Anyone Ever Reverse Engineer A Korg MS-01 Footpedal?

[diffeq]

Is there an easy way to get a +4V / -4V power supply? I know there's a way to do it with a 741 opamp to get +4.5V & -4.5V with a 9V battery, but how do I knock off another 1/2 volt? With some resistors?

My other problem is that I have no clue as to how much current is involved. I can't imagine it's enough to be hurt by a couple of resistors but I really have no idea. I would be using it to control the VCF section to get wah-wah like sounds, so I think it's just taking the place of a panel mounted control. But you could connect the pedal in other ways, like between the VCF section and MG section, so I don't know what happens in that case. This would be for use with my Korg PolySix.

Anyone have any thoughts?

Quick search revealed this sale, which shows the gutshots of the original:


Dual gang pot, three resistors and a battery. So at least that's the way it was done back then. If I'm not mistaken, resistor colour codes read 4.7K - gives the impression that currents are small.

So it's only like almost six years later now, but I'm finally going to do this project. 

Makes me feel better that my projects take only 6 months to finish.   

[Rob Strand]

Maybe like this:

[See post below for fix]

Sorry I screwed-up the attenuator, the pot/jack connections are shown going to the ring, they should go to the tip - but you get the idea.

[RickL]

I might be the only one that thinks this, but I think that pedal provides either 0 to -4 volts or 0 to + 4 volts, depending on what jack you use. That's how I'm reading the legends beside each jack.

It should be relatively easy to replicate this using a 9 volt battery, as already described. You'd just have to use insulated jacks so the "ground" of each jack wasn't the ground of the battery (or power supply).

Or is that what everyone is saying and I'm just misunderstanding?

[Rob Strand]

I might be the only one that thinks this, but I think that pedal provides either 0 to -4 volts or 0 to + 4 volts, depending on what jack you use. That's how I'm reading the legends beside each jack.

That's my understanding and it's how I drew the schematic.    Pedal up is 0V & 0V pedal down is -4V and 4V.   I have read that elsewhere.

As best as I can work out, if you plug into either of the controller jacks and it connects the battery.  And if you probed the internal points both -4V and +4V  signals would be present.  Obviously if you connect a jack into each you would see -4V on one and +4 on the other simultaneously.

The main unknown is the resistance of the pot.   I'm pretty sure each gang will be the same impedance.  A 10k attenuator would be fine on a keyboard.    I noticed one of the Korg FV-x (x = 1 to 5) pedals used 10k linear and the keyboards use 10k linear.  The 10k also gives a good approximation to the +/- 4v.  Anyway it's  my best guess with info I have.  I also suspect it's linear but it might not be.

Korg provide no impedance or battery current specifications to derive the pot impedance.

[Rob Strand]

Fixed:

[Paul Marossy]

Thanks for the replies everyone!

I might be the only one that thinks this, but I think that pedal provides either 0 to -4 volts or 0 to + 4 volts, depending on what jack you use. That's how I'm reading the legends beside each jack.

That is in fact what the manual says.

Quick search revealed this sale, which shows the gutshots of the original:


Dual gang pot, three resistors and a battery. So at least that's the way it was done back then. If I'm not mistaken, resistor colour codes read 4.7K - gives the impression that currents are small.

So it's only like almost six years later now, but I'm finally going to do this project. 

Makes me feel better that my projects take only 6 months to finish.   

Well this has a long story attached to it. When my dad died in Nov 2011 this Korg PolySix that they loved to play at one time was down in the basement dismantled. Apparently he tried to fix it but gave up on it. I took it home and found out what was wrong with it - the NiCad battery leaked and ate some tracks on the KLM-367 Patch Assigner PCB. It had a few other problems too, which I fixed. Then my mother was in assisted living place for 18 months from 2015-2016 essentially slowly starving to death. Then I got shingles IN my left eye and it took me about a year to get better after that (medications screwed up my liver, clogged my kidneys and then I got cataract in infected eye which needed surgery). During this time I didn't even play guitar much, which is my main instrument. Then I was going thru a bit of deression this last summer, so more time lost. Anyway, if it weren't for all that stuff I would have had it done in a week or two... but it just got forgotten with all that c.r.a..p. going on.

Fixed:

That's another way to do it... and actually I think that's how it was done in the original based on the one picture I have found of the inside. Yesterday I made a circuit that uses two batteries and a dual opamp and put in a CryBaby shell. I'm going to make a replica of the real Korg one so that I have a little more than 10-12mm of travel before I got thru the whole range. Korg has their pot more towards the heel end of the treadle where there is less movement per degree of movement of the treadle. I got around this by making a stop in my shell. It works but I'm not totally happy with it. But I am happy with the things I can do with this simple little control pedal. I'll make a YouTube video on it eventually in case anyone ever wants to build their own and save $200.

I believe that the dual pot is a 10K linear for the voltage control side and a 10K log for the attenuator side.

[ElectricDruid]

I believe that the dual pot is a 10K linear for the voltage control side and a 10K log for the attenuator side.

Ooh, that's kind of cool.

T.

[Rob Strand]

That's another way to do it... and actually I think that's how it was done in the original based on the one picture I have found of the inside.

The schematic was my attempt at tracing the circuit from the pic diffeq posted and another one I found.

NiCad battery leaked and ate some tracks on the KLM-367 Patch Assigner PCB.

Great!

I believe that the dual pot is a 10K linear for the voltage control side and a 10K log for the attenuator side.

Makes sense.    Big companies are funny sometimes they use the "right" thing even though it is non-standard, other times they cut back - like using the same value linear pot in 10 positions throughout a circuit.

One thing that wasn't so clear is if thecase is conductive or plastic.   It makes a difference in that the grounds of the Attenuator jacks get shorted to the grounds of the Control Voltage jacks through the case.   I noticed there's wires connecting the jack-grounds on each side but the case looks like metal (cast).

[Paul Marossy]

I believe that the dual pot is a 10K linear for the voltage control side and a 10K log for the attenuator side.

Makes sense.    Big companies are funny sometimes they use the "right" thing even though it is non-standard, other times they cut back - like using the same value linear pot in 10 positions throughout a circuit.

One thing that wasn't so clear is if the case is conductive or plastic.   It makes a difference in that the grounds of the Attenuator jacks get shorted to the grounds of the Control Voltage jacks through the case.   I noticed there's wires connecting the jack-grounds on each side but the case looks like metal (cast).

Looking at some pictures of the MS-01 and MS-04 pedals I found online, I'm about 97% sure they must be castings. All that Korg stuff from the early 80s is built like a tank.

[Rob Strand]

I'm about 97% sure they must be castings. All that Korg stuff from the early 80s is built like a tank.

I got that impression as well but I was at about 90%.  I'm sure I could see over-spray on the insides of some pics.

[Paul Marossy]

I'm about 97% sure they must be castings. All that Korg stuff from the early 80s is built like a tank.

I got that impression as well but I was at about 90%.  I'm sure I could see over-spray on the insides of some pics.

I saw one picture where you could clearly see silver inside the screw holes. It appears that Korg used the same pedal for four different products - the Mr Multi, the MS-01, the MS-04 and one other thing I can't remember the name of right now.

Anyway, the neg voltage is quirky... it reverses the heel and toe down, so when it's it's in neg mode heel down goes up and toe down goes down, but the sound completely cuts out way before I get to full toe down.  OTOH that makes sense but OTOH, according to the manual, the neg mode is supposed to give a more rounded sound, which it does do but it's like compressed into a very small range of the pot's rotation. I'm not sure if it's working as intended, or maybe when they said that they had the MS-04 in mind, I dunno. When in positive mode it works exactly like I would expect it to though. But still is weird to me that it works so differently between the modes. Maybe I could put a switch in to reverse the leads to the pot? Makes me wonder if there's a little more going on with those switching jacks... but there's not that many wires inside of there that I can see. 

[Rob Strand]

saw one picture where you could clearly see silver inside the screw holes

I guess that answers it.

Anyway, the neg voltage is quirky...

From my understanding the voltage is going too negative?  So maybe -3.3V or -3.5V would be better?
If you decreased the 4.7k resistor going to the negative output to say 3.9k that would decrease the negative swing.  However in order to keep 0 to 4V on the positive swing you might need to increase the 470R to a higher value to compensate.

If you want to change the direction of the pedal that will require different changes.

[Paul Marossy]

Anyway, the neg voltage is quirky...

From my understanding the voltage is going too negative?  So maybe -3.3V or -3.5V would be better?
If you decreased the 4.7k resistor going to the negative output to say 3.9k that would decrease the negative swing.  However in order to keep 0 to 4V on the positive swing you might need to increase the 470R to a higher value to compensate.

If you want to change the direction of the pedal that will require different changes.

I am using a dual battery arrangement with a dual opamp to get the pos/neg voltages (not sure it's necessary but that's what I have at the moment). I suppose it could also be something quirky with the keyboard itself - but you might be right about that, the foot controller is going too negative. That VCF input part of the PolySix could also be a little out of calibration or something, but I might be able to work around it. I'm still trying to find how the heck that VCF IN even interfaces with the rest of the circuits.... had to pop the hood just to see how that jack is connected and where the wires go. But finding it on the very complex schematics is another story!

[Rob Strand]

I am using a dual battery arrangement with a dual opamp to get the pos/neg voltages (not sure it's necessary but that's what I have at the moment). I suppose it could also be something quirky with the keyboard itself... but I think I'll try adding a resistor in series with the neg out - you might be right about that, it's going too negative. That VCF input part of the PolySix could also be a little out of calibration or something, but I might be able to work around it. I'm still trying to find how the heck that VCF in even interfaces with the rest of the circuits.... had to pop the hood just to see how that jack is connected and where the wires go. But finding it on the very complex schematics is another story!

Yes it could be a side-effect of a bigger problem!
I'd try to keep the opamp output and adjust the -ve range with the opamp gain or restricting/changing  the opamp input voltage.   The series resistor will work but it depends on impedances in the other unit.  It (The resistor)  could even affect 0V.

[Paul Marossy]

Yes is could be a side-effect of a bigger problem!
I'd try to keep the opamp output and adjust the -ve range with the opamp gain or restricting/changing  the opamp input voltage.   The series resistor will work but it depends on impedances in the other unit.  I could even affect 0V.

Maybe. The VCF Cutoff control on the keyboard does work perfectly, and this foot controller sort of replaces it when plugged in but kind of not. That's why I need to find this VCF IN on the schematics where it connects to whatever board is inside so I can attempt to understand just what happens when you use the foot controller. Basically the foot pedal kind of enhances the VCF and seems to expand its range. Depending on the patch used, it can make some very cool sweepable sounds.

[ElectricDruid]

The VCF Cutoff control on the keyboard does work perfectly, and this foot controller sort of replaces it when plugged in but kind of not. That's why I need to find this VCF IN on the schematics where it connects to whatever board is inside so I can attempt to understand just what happens when you use the foot controller. Basically the foot pedal kind of enhances the VCF and seems to expand its range. Depending on the patch used, it can make some very cool sweepable sounds.

The "VCF In" is on the left-middle at the bottom of the page for the "New production" KLM-366 board. The incoming CV (from the footpedal in your case) is added to all the other sources of CV. It doesn't "replace" anything, but with a range of +/-4V, it can "override" pretty much everything else. As you say, it will expand the range of the VCF - you can push the maximum CV four volts higher, and the minimum CV four volts lower too. That's probably way off the top and way off the bottom in some cases, but that's the price of flexibility!

HTH,
Tom

[Rob Strand]

Maybe. The VCF Cutoff control on the keyboard does work perfectly, and this foot controller sort of replaces it when plugged in but kind of not. That's why I need to find this VCF IN on the schematics where it connects to whatever board is inside so I can attempt to understand just what happens when you use the foot controller. Basically the foot pedal kind of enhances the VCF and seems to expand its range. Depending on the patch used, it can make some very cool sweepable sounds.

Probably the best way forward.   I like all the stuff from that era (and before).  When digital stuff like the Yamaha DX7 came out something was gained but something was lost.

[Paul Marossy]

The "VCF In" is on the left-middle at the bottom of the page for the "New production" KLM-366 board. The incoming CV (from the footpedal in your case) is added to all the other sources of CV. It doesn't "replace" anything, but with a range of +/-4V, it can "override" pretty much everything else. As you say, it will expand the range of the VCF - you can push the maximum CV four volts higher, and the minimum CV four volts lower too. That's probably way off the top and way off the bottom in some cases, but that's the price of flexibility!

HTH,
Tom

Thanks! I was looking at the original schematic and couldn't find it anywhere. I'll have to look at the "new production" one now that you've pointed that out. Yeah, I guess the word "override" is more like what it does.

Maybe. The VCF Cutoff control on the keyboard does work perfectly, and this foot controller sort of replaces it when plugged in but kind of not. That's why I need to find this VCF IN on the schematics where it connects to whatever board is inside so I can attempt to understand just what happens when you use the foot controller. Basically the foot pedal kind of enhances the VCF and seems to expand its range. Depending on the patch used, it can make some very cool sweepable sounds.

Probably the best way forward.   I like all the stuff from that era (and before).  When digital stuff like the Yamaha DX7 came out something was gained but something was lost.

I figured out my problem, it was a dodgy ground connection. Now I get full range in neg mode as well, it just behaves opposite of pos mode, which is kind of what I suspected it did before I built it.

[Rob Strand]

I figured out my problem, it was a dodgy ground connection. Now I get full range in neg mode as well, it just behaves opposite of pos mode, which is kind of what I suspected it did before I built it.

Good to know.

[SPOD]

Few years down the track, but how did you go with this Paul?  You ever get it finished?  Would love to see the final thing if you did as i'm keen to have a go myself.