Modding the Marshall Ms-4/ms-2 micro-amplifier!

Modding the Marshall Ms-2 / Ms-4 amp:

 This is a modification tutorial for the marshall ms-4 micro-amp.
This little (and quite cheap) amp can produce a unique marshall sound at high volumes,
especially through a guitar cab (1x12", 2x12", 4x12", 8-16 Ohm)!

 Mods!
First of all, you ll need to open the amp by unscrewing the 8 (!) screws in its back.
Then, remove the out Jack and then remove the amp board which is held in place by the 4 knobs of the front panel.

-Cab out Jack!
 The "out" stereo headphones Jack could be easily turned into a "guitar cab" out. All you have to do is to place a jumper across the "R12" marked resistor(the only resistor of the "out" board).
In order to connect the cab's mono cable to the stereo output jack, you should slide the cable's tip only halfway in the Jack. This will mute the internal speakers, providing a "mono" output signal to the cab.
 Be careful! Don't connect any headphones or pedals/amps on the  "cab output", the ms4's output is loud enough(up to 2W) to destroy them!

-More gain!
 You could modify the ms4's amp board to produce more gain! All you have to do is placing 2 jumpers across the "R1" and "R8" marked resistors of the board.
 An extra cap could be used in parallel with the 470pF cap(across pins 1 and 4 of the IC/chip) to reduce the "nasty" highs(that's an EQ issue).
  In case you don't need the gain pot, you could put an extra jumper in parallel with the R4 resistor for even more gain (Hard rockin'!). The amount of gain is controlled by the volume pot in this case.
 These mods will give the amp more gain at full output volume.
But, what if the output volume is a little bit too much for bedroom use? Then,...

-Built-in Attenuator!
 You could put a built-in attenuator in the amp, to keep the volume low at maximum gain settings!
You 'll have to cut the red wire that connects the "O1" pin of the amp board and the "O1" pin of the out Jack board.Cut it in the middle and strip the 2 cut ends of the wire. Solder the ends to a 4,7kOhm,1W pot (lugs 2 and 3). It would be really good to connect a 47Ohm, 1W resistor to pins "O1" and "O2" of the amp board to serve as a Load and keep the IC's temperature low.

Simple alternative attenuator:

 
Then, mount the pot on the plastic chassis of the amp, close the amp and you 're ready to play!
Here's a pic of the mods:

                                  Now, I can plug my MS-4 to a 1X12" Randall cab and rock out!
                                  It's loud!


MS-2 amp:
 The same mods could be applied to the marshall ms-2 amp.The only difference between the MS2 and MS4 is the extra speaker of the ms4 and its gain pot (which is constantly set to max. on the ms-2 amp).


Here's the MS-2 amp schematic for reference:

http://i31.tinypic.com/rcq89j.png

AUX input mod for the ms-2/ms-4 amplifier:
The "auxiliary audio input" mod is now posted as a schematic diagram below. You could build an ultra simple audio mixer to add an auxiliary audio input for the ms-2. You could also use this aux input to connect a 2nd guitar, an external stomp-box, an mp3 player or a microphone on the ms-2.

The mixer consists of a transistor, a couple resistors/caps and two level pots (guitar/aux).
If using an mp3 player, keep the aux in level low to ensure the music is not totally overdriven due to the ultra low headroom of the ms-2 750mW power amp.


The schematic:
 (click to enlarge)

Rock on!