#1
Hello everyone. A friend has given me a 100w DSL JCM2000 that no longer works. From what we can tell, the previous owner played it through a mismatched speaker impedance, or without speakers at all. The amp no longer functions, but if I can economically repair it, we have a free amp. From my research, it looks like the output transformer, power tubes, and/or the resistor grid need to be replaced. Since it could be any combination of things, is there a way to safely determine which components are in need of replacement?
Thanks
- Lord Bosch
#2
Is this your first experience with tube amps? If so, this is not the kind of repair I'd suggest to start on. The amp tech bill maybe be a few 100's, but likely less expensive than a used properly working model, and you will save a lot of time, frustration, and possibly your life.
#3
Will Lane This is my first foray into amp repair, but I am planning on working alongside some of my EE friends who will know basic circuitry safety, though not tube amps specifically. The component I suspect most is the OT. If there is a safe way to test that with just a multimeter or other tools, I can at least begin to acquire parts, and then go to a repair place and pay for just labor. 
- Lord Bosch
#4
It appears the OT is working fine. At least one preamp tube is bad, so I used a preamp from another amp, and linked them via the effects loop. The sound from the preamp was barely audible, while the JCM2000 was screaming and wailing full volume with random noises. I might be able to get my hands on a space 12AX7, but could this be the sign of bad power tubes as well?
- Lord Bosch
#5
That does sound like a preamp problem. But first, describe these random noises
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#6
So, did some further testing with ear plugs. The preamp seems dead, still need to find a spare 12AX7. When I plug into the front input, nothing. When I use my other amp's preamp and loop it in, with the Loop Level button on the poweramp out, I get manageable sound, but none of the volume knobs work. When I press it in, I get a full blast of volume, kind of like a chainsaw cutting into concrete. One of the loudest and most unplesants sounds I've ever heard. One of the power tubes glows seems to brighten up up a bit in the center as well. 

I am starting to think plugging into the effects loop uses full power from the power amp, and the volume knob is earlier in the signal chain. However, this does not solve the preamp problem.
- Lord Bosch
#7
Just buy a JJ ECC83S and try it in every position. Chances are that on;y one of them is blown.
And yes, the volume is in the preamp section.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
Last edited by Cathbard at Apr 27, 2017,
#8
I'll look into that. That would, however, not explain the screeching noise when plugged into the preamp on the line level setting of the effects loop, or the noise when playing at instrument levels. 
- Lord Bosch
#9
All of those things are symptomatic of a dodgey tube. You may be wise to buy two ECC83 tubes.
If you can afford it, give it a full retube. It's what you should do with any used amp. If you can afford it, do the power tubes too. What does that take? They are all EL34 amps over here but I think some American ones were 6L6.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#10
it takes four 12 AX7's in the preamp, and four EL34's in the power section. I have two preamp tubes in an amazon prime box on the way. I would like to hold off on a power section retube until I know if it is the preamp section or not. Sadly, my other amp takes 5881 power tubes, so I can't test those out just yet. I tested the power tubes, and it seems none have gone microphonic, which I thought would explain the nightmare banshee sound from the power amps when my other preamp was effect looped in. 
- Lord Bosch
#11
Preamp tubes can go microphonic as well.
When you do get around to doing the power tubes buy a set of JJ E34L tubes. That wasn't a typo, E34L tubes are a bigger and better EL34.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#12
Thanks for the help and suggestions so far. I'll look into E34L's once I know this amp can work again.
I've switched out the old preamp tubes, and it still does not work on either channel. When I do switch channels on the amp, I do hear an audible "click" coming out of the speakers quite clearly, and if I max out the gain and volume on the amp, I do hear a slight increase in noise coming off of the speakers, though my guitar is still plugged in and nothing is happening. At this point I am at a loss, could this be the resistor screen grid, a loose component, a broken input jack, etc.
Any ideas?
Thanks.
- Lord Bosch
#13
If the amp turns on less likely the tranny. Issue may be slow burn fuse , power tubes or pre amp tubes. Issue could also be in the FX loop( which u can test with jumping the FX loop, ie patch cable them together after some contact cleaner ). Fuses should be easy to check .
I traded in my Real Books for Robbins and Cotran Pathology Textbooks
#14
It won't be the screen resistors because the power amp is working ok. It could be a joint or jack but you are into territory that can't really be diagnosed over the net. I'd need the thing in my hands from this point onwards.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#15
Tried plugging in send to return on the effects loop and nothing. I know the return jack works. I tried plugging the send of the JCM2000 to the return of the my other amp, and nothing. In other news, does anyone know of a good amp servicing place in the NYC metro area?
- Lord Bosch