Marshall JCM 2000 DSL 401 - Board Pull?

[jjasilli]

OK, I'm feeling really dumb.  Have a Marshall JCM 2000 DSL 401 Dual Super Lead on the bench for repair.  Need to replace the bridge rectifier which feeds DC to the filaments of the preamp tubes.  Got all the knobs & nuts removed from the controls & jacks on the front & rear of the chassis.  Removed the mounting bolts for the PCB.  But the pot stems and jacks protrude through the face & rear of the chassis with no room to maneuver.  How do you pull the PCB board?

[DummyLoad]

is the front or rear panel detachable?

pics? 

[jjasilli]

Front panel is detachable.  Thanks!

[jojokeo]

Not quite sure what you mean? I just worked on one two weeks ago and was able to get everything apart okay. I removed the power IEC/fuse board, the little reverb board, then the rest one at a time and didn't have to unconnect too many of the small connectors going to each other's boards. Take pics before you start removing everything and mark connectors if necessary to assure it all goes back together properly.
There's a lot of bad contacts on those boards and mine was way out of bias adjustment. Got it all back running properly again and sounding really sweet.

[Tone Junkie]

Pics would be cool if for no other reason than for good amp porn that and I will probably never see the inside of one otherwise.
Thanks Bill

[jojokeo]

Here's a few

[jojokeo]

and the boards being removed

[Tone Junkie]

Thanks, glad you guys have to put those back together and not me, I could see were a digital camera is your friend, Im gettin old and forgetfull.
Bill

[Jack1962]

Myself I leave as many of the sub-boards attached as possible , the majority of the problems i have encountered and have read about where cable not being connected to the right board.

[jjasilli]

I think my version is, thankfully, a bit simpler.  The front panel is lying face down in the foreground; it attaches with 3 screws.  The only connections are: a) to the power & standby sw's; b) the small PCB mounted to the fr panel which uses a snap-on multiconnector; and c) a smaller multi-connector to PCB mounted to a side panel.  Then, after removing tubes, knobs, nuts and the set screws to the main PCB, the main PCB can be partially pulled and easily worked on w/o any more disconnects.

My culprit is the black square -- diode bridge -- in the top, center left of the main PCB, near 2 tall el. caps.  The bridge and those 2 caps are the DC filament supply to the preamp tubes. 

[plexi50]

The worst part of working on this amp is the power tube socket mounting screws usually strip out very easy when putting it back together

Also remove C46 22uf/500VDC tan Ceramic cap if that amp has one. Just snap it off. If you get a surge over 500VDC and the cap shorts out to ground it will take out a power tube or two. It is not needed and can cause big trouble down the road. This problem was suppose to have been resolved by 1999 but i have still seen them from time to time on newer boards
There have been many schematic revisions of this amp so your cap may not be labled C46. It's on the right side end of the board next to the last power tube socket

The newer boards will have a Blue 1KV cap. To me it is not needed and just a problem waiting to happen

[Tone Junkie]

Not to hijack the thread but if your making a DC filliment and using one of those bridges what would be the proper rating of voltage and amperage I have some that are 50volts and 4 amps I think is that enough or should I use bigger. Just a hypothetical question.
Thanks Bill
PS I guess its also tyed to this what size is that one and why do you think it failed in the first place,
 would a voltage regulation chip have helped, if it doesnt have one anyway.?? just curious

[jjasilli]

Good question.  The stock diode bridge in this amp is 6A 200V.  The scuttlebut on the internet is that this part tends to blow, and so do replacements.  I believe that 3X 12ax7 tubes are supposed to be getting 12-0VDC @ 150Ma = only 450mA in this circuit.  Even @ 6V that's only 900mA, so it isn't clear why a 6A bridge should fry.  But the bridge is fried, confirming the internet chatter, because the filaments have less than 3V on them, and I get both an AC & a DC reading on the filament pins!  Clearly the bridge is no longer rectifying. 

Anyway, my brilliant solution is a 25A replacement bridge.  Let's see Jim Marshal fry that!     Haven't decided yet between a board mount or a chassis mount.  Got both, but have a gig tonight. 

[phsyconoodler]

The three bridges I replaced I upped the amperage handling,lifted it a bit off the PC board and they seem to be fine now.They get awfully hot.I think they were cheap quality parts and mounting them on a PC board is a recipe for disaster.
  One thing to bookmark is when you put the chassis back together,make sure the rear panel nuts are tight because the ground for the channel switching is on the rear panel behind one of the jack nuts.Duh!
  Poor design and execution of a fine sounding amplifier.

[DummyLoad]

substandard asian part w/ undersized dice? no surprise if it were. a 6A bridge should have no problems w/ 900mA filament load.

[jjasilli]

The amp is repaired; working & ready for pick-up.  Below is a shot showing the new 25A bridge installed on the board.  Thanks again for the help & suggestions.

[chabby-ao]

The worst part of working on this amp is the power tube socket mounting screws usually strip out very easy when putting it back together

Also remove C46 22uf/500VDC tan Ceramic cap if that amp has one. Just snap it off. If you get a surge over 500VDC and the cap shorts out to ground it will take out a power tube or two. It is not needed and can cause big trouble down the road. This problem was suppose to have been resolved by 1999 but i have still seen them from time to time on newer boards
There have been many schematic revisions of this amp so your cap may not be labled C46. It's on the right side end of the board next to the last power tube socket

The newer boards will have a Blue 1KV cap. To me it is not needed and just a problem waiting to happen

Ha! That would be about the best thing aboutworking on one of these to me-lol.
They were when Marshall became play and toss, or play and convert amps.
The thing is, even if you fix them......why? They'll just be back on the bench soon enough anyway.
Fans should be standard issue on the tube boards. That entire amp is a problem waiting to happen.
They sound great while they run though, i'll give em that. It's really sad what quality builders have had to do to stay competitive and sell units. Looks like the flight deck of a DC-10-lol.