Marshall JCM 900 DR 4100 restoration and mods.

Henry Sabbath

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Hi Everyone,

Just willing to share my experience and confirm some mods circulating around on the internet around the JCM 900 DR 4100. Bought a neglected, cheap, not fully working DR 4100 head made in 1992 for 350 euros.

It´s symptoms where:

- preamp working with a very low faint volume, like being a 10 watt transistor amp, nearly no sound....preamp not working basically. Original preamp tubes.
-Reverb not working
- 4 front panel pots with the stem loose and banged many times
- switching issues between channels
- Power amp with bias drift and original power tubes.
-over all white noise / ground noise

Preamp issues:

- It had 2 IC´s fried, IC4 M5201 was shot, and the M1458 in the reverb section was shot as well. Did change all M5201 IC´s for JM2121D / JRC2121D, 5 in total. Had an overall better definition and feel, maybe due to better slew rate compare to the M5201.

- Recapped all the electrolitics and polyester caps for Polyprop on the JMP52C preamp board, replaced 100k preamp resistors as they where noisy, for 1/2 watt ones, TR3 was changed resolving weird channel switching issues.

- Replaced input jack for Neutrik.

-Replaced all front panel potentiometers for alpha ones.

Power Amp Issues:

Replaced all electrolytic caps for TAD filter caps, 2.2k resistors, bias pot and 150 k
Replaced 100ohm resistor as choke for a vishay 5w rest
Replaced all polyester caps for poliprop like orange drops and XIcon / phillips.


The amp will have worked fine just replacing 2x IC´s , 2.2k Resistors, 150k, and bias pot network, re-tight ground network, cleaning pots and jacks. But had the original filter caps LSR and had a loosse feel overall with background noise, and, suspect it did work as a power amp for many years having the preamp IC´s dead for a very long time., so Iknew the power amp section has been mis treated and abused somehow.

At this point, as I did perform replacements stage by stage, did have a restored head with new JJ tubes fully working with no issues, sounding better than brand new, as I did notice the changes from unpleasent chewwy and loose, to tight and focused with better gain and overall feel, as I moved along from the polyester cheap marshall factory caps on the preamp section and dried filtering caps in the power amp section....

But things where not ideal for my tastes tonewise, and lack of gain (being advertised/sold and marketed as HIGH GAIN, its a tad bit more than a jcm 800, but no high gain at all ) as we all know from this line up of marshall models. so there are few mods around internet, as we know, regarding the JCM 900. Now, there are minor discremptancies issues, like different revision boards for the DR4100 and related family jcm 900 members. Some People posted mods for what revision board you have on hand and production year. Mine is a JMP52C. SO you will find places where is printed in the board a resistor in place and its actually a cap being placed at factory. Schematics will be another headache, as you see this discrepancies being noted and some others not.

I gathered the info around the net, and did perform the follwing stuff, giving full thanks to the fellow tweakers and wizards that posted all the mods around, just confirm that they work and I´m happy with this mods:

1] C27 spot in the board is a resistor in place from factory. Lift one leg and will disable the pre EQ feeding the IC. Must do if u don´t like the boxie nasal shrill tone and upper frequencies feel., which takes away a bit of perceived gain, as those freq are not pushed at all now. It feels thicker and relaxed gain structure- wise so to speak. more natural gain cascade feel and clear er note separation.

2] No brigth cap mod yet, have dsicrepancies between schematics and board, as I only could find JMP50C schematics as the closer one to mine.

3] R42, (10k) piggy back another 10K over it. Must do for More gain and leading the head to a real High gain amp. From factory, the amp has a bit more gain than a JCM 800, but by no means I consider a High gain amp. so this is the way for a fire breathing marshall.

4] D10, snipe the whole diode. Get rid of the Diode feel and gain structure that bears with it. A Must do. this disables the diode bridge clipper..little more organic tone

5) R60, (470k) Pickybag 47K over it. Msut do for a high gain territory and compensation. increases the signal pad, necessary because you will get a big boost from losing the voltage drop across the diode bridge you clipped

6) R13, (220k) snipe one leg off... .it's a negative feedback resistor ..snipping it will give a big gain boost... Must do along the previous 2 steps.

Bias setting -40v at 150k junction / 43mA with bias probe per tube

The Eq section now works for real, it sweeps beatifully with loads of bass, middle and treble. it respnds great to minor eq changes with huge range... feels like a real cathode follower with like active eq feel somehow... Bass knob really kicks in some low end without being muddy...

The volume pots have better transition and sweep, with a more open organic growl as u open up.

Presence knob works great now with better sweep after the mods...

It transforms the whole amp into a very different beast, a real marshall high gain pleasent amp that would have compete and stand the years along it´s competence back in the day.... don´t know why Mr Grindrod, designer, did not scrap off this few componets and value changes on resistor in the preamp section. I do know the constraints tube wise back in the 90´s on avalaibility, but hey... stilll... don´t get it, how this minor changes done at factory from the gate go... would have produce a really nice sounding amp... lots of gain, thick, natural feel, punchy,

Overall, now, I have a decent marshall tone for my Black Sabbath, zack wylde, Iron Maiden, early exodus and megadeth thrash bits, and a modern high gain marshall type of thing..... it performs great on weekends and local gigs for my cover band.

Had great fun restoring and for a couple hundrerd more euros including tubes and spare parts, have a great marshall tone for 500 euros approx at the end. great bargain-great tone for me. I´m keeping this amp forever, and had killed my JCM 800 GAS for ever... Gret alternative for a sleeper amp in the market.

Suggest to everyone on a tight budget to try out and perform this mods on a JCM 900 for a fire Brething monster, and with a BOss SD-1 for Modern High gain low tuning chugs and blistering harmonic squeals...

Mods confirm that are reliable with no oscillations, noises, weird behaviour or wrong doing.

Disclaimer, I dont take responsabilities on your amp or your well being if you do or dont know what you are doing... it´s your amp, your body, you take the chance.. if you Zzzapp yourself, then should have look for an amp tech...
 

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Tatzmann

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Good effort.

I did a full tube 2203 preamp mod on
my 4100, didnt wanted to doctor around
on this precious piece of shit opamp
garbagefest.

Still has the 4100 pre intact parallel, modded
it just for more bass.
 
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Henry Sabbath

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nice Tatzmann !! ... it´s a great platform for many mods and hot rodding, at the end of the day.... better than many other options out there... they sell for affordable price and good condition, despite mine, obviously..... knew what was buying... i like twaeking and experimenting stuff...

yours should sound great by your comment... interesting... in parallel.... clever approach..

cheers
 

Spooky88

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Hell yeah, I have a 4102 that I’m working on. Looks like you did yours right!
 

Tatzmann

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If they simply had used the 3203 preamp
design with a standard tube cathode follower
the 900DR would have never gotten its (righteously so!) bad reputation.

But cool thing is that they can be had every
now and then for very little monies and can
be tweaked in hundreds of ways.

I usually tend to skip all the transistor stuff
and make it a real tube Marshall, why spend
all the monies on tubes to idle them away on
lesser tasks like effects or reverb.

The Reverb and Effects on all 80s transistor
Marshalls were sounding better than on the
900DR, without use of tubes.
 

clarkleach

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FWIW, C27 is in fact a capacitor not a resistor as you stated. There are several capacitors in this circuit that look suspiciously like resistors. They are typically light green.
 

jagrenet

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Hi Everyone,

Just willing to share my experience and confirm some mods circulating around on the internet around the JCM 900 DR 4100. Bought a neglected, cheap, not fully working DR 4100 head made in 1992 for 350 euros.

It´s symptoms where:

- preamp working with a very low faint volume, like being a 10 watt transistor amp, nearly no sound....preamp not working basically. Original preamp tubes.
-Reverb not working
- 4 front panel pots with the stem loose and banged many times
- switching issues between channels
- Power amp with bias drift and original power tubes.
-over all white noise / ground noise

Preamp issues:

- It had 2 IC´s fried, IC4 M5201 was shot, and the M1458 in the reverb section was shot as well. Did change all M5201 IC´s for JM2121D / JRC2121D, 5 in total. Had an overall better definition and feel, maybe due to better slew rate compare to the M5201.

- Recapped all the electrolitics and polyester caps for Polyprop on the JMP52C preamp board, replaced 100k preamp resistors as they where noisy, for 1/2 watt ones, TR3 was changed resolving weird channel switching issues.

- Replaced input jack for Neutrik.

-Replaced all front panel potentiometers for alpha ones.

Power Amp Issues:

Replaced all electrolytic caps for TAD filter caps, 2.2k resistors, bias pot and 150 k
Replaced 100ohm resistor as choke for a vishay 5w rest
Replaced all polyester caps for poliprop like orange drops and XIcon / phillips.


The amp will have worked fine just replacing 2x IC´s , 2.2k Resistors, 150k, and bias pot network, re-tight ground network, cleaning pots and jacks. But had the original filter caps LSR and had a loosse feel overall with background noise, and, suspect it did work as a power amp for many years having the preamp IC´s dead for a very long time., so Iknew the power amp section has been mis treated and abused somehow.

At this point, as I did perform replacements stage by stage, did have a restored head with new JJ tubes fully working with no issues, sounding better than brand new, as I did notice the changes from unpleasent chewwy and loose, to tight and focused with better gain and overall feel, as I moved along from the polyester cheap marshall factory caps on the preamp section and dried filtering caps in the power amp section....

But things where not ideal for my tastes tonewise, and lack of gain (being advertised/sold and marketed as HIGH GAIN, its a tad bit more than a jcm 800, but no high gain at all ) as we all know from this line up of marshall models. so there are few mods around internet, as we know, regarding the JCM 900. Now, there are minor discremptancies issues, like different revision boards for the DR4100 and related family jcm 900 members. Some People posted mods for what revision board you have on hand and production year. Mine is a JMP52C. SO you will find places where is printed in the board a resistor in place and its actually a cap being placed at factory. Schematics will be another headache, as you see this discrepancies being noted and some others not.

I gathered the info around the net, and did perform the follwing stuff, giving full thanks to the fellow tweakers and wizards that posted all the mods around, just confirm that they work and I´m happy with this mods:

1] C27 spot in the board is a resistor in place from factory. Lift one leg and will disable the pre EQ feeding the IC. Must do if u don´t like the boxie nasal shrill tone and upper frequencies feel., which takes away a bit of perceived gain, as those freq are not pushed at all now. It feels thicker and relaxed gain structure- wise so to speak. more natural gain cascade feel and clear er note separation.

2] No brigth cap mod yet, have dsicrepancies between schematics and board, as I only could find JMP50C schematics as the closer one to mine.

3] R42, (10k) piggy back another 10K over it. Must do for More gain and leading the head to a real High gain amp. From factory, the amp has a bit more gain than a JCM 800, but by no means I consider a High gain amp. so this is the way for a fire breathing marshall.

4] D10, snipe the whole diode. Get rid of the Diode feel and gain structure that bears with it. A Must do. this disables the diode bridge clipper..little more organic tone

5) R60, (470k) Pickybag 47K over it. Msut do for a high gain territory and compensation. increases the signal pad, necessary because you will get a big boost from losing the voltage drop across the diode bridge you clipped

6) R13, (220k) snipe one leg off... .it's a negative feedback resistor ..snipping it will give a big gain boost... Must do along the previous 2 steps.

Bias setting -40v at 150k junction / 43mA with bias probe per tube

The Eq section now works for real, it sweeps beatifully with loads of bass, middle and treble. it respnds great to minor eq changes with huge range... feels like a real cathode follower with like active eq feel somehow... Bass knob really kicks in some low end without being muddy...

The volume pots have better transition and sweep, with a more open organic growl as u open up.

Presence knob works great now with better sweep after the mods...

It transforms the whole amp into a very different beast, a real marshall high gain pleasent amp that would have compete and stand the years along it´s competence back in the day.... don´t know why Mr Grindrod, designer, did not scrap off this few componets and value changes on resistor in the preamp section. I do know the constraints tube wise back in the 90´s on avalaibility, but hey... stilll... don´t get it, how this minor changes done at factory from the gate go... would have produce a really nice sounding amp... lots of gain, thick, natural feel, punchy,

Overall, now, I have a decent marshall tone for my Black Sabbath, zack wylde, Iron Maiden, early exodus and megadeth thrash bits, and a modern high gain marshall type of thing..... it performs great on weekends and local gigs for my cover band.

Had great fun restoring and for a couple hundrerd more euros including tubes and spare parts, have a great marshall tone for 500 euros approx at the end. great bargain-great tone for me. I´m keeping this amp forever, and had killed my JCM 800 GAS for ever... Gret alternative for a sleeper amp in the market.

Suggest to everyone on a tight budget to try out and perform this mods on a JCM 900 for a fire Brething monster, and with a BOss SD-1 for Modern High gain low tuning chugs and blistering harmonic squeals...

Mods confirm that are reliable with no oscillations, noises, weird behaviour or wrong doing.

Disclaimer, I dont take responsabilities on your amp or your well being if you do or dont know what you are doing... it´s your amp, your body, you take the chance.. if you Zzzapp yourself, then should have look for an amp tech...
I'm just curious, when you say "piggyback" do you mean series or parallel ??
 


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