JMP-1 preamp mod for a rear input

mich · Post by **mich** » 04 Jan 2011, 00:00

I don't know if this is of interest to anyone but I run two rigs, an all tube (Vox AC30 and ac50 A/B'd) with analog stompboxes (sans a gigadelay in the effects loop) based rig and an all solid-state rack based rig with a JMP-1, line6 pod pro and an ada mp-1 (switchable) with bbe sonic maximizer and mostube power amp. Anyways, the JMP-1 is notorious for a front input jack that goes south after a while. Additionally, it does not have a rear input jack like the other preamps i mentioned which is a little annoying when setting up the cabling. So my front input jack went south and I when I opened it up, it just looked like a major PITA to pop out the board and replace the jack. So I drilled a hole about 2 - 2.5 inches right of the power jack on the rear of the unit, installed a mono-jack and ran wires the the input jack and soldered wires to the signal and ground pins on the jack housing. I figured this would only work if the jack was electronically sound but the physical connection of the instrument cable to the jack was what was causing the intermittent connection. Well this seemed to be the case b/c the mod worked. I thought the proximity to the power line may introduce some noise but I don't really think its the case ... I haven't noticed that much additional noise in the line .... may redo with some shielded wire to see if that helps at all. Anyways ....

Ice-9 · Post by **Ice-9** » 11 Jan 2011, 21:23

If you replace the wiring with shielded cable can you take a photo of the mod and post it.
I recently had a broken jack on my JMP1 as well, I have had it for 20 years from new, I contacked Marshall and they sent me both front jacks FREE of charge, it wasn't such a big job to pop the PCB out as its only held in place by the rear jack nuts.
So now i have a new lloking repaired JMP1

BTW There used to be a forum about that had some circuit mods for the jmp1 I cant remember what it was called but a google search should find it pretty easy.

mich · Post by **mich** » 19 Jan 2011, 06:25

I will. I just got some shielded wire from small bear. Probably do the job this or next week

mich · Post by **mich** » 26 Jan 2011, 17:22

I redid the wiring last night. I actually used a simple coaxial cable from an old TV cable hookup. Happy to say that for high gain settings the noise has dropped to pre mod levels

OLd (unshielded wiring) mod:

: Jack, old wiring

: Input jack relocated

: Old wiring, run

New, shielded:

: Shielded wired jack

: Shielded wire soldered to old input jack

: New wire run

Sorry, crappy iPhone pics

brain_lp · Post by **brain_lp** » 08 Feb 2011, 05:29

the thing sounds crazy....

awesome!!!

peoplesguitar · Post by **peoplesguitar** » 09 Jul 2011, 14:45

New to the forum. Have been using the JMP for 10 years...best way for me to achieve cranked old Marshall tone without having the soundman after me the whole evening. I use only the plexi 50 patch. A guest sax player stepped on my cord, breaking the input. Got replacement. Four hours later, (the PC standoffs!!!!), got it installed and find the noise level, hiss to be increased.

Like to try your mod, but do not have permission to see the photos that you've posted. Any folks out there know what I have to activate to see the photos? Thanks

JiM · Post by **JiM** » 24 Jul 2011, 18:59

peoplesguitar wrote:Like to try your mod, but do not have permission to see the photos that you've posted. Any folks out there know what I have to activate to see the photos? Thanks

Usually, becoming a member is enough to see any material posted on the forum. if you still can't see the images after being able to post a message, you should report the problem in PM to a moderator. Did you refresh the page, clear your browser cache, etc. ?

By the way, welcome on board !

lolbou · Post by **lolbou** » 24 Jul 2011, 19:01

Or reach a post count of 5 by posting in a thread in the Testing section...

DeanM · Post by **DeanM** » 24 Jul 2011, 23:23

lolbou wrote:Or reach a post count of 5 by posting in a thread in the Testing section...

Is that what thats for?

wondered about that! lol

lolbou · Post by **lolbou** » 24 Jul 2011, 23:40

It's basically for testing.