Marshall JCM 2000 repair issues

[oc disorder]

Welcome to the forum hemorrhage.
You can find the schematic on this page.
historiclespauls.com - piller.at MUSIC - Marshall DSL201/401
It's not necessary to fit a heat sink but it worked for him..
Start a fresh topic with 401 in the subject.
I havn't seen one for a while so hopefully someone in the USA
can chip in.
If you have limited experience and minimal tools it may be
rather challenging for your first time major project.
Its up to you to weigh up the pro's and con's but if something goes awry
it may cost a lot more to get it fixed.
eg Small champs etc are much eaiser to get into as you can see all the components and there is tons of info about these.
Maybe a kit for a small practice amp would be good where you can take your time and enjoy it .
Just my thoughts... anyway I've been over posting today and I need some decent sleep.
Good luck !

[Mark Black]

1. do you know of any DSL techs in the Seattle area?

The guys at Aviator Audio in Edmonds have extensive JCM2000 (and specific DSL401) experience.

Disclaimer: Yes - I have worked there. That's how I know...

[shibbz]

Hi, thanks for all this useful info!
My dsl 50 was doing the same thing so added the jump and tried it and indeed, it works!
I feel like I may have done a bad job soldering the wire though, I took a picture.
http://i19.photobucket.com/albums/b1...r/DSCF6801.jpg

On the subject, while inside the amp I notice all the internal fuses, and noticed one was missing?
I've had this amp for almost 2 years, and minus this weird output thing haven't had any problems.
Is this an active place for a fuse, or is this something like Marshall using the same board for the dsl 50 and 100 and it is unused in the 50 but used in the 100?
Here's a picture.
http://i19.photobucket.com/albums/b1...cular/fuse.jpg

[JerryP]

Hi, thanks for all this useful info!
My dsl 50 was doing the same thing so added the jump and tried it and indeed, it works!
I feel like I may have done a bad job soldering the wire though, I took a picture.
http://i19.photobucket.com/albums/b1...r/DSCF6801.jpg

On the subject, while inside the amp I notice all the internal fuses, and noticed one was missing?
I've had this amp for almost 2 years, and minus this weird output thing haven't had any problems.
Is this an active place for a fuse, or is this something like Marshall using the same board for the dsl 50 and 100 and it is unused in the 50 but used in the 100?
Here's a picture.
http://i19.photobucket.com/albums/b1...cular/fuse.jpg

Same board for the 100 and the 50 watt so that fuse is not used in your amp.
Jerry

[PepeSilvia]

I've got a busted DSL100 that I'm digging into, and was wondering if the symptom I'm seeing is a result of the dirty 16-ohm jack. I open it up and the resistor outlined in red in this figure was complete roasted. We're talking big black spot, nothing left except the leads. Can someone explain to me the function of this resistor, and what could have caused that kind of damage? Also, I've read a bit about "flyback voltage" in this thread, but haven't really found a good explanation of what it is. Is that similar to what would be called a 'reflection' on a transmission line, caused by a mismatched impedance? Thanks!

[oc disorder]

hi PepeSilvia
That resistor is the negative feedback connection which returns via W2 on the main PWR board and back to the phase invertor.
Better check R56, C30 and R59 on the main board as well.

As to what would cause it um component failure or forgetting to connect the speaker
or possibly ye olde 16 ohm jack socket internal switch failure.

For it to burn like that it must have conducted a fair whack of the output much more that the quarter watt resistor was designed for.
I guess it went to ground via some or one of the components mentioned above.

Regarding "flyback voltage" it is similar to transmission line reflection in that it is an undesirable effect interferring with the main signal.

I think its more related to Back EMF which is eaiser to understand with a relay.

When power is applied to the relay coil, a magnetic field is created and energy is stored in the coil. When power is removed, the magnetic field collapses causing a reverse voltage to be generated (it's called inductive kickback or back EMF).

Got that quote from Relays
Some entertaining interactive diagrams there if anyone wants to understand relays.

So the same thing happens in a transformer when the magnetic field collapses it generates a reverse voltage. In some modern circuits for TV's and power supplies
this is taken advantage of and part of the design but not in the Marshall.

Usually that kind of failure (R19 47K feedback resistor burn out ) unfortunatly points to more serious problems like the output transformer itself .

Hope the above is enough there are many brains on this forum that are better equipped
to answer this than mine and hopefully they will chip in shortly !

[AmpFix]

The early amps had a service bulletin regarding the channel switching LDR which can cause a drop in volume.
Jerry

Interesting. Do you have a copy of this service bulletin? I'd love to see it. THX.

[PepeSilvia]

hi PepeSilvia
That resistor is the negative feedback connection which returns via W2 on the main PWR board and back to the phase invertor.
Better check R56, C30 and R59 on the main board as well.

As to what would cause it um component failure or forgetting to connect the speaker
or possibly ye olde 16 ohm jack socket internal switch failure.

For it to burn like that it must have conducted a fair whack of the output much more that the quarter watt resistor was designed for.
I guess it went to ground via some or one of the components mentioned above.

Regarding "flyback voltage" it is similar to transmission line reflection in that it is an undesirable effect interferring with the main signal.

I think its more related to Back EMF which is eaiser to understand with a relay.


Got that quote from Relays
Some entertaining interactive diagrams there if anyone wants to understand relays.

So the same thing happens in a transformer when the magnetic field collapses it generates a reverse voltage. In some modern circuits for TV's and power supplies
this is taken advantage of and part of the design but not in the Marshall.

Usually that kind of failure (R19 47K feedback resistor burn out ) unfortunatly points to more serious problems like the output transformer itself .

Hope the above is enough there are many brains on this forum that are better equipped
to answer this than mine and hopefully they will chip in shortly !

So if I replace those components, what are the chances it blows up again the first time I turn it on? Will I have a chance to troubleshoot it?

[PepeSilvia]

So I replaced all the components that were fried, and fired it back up. Started smelling that familiar smell of a burning IC, shut her down and flipped her over to take a look. R19 on the rear output board (the 47k mentioned earlier) is black, though it didn't get to point of completely blowing up. I should mention something I discovered after my previous post. IC2, R41, and R42 on the 'channel select' board (the one with all the front controls) were also blown up, I believe that's where the source of the smell is now coming from, I wouldn't be surprised if it blew up if I left it on for too long. These are connected to the same circuit as R19 through W4->W2 from the rear output board to the main power amp board, then from CON13 on the power amp board to CON12 on the control board. I used the 16 ohm output into a 1960 cab. Is that IC a part of the feedback you were talking about? What exactly does this circuit do, I noticed that the Presence knob is a part of it? Does this still point to the output transformer or could it just be IC2 (Opamp) blowing up and taking everything out with it, I'm not sure it would source enough current before that happened?

[ampman755]

this guy is TAKING YOU FOR A RIDE !
el83 = 7 ea
el84 =15 ea
total 51 L
you can put tubes in your self easily , can you change a lightbulb ? just a tiny bit harder with aligning pins in socket
biasing is NOTHING a quick 10 second adjust on trimpot AND NOT CRUCIAL

[ampman755]

good show , that quack attempting ripping 120L outa somebody for NO reason , you're correct the 1st inline gets weak and whole amp fades , move it to another stage and get a strong one in ...
the the OT idiocy , "it's hot , its gone "?????!!!!!!
amateur con man!

[Leo_Gnardo]

ampman, you have a good point BUT you're sending a message to someone who stopped paying attention to this thread 3 years ago. Sorta shouting down the well.

[ampman755]

not at all .
this is up as a 1st page hit by google , many young guys can derive the benifit of experience of the instigator and an opinion stating reinforcement to his suspicions of being rolled over by the fake "electronics" guy were correct .

[Leo_Gnardo]

this is up as a 1st page hit by google , many young guys can derive the benifit of experience

Well, I s'pose, if that was your aim. Young guys & not so young. (Don't leave out the women! May be sort of rare but there are some and a shout out to them, hey! The more the merrier. And bring your friends.) Warn as we may, the quacks still get a lot of biz and hard working honest techs like you & me & many others here wind up having to clean up the mess while dealing with clients who have ruffled feathers & worse. Imagine their attitude "I've been ripped off & I know it & I don't trust anybody."

I've had a notion to demand an invoice from clients who have had quack mods done, $500-1000-2000-more, and charge them the SAME to make their amps right. Burns me up that the quacks walk away with big cash while I fix it for pin money. Really tired of being the underpaid amp janitor. Some of those quacks are famous ya know. Articles in Guitar Holder magazine, "How I saved - insert rockstar name here." Hope they don't hurt their arms, patting themselves on the back. Attaboy!

In this thread, haven't addressed "bias drift" problem common to 2000 series Marshalls made prior to 2006. BUT that's been well covered in other threads here on MEF as well as other good sites. A vexing problem that can leave amp owners spinning in circles, and even well-meaning techs doing the same until they get hip to it. OTOH there are a couple of 2000 series problems and solutions offered. To make one work reliably, you have to do 'em all. They are capable of some very good tones when working right.

There are many sad tales of woe and intrigue if you care to look. See what poor dumbassbob has been going through with his Bandmaster. Such quackery. Ducks could have done better for him. We'll see ya through it bob!

In any case allow me to offer a welcome aboard ampman755. You've had a look around I'm sure - good bunch of folks here.

- - - - - -
edit: after trying google search

2000 series bias drift well covered by hullerum, a few entries below (now) this one on Google. Hey we're #2 hooray! Almost famous, ain't that somethin'. All thanx to ampman! Search "Marshall 2000 repair." Break out the dremel & eye & ear protection, goodbye drift.

[dcline]

Hello,
Is this the anti-oscillation disc capacitor that was referred to earlier?
I'm working on a DSL 50 myself that had major bad solder joints. I fixed them but am still getting low power with a hum.
OT checks out good. Plate and bias voltages good... Ruled out tubes...
Could someone please tell me the schematic number of this cap?
Or any other suggestions. I've worked on quite a few of these myself and have seen many of the problems mentioned in the forum but this disc cap thing is a new one for me.
Thanks in advance