaments are drawing too much current than they should and you will probably need to bring it to a tech to check the tubes and the filaments circuit
#5
ArturPr Well, I've come to the conclusion that the fuse itself is faulty. It tested dead last night, and for whatever reason I tested it again this morning, and checked good. Popped it back in and the amp was alive once again. Put the chassis back in, and nothing, pulled it back out, and bam, working again. Played for a while because I was so happy to have the amp back, turned it off to let it cool, turned back on to check one last time before putting it back together, not working again. Pulled the fuse out, checks as not working. I guess the fuse wire is making a bad connection with it's housing or something. If I give it a few good whacks it starts checking as good again. I ordered a 5 piece set of Bussman fuses so I should be golden
#6
This is a known thing, the tube filament rectifier is the one that is quirky in these amps. Even that it has good ratings, it still overheats. I have done this one or 2 times I think.


There are sites that you can find by googling DSL401 bridge rectifier, but in short: replace the rectifier with a new one and install it higher than the old one, this dissipates more heat. And check the traces that they are OK and the solder sticks on it.

But if you've found the problem and you think you got it fixed, that's a good thing, I just posted this like a precaution or something
#7
SacredJones Sorry for the late response, yep it wound up just being a crappy fuse, it didn't fail, at least not in the traditional sense. I guess vibrations broke the filament or something, causing it to test good at times