Newer JCM 800 (w/ factory effects loop?) wiring question

[mhuss]

I have an ST1 Marshall board (NOT a complete amp) which I think was used in the later 'vertical input' JCM800 models (just before the PCB mounted pots and jacks versions). I have not been able to locate a schematic for it. (I found lots of generic/older JCM800 schemos.) Most of it is 'self evident' but I have one question - in the middle of the board on the pot side there are three brown wires. One goes to the Presence control, one (might) go the effects loop board, and one I have no idea. All three are hooked into the 'lower' end of the PI circuit; two are obvious (presence control, ground) but one connects to the 10k/4k7 resistor junction -- the PI AC bypass cap and the NFB resistor that goes to the OT also connect here. Does anyone know what else might be connected to this node?

https://mhuss.com/MyJCM/JCM800_2204.pdf

[Pedro Vecino]

The amp must be a 2203 reissue. Those cables must go to the loop input, the presence potentiometer and the one connected to the 4K7 resistor should be connected to the ground bus.

EDIT: to the loop output. The input to the loop is taken from the center pin of the master volume.

[mhuss]

Thanks; reissue makes sense, as I'm pretty sure they weren't doing effects loops in the early 80s.

Those two (presence pot and bottom 4k7 going to ground) make sense, but I can't figure out why that node would go the the loop. With the presence pot at the zero ohm setting the signal at that node would be somewhat high-passed with 100n to ground (~150hz?).

[Helmholtz]

To avoid misinterpretation, can you post an edited schematic showing where each wire connects?

[mhuss]

The presence cap and pot are reversed, but functionally the same.

[Helmholtz]

The first 2 wires from the left might both be ground wires connecting to the ground bus at the pots. The third wire I don't understand but I agree that it makes no sense to use this node as effects return.
Are you sure the rest of the schematic is correct?

Unfortunately the picture is not very clear and I can only recognize a brown wire that goes to the presence pot.

[mhuss]

OK, I'm a dummy. The node I was going on about is not connected! See attached. (The third brown wire is the return from the loop, going into the PI)

Thanks for the responses.

[mhuss]

(Added missing attachments)

[g1]

Attaching complete 2203 re-issue schematic for reference.

[mhuss]

I could have sworn I saw your upload, but it seems to have disappeared.

[g1]

I could have sworn I saw your upload, but it seems to have disappeared.

Check your settings for the issue mentioned here: https://music-electronics-forum.com/...327#post926327

[mhuss]

Check your settings for the issue mentioned here: https://music-electronics-forum.com/...327#post926327

Good grief, I hate this latest vBulletin forum software. And I hate Post mode too (even though it's still my default)!
Thanks for the pointer

[g1]

Good grief, I hate this latest vBulletin forum software. And I hate Post mode too (even though it's still my default)!
Thanks for the pointer

Are you able to see the attachment now? You also have the ability to toggle between modes at the top of each page, if you do not want to set the default to 'posts' mode.
But you will still have to toggle it every time you want to see an attachment.

[mhuss]

Yes, thanks.

I guess this explains why Post mode is the default.

[g1]

I think activity mode is so you can see a lot more posts on the page without any extras. It would be nice if it at least showed a symbol that there is an attachment available, or something.