JMP1: sound stops and dashes on the display

[Dogs of Doom]

the trans' do fail in these, that's why you can buy them online pretty easily...

this guy makes a replacement:

Marshall JMP-1 Hum - Plasma Music Limited

Many users of this well known pre-amp, will be familiar with the infamous Marshall JMP-1 hum! My PML-TX01 transformer addresses this issue.

plasmamusic.com

w/ some explanation about the unit. He shows how to jumper the trans' for EU, vs US...

so, you guys in EU can buy US models & vise-versa...

from TAD:

Mains transf.for Marshall JMP-1 Preamp

Power Transformer for Marshall JMP1 Preamp replacement for the T-6001 does NOT fit for the 9001 model. L x W x H:…

www.tubeampdoctor.com

Mercury Magnetics (expensive)

Mercury Magnetics Transformers

Best guitar amplifier power transformers, output transformers and chokes. Far more than tubes, speakers or other amp components, the transformers establish the platform for shaping your amp’s sound.

www.mercurymagnetics.com

 

[Gutch220]

Somebody with much more experience on these than me might be able to show you specific locations on the circuit to measure voltage/etc to perhaps narrow down the problem. just a thought. Looking at this circuit board makes me dizzy but at least it's all through-hole components. I'm used to relatively easy "normal" tube amps like JCM800's which have 1/10th the components. The last thing I would do is A) give up, or B) do anything to make it worse. It's better than spending a $1,000 or euros on a used one.

 

[Tunesketcher]

OP, When you find a technician to repair your unit, just know that these are worth repairing/keeping and can last a lifetime. I had one for 12 years and was super reliable with a great sound just had to replace a preamp tube. A friend of mine has had his for 20 years (still making $$ with it) and just replaced the battery/preamp tube. Marshall hit a home run with these units, especially for the time.. Good luck!

 

[anitoli]

Someone was definitely planning ahead for an EPROM failure as the 27C64 is in a socket. These units had these soldered to the boards and are a bitch to remove. If i were you i would make a copy of that program cause if that chip loses any memory block you are up the creek.

 

[PelliX]

Someone was definitely planning ahead for an EPROM failure as the 27C64 is in a socket. These units had these soldered to the boards and are a bitch to remove. If i were you i would make a copy of that program cause if that chip loses any memory block you are up the creek.

Not necessarily. While us techs appreciate this convenience, plenty of kit has sockets for potential upgrades later down the line. For example, Digitech's Quad series for the most part had sockets allowing ROM upgrades. This was probably cheaper/easier/more desirable than attempting to (ab)use MIDI SysEx or integrating an additional dedicated interface for upgrading the ROM. Less circuitry involved, too, of course.

 

[anitoli]

Not necessarily. While us techs appreciate this convenience, plenty of kit has sockets for potential upgrades later down the line. For example, Digitech's Quad series for the most part had sockets allowing ROM upgrades. This was probably cheaper/easier/more desirable than attempting to (ab)use MIDI SysEx or integrating an additional dedicated interface for upgrading the ROM. Less circuitry involved, too, of course.

As my experience with the JMP-1 is limited they came out at roughly the same time as the 6100 and were based on the same midi idea. Although most 6100's have all the chips in sockets i have had several on the bench that did not and they were 92-93 in manufacture years, they were soldered directly to the boards. And i can attest to the fact that when you have a blown out midi board with soldered on chips you got a real problem on your hands. I developed a technique for getting these off of there without damaging the trace and it works pretty good. Every JMP-1 i have seen did not have sockets.

 

[PelliX]

Maybe it was a 'leftover' from R&D where it would make sense that got put into production as nobody questioned it, and then later they realized it would save them 2 pennies per amp. They weren't planning on improving the logic in the controller, I would assume. Or maybe they wanted to have an easy fix ready in case the programming appeared problematic in the field.

And i can attest to the fact that when you have a blown out midi board with soldered on chips you got a real problem on your hands. I developed a technique for getting these off of there without damaging the trace and it works pretty good. Every JMP-1 i have seen did not have sockets.

Hot air and/or a solder sucker and some desoldering wick do it for me with IC's. What's your method of attack?

 

[anitoli]

Maybe it was a 'leftover' from R&D where it would make sense that got put into production as nobody questioned it, and then later they realized it would save them 2 pennies per amp. They weren't planning on improving the logic in the controller, I would assume. Or maybe they wanted to have an easy fix ready in case the programming appeared problematic in the field.



Hot air and/or a solder sucker and some desoldering wick do it for me with IC's. What's your method of attack?

Clip the pins off the chip then the board as close as possible. The remainder in the through hole usually pulls right out with the solder. If it wants to be stubborn you can usually grab it with fine point pliers and a dab of heat and it pulls right out. I try to get in and out as fast as possible because the trace on these boards is very flimsy.

 

[durante]

Hello, some (not great) updates after a couple of months..

I got a recommendation for a amp tech in Berlin and reached out. He told me that he is more on the analogue side of things rather than digital equipment such as the JMP-1 but was happy to have a look at it for me.
Some days later he mentioned that he couldn't find the problem, but would guess it is something with the processor, however, out of his knowledge area.
He was very humble and open to discuss about the problem, and in the end didn't charge anything as he couldn't help me. He even gave me a missing bezel for the front phones output. The amp was returned just like it was when I delivered it to him. Good guy in Reinickendorf.

Next I found a list of authorised Marshall repair shops in the Marshall website. Called one of the guys there around Berlin.. "yes, I deal with the JMP-1, bring it in and I'll check it out". Sounded promising.
This guy though charges for any checks, repairing it or not. I also asked for a battery holder installation together with the fix for the problem.

More than a month later I called him to know if he got updates. The next day he wrote me "hey, it's probably something with the RAM, I will order some new ones."
Another couple of days later... "ok, the RAM didn't solve it, it is probably something with the 8031 processor or the EPROM, but those need programming and aren't produced anymore. I can't fix the amp."
I have a spare EPROM that I bought just in case some months ago when someone was selling it from an old unused stock (for an old equipment sometimes it's good to have spare parts right..). However, the tech told me that if I wanted him to try it, I'd need to place another paid order and wait several months.

So here I spent 100eur to know that my amp has a problem and that the tech can't fix it (I knew about this risk from the beginning, all fine). The worse though, is that I received the amp with several loose nuts and screws, now only 4 of the 25 built in presets output any sound (not sure if related to any repair or it just got worse as it would in any case) and I got a stripped head screw which I couldn't extract and therefore I can't open the case to try anything at this point - I could verify the battery holder has been installed though.

I might try removing that stripped head screw (tried screwdriver with rubber band, glue, tried to gently hammer a driver on the screw head to make a new groove... all with no success), and then installing the new EPROM to check the results - however I don't have anti static protection and am afraid of creating even more problems in the amp.

Does this bring your thoughts to a more conclusive path?
What do you suggest?

Thanks!

 

[Purgasound]

Replace the battery then factory reset. Sounds too simple but that's the fix. When the battery dies it does funny things to the amp. Mine died on the road and when I replaced it I was getting weird whining noises and some of the controls didn't work. Performed a factory reset and all went back to normal. You can do a Google search for the manual and it's explained in there. I think it's just holding a couple buttons while powering it on. Try the simple things first. If a tech doesn't have first hand experience with these specific units they're likely going to try a lot of other methods first that may cause more problems than they solve.
Give it a shot then report back.
Make sure there's a fresh battery first. It does more than just retain the memory for user settings. An MP1 can still work with a dead battery. A JMP-1 will not.

 

[durante]

Replaced the battery and did a factory reset today.
The result is the same, the unit still reboots.

 

[Michael Roe]

I had this screen shot saved to my computer.
I don't remember where I got it....maybe here on this forum.
But did you try this reset procedure?

 

[durante]

Thanks Michael, it was indeed locked and I didn't notice it before.
However, even after unlocking and resetting, the problem persists.

While resetting all the front lights turned on and blinked endlessly until I released the buttons (don't remember this behavior from back then when I last reset it).
After I released the buttons I got an "E" on the display and then it turned on.
According to the manual, the E means there's an error, likely a dying battery - but I just installed a new one. Probably another error?
Interestingly it seems the reboots are more frequent now.

Any more ideas to try?

 

[Michael Roe]

Thanks Michael, it was indeed locked and I didn't notice it before.
However, even after unlocking and resetting, the problem persists.

While resetting all the front lights turned on and blinked endlessly until I released the buttons (don't remember this behavior from back then when I last reset it).
After I released the buttons I got an "E" on the display and then it turned on.
According to the manual, the E means there's an error, likely a dying battery - but I just installed a new one. Probably another error?
Interestingly it seems the reboots are more frequent now.

Any more ideas to try?

Did you have the power transformer replaced?
I had mine replaced several years ago. I didn't however have all the problems you had.
Maybe I caught mine before it got worse?
With mine, the first thing I noticed was a drop in volume. I believe it was only on the OD channels at that time. I had mine immediately checked out, replaced the tranny with a Mercury magnetics one and the battery replaced.
I would recommend that if the tranny has not been replaced to do it even if you have no problems. It is a known issue for these old devices.
It could be possible to have purchased one and not know about that volume drop. IOW, maybe yours was already doing that? One might think that is just how they operate normally. It wasn't drastic but just enough that an experienced user would notice. Who knows how long it could have gone in that state before more problems arose?
The tech that did the work on mine said, if you have any issues with a JMP-1 the first two things to do is replace the tranny and battery.

 

[Michael Roe]

I should also add another known issue is the "Data knob" can go bad on these. I had one once that did that and at the time was able to order a replacement. I did the work myself on that. It was fairly simple.
That issue just caused the numbers to not move or just jump around randomly. If you couldn't get a replacement, it would be an easy mod to just add a "+" and "-" buttons.

 

[tmingle]

I am going to assume that the unit has some type of DC power supply, which would indicate the possibility of DC filter caps in need of replacement. My Digitech GSP-1000 unit did something similar & 1 of the caps had failed. The unit worked fine after I replaced them.

 

[durante]

Did you have the power transformer replaced?
I had mine replaced several years ago. I didn't however have all the problems you had.
Maybe I caught mine before it got worse?
With mine, the first thing I noticed was a drop in volume. I believe it was only on the OD channels at that time. I had mine immediately checked out, replaced the tranny with a Mercury magnetics one and the battery replaced.
I would recommend that if the tranny has not been replaced to do it even if you have no problems. It is a known issue for these old devices.
It could be possible to have purchased one and not know about that volume drop. IOW, maybe yours was already donning that? One might think that is just how they operate normally. It wasn't drastic but just enough that an experienced user would notice. Who knows how long it could have gone in that state before more problems arose?
The tech that did the work on mine said, if you have any issues with a JMP-1 the first two things to do is replace the tranny and battery.

I have the original transformer.
Now that you mentioned it, I remember I also had some volume drops before the reboot cycles started. I even emailed Marshall to ask about it but never got a reply..
With the reboots, the volume drops are gone - or at least can't be noticed anymore because of the frequent reboots.

 

[Michael Roe]

I have the original transformer.
Now that you mentioned it, I remember I also had some volume drops before the reboot cycles started. I even emailed Marshall to ask about it but never got a reply..
With the reboots, the volume drops are gone - or at least can't be noticed anymore because of the frequent reboots.

I forgot, as @tmingle said, I had my tech replace all caps in the unit as well.