what to replace next on my JMP to fix this issue?

glpg80

Member
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186
Sounds to me like intermittent cold solder joint or a bad ground reference from the 2203 conversion. It needs to be chopsticked to find the problem that isn’t electrically or visually visible.
 
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cal_83

Member
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82
Sounds to me like intermittent cold solder joint or a bad ground reference from the 2203 conversion. It needs to be chopsticked to find the problem that isn’t electrically or visually visible.
Yeah, the last time it cut out it was actually the worst its been, I had to turn the amp off after about a minute and grab a spare. Normally it was just kinda dipping in and out occasionally not like this bad.
 

Geetarpicker

Member
Messages
2,908
About few years ago I worked on a ‘68 Marshall JMP 50 that stumped me for a bit. Random noise and drop out issues. The amp was original dead mint cosmetically, inside and out. That said it took a while to figure out the issue. Marshall liked to use brass screws and nuts to hold the boards in place, plus the same holding several ground tabs and tube sockets to the chassis. On the output stage each tube had a ground connecting held in place by the copper screws. Once I undid the screws there was considerable hidden corrosion causing bad ground connections. You may have heard dissimilar metals causing corrosion problems, but I wasn’t expecting this on an otherwise mint looking amp. Copper hardware, steel chassis, and steel ground tabs. You could not even see the corrosion until the copper screws/bolts were taken apart. Anyway, a little cleaning in just the right places with some fine sand paper was the fix!
 
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j_el_jee

Member
Messages
1,445
You sure the Boss, boss power supply, and cables aren’t the problem? A pedal losing power might exhibit a similar result to what you describe. Is your tech using the pedal when he’s troubleshooting? Do you use it with your other amp? Loose connection on the power plug or power supply?
 

cal_83

Member
Messages
82
You sure the Boss, boss power supply, and cables aren’t the problem? A pedal losing power might exhibit a similar result to what you describe. Is your tech using the pedal when he’s troubleshooting? Do you use it with your other amp? Loose connection on the power plug or power supply?
Hi, no the tech hasn't used my pedalboard with the amp but has run it with my box and checked my speaker cable and replaced the socket on the speaker box ( it was worn). I have been running the pedalboard with my Soldano SLO - 100 and it normally works fine, there have occasionally been similar sounding niggles on the board over the years, but once I used contact contact cleaner it sorts it out.

I will go through my board again and give it a once over, and my tech said the next time it happens I need to unplug and plug my guitar straight into the LOW input, and test it there to see if it still does the same thing. I'm actually using a multi power supply in my pedal board which powers all of them at once
 
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cal_83

Member
Messages
82
About few years ago I worked on a ‘68 Marshall JMP 50 that stumped me for a bit. Random noise and drop out issues. The amp was original dead mint cosmetically, inside and out. That said it took a while to figure out the issue. Marshall liked to use brass screws and nuts to hold the boards in place, plus the same holding several ground tabs and tube sockets to the chassis. On the output stage each tube had a ground connecting held in place by the copper screws. Once I undid the screws there was considerable hidden corrosion causing bad ground connections. You may have heard dissimilar metals causing corrosion problems, but I wasn’t expecting this on an otherwise mint looking amp. Copper hardware, steel chassis, and steel ground tabs. You could not even see the corrosion until the copper screws/bolts were taken apart. Anyway, a little cleaning in just the right places with some fine sand paper was the fix!
yeah well it still could be something really minor like that, I really hope its not a transformer but my tech seems to think not as they tend to go out really quick not linger for 3 years like this.
 

cal_83

Member
Messages
82
well i went through every input/output on the front and back with a Q-Tip, cleaned out the fuse holders and realised there was actually a fast blow fuse in the 1Amp socket, so put a slow blow in it as per what it should be. Went through my pedalboard and tightened every jack and cleaned every sock on all pedals. And I've been trying all day to get it to play up at home, and its been totally fine.

This is of course at bedroom volumes, but I can actually remember getting to gigs, and even just when I turned it on it would act up straight away.

It really has a mind of its own, but could also be getting knocked around in transit?
 
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Motterpaul

Tone is in the Ears
Messages
12,983
About few years ago I worked on a ‘68 Marshall JMP 50 that stumped me for a bit. Random noise and drop out issues. The amp was original dead mint cosmetically, inside and out. That said it took a while to figure out the issue. Marshall liked to use brass screws and nuts to hold the boards in place, plus the same holding several ground tabs and tube sockets to the chassis. On the output stage each tube had a ground connecting held in place by the copper screws. Once I undid the screws there was considerable hidden corrosion causing bad ground connections. You may have heard dissimilar metals causing corrosion problems, but I wasn’t expecting this on an otherwise mint looking amp. Copper hardware, steel chassis, and steel ground tabs. You could not even see the corrosion until the copper screws/bolts were taken apart. Anyway, a little cleaning in just the right places with some fine sand paper was the fix!
This is really logical. I have been working in a 1980s Marshall 2204 and I see a lot of cold solder joints from apparently anodized wires. Just trimming to old wire off, cleaning (de-soldering) the tube socket, and re-flowing with new solder has helped this amp out a lot. Changing out old tube sockets helps as well.
 

Geetarpicker

Member
Messages
2,908
As I mentioned before the corrosion I found on the ‘68 JMP was not visible until I actually removed the copper screws. Worth taking the time to undo them and see what you have.
 
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Thinline_slim

Member
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3,269
I had the same issue with my JMP 2204. Turned out to be the standby switch. It has the rocker type switches. Some deoxit fixed it.
From the symptoms I was thinking the same thing.
The amp sounds fizzy and it like im playing through a 5 watt transistor amp almost
Have you ever flipped the amp into standby and kept playing your guitar? Is that pretty much the “transistor amp” sound?
 

cal_83

Member
Messages
82
From the symptoms I was thinking the same thing.

Have you ever flipped the amp into standby and kept playing your guitar? Is that pretty much the “transistor amp” sound?
Yes thats the sound exactly, that second or two when you hit standby and you guitar is still ringing out. Last time it happened at band practice, I turned the master volume all the way up and it had no effect to the volume at all. So I just had to switch it off and use a spare, then when I came back to it at the end of jam it was fine.

I did however also go through my pedalboard and noticed the nut surrounding the switch on one of my MXR pedals was loose so I tightened. There have been occasional times on both amps where I've hit this pedal, and it caused a cutout. But the marshall always seemed to play up more than the other amps so i assumed it wasn't that. Lets hope it's fixed it.
 
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cal_83

Member
Messages
82
As I mentioned before the corrosion I found on the ‘68 JMP was not visible until I actually removed the copper screws. Worth taking the time to undo them and see what you have.
well, next time it's at my tech I'll get him to double-check as I'm not comfortable poking around the amp insides. He did recently work on the board though and inspected it.
 

J M Fahey

Member
Messages
2,433
These are the kind of problems techs hate. You find something that seems the to be it, only to have the issue pop up again.
...........
Really.
Sometimes a customer brings an amp in, says "smoke came out - I see sparking - tubes redplate - whatever" and I smile.
Why? Am I a Sadist? A greedy guy happy with the massive damage? No: because it´s relatively "easy".
Problem sticks out like a big sore thumb, you replace what is obviously bad, search for possible cause, it´s a relatively straightforward path.

Now when customer says "hey Juan, it should be easy, Amp works well, sort of, BUT ..... " .... then it´s when the PITA Pandora´s box gets open.

Often a time sink black hole chasing phantoms.
 

zenas

Member
Messages
8,768
Really.
Sometimes a customer brings an amp in, says "smoke came out - I see sparking - tubes redplate - whatever" and I smile.
Why? Am I a Sadist? A greedy guy happy with the massive damage? No: because it´s relatively "easy".
Problem sticks out like a big sore thumb, you replace what is obviously bad, search for possible cause, it´s a relatively straightforward path.

Now when customer says "hey Juan, it should be easy, Amp works well, sort of, BUT ..... " .... then it´s when the PITA Pandora´s box gets open.

Often a time sink black hole chasing phantoms.
Yeah like tires when I worked in a gas station in high school. A flat that goes flat fast, easy peasy. Put some air in it, you either hear or feel the leak or a little soapy water and you spot it straight away.
The folks that came in with the tire that lost air slowly, a total pain in the arse. Fill the tire with air, drop it in a tank of water and wait to see a bubble.Then the bell rings and you gotta run out and pump gas for some old fart too lazy for self service, you miss the bubble. . . . . .
 




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