Jcm800 4210 Mods?

Discussion in 'The Workbench' started by brando69, Jul 2, 2018.

  1. brando69

    brando69 New Member

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    hi there. I'm looking for some help on distinguishing the mods on my 4210. Its dated 1984 on chassis. I picked this amp up last week with then intention of possibly doing some minor mods. When i tested it out, it was just what i was looking for, it works, but not too great. It definately needs a tuneup, possibly cap and tube job. The reverb is very weak and the amp isnt very loud (my dsl5 blows it away). Pots are scratchy.... etc. Etc... so now to today. I pulled the chassis and started investigating. The amp has some mods done to it already. 2 of the clipping diodes on the gain have been lifted, but it looks like nothing else was done for this mod-no other wires added to change the circuit? It also has a small board installed in the normal channel with a 5vdc switch some resistors and some caps. I believe this would be a mod to correct the channel bleed, but im not sure. There is also a piggy backed resistor on r5? Any help on this would be appreciated. Thank you! :)


    Ps i have some pics of the board, but i cant seem to figure how to post them. It keeps saying they are too large.
     
    Last edited: Jul 4, 2018
  2. brando69

    brando69 New Member

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    Here are some pics of the board... btw the amp is a canadian versian with el34s and extra fuses
     

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    Last edited: Jul 7, 2018
  3. brando69

    brando69 New Member

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    Anybody? Any kind of feedback is welcome. Am i posting in the right area?
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    We could never know what mods have been done without seeing the actual amp / boards.
    Post plenty of close up pictures with plenty of light.
    Post pictures in PNG or GIF format if the file is to large.
    JPEG pictures, PDF or bitmap may be too large a file to post in he forum.
     
  5. brando69

    brando69 New Member

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    I did post some close up pics. Are they not showing up here? They are in the second post of the thread.
     
  6. brando69

    brando69 New Member

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    Here is a little bit more info, please bare with me as i am a newb in reading circuitry.

    Looks like the 3300 cap runs into 470cap. Then into 2 sets n4007 diodes. Also its branched off a small board with a transistor and a few resistors. Also ties into T24. The small board ties into a hb2-dc5v switch with a N4007 diode. Also ties into R45,R46,R48.

    D3a +D4a each have one leg lifted
     
    Last edited: Jul 7, 2018
  7. RickyLee

    RickyLee Well-Known Member VIP Member

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    I have yet to see a first generation Split Channel made in 1984. By your description of the clipping diodes, it sounds like you could possibly have the first generation preamp.

    You will need to pull V2 to find out which version you have. I do not know if you already know about the early/later preamp designs?

    You need to pull out V2 and see if you still have the Normal channel basically.
     
    Last edited: Jul 5, 2018
  8. brando69

    brando69 New Member

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    Ya, the normal channel still works with V2 pulled. Is that a good thing or bad? I also noticed a white sticker on the board with the letter R on in (1983?) but the chassis is labelled 20/08/1984. The serial number also starts with an S on the back.
    With the V2 pulled, one channel works and one doesn't

    I also just noticed that one of my el34 is 1/4" longer than the other one, strange...

    Thanx
    IMG_1073.jpg IMG_1074.jpg
     
    Last edited: Jul 5, 2018
  9. RickyLee

    RickyLee Well-Known Member VIP Member

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    On the later design preamp, V2 is only for the Normal channel. So it seems you do have the first early design. Even more bizarre to me, is you have a late 1984 made and still have the earlier preamp design. What I have noticed is with the Canadian made amps, all the set in stone manufacture/factory amp models is thrown out the the window. What part of the World are you in?

    Good or bad? That is up to your ears really. The two versions are different amps circuit wise for sure. Mainly in the preamp where your early version is more like a hot rodded Fender than a hot rodded Marshall. That is due to where the Boost channel tone stack is located in the preamp. The later versions have the tone stack moved later in the gain stages and many other changes as well. I have two later 1988 2205 amps and have been wanting to find a 1982 built 2205 or 2210 to trade for it, just so I can try the early design one more time. And being I have two of the same I would like to have something a bit different for one of them, even though I might be trading down in value

    As for that "R" on the PCB, that might just be the initial or a code by someone that checked it or installed it. I think I have seen that letter on the PCB of some of my 800's with different years than 1983. How about your transformer date codes?

    Your amp should be somewhat close to this schematic:

    https://drtube.com/schematics/marshall/4210.gif

    That schem shows the last changes Marshall made in the Fall of 1983. And they were changing things many many times from early 1982 up until they finally canned that design and re-worked the preamp mainly. That goes for the 2205 and 2210 as well. But the most changes and tweaks might possibly have been made on the 4210 combos.

    I had a late 1983 built 4210 that was a lemon big time. Very bad tone on the Boost channel. Then it had terrible channel bleed and also odd noises that were intermittent and I could not figure out after countless parts swapping and chasing my tail. Basically, it was the amp I first tried to work on and modify and the fragile PCB went south from the solder iron heat. Never did finish finding out some of the odd ghosts and noises that amp had due to the PCB being ruined. So every person I hear of thinking of working on or modifying these amps or other early Marshall PCB amps, I just tell them to be careful with the soldering and try not to do any unnecessary work. With these amps, it is almost best to take it for what it is or get rid of it.
     
  10. brando69

    brando69 New Member

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    Thank you very much for your response and advice! I really appreciate it. I am located in ontario canada. The transformers are all stamped 84. Today i had some time to clean the pots, inputs, tubes, and sockets. I do notice that the amp isnt really that loud. The amp does sound more like a silver face deluxe reverb than a marshall. I think im going to change the filter caps, re tube and re bias. Then i will re evaluate.


    It looks like someone changed the circuit around by the
    5v switch? Also 2 of the legs are lifted on d3 and d4?
    When changing the caps do i need to replace all of them or just the 2 big ones, and the 2 near the bias pot?
    Thank you again
     
  11. mickeydg5

    mickeydg5 Well-Known Member

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    Does the switching system work? Is it switching levels of distortion or something?

    It looks like a mad scientist did that work! :wow:

    That is no relation to Amp the MadScientist.
     
  12. brando69

    brando69 New Member

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    Ya the channel switching works fine. The amp has a "premium sound" badge on the front. I dont think that its been played in a long time. It has ehx el34's dated 03, and by the looks of it all the 12ax7s are original. The prongs on the tubes were pretty gunked up. I had a couple of jj 12ax7's and a ruby ax7 that i threw in. The amp doesnt sound terrible, just different than i expected. Volume level my dsl5 is almost as loud.

    The soldering on the 2v switch looks pretty good, doesnt look amateur... Im just trying to figure out what he was trying to accomplish..... like if the diodes were changed to germanium or leds or even removed i would understand the direction.

    I dont even know the direction i want to go with it. Lol, if that makes any sense. My dual rectifier and dsl5 are high gain already, and my tweed deluxe has some good gain when i jumper the 2 channels together. Maybe i will do what i learned from you and micky on the dsl5 thread, and put some lower gain tubes in V1,V2, and a 12at7 for the reverb, and then crank it through my hot plate.

    i am learning alot from this forum, i do want to thank everybody that posts and offers their knowledge in all the threads. Without you senior guys, guys like me would be completely lost. Thanks :)
     
  13. RickyLee

    RickyLee Well-Known Member VIP Member

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    Where are you at with all the settings/controls? The Master Volume on these amps is also a different animal, as in it is a PPIMV type.

    You would want to run the Boost channel Volume a bit more as your master volume and try setting that MV higher. These amps do have quite a bit of fizz to them due to that circuit.

    I would replace every cap that is an electrolytic type, more so those filter caps.

    If someone did remove that clipping circuit, then they more than likely had to do some other changes to compensate for that. That is because the signal would end up much too hot/over loaded by just removing the clipping diodes.
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    [

    QUOTE="brando69, post: 1732136, member: 53977"]hi there. I'm looking for some help on distinguishing the mods on my 4210. Its dated 1984 on chassis. I picked this amp up last week with then intention of possibly doing some minor mods. When i tested it out, it was just what i was looking for, it works, but not too great. It definately needs a tuneup, possibly cap and tube job. The reverb is very weak and the amp isnt very loud (my dsl5 blows it away). Pots are scratchy.... etc. Etc... so now to today. I pulled the chassis and started investigating. The amp has some mods done to it already. 2 of the clipping diodes on the gain have been lifted, but it looks like nothing else was done for this mod-no other wires added to change the circuit? It also has a small board installed in the normal channel with a 5vdc switch some resistors and some caps. I believe this would be a mod to correct the channel bleed, but im not sure. There is also a piggy backed resistor on r5? Any help on this would be appreciated. Thank you! :)


    Ps i have some pics of the board, but i cant seem to figure how to post them. It keeps saying they are too large.[/QUOTE]

    Considering the mess that's in there...
    I would put it back to 100% stock.
    I would make sure that it works solid, 100% at all functions.

    Then if your'e going to modify it, do this after you prove that it works 100% correctly stock first.

    I wouldn't even try to mess with it, with that slop inside the chassis. I would get rid of that abortion first thing.

    I mean let's face it:
    you don't even have a schematic to what's in there now. (WTF who knows?)

    OR
    If I just bought that, I would take it back and get a refund.

    slop.png OH, how perfectly hideous.
     
    Last edited: Jul 6, 2018
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    "Ps i have some pics of the board, but i cant seem to figure how to post them. It keeps saying they are too large."

    Post pictures in GIF or PNG format. PDF, bitmap or JPEG files will be too large to post usually.
     
  16. mickeydg5

    mickeydg5 Well-Known Member

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    I have to agree with Ampmad at this point. Either put it back to stock and work with that or at the least draw a schematic of what is there for you and us to work with and make deciphering easier.

    P,S.
    I do not think pictures are going to help much over the internet. There is too much that cannot really be seen. Who knows what may be under the board. Maybe, maybe not.
     
  17. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    There could be a reason why it's not working right................
     
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  18. mickeydg5

    mickeydg5 Well-Known Member

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    The question is "Which reason?" then.
     
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  19. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    There are a few too many "apps" in there............
     
  20. mickeydg5

    mickeydg5 Well-Known Member

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    Are you saying this thing is modded Bluetooth?
     

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