Modifications to the Polysix

A few modifications must be made to the Polysix in order for it to accept the retrofit. Apart from the obvious tasks of installing the MIDI connectors and mounting the retrofit boards, a number of tracks must be cut on the existing PCB's and a number of jumpers patched in. All in all, I'd suggest the following building order:

  • Build the CPU board, including the 40-pin ribbon cables.
  • Build the S/H board.
  • Drill holes and mount the MIDI DIN connectors on the rear panel. Don't wire the MIDI connectors just yet though.
  • Perform the modifications to the Polysix boards and wheel assembly described below.
  • Connect the S/H board 8-pin connector to the appropriate points on KLM-367.
  • Mount the S/H board and connect the wire for the BEND CV.
  • Mount the CPU board. This involves building small holders out of piano wire, removing the existing 8048 and 8049 CPU's, connecting the wire to the battery (Item #3 in the modification descriptions below) and plugging in the DIL headers to the 8048 and 8049 sockets. (The old 8048 and 8049 CPU's are no longer needed.)
  • Wire up the MIDI connectors and plug them into the CPU board.

    At this point, you'll be rather nervous that you've done something wrong, so to make sure that you won't fry anything at least, get out your multimeter and using an ohm range, verify that +5V on the CPU board is connected to +5V on the Polysix boards, with the same for GND. Do the same for the +15V,-15V,-5V and GND wires on the S/H board. Once you've assured yourself that the the power wiring is properly connected, you can turn the Polysix on.

    MIDI connectors

    The MIDI connectors should be mounted in the location shown in the picture below. Although there appears to be a lot of free space on the rear panel, in practice one has to make sure that the connectors don't get in the way of anything on the inside. The indicated location has been selected with this in mind. I mounted the DIN connectors at an angle to make my bad drilling skills less obvious (I would never be able to line all the screws up in a straight line), but that's all up to you.

    Picture of MIDI DIN conectors

    The exact measurements are given in the figure below (measurements in mm).

    Measurements for DIN connector mounting

    When it comes to labelling the connectors, I used Letraset for the actual label work, then masked off the surrounding area and covered the labels with spray-on lacquer. It's not visible in the photo above, but the laqcuered area does look different from the rest of the synth and you might opt to do it differently for better looks. Martin Wise has produced artwork for labels for the connectors, together with labels for the PCB's and also for the new functions available, to stick on the front panel.

    Other modifications

    Here you will find text descriptions of the various modifications that must be made to the Polysix boards, including the wheel assembly. Not much remains to be said regarding the modifications, except to take your time and make sure you know what you're doing.

    If you're wondering what the mods actually do, here's a short description:

  • Items #1 to #3 disconnect the battery from internal RAM and support circuits and connect it to the MIDI retrofit CPU board RAM.
  • Items #4 and #5 disable reset of the voice Gate signals in Tape Enable mode. (This allows the synth to be played in this mode, which is used for setting up MIDI parameters).
  • Items #6 and #7 separates the Chord Memory Pedal from the front panel pushbutton (so that the pedal can be used independently as a sustain pedal if necessary).
  • Items #8 and #9 provide equal brighntess on all A to D leds even when several are illuminated at the same time. (This is used when setting up the MIDI parameters).
  • Items #10 and #11 disable the muting circuit in Tape Enable mode. (This allows the synth to be played in this mode, which is used for setting up MIDI parameters).
  • Items #12 to #14 separate the Unison function from the front panel Unison LED. (This allows the Unison LED to light up without the oscillators detuning. The Unison LED is used in Tape Enable mode to indicate certain MIDI parameters.)
  • Item #15 disconnects two pullup resistors that would otherwise double the resnets on the new CPU board.
  • Items #16 to #20 disconnect the BEND and MOD wheels from their respective CV inputs, and instead connect them to two unused channels of the front panel CV mux.
  • Items #21 to #24 enable the wheels to output CV's of the proper polarity and magnitude.

    This page is (c) Copyright 1999 by Ricard Wolf

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