need help with JCM800 restoration

[rfgordon]

A very early (I think) JCM 800 came to me to be restored back to the original topology. I think the date code on the choke is 11-82, which puts this as a late 82, early 83 amp. The back plate is long gone, so any serial number or other model info is gone.

Back in 1986 the amp was modded in the extreme by Rick St Pierre (Wizard amps?), if that's whose signature is on the back. I couldn't find a pic online to verify that. You can see in some of the pictures that there were a bunch of rectangular holes cut in the chassis and one extra on the front. The end of the cab has a hole cut in it and a fan, though that is no longer wired.

Anyway, I'm not much of a Marshall guy, having only worked on a few. I can't find any pics online of this same amp guts. This amp seems unusual, with no less than NINE fuses! But I don't really know old Marshalls, so maybe that's fairly normal.

It seems as if it's been roughly put back to a JCM 800 signal flow, though with the post-PIMV.

Can anyone help me assess what might need to be done here, if anything? It sounds good in the Low input with almost any guitar. The High sensitivity is definitely more humbucker friendly than Strat friendly. The single coils sound very fizzy and thin in that input. But, like I said, I don't have that much background in Marshalls.

I think the Master is good, but the caps may be a bit small, and that accounts for some of the thin sound.

Opinions? Suggestions?

Thanks

[rfgordon]

more pics

[rfgordon]

more

[dcribbs1412]

Cool amp
not a Marshall guy here either
but I like cool loud amps.
First question is how are the voltages looking?
Is the added master the only mod done to it?
There is a mod thread in the Marshall section that has some good ideas
if you are going to mod it more.

D

[Turret]

It's Canadian export version

I'm going to bed now but I will help you as good as I can tomorrow

Do you think you could take direct shots of that fuse board track side for me?

[Turret]

The only mod I can see is funky looking PPIMV

Other than that the amp is dead stock

[rfgordon]

Turret,
Thanks for your help. Functionally, the amp acts just like a JMC800, except that the PPIMV gets the PI overdriving more.

It seems that it was put back to stock, but without using the original component spots on the board.

I think I'm gonna get back to the owner to see what he's really after in terms of sound, rather than start rearranging things just for the heck of it.



thanks

[Turret]

Honestly I can't see other mods than ppimv mentioned above. All resistors are in correct value of the same brand (piher), typical era correct boxed caps except PI coupling caps. The ppimv itself is one of the variants found in Ken Fisher's article

Preamp board is almost the same as in British and US versions, it's just shorter (9 inch long) - rectifier diodes and bias feed circuit is relocated to the fuse board.

The amp itself does not have 16r speaker output, all these due to Canadian safety regulations set by CSA organisation. This is why Canadian export models are often nicknamed CSA - ie., 2203 CSA, 2210 CSA

Fuse board for 50w is slightly different - less fuses

I'm fine without the picts of the fuse board, I'm 75% done with layout now and I should have it up on my FB page soon along with 50w fuse board layout (less fuses)