Help with channel switching on the dreaded JCM2000DSL 100

[Wes]

The other day I was working on this amp checking the solder joints around
the bias section and various other places. I got it all back together and
working fine and went to switch from the "clean" to the "ultra" channel and
got no response. I unplugged the foot switch and tried just the panel switch
and still nothing happened.

I've opened it up and tested the transistors around the switch, tested the
switch, and checked/touched up a bunch of crummy looking solder joints
around there. All of that seems ok. Again, the amp works just fine on the
clean channel, it just won't switch to the dirty channel.

Could it be any of the three relays in the circuit? When I fire up the amp
I hear a click, that I assume is the relays, and the click occurs when I power
it down as well.

Hitting the switch- the LED for the dirty channel doesn't light, I hear no
click, it just stays on the clean channel.

I've been working on amps for a little bit now, but I have never dealt with
channel switching circuits yet. It seems a bit overwhelming so if anyone
can give me a little advice it would be greatly appreciated.

[Jazz P Bass]

Jcm 2000

Do you have a schematic?
Can you identify the jack number on the board for the channel switching footswitch.
(silk screened on the board ie: JS#)
Can you also identify the switch number (ie: SW# Letter)

[Wes]

I have the schematic from schematic heaven for this amp. There are
differences in this schematic vs the one available at Dr. Tube. It seems
that the Dr. Tube one is newer, while my amp is older and matches the
SH layout.

Mine doesn't have that cap that's connected between plate and screen
on V8 and it has 220k grid stoppers vs. the 5k6 ones in the newer models.

Anyway, the jack number is JS9, the switch number is SW2.

[Don Moose]

You might want to recheck that all those little cables are still seated and not damaged - especially the one to CON4 where the relays get driven.

[Jazz P Bass]

Jcm 2000 dsl

Yeah, what Don Moose said.
The schematic from Schematic Heaven seems to have an error.
(Not uncommon from Marshall)
On rear output board JCM-62-00 , I think they mislabeled the 6 pin connector.
It states Con 5 goes to Con "2" JCM-61-00.
I do believe it goes to Con 4 JCM-61-00.
At any rate, you should read +24 Vdc on pin #3.
Pin #6 is ground.
Both ends of the connector cable.
Good luck.
John G

[Wes]

Thanks, guys. I'll have a look at that this evening and post, back, what I find.

[Wes]

Ok, I checked the voltage of #3 on that connector, both ends, and I'm
getting about +23V, so that's good right?

Looking around and testing I notice that Transistors 2 and 4 have about
half a volt on their collectors. Beyond that...I'm at a loss.

I'm so frustrated. Are there places that I could check for voltages that
would be indicative of where the problem could be?

[Wes]

Doh, I had CON6 and CON4 connections of the back/outputs board #62
crossed up! Son of a! It's my first time dealing with this rat nest Marshall
styled layout. What a pain in the ass.

Thanks for the help, guys.

[Phostenix]

It sounds like this one gets filed in the "What changed last?" category.

[Jazz P Bass]

JCM 2000 rats nest

I had a customer that brought in a 2000 DSL with every board & connector removed.
The fun part was wiring up the power transformer.
Marshall schematics are really fun to try to follow.
A layout diagram would have helped.
Then there are misprints.
Yeah, what a rats nest.

[Phostenix]

Marshall schematics are really fun to try to follow.

My boss has a JTM30. That's one of the most confusing schematics I've ever seen. VERY different.