Marshall JMP 2204 issues and replacement parts

Astro2069

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I have an old 1976 JMP 2204. It sat for a while and when I plugged in it’s volume level is nowhere near where it should be. Sounds a little off. Any ideas? Filter caps can use replacing. I’m curious if this could be a symptom of a bad master volume pot, bad input jacks? Does anyone know the potentiometer values for the amp?
 

gregr

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Tubes and tube sockets.

Give the pots a good cleaning with D100. Jacks too, but you can use regular contact cleaner on those.
 

neikeel

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I agree.
Clean and lube the pots (D100 followed by G100).
Clean and lube the switching Jack contacts with the same.
Reseat all the tubes in their sockets (if you have good spares to test do so).
Don’t just randomly replace jacks and parts out of ignorance. Test, work out what is not right and then replace. Same with filter caps. If they are leaking, the amp is flubby and lacks punch there is audible mains hum then they may will be the issue (in which case don’t forget new bias caps too).
We see too many poorly implemented hack jobs.
Do it properly.
Ps realised I really sound like my father now (retired GEC design engineer)!!!!!
 

Pete Farrington

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With cleaner lube sprays and non tech users, it’s worth noting that a tiny squirt is plenty. ie don’t go randomly blasting away at things for second after second.
So a tiny squirt into the pot casing or onto the jack’s contacts, exercise the pot / jack, then another tiny squirt and exercise for good measure. Then wipe up any excess.
 

Astro2069

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I had tried spraying jacks and pots and sockets with crc electronics cleaner. I do need to get a lube. I tested cables with another head, cables are fine. I tried a quick swap of tubes, new 12ax7’s, and tried power tubes from my other head, no change. Tried to retension the 12ax7 sockets. The filter caps are original. Can’t see any obvious bad solder joints. Previous work in the amp does show a good bit of flux which was never cleaned up after soldering.
 

Dblgun

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Just so we're clear, the amp was good before it sat for a while? No work/changes in the interim?
If this is the case as said before a little maintenance will go a long way to getting it back to where it was. If you are still having issues you may want to check some voltages to see if any are way out of range. This could go a long way in determining if there is a problem.
Again only after you have done the cleaning/maintenance and given the amp a complete look over for something obvious.
 
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AndyD

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Have you set the speaker ohms correctly on the amp? A mismatch between amp and speaker could affect the perceived volume.
 

Spanngitter

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I had PreAmp tubes go foul from one nite to the next so before ripping everything apart and creating a huge list of possibly required parts I would tale an known good 12AX7 and test it in each of the PreAmp positions (Pull 1st, install replacment, test => if NOK reseat old tube, move new to next socket)....
However, I would not neglect the fact that 45yrs it might be a good idea to plan at least replacement of the main filter caps and bias cap and also check all GND connection for any contact resistance as they use(d) only passivated iron washers at the screws which do tend to corrode over time and then your GND is floating..
 

neikeel

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I had PreAmp tubes go foul from one nite to the next so before ripping everything apart and creating a huge list of possibly required parts I would tale an known good 12AX7 and test it in each of the PreAmp positions (Pull 1st, install replacment, test => if NOK reseat old tube, move new to next socket)....
However, I would not neglect the fact that 45yrs it might be a good idea to plan at least replacement of the main filter caps and bias cap and also check all GND connection for any contact resistance as they use(d) only passivated iron washers at the screws which do tend to corrode over time and then your GND is floating..

I agree absolutely that we are heading that way but I am encouraging the OP to do things for the right reasons. You and I have the right testing gear (scopes, cap testers, high quality DVMs and tube testers) and he probably does not. So looking at methodical logical approach.
 

Tatzmann

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If i would be a gambling man with only
one shot i'd say its the low input switching
jack.

How exactly did you clean it again?
 

Astro2069

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If i would be a gambling man with only
one shot i'd say its the low input switching
jack.

How exactly did you clean it again?


I had sprayed crc electronics cleaner in to the jacks working back and forth with a patch cable. I tried spraying the terminals that grasp the patch cable. Going to try a couple of other tricks when I get some time. I’ve seen a tech gently raise the terminal, sprayed cleaner on to a piece of paper, inserted in to it allowing the paper paper to be grasped then slowly pull it out to help clean the terminal.
 

Tatzmann

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Yes exactly this you must do. Spraying the
shit into the holes doesnt do anything to
the switch contacts, the whole Signal from
your high Input goes through this Low-Input
Switch, and they build up oxide layers
over time, because they dont clean themselves
through friction.

Do the paper thing or change the jack
because you have to partially disassemble
the Inputs for precise cleaning anyways.

Do the proper cleaning procedure to
ALL jacks!
 
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neikeel

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Blotting papier or sugar paper work best.
 

Astro2069

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Does anyone have a correct schematic for the JMP 2204? Seems to be a bit of conflicting info on that. I plan to eventually update and replace the potentiometers. Does anyone know the proper values-audio/linear taper etc? Would love to find a step by step video/pictures instruction for updating to the cascaded gain update.
 


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