Korg Monotron Euro Module DIY

[lionelfischer]

are there MS20 modules for Euro already?

[Kodama]

ASOL SY02
The 106-1 is wonderfully friggin weird and lovely and evil in it's own right though!

[mark_snipz]

Back in March...

viewtopic.php?t=14577&start=0&postdays= ... t=monotron

[phono1337]

This guy added CV jacks to his:

http://www.dinsync.info/2010/06/how-to- ... otron.html

That was me

[phono1337]

UNLESS there are already MS20 modules for Euro already ?? I have the doepfer xtreme filter which I thought was supposed to be similar but I think its pretty different right

I have the 106 also and it's similar but also extreme at the same time. The main difference is that the ms20 has two filter stages and controls but the 106 has only one with a single set of controls.

Likewise the monotron only has the LP, their marketing should have really said it was the ms10 filter, which would have been more accurate.

[anselmi]

This guy added CV jacks to his:

http://www.dinsync.info/2010/06/how-to- ... otron.html

That was me

excelent!

[TWINSTATES]

polyphonic monotron module could be very cool. Five voices that i could use to play chords on my modular, oh that would be quite exciting...

I'd buy one of those and make a contribution for development quite happily...

[Tubeampguy]

I could easily design a front panel for it if I had one to take measurements from. The main problem would be how to mount the "keyboard". I would imagine it would have to be vertical to save HP. The unit runs off 3V too (2 batteries) so that would have to be sussed out and a circuit designed to convert the +12v to that (unless it actually can be run off of +5 which would make it much easier). The project would be pretty simple otherwise. I have never seen the guts of the unit but if the jacks are PCB mounted than they would have to be replaced, no problem there either. I probably could make a front panel express design and let the plans out for free if I could get my hands on one for a while. Someone could dis-assemble one and scan the front panel with a decent ruler next to it and I might be able to scale it accurately enough to make plans from that. Also scan the guts for possible PCB mounting holes etc. - or you can send me one! If I go and buy one I will have to be charging for the panel plans!

-James

Here ya go.....

[bsmith]

I added jacks to mine super easily after being encouraged by watching the dinsync (thank you phono1337!) vid a while ago. It's clearly marked what is what on the back side of the pcb.
Also, here's the schematic that korg released:
http://www.korg-datastorage.jp/Manual/monotron_sch.pdf

[Tubeampguy]

I could easily design a front panel for it if I had one to take measurements from. The main problem would be how to mount the "keyboard". I would imagine it would have to be vertical to save HP. The unit runs off 3V too (2 batteries) so that would have to be sussed out and a circuit designed to convert the +12v to that (unless it actually can be run off of +5 which would make it much easier). The project would be pretty simple otherwise. I have never seen the guts of the unit but if the jacks are PCB mounted than they would have to be replaced, no problem there either. I probably could make a front panel express design and let the plans out for free if I could get my hands on one for a while. Someone could dis-assemble one and scan the front panel with a decent ruler next to it and I might be able to scale it accurately enough to make plans from that. Also scan the guts for possible PCB mounting holes etc. - or you can send me one! If I go and buy one I will have to be charging for the panel plans!

-James

Here ya go.....

And here's the schematic in a PDF format......

[ersatzplanet]

Here ya go.....

Looking at the PCB brings up a few problems for panel mounting - The PCB is snapped into place in the plastic chassis so to mount it to the panel means you will have to find a place on the PCB to drill holes for standoffs unless the two holes in the PCB (near the "A" and near "Lead-Free" can be used (or the smaller two holes in the far right upper corner and just above the "keyboard" connector. Theses are still on the opposite side of the board from the pots so you might get flex when turning the pots etc. Too bad the pots don't have bushings. The jacks are surface mounted so that is no real problem - you can remove them and wire panel mounted jacks to those pads (nice and big ones). I would say leave them there but the headphone/output jack is normalized to the speaker and that would have to be defeated. The "keyboard" seems glued to the plastic chassis so it might be useable if removed carefully but I think it can just be discarded. I assume the Red/Black wire harness connects to the speaker. That output can be used as an audio out if it is hot enough to use in a modular - probably closer to modular levels than the line out. I also am guessing the circular dial near the "A" is the volume pot and that will have to be replaced also since it won't stick through the front panes as it is now.
Surface mounted parts makes the addition of CV and different outs a little harder. You just have to find the places on the schematic and hope there is a good trace you can scrape the mask off and solder to.
THe "keyboard" looks like it can be replaced and it's connector can be used for a CV input - it looks like a mini ribbon connector that is just appears as a resistor between ground and +Vcc so that looks like 0 to +5v CV inputs.
It really looks like it would be easier to just mount the whole unit to a bracket holding the extra CV and audio ins and outs than to re-mount it onto a new panel. I can't get dimensions off this picture except by roughly guessing from the pin spacing on the ICs but a decent metric ruler or calipers (I can bet it was done in metric) measuring the removed front panel will get you very close to the data needed for front panel express to bang out a panel. The power supply and circuit hacking is another thing.
I don't think the modularizing of this unit is very cost effective though. I would imagine the panel alone would be in the $30 range from FPE.
-James

[4mspedals]

This guy added CV jacks to his:

http://www.dinsync.info/2010/06/how-to- ... otron.html

That was me

Hi! I hadn't seen your site until now. lots of great info!

[phono1337]

This guy added CV jacks to his:

http://www.dinsync.info/2010/06/how-to- ... otron.html

That was me

Hi! I hadn't seen your site until now. lots of great info!

Cheers, keep your eyes peeled for some modules coming up in a month or so

[Tim Stinchcombe]

... unless the two holes in the PCB (near the "A" and near "Lead-Free" can be used

Two of the screws holding the case together pass through those, so they should be reasonably suitable.

I would say leave them there but the headphone/output jack is normalized to the speaker and that would have to be defeated.

I haven't quite fathomed out the totality of what the output amp is doing, but one channel is used for the output, the other for the speaker, so no real problem there I suspect. However in the other threads people have commented how noisy it is, a lot of which is liklely to stem from the 1.2MHz switching of the DC-DC converter, but some possibly also from this audio amp, so bypassing it altogether is probably worth looking into.

I assume the Red/Black wire harness connects to the speaker.

Yes.

I also am guessing the circular dial near the "A" is the volume pot and that will have to be replaced

Yes and yes.

Surface mounted parts makes the addition of CV and different outs a little harder. You just have to find the places on the schematic and hope there is a good trace you can scrape the mask off and solder to.

Many of the usable 'hook in' points in the circuit are brought out as 'test points' on the PCB, so this is very easy. There are many pictures around of these, e.g. one of those in the link I gave above (and possibly one on din sync's blog?):

and all these 'TP's are marked in the schematic.

THe "keyboard" looks like it can be replaced and it's connector can be used for a CV input - it looks like a mini ribbon connector that is just appears as a resistor between ground and +Vcc so that looks like 0 to +5v CV inputs.

This is where the fun begins. It is between +5V and the 1.35V reference, not ground, and the summing node (TP1= 'pitch') is also at 1.35V, thus all CV ins are referred to this 1.35V level. It would be nice to hear from someone who has hooked into this point just how much of an issue this really is...?? Din sync/phono1337, can anybody here comment?

Tim

[phono1337]

the pitch certainly doesn't track very well at all and the gate point also affects the pitch, which is also dependent on the level of the gate. The easy solution is to use silent way since you can calibrate this all away, doing it standalone would be a lot more of an issue.

[darenager]

^ I think I read somewhere there is a resistor to remove to make the pitch CV and gate work properly, think it was on a Japanese site.

[wetterberg]

Or perhaps stick a much longer cable on it (it is separate from the main PCB), so that it could be plugged in a la traditional 'ribbon controller'

this is the most elegant solution.

[Tim Stinchcombe]

^ I think I read somewhere there is a resistor to remove to make the pitch CV and gate work properly, think it was on a Japanese site.

Indeed. Now that I look at the schemo more closely, it is obvious - removing R11 will prevent a gate applied to the 'gate' pad from influencing the pitch (it will also of course prevent the ribbon from operating the unit...!). I've taken a snap and highlighted it - a '102' (=1kohm), next to the LM324 under the volume pot.

Tim

[darenager]

I suppose a switching jack and a small through hole resistor could be utilised to allow the keyboard to function, may be fiddly though

Personally I will probably do away with the keyboaed entirely - I find it pretty useless for any kind of accuracy.


I wonder if the gate input will retrigger the LFO, did you look at that yet Tim?

[Tim Stinchcombe]

I wonder if the gate input will retrigger the LFO, did you look at that yet Tim?

Yes, it should do. I've not tried that yet myself, but the 'gate' testpad to which an external gate may be applied is the same point as the voltage coming off the ribbon, and so an external gate will cause the same events as an internal one: enabling the VCO (it is not running unless you touch the ribbon, presumably to save on battery power); resetting the LFO; and gating the VCF.

Tim

[Tim Stinchcombe]

OK, I seem be totally distracted by this now, so here are some pitch-tracking plots. First up I injected a CV through a 100k resistor to the 'pitch' summing node (100k so it looks the same as the ribbon voltage). As I received it, it looks like my Monotron is set roughly to 2.5V/octave (the plots show actual frequency, red, against left y-axis, and log2 of frequency, blue, against the right y-axis: log2 so 1V/octave is a 'gradient 1' straight line). It is covering around 2 octaves, which is about right, as the ribbon has 18 'keys' on it, and here I'm taking it lower due to lower CVs input being below the 1.36V reference!).

I then plotted the pitch pot wiper voltage vs. frequency, expecting a nice linear plot, and that is what I got!

Working out the scalings of the circuit, the range of adjustment of trimpot VR1 suggested a resistor between about 25k and 50k would allow tuning to 1V/octave - I chose to use a 39k one, as this is close to mid-range (so this replaced the original 100k I started with). I only did a rough-and-ready setup, as the waveform is pretty jittery, so taking accurate readings is quite tricky (which really makes me think that sorting out the power supply should be the first thing to tackle...). Anyhow, I then took readings for the pitch pot fully CCW, fully CW, and somewhere in between. The log plots are close to gradient 1, but I wasn't listening to the output, nor did I have a chromatic tuner hooked-up to see just how good it really was, but it is now giving a 5 octave spread for 5V input. So, take 'em at face value!

Note also it really can oscillate at quite high frequencies, not pegging-out until beyond audible!

Tim

[darenager]

Very interesting, might be worth turning it into a module yet, wonder if CV for VCF cutoff will have a similar response.

Thanks for sharing your findings anyway

[pulplogic]

^ I think I read somewhere there is a resistor to remove to make the pitch CV and gate work properly, think it was on a Japanese site.

Indeed. Now that I look at the schemo more closely, it is obvious - removing R11 will prevent a gate applied to the 'gate' pad from influencing the pitch (it will also of course prevent the ribbon from operating the unit...!). I've taken a snap and highlighted it - a '102' (=1kohm), next to the LM324 under the volume pot.

Tim

It looks like R42 should also be removed to avoid having the cutoff affect the pitch.

[Tim Stinchcombe]

It looks like R42 should also be removed to avoid having the cutoff affect the pitch.

That will certainly stop the pitch affecting the cutoff - Korg appear to have deliberately done that so that the filter cutoff 'tracks' the pitch changes from the ribbon in some way. I think Din Sync mentions it in his video, and his mod certainly shows a (switchable) resistor from the (new) incoming pitch CV to the cutoff in order to mimic that if so desired. I haven't paid much attention to the filter so far, but did run some simulations this morning on the cut-off voltages - the means of 'gating' the VCF is quite subtle, with only small changes in voltage apparently having a large impact. I suspect that 330k is really only a 'token' effort though, as altering the pitch pot has such a large effect overall (but I haven't studied all the scalings thereabouts that much yet, nor taken any readings to speak of). However there shouldn't be any impact of the cutoff on the pitch through that resistor (R42), as the node it is connected to at the filter end is a 'virtual' one, at the Vbias point, and that is the same ref point used by the pitch CV amp, IC2A (so there is no potential difference to cause any impact on the pitch...).

And whilst I'm here, I am currently hacking mine to bits, in that I have mounted it on an upturned 'Tupperware' container so that I can stick a load of switches and jacks on it, to more easily try things and take some measurements etc. (I find it really annoying trying to faff around with loads of wires and hook-clips etc.). I'm currently planning to do the following:

1. disable the switching DC-DC converter, and power the +5V rail from a linear regulator (a 78L05) instead, so it runs off 12V. I'll be making it switchable, so that I can see the affect of switching noise on everything (hopefully)
2. add jacks + resistors/whatever for inputs:
a) pitch CV in, plus a switch for filter tracking (if I can work a sensible scaling out)
b) cut-off CV in
c) gate in, but still have the ribbon gate switchable so I can run as originally intended
d) LFO reset in
e) replace the 'aux in' jack with 2 jacks for both channels in, plus a switch for the 'filter enable'
3. add jacks for outputs:
a) VCO out
b) LFO out
c) replace jack for 'headphone' out, and add a switch to mute internal speaker
d) (I might bring the filter output out if it really is the headphone amp contributing to the noise, which currently I doubt...)
4. add a switch to enable the VCO (but I'm seeing a weird effect I can't account for at the moment, so this may not be successful)

Some of these may not have much utility in the day-to-day use of the thing (e.g. the VCO and LFO outs are unipolar, so have a large DC offset in them - a cap may help here though of course). It is looking like I will have another snow-enforced day off work tomorrow, so it may be up and running at some point in the day...

Tim

[glitched01]

You guys have really made some progress! Very interesting thread. Keep hackin' away!