Marshall JMP 2204 issues and replacement parts

Spanngitter

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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.
You need

3x A1M log
2x B25K lin
1x B(220)250K lin
I'll recommend to get either Alpha or CTS 24mm with lugs (no PCB mount), 6.3mm solid shaft (as your Amp should have knobs with set screw, later models with PCB mounted Pots need knurled shafts as the knos are press on) and 3/8" Bushing. Depending on where you are from there is e.g. complete Sets available from vendors like e.g. TAD:
https://www.tubeampdoctor.com/en/poti-set-for-marshall-jcm800-2203-2204
https://www.tubeampdoctor.com/en/cts-poti-set-upgrade-for-marshall-jcm800-2203-2204

PS: Also I would recommend to check where your Bias Supply is branching off, some stupid guy once decided to put it behind Standby to save 4" of wire which can cause the tubes to run away due to no bias supplied for a short time when Stndby is turned off. Alternatively just bridge the Standby with a 220k 3W Resistor to have a small amount of current constantly flowing, keeping the Bias Supply alive and preCharge the Filter Caps.
 
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Seanxk

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the whole Signal from
your high Input goes through this Low-Input
Switch,

Are you sure?, as I thought this too, but have another look, the switch just takes out that jack sockets input path and leaves the jack socket that is being used with a hard wired path.

I'm not saying don't clean them though......
 

Pete Farrington

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Bournes seem the best current choice for 10% taper pots.
For about the past couple of decades, the track CTS use doesn’t have the wiper move off its CCW end stop until it’s rotated maybe 15% of the available range.
So on a typical Marshall faceplate, that’s 0-2, ie the wiper doesn’t reach the resistive part of the track until after 2.
 
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Tatzmann

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As i told him about the low jack i
forgot that the early 2204s were not
cascading the Inputs.

That change occured 1978 or something,
the 100 watter MVs had the cascading preamps
right from the start.
 

Astro2069

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I figure I need to get the amp working right before doing the cascaded gain update. Does anyone have any instructions/pics/video of exactly how to do it? I saw an old thread that had a link with that info but the web address is no longer valid.

I did a quick replacement of the low input jack last night. That didn’t seem to correct the issues. I’m starting to wonder if it is the fx loop that was previously added. Got some star washers at work to rework the ground connections to the chassis at the pots.
 
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Tatzmann

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Well effects loop could be another jack
thing, clean em or try a patchcable from
send to return if havent done yet,
see if it makes a difference.
 

Astro2069

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Bournes seem the best current choice for 10% taper pots.
For about the past couple of decades, the track CTS use doesn’t have the wiper move off its CCW end stop until it’s rotated maybe 15% of the available range.
So on a typical Marshall faceplate, that’s 0-2, ie the wiper doesn’t reach the resistive part of the track until after 2.

Do you have a recommended place to get these from?
 

Astro2069

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I think I did find the instructions for the cascaded gain update. May need to source some resistors.
 

neikeel

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Seanxk

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Over use of Deoxit and the like (in it's previous life ) can actually mix with the grease that's used in the pot and spread it on the tracks.
Before replacing all of them, try dismantling them and cleaning the tracks by hand, if this is really the problem that is.
 

Pete Farrington

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Looking at mouser and they all seem to be 20% tolerance.
That’s pretty typical for pots, I suspect that almost all production amps ever used pots with that tolerance spec.
Of course there’s a few boutique amps with PEC pots which may have a tighter tolerance spec.
Bear in mind that tolerance is just in regard of the total track resistance. The conformity to the taper nominal almost never has a tolerance spec, ie it could be anything.
 

Astro2069

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Did a quick measurement of pots in circuit, most are off. Need to take a minute and unsolder from circuit to measure again. I suppose that this could be my main issue.
 

Pete Farrington

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It’s normal for the track resistance of old UK pots to tend to drift up in value. There’s no reason that should cause a problem per se, it’s rather unlikely to be the cause of the loss of volume mentioned in post #1.
 
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neikeel

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Of course I have not got the amp in front of me to test but this issue with pots is a weird one. A carbon tracked pot will drift up a little - in the vintage amps I fiddle with most of the best amps have high testing pots (often >20% in the case of the old AB/RS labelled ones. Sure if they have damaged tracks that can’t be cleaned/lubed to be quiet then of course replace but I really doubt that it is the issue here.
 

Astro2069

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Of course I have not got the amp in front of me to test but this issue with pots is a weird one. A carbon tracked pot will drift up a little - in the vintage amps I fiddle with most of the best amps have high testing pots (often >20% in the case of the old AB/RS labelled ones. Sure if they have damaged tracks that can’t be cleaned/lubed to be quiet then of course replace but I really doubt that it is the issue here.

They values have actually dropped. Off the top of my head the presence pot dropped down to something like 12ohm. Not even 12kohm. The pregain was slightly above 1M, the master was quite a bit below, I believe the middle was pretty close, bass was well below 1M and I believe treble was off a good bit too. I wrote it down but we have left for the night, don’t remember exactly what the values were.
 

Pete Farrington

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The low readings will almost certainly be due to parallel paths, eg the master has a parallel path via the tone stack.
The presence might be wired as a pot or a variable resistor, if the latter then a high setting may cause a low resistance.

How about compiling a voltage survey? I suspect that may be a more useful exercise. Load connected, all controls zeroed. But only if you’re comfortable / safe probing around high voltages.
Ensure that the mains supply is via an RCD / GFI protected feed.
 
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