Epiphone EBM-4

Pickups, wiring schemes, switch techniques and onboard active electronics for guitars and basses
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CheapChip
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Post by CheapChip »

Hi there,

A friend gave me a Epiphone EBM-4 he just bought used in pretty good condition.
The only problem is that despite what the seller told, it doesn't make any sound but a slight noise when it's plugged into an amp.
I don't really worry about that: I think a JRC082 on the preamp board has to be swapped, or something like that.

But since there's no infos on the web about this preamp, I decided to trace a schematic from the board and share it there. :wink:

After a double check, everything seems ok but I'm quite confused by the preamp design:
  • - There's many more inputs than pickups (2) and I'd like to know if custom wiring could possibly be done,
    - 3 switches are on the board but what are their purposes?
    - More globally, I doesn't remind me any other preamp I've seen before...
So, if any of you can enlighten me, it will be really appreciated.
And and if my documents can help any of you, all the pleasure will be mine. :wink:

Cheers!
Attachments
Epiphone EBM4 Preamp Schematic.pdf
Seems fine but feel free to take a look...
(69.68 KiB) Downloaded 154 times
EBM4 Preamp Layout V2.pdf
(1.82 MiB) Downloaded 24 times

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Ichabod_Crane
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Post by Ichabod_Crane »

I tried to draw the schematic in the software emulator I have. I don't know the value of the pots, so I inserted ramdon value.
I got some loss signal, not so flat. I see in the schematic a sort of dual band baxandall eq, right? Anyway I got just a minor action, but, the value of the pot could be wrong.
That switch seems to be a sort of mid boost.
Maybe there's something wrong in the schematic you draw or in the one I drawn in my software, I can check again my drawn later.

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CheapChip
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Post by CheapChip »

Hi Ichabod_Crane,

Thanks for your help and sorry for my oversight.
According to another user, pots are 100kB (3 pots values are hidden by PCB and I've not unsoldered them yet).
https://forum.gibson.com/topic/86067-ebm4-schematics/
Some say they are crappy and replacing them may be worth it.
VR4 is the only pot with no center-detent. Therefore I assume VR1 is pickups balance, VR2 and VR3 are 2 bands EQ and VR4 is volume.

What switch are talking about for mid-boost?
And what do you think the others are intended for?

Thanks again for your help! :D

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Lani
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Post by Lani »

They do look like different inputs. Maybe they had different versions and all they had to do is connect the inputs up accordingly? :hmmm: I take it pins 1, 2, 5 & 6 go nowhere/do not have any wires installed? Any chance of a shot of the other side of the PCB? Thanks for the trace & schematic!!

One more thing, I've also read on another forum somewhere that the pots used in these are not of the greatest quality and are usually suspect.

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Ichabod_Crane
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Post by Ichabod_Crane »

CheapChip wrote: 11 May 2024, 13:11 According to another user, pots are 100kB (3 pots values are hidden by PCB and I've not unsoldered them yet).
VR4 is the only pot with no center-detent. Therefore I assume VR1 is pickups balance, VR2 and VR3 are 2 bands EQ and VR4 is volume.
VR1 seems a pickups balance to me, too. VR2 should be bass and VR3 treble control. VR4 the volume. It doesn't go to the ground but in the voltage reference (a 100k to the 9v and a 100k to the ground).
I seen an image EBM-5, actually, where the one we guess is the volume pot is a log (A100k).
CheapChip wrote: 11 May 2024, 13:11 [...]
What switch are talking about for mid-boost?
And what do you think the others are intended for?
[...]
The switch could be a sort of mid boost is the SW3.
The other two switches seem pickups selector, on/off.

I think you do a good work with the tracing, but did you check it twice or more? In case of some mistake. Maybe some resistors value? In my emulator I don't get enough volume.

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CheapChip
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Post by CheapChip »

sorry for the wait: a bit of a complicated start to the week... :wink:
I think you do a good work with the tracing, but did you check it twice or more? In case of some mistake. Maybe some resistors value? In my emulator I don't get enough volume.
Thanks for your comment. I checked it many times before and after tracing. But I can check it once again, no problem.
I'll try to have a look at it either today or tomorrow.

By the way, here are some pictures.
Component side and wiring.

Cheers.
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Wiring. Sorry for the poor quality...
Wiring. Sorry for the poor quality...
Component side
Component side

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george giblet
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Post by george giblet »

It's always good to document these things. When I see preamps which drive the Baxandall EQ from a pot it makes me think someone hasn't thought about the design.

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