Ricard's MIDI retrofit for the Korg Polysix

Polysix-M top view

Please note, these pages describe my original retrofit, built either on veroboard or using Martin Wise's PCB designs. If you want additional features not included in my original design, or don't want to make the boards youself, consider getting Johannes Hausensteiner's kit instead.


  • MIDI IN functionality: note on, note off, all notes off, bend wheel, mod wheel, program change, sustain pedal, chord memory pedal, arpeggio switch, clock, start and stop (for the arpeggiator), sysex program dump, mode messages, active sensing. See the MIDI specification for details.
  • MIDI OUT functionality: note on, note off, all notes off, bend wheel, mod wheel, program change, sustain pedal, chord memory pedal, arpeggio switch, sysex program dump, active sensing. See the MIDI specification for details.
  • Individually selectable MIDI in and out channels 1..16. Omni on/off, local on/off and poly/mono mode messages received.
  • Arpeggiator can sync to MIDI clock at one of eight different clock rates. Arpeggiated notes sent over MIDI OUT.
  • Expands memory to 128 programs, grouped in 4 groups (1..4) of 4 banks (A..D) of 8 programs (1..8)
  • Edit recall functionality.
  • Chord memory pedal can now function as sustain pedal (switchable). Pedal polarity automatically detected when synthesiser is switched on.
  • Sysex dumps in banks of 32 or 128 programs can be sent and received.
  • 128 program tape dump available, as well as 32 program format compatible with the original Polysix.
  • Dual and triple voice capability: up to three voices can be layered on the same note for fatter polyphonic sound.
  • 8 selectable key assign modes.

    What it doesn't do is

  • No real time parameter update, i.e. no sysex or control change messages mapped to the knobs. If you want this, you should get Johannes' kit instead, as I'm not planning any future software upgrades.
  • No velocity sensitivity. I've heard the Kenton interface can do this, but only monophonically for all voices at the same time which seems a bit pointless to me.

    How to build it

    Ok, cut the small talk and lets get over to the more technical details. First, some potentially bad news: There are two versions of the KLM-366 circuit board within the Polysix, referred to as 'old production' and 'new production' in the service manual. (Unfortunately, it doesn't list the serial number when they switched from one to the other.) The 'old production' version is easily distinguished by the addition of a small extra board called KLM-396. This retrofit is designed for the 'new production' KLM-366, i.e. without KLM-396. While there circuit is essentially the same, and the MIDI retrofit should work in both cases, it is likely that some of the patching descriptions won't be correct for the 'old production' board. Also, judging from an 'old production' machine I saw once, you might have to assert a bit more inginuity to fit the board in place, as the KLM-396 boards partly occupies the space where the retrofit CPU board should go. (Better yet, sell the 'old production' Polysix to someone else and get a 'new production' one instead. :-)

    Ok. To start with, some user documentation, namely the detailed MIDI spec and the user's manual. (Det finns också en svensk manual tillgänglig for those of you who prefer it in Swedish.)

    Basically, there are three main steps to putting all this together. First, there's the CPU board, the small S/H (analog) board, and finally a number of modifications must be done to the synth itself. You'll also find Martin's PCB artwork on these pages; please note that he has made some slight changes to the design, so be sure to read his accompanying notes carefully.

    Check here for a list of the components needed. (If you choose to build Martin's PCB design, you'll find a better list together with his artwork files). After all the hardware has been built, you still need some software to make it run. Email me to get the EPROM file when you get that far!

    Some conventions and pointers:

  • The Polysix circuit boards all have names like KLM-367, KLM-368 etc. All references to existing components and connections are made using these names. I've written a short description of the various boards so you can understand what's going on.
  • On pots, CW means Clockwise, and CCW means Counterclockwise. I have used these abbreviations to refer to the two outer connections on pots.
  • In my schematics, I use a ground symbol (thick horizontal line) not to actually indicate chassis ground but as a 0V reference designator. All ground symbols an a schematic are to be considered connected together and brought out to a common 0V connection for the board, and not just wired into any old ground point around.
  • Do you need the service manual? No, not really, allthough it's nice to have around if you want to understand what's going on. Besides, you'll find the adjustment procedure (most importantly, the tuning procedure) here. Can I copy it for you? No, sorry, my copy is bad, and copying it would just make it completely illegible. However, both the adjustment procedure and other userful information, including battery replacement (and PCB cleanup if the old one has leaked) is available on the Old' Crow's Synth Shop page.

    Comments? Ideas? Questions? Mail me!

    This page is (c) Copyright 2000 by Ricard Wolf

    Go to my Polysix page.