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Sunday 8 April 2018

The solution(s) to my old Marshall JMP-1 issue has been discovered

If you remember the jolly escapades involving my JMP-1 preamp, its death, and its internal organs transplant, all following my soldering-in of new capacitors and a battery holder and then encountering issues:

... well, I had made a YouTube video demoing the issues, which I left unlisted, while sharing the link in some places where I was asking for tech advice. This is that video:

At the time, nobody I asked had any idea what the issue was, and so I ultimately replaced the JMP-1's entire PCB and front control panel by purchasing a new one from the Marshall distributor for Canada.

And this is what the replacement board looked like before I broke it into its pieces, and then installed it into my original JMP-1 chassis:

So, recently, some comments were made for the video I uploaded, coming from other people who are familiar with the issues shown, and who mentioned their solutions.

Posted in the comments by Olivier Vaillancourt-Girard:

"You probably got this sorted but I'll share my solution. *You should make this video public to help anyone who encounter these problems!

Got this exact problems after changing the battery. Look like the preset memory got corrupted by lack of sufficient power.

Values jumping around when pressing buttons, data would not always change or respond slowly, prest loading was buggy. High pitched noise when messing around for a bit, etc.

You need to reset the unit (Hold OD1+Clean1 then power the unit) but the unit need to be UNLOCKED first.

You can tell yours is locked when you hit Save: the display show "St L" That mean the save fonction is disabled so you can't write on the memory.

To unlock, press Save and while you see "St L" press the "Channel" button. The unit will unlock and display the current channel to save to ex: "St 01".

Now perform the reset function Powerup+OD1+Clean1 and the memory will refresh itself and all glitch, lag and weird sounds will be gone.

Hope it help someone!"

And posted in the comments by Tony Montana:

"The problem is the contacts to Battery, not the motherboard.
If you've changed the battery, your new battery will not get that good contact. You have to work the battery in. Make very good contact.
JMP-1 is very old box, on the battery contacts is a thin invisible oxidized and corosion coating. You have to sand it down then work with contact spray and press the contacts firmer. If you've done everything well, I mean the contacts on the JMP-1 holding the battery. Put the new battery in and press the contacts firmly with your fingers then turn on the JMP and you will see your JMP is working, as soon as you leave finger loss the problem is back. The problem is bad contact, you have to eliminate that.
I repaired a lot of JMP-1 like that. I am a technician, the problem is exactly as I say. It does not look like it at first glance but it does."

So, I now made the video public, so that other people with the same issue can see their reported solution. And thanks to Olivier and Tony for their experienced solutions!

I guess this means that maybe I didn't botch my work on my JMP-1 after all (bottle of isopropyl, oven-cooking, and all), and if I had known about needing to unlock the firmware, then I could have avoided an expensive replacement mainboard purchase.

I still have my original JMP-1 mainboard and front control panel, and so I could test the reported solutions, and likely fix its operation. However, I moved my replacement JMP-1 mainboard and front control panel into the old JMP-1's chassis, and I don't have a second JMP-1 chassis to put the old JMP-1 into.

So, while I likely can get the old JMP-1 running as normal (and with all-new high quality capacitors and a battery holder!), I don't know what I could do to house it, to make it usable in a rack setup.

If anybody comes across somebody selling a non-working JMP-1 preamp for cheap, maybe let me know? I could use its chassis to install my old JMP-1 board into!

C~ Soundscapes


  1. I hope you got the power link jumpers all installed properly with the appropriate links connected for 117v.

    Looks like that replacement pcb came with all 3 power jumpers installed and if you're using it in the USA, the middle jumper needs to be clipped out. If you're using it in the U.K., the outer jumpers need to be clipped out and only the center jumper should remain.

  2. Weirdly enough, I was 99% sure that I’d done that unlock/reset maneuver by now. The original battery was in the early stages of total meltdown onto the motherboard, but I managed to clean up the mess and installed a new battery holder. The previous owner didn’t know there was such a thing as a backup battery…go figure. Neither did I until the amp started acting strange.
    Anyway…after the battery holder was replaced, it would lose everything in memory if I left it with the power turned off for more than a hour. If I turned it on, the "E" would briefly appear, and it would do all the crazy things you are describing until I reset it again, but there was always this feeling of unpredictable behavior because I knew that it could burst into that well known high pitched squealing pig mode at any time…or any of its other specialties. So I was desperately looking for clues and insights from other JMP owners, and I found this page. When I read the thing about the factory reset not being able to complete because the memory was write protected, a missing piece of the puzzle clicked into place in my head. I was aware of the write protection, but I didn’t connect the dots. So I unlocked the memory and reset it earlier this morning, and it’s still working, 12 hours later. I think it’s still a bit too early to make any conclusions (I’ve learned that the hard way of course), but the amp is back in business for now, and I just wanted to say thanks (👍🏼) and share my amp adventures with someone else in the same ridiculous boat.

    Thank you, mister! I hope you’ll find the parts you’re looking for. (Backup battery still working here…and no signs of random erratic strangeness…💖 ). Awesome...

  3. I just saw the video. it’s acting EXACTLY the same as my amp. Just wow.:O