Korg Micro-Preset M-500SP
Korg MicroPreset

Korg Micro-Preset M-500SP

 

This project wouldn't have been possible without Gene Jans, a former colleague, who gave me a Korg Micro-Preset. Thanks Gene!!!

 

When I received the Korg Micro-Preset I was quite impressed with the sounds coming out of this little machine. But after a few jam sessions it disappeared in the background due to the lack of controllability of the synth.

 

After searching the web, I found a nice page by Tony from Oakley Sound who modded the hell out of a Micro-Preset. I decided to have a look to see what I can come up with.

 

I documented my attempt below, enjoy!

 

As usual, these are just my experiments. Use at your own risk. If you don't feel conformable doing these kind of things, ask an experienced electronics expert to do it for you.

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Day one

 

I opened the box, piece of cake. I studied the schematic. Not easy as only a low res version is available. And I started to probe around a bit to see what is happening inside the circuit. I also made a list of all the mod that seem feasible for now:

1) Add 2 EG's for VCA and VCF of the "synthe" sounds. (DONE!)

2) Add EG selection switch for "voice", "string" and "wood", Add AD/AR switch. (DONE!)

3) Add "synthe 1" waveform switch. (DONE!)

4) Add slider (second traveler) for "synthe" VCF (DONE!)

5) Add external Gate and CV (Hz/V) input.

6) Add switch to select external input on "synthe 1".

7) Remove internal speaker (DONE!)

8) Add EG selection switches on "brass" section for VCF and VCA. (DONE!)

 

Day two

 

I started doing the first mod, removing the internal speaker and the speaker PCB. Easy, un-soldering a couple of wires and the speaker is out.

I checked how the 5 position selector worked, and how the sound selector switch worked. As the schematic is hardly readable, I noted down my findings:

 

X1 = Pulse waveform

X2 = Synthe 2 mode (waveform select, input for VCF)

x3 = Mix out

x4 = Wood out

x5 = Voice out

x6 = String out

x7 = Brass out

x8 = Synthe out

X9 = Bender

X10 = Trigger input for AD EG Synthe section

x11 = Ground

x12 = Trigger out (Connected to x10 or x13)

x13 = Trigger input for AR EG Synthe section

x14 = Bright

x15 = Synthe in (does to VCF input)

 

Pushing the sound selector switch makes following connections:

Synthe 1: x10 - x12 (on) / x15 - x1 (on) / x8 - x3 (on) / x13 - x12 (off)

Synthe 2: x4 - x14 (on) / x8 - x3 (on) / x15 - x2 (on)

Brass: x7 - x3 (on)

String: x6 - x3 (on)

Wood: x4 - x3 (on)

Voice: x9 - x11 (on) / x5 - x3 (on)

 

I also checked the signal @S7A (EG Brass)and S7C (EG synthe). They go from 0V to 5V. The raw gate signal can be found @S5C and that's a 0V to 10V signal. This is interesting as I have a MFOS dual AR/AD EG laying around.

 

So I quickly assembled the MFOS dual AR/AD EG and tested if I could trigger it from the signal present @S5C. This works. So the first step for mod 1 is done.

Day 3

Today I did the first mod. I added a switch for the "voice, "string" and "wood" section.

Just locate the resistors indicated below with a red circle and remove them from the PCB. Put 2 wires in the holes instead. Take note to see which wire does directly to the transistor.

Now take a switch. The central pin of the switch will feed an EG signal to the VCA, so this should be the wire going to the transistors (TR22/TR23/TR24/TR25). In the drawing below, this is the purple wire.

On another pin of the switch, you solder a 1.5M resistor and this goes next to the wire you just soldered. This is the yellow wire in the drawing below.

Now solder a 820K resistor on the last pin of the switch. Now connect the green wire coming from the internal EG. This is the green wire.

Now repeat these steps for all the voices. Pay attention to the switch for the Wood section. Use a dual pole switch here. And the switches are installed and operational.

I also did a second mod, two switches for the "brass" section. The first switch switches the EG going to the VCF, the second switch is for the VCA. Both switches switch between the original EG and the EG from the "synthe" section.

The principle is the same, but mind the resistor values.

Click on the image for a larger view.

 

Korg MicroPreset Mod1

Day 3 continued

 

It was a productive day :-)

 

This mod allows you to change the type of the EG original used on the "synthe" section. The switch changes the EG from AD to (A)R mode. This actually requires no soldering on the PCB. Only a few wires need to be cut. See the diagram below.

Make sure to connect the right side of the cut cable. The picture below is not correct. the yellow, green and purple wire should be connected to the wire going to the PCB, not the connector!!!!

Korg MicroPreset Mod2

Day 3 continued

 

Is was a very productive day :-)

 

I finished the MFOS Dual EG PCB I had lying around. Than I removed the resistors in the red circles.

The trigger/gate input for the MFOS is connected to the purple wire of the mod above (switch between AD/AR for the internal EG). The output of the MFOS EG's are going to 2 switches, as shown in the pictures below.

The first switch goes to the VCA section. It requires a 820k resistor. The second switch goes to the VCF section. I used a 33K resistor. When using the original 10K resistor, I noticed that the traveler had almost no impact on the sound. So I increased the value until it sounded like I wanted it to.

 

I also added a slider (as I had one left over from the Klee project). I tapped in on the original slider to get a ground and +12V signal. The center tap of the slider goes to a resistor of 68K , then to the PCB.

Be careful when removing the resistor. If the traveler stops working for the "brass"section this means that the signal is cut somewhere. I tell you where: the second leg of the resistor is not used after this mod. You should put a resistor leg piece of clip off in this hole and solder it on both sides. this way the signal is continued when going from the top to the bottom side of the PCB.

 

Korg MicroPreset Mod3

Day 3 Bla Bla Bla

 

Last mod for today!

 

Another switch, this time it switches between the square and the pulse waveform as input for the "synthe 1" section. Again, it only requires some wires to be cut. See diagram below. The central pin goes to the "synthe in" signal, just cut a wire from the connector. The top pin goes to the square wave output of the VCO. I actually soldered the wire on the bottom of the PCB, on the middle pin of S6. The pulse wave form I take from the connector.

Korg MicroPreset Mod4

Day 4

 

Time to document the mods a little better.

Below you can find a picture with the indication of the position of the mods. You'll need to see this full screen and maybe zoom in a bit.

So here we go:

 

1) Add 2 EG's for VCA and VCF of the "synthe" sounds. (DONE!)

Position 16: EG to VCF

Position 17: EG to VCA

 

2) Add EG selection switch for "voice", "string" and "wood", Add AD/AR switch. (DONE!)

Position 1: "string" switch

Position 2: "voice" switch

Position 3: "wood" switch part 1

Position 4: "wood" switch part 2

Purple wire goes directly to the transistor (input of VCA), Yellow bring the original VCA signal to the switch. The internal EG signal (originally used by the 'synthe' section) is available at position 17.

Position 5: AD/AR selection switch (green wire) X13 (AD position of switch)

Position 6: AD/AR selection switch (red wire) X10 (AR position of switch)

Position 7: AD/AR selection switch (yellow wire) X12 (middle pin of switch)

 

3) Add "synthe 1" waveform switch. (DONE!)

Position 8: Back of PCB middle pin of connector S6, this goes to the upper or lower pin of the switch.

Position 9: X1 cable to be cut. The side connecting to the PCB goes to the upper or lower pin of the switch, the wire going to the connector goes to the middle pin of the switch.

 

4) Add slider (second traveler) for "synthe" VCF (DONE!)

Position 10: +12V for the second slider

Position 11: Ground connection for the second slider

Position 12: Connection of the slider to the filter. (red wire)

 

5) Add external Gate and CV (Hz/V) input.

6) Add switch to select external input on "synthe 1".

7) Remove internal speaker (DONE!)

 

8) Add EG selection switches on "brass" section for VCF and VCA. (DONE!)

Position 14: "brass" VCA section

Position 15: "brass" VCF section

Purple wire goes directly to the transistor (input of VCA or VCF), Yellow bring the original VCA signal to the switch.

 

Korg MicroPreset Mod5

Day 4

 

Samples!

These were recorded while testing the mod's so nothing fancy. But it will give you an idea of how it all sounds at the end. (or is this just the beginning) Love it or Hate it!

 

Voice sample

 

Wood sample

 

String sample

 

Brass sample

 

Synthe sample

 

Click on the pictures below, full screen is nicer!

 

korg after mod1
korg after mod2
korg after mod3

Day 5

 

Today I decided to have a look at the synthe VCF. I studied the schematic some more and compared it with the LPF of the Korg MS-10. This mod should be easy.

First I decided to remove some components as indicated in the picture below. I replaced some of the resistors and capacitors to get a faithfull copy of the MS-10 VCF.

I used a MUUB-2 board from Tellun to replicate the missing circuitry. As I used a double opamp, I decided to add the Cutoff knob and additional parts.

Next thing to do is to interconnect the MUUB board with the MicroPreset. THis step needs some examination of the Micrpreset PCB. Make sure you connect the wires into the right hole of the removed component.

As I forgot the 33K resistor near point C at first, I found out it is not working. The 220ohm resistor going to ground, near point C, is already available on the micropreset PCB.

korg after mod5
korg after mod6

Day 6

 

I made a big mistake today! I cut the hole in the front panel totally wrong. There's no space for the switches and pots, these components thouch the keyboard.

But no worries, this things happen. Eventually I found a solution. It does change the look of the synth a bit.

Anyway, today I first finished the Hz/V and Gate mod.

Cut the orange wire coming from the keyboard. Connect the part going to the PCB to the signal lug of the Hz/V input jack. Connect the other end of the orange wire to the switch lug of the Hz/V input jack. Don't forget to connect a ground wire!

For the gate jack, R65, 220ohm, needs to be removed. Put an orange wire in the hole that connects to the 2 transistors, a purple wire in the second hole. Now connect the orange wire via a 220ohm resistor to the switch lug of the gate input jack. Connect the purple wire to the signal lug of the gate input jack. Again don't forget to wire the ground connection.

That's it.

Day 7 & 8 & 9

Wow, this turned out to be a big job...

Last 3 day's I've been drilling holes in an aluminum plate, printing front panel overlays, rerouting wires and resoldering connections. A true spaghetti mess inside.

To be honest, I didn't take any pictures of the inside of the synth because I didn't get it as nice as Tony did on his.

Anyway, here are the pics!

Korg MicroPreset M500 Modded 1
Korg MicroPreset M500 Modded 2
Korg MicroPreset M500 Modded 3
Korg MicroPreset M500 Modded 4

Future!

 

Well, I think I'm finished here.

 

I'm planning to upload some more interesting samples soon!

Today I recorded some additional samples, enjoy (and excuse me for playing bad)

Sample 1

Sample 2

Sample 3

 

Stay Tuned!

 

Questions? Remarks?

 

You know the drill!

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