82 JCM800 2204 Overhaul

Coolidge

Gold Supporting Member
Messages
2,108
Scored another used Marshall and decided to breath new life into this 38 year old amp.

Before...



After - new transformers and choke, new tube sockets and filter caps. Added an external pot for biasing lower/right top of chassis.



All new...well everything. New Marshall power/standby switches, indicator light, pots (PEC), jacks, fuse holders, IEC socket, Marshall output and mains selector switches.







Finished the new JCM800 board last night. I'll start wiring it into the chassis this evening.



 

louis

Member
Messages
4,221
Coolidge ,..........I was about to start another thread on where to have a board
made like the beautiful one you have but is it ok if I ask here instead ?
Btw,....I love how clean your work is ,...........I know techs that are sloppy Joe's
and always worry when I bring an amp there .
Gotta leave but will be back .;)
 

Stormin

Tele's and Plexis
Staff member
Gold Supporting Member
Messages
4,123
That looks amazing and the quality of your work looks top notch, but you now have a 2204 clone in a vintage chassis and shell. I don’t understand why you’d choose to rip out the iron and pcb from a vintage amp - it couldn’t have all been junk.

I don’t mean you any disrespect - I just don’t know your rationale for the project.
 

Coolidge

Gold Supporting Member
Messages
2,108
Beautiful work. What was wrong with the transformers ??
Thanks. Several of us have been researching and discussing the Canadian government bastardized version of the JCM800 vs UK/USA version. The more we looked the more differences we found. One difference is the Canadian version uses different Drake output and power transformers. So far they look specific to the Canadian market, due to government safety regulations at the time.

Mine is a Canadian version so it has the smaller OT Drake 783-257 vs the UK/USA Drake 784-139. The Canadian OT does not have a 16 ohm tap for 16 ohm cabs, the primary reason I replaced it plus I had a full set of MM transformers and choke sitting on a shelf. All the JCM800 aftermarket OT's seem to be modeled on the UK/USA Drake 784-139. The oddball Canadian 783-257 which has a smaller chassis bolt hole pattern is unobtanium in the aftermarket. I can't even find specs on the Canadian transformers. The Canadian version PT is a Drake 1202-304 vs the UK/USA Drake 1202-324.
 

Coolidge

Gold Supporting Member
Messages
2,108
Coolidge ,..........I was about to start another thread on where to have a board
made like the beautiful one you have but is it ok if I ask here instead ?
Btw,....I love how clean your work is ,...........I know techs that are sloppy Joe's
and always worry when I bring an amp there .
Gotta leave but will be back .;)
You know what I hate, solder and flux spatter all over my shiny new board. :mad: So I adopted a drape approach like they use during surgery in operating rooms. Recommend you start a separate thread on PCB over in the Amp/Cab Tech Corner.

 

jdel77

Supporting Member
Messages
9,835
Man you have the skill and dedication, but to me that’s a whole new amp in an old headshell and chassis.
I respect your work ethic though!
 

Coolidge

Gold Supporting Member
Messages
2,108
Looks fantastic but I just have to ask...why not just build it from scratch as it looks like a complete rebuild?
I was disappointed how different this Canadian was from the UK/USA version, that took all the shine off owning a 'vintage' JCM800 so since I had all the components needed to re-craft it in my image I went for it.
 

Badstrat

Silver Supporting Member
Messages
1,314
I’m sure you could have sold it for a pretty penny and bought enough components for 4 clones but hey it was your amp to do what you want with. Looks great and I’m sure it’ll rock the house. Love your work.
 

Badside

Member
Messages
1,225
Scored another used Marshall and decided to breath new life into this 38 year old amp.

Before...



After - new transformers and choke, new tube sockets and filter caps. Added an external pot for biasing lower/right top of chassis.



All new...well everything. New Marshall power/standby switches, indicator light, pots (PEC), jacks, fuse holders, IEC socket, Marshall output and mains selector switches.







Finished the new JCM800 board last night. I'll start wiring it into the chassis this evening.



Once again, where did you get your board made?
 

Coolidge

Gold Supporting Member
Messages
2,108
More on the boards...

- .125 inch thick FR4 board, that is quite thick. Where you have radial components like the WIMA 470pf film/foil caps take care to order components with lead lengths long enough to reach through a .125 board with enough lead length sticking out the other side for soldering.

- 2 oz copper. This is also quite thick vs the typical 1/2 oz copper. About as thick as you can go. I ordered 4oz copper once but they couldn't etch it, the copper was so thick the etching was undercutting the pads before the waste copper was etched away.

- Black solder mask. There are other colors, the blue kind of sucked honestly it was more of a greenish blue. Red is a bit much. White looks great but any solder flux spatter will show up as brown gunk.

- White silk screen top only.

- Plated through holes which connects the top and bottom solder pads together in a single unit.

- Full tab routing. Normal tab routing leaves tabs connecting the boards together. This board material is quite tough, full routing completely cuts the boards apart into separate boards. EXPAND YOUR THINKING...you may have noticed the little mini turret boards at each end of the amp. One for preamp grounds, another for filament leads. In fact I created quite a number of mini boards both turret and PCB. Full tab routing separates all these boards cleanly.



This is DesignSpark PCB a free download. Fairly user friendly tool for designing boards like this though some learning should be expected. I have used industrial CAD/CAM software so I was able to jump right in and start drawing. Below you see the main board and all the mini boards. Board borders in green, those red lines are the tab router paths. You need to leave a minimum of material between the boards for the router bit.

 

fusionbear

exquirentibus veritatem
Gold Supporting Member
Messages
10,680
Your work is no doubt beautiful and functional. But, dang! Those Canadian models sound really good to my ears and I've been working on old Marshall amps since 1979! The differences are very minor IMO.... I dunno, I would have left it alone and built a clone instead....
 

Coolidge

Gold Supporting Member
Messages
2,108
Mini boards can be useful. The simple 2 turret mini board for the transition from PT filament wires out to the tube sockets. Those wires are always large and bulky. You can run them directly to the first tube socket, instead I'm running them to the 2 turret mini board and from there using Teflon plated wire out to all the tube sockets. Easier, neater.

Another mini board idea is for the input jacks. The jacks are still chassis mounted but I'm using the UK Cliff brand PCB pin jacks and mounting them on a mini board on the back side. This bolts the two jacks together, very stout vs the jacks floating in mid air held only by the chassis nut. The board includes space for the 1M, 470k, and 470pf components and solder pads for the grounds and signal wires. Neater, faster, and avoids components hanging in mid air between the jack and pot.

 




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