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Hey guys, new to the form. I just bought a 100w DSL JCM 2000 from a buddy. The thing sounds friggin great, but after I've been playing for a little while, about 20 mins to a half hour, the audio will cut in an out randomly. Usually it only lasts for a few seconds, but I'm smelling a weird electrical burning smell in the back too. The power light doesn't flash or anything, just the audio cuts. Is this a fuse issue or a tube issue? And if its a tube, is it easy to identify the shoddy tube? I'm playing through a 1960 lead 4x12 cab, using a great speaker cable and the 16ohm output and input, so I'm sure I'm doing everything correctly. Any help would be greatly appreciated. Thanks.
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Unplug it, pull the chassis. Look on the main circuit board. There are four power tube sockets, I am interested in the one closest to the power transformer - right next to it. There is a small disc capacitor next to the socket pins - next to pins 3 and 4. does it look blackened? Are pins 3 and 4 shorted together?
Also, there should be four rectangular power resistors - 1k ohms 5 watt - standing up off the board on long legs - one by each power tube socket. Are any of these cracked or discolored?Education is what you're left with after you have forgotten what you have learned. -
what a coincidence
my dsl 100 does the same thing; I also noticed that the burning is coming form what appears to be the tube that is the farthest outward, it is also causing the plastic vent above the tubes to become hot and pliable.
I look forward to receiving any adviceComment
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Well, DSL, may I direct you as well to look at that little cap next to the very tube you indicate.
Without me looking it up, I think it is C46.Education is what you're left with after you have forgotten what you have learned.Comment
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Make sure you check the 1 ohm Cathode current sense resistors going from Cathode pin 8 to ground. If they open up the tube will go cherry and this happens a bunch with this amp that causes that.KBComment
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As fate would have it, I have a post about a similar but more exteme issue on a 2205 JCM800. I just replaced one the 1K 5W resistors and this amp is rolling along.
The resistor was measuring 750 ohms and causing that particular tube to burn up. The genious that sold it to my customer had a 2A fuse in the HT slot where a 500mA would normally be.
So... This guy was drawing way too much current and being allowed to do so by the 4x over fusing.
Take Enzo's suggestion. and check the resistance on them.Comment
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I have exactly the same problem with 2205 JCM800. The 1K resistors have signs of high temperature. I replaced them although they had correct vlaue. I also replaced both EL34. The amp plays OK for some time and then one of the valves starts glowing. It seems to me that the valves are OK and the resistors are OK. Does it happen when the bias voltage is too low? Currrently it's -36V. I suspect that the sockect is not contacting. What is the best way to check the socket?
MarkComment
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About JCM 2000 DSL 100s... a really odd one came in today. Dead shorted output transformer primary. Board burned between pin2 and pin3 of V8. (the pic below was taken after I cleaned all the loose carbon off) But whats interesting is... tubes are all good, sockets are good, no caps are shorted including the little one on V8, no resistors are damaged, and... not a one of the six fuses blew.
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That light colored board (oem part) is the one that causes all the bias drift problems. Its made of shitty material that causes it to become slightly conductive over time and heat makes it worse. Contact a Mar$hall service center near you and see if they'll sell you the replacement board (darker green color, made of FR4 material, better quality). Its the only way to make these amps reliable.The farmer takes a wife, the barber takes a pole....Comment
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Here's a link to the problem discussed at length!
You may have the faulty bias supply parts as outlined in the link and that "may" be enough to fix your problem.
I've encountered this problem on several of the DSL/TSL amps and after rebuilding the bias supply they seem alright, at least none have come back yet!
The Marshall TSL122 JCM2000 Repair/Mods PageComment
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