Discussion about the Korg PolySix synthesizer

Hi all,

I do a lot of analog synth service here in Norway. One customer  
brought in no less than 4 Polysix´es for service, and in the end I was  
given one of them as payment for the repairs.
The one I ended up with was quite good-looking (front panel complete  
with all original parts, keyboard nice, case quite all right etc.). It  
did have some technical issues (one dead voice, several controls not  
working properly, memory system out of order etc.). There was some  
damage after the usual battery problem, and someone had tried to fix  
it, but it was not a professional job. I started repair on the  
processor board, but as I did so, I also searched the web for info,  
and at the same time (good timing!) there was a discussion on the  
Yahoo Polysix discussion list regarding some modifications. This got  
me thinking, and after some reading and considering, I decided to "go  
crazy" with the Polysix as I once did with my old ARP Axxe -  
installing all kinds of modifications. However: all of them should be  
useful, musical and well done. So this is what I did with my newly  
acquired Korg Polysix:

- Completely took everything apart
- Removed rust
- Cleaned and dusted everything
- Replaced missing rubber foot with new original
- Replaced old hardware with new hardware (screws and spacers)
- Repaired two broken voices (bad SSM2056 chip on one voice, and bad  
SSM2044 chip on the other)
- Removed the old power supply board and installed the new power  
supply board from Kiwitechnics (brilliant kit!)
- Removed the damaged old processor board and installed the new  
processor board from Kiwitechnics: the Kiwisix (even more brilliant!)
- Removed the two rear cable holders (not needed anymore because of  
the Kiwisix upgrade)
- Improved grounding as suggested by Kiwitechnics (new wires, some  
cutting in the aluminium foil)
- Installed the Polysex modulation kit
- Made a new LFO (MG) as a substitute for the PWM MG to the Polysex  
mod kit, complete with speed pot and dual-colour LED
- Installed a toggle switch, so that the modwheel control to VCO can  
be selected between internal MG (as original) or the new extra LFO/MG  
installed for the Polysex kit
- Recapped the 3 electrolytic caps per voice (x 6) in the analog voice  
chain on the voice card with very high quality "gold audio" el. caps
- Made the FX in/out modification, so I can have direct out, and use  
the FX board as an pure effects board, or use the send/return for  
additional treatments
- Made the post-effects modification for the effects board, with on/ 
off switch (complex multiway switching)
- Made the bass modification on the effects board, with on/off switch
- Made the treble modification on the effects, with on/off switch
- Installed the "VCO to VCF" modification as suggested by the late  
Jürgen Haible (rip) with individual on/off switches per voice, and  
with a total amount pot for all voices. An extra veroboard with 6 VCA  
was needed for this, with a lot of flying wires. This mod was a lot of  
work.
- Swapped out the 6 tiny LEDs per voice on the voice card, and wired  
the new switches (same switches as the VCO-VCF mod above) with  
internal LEDs on the front panel (these were quite expensive)
- Changed all 6 LED driver resistors for more brightness on the voice  
LEDs (originally 4.7k, now 1.2k)
- Did the final touches, like mounting the new Kiwisix front panel  
sticker and manual marking of the various other mods
- Mounted everything properly
- Checked, debugged and calibrated everything

Some comments:
- The Polysex modification kit use several analog signals, including  
two analog signals from the old Korg processor: MG and the PWM LFO.  
They are not present on the new Kiwisix processor board, because  
everything is processed internally before CVs are sent to the analog  
voice card. However; there is one analog signal called "LFO" coming  
from the Kiwisix processor card, and on the Polysex web site it state  
that you can use this signal, but that is only partially true. There  
is an analog signal there, but it is only for turning the front panel  
LED for the MG on and off. So there is no smooth modulation, only a  
square wave. Useful in some settings, but not as intended. Solutions:
a. Use it as it is, which gives you "trills" and not sliding detune  
(spread). Works for me, and I will use this alternative for now
b. Modify the squarewave LFO signal into quasi triangle signal, either  
with a crude RC network, or with a 4046 chip
c. Make your own LFOs

- Since I made an extra LFO for the Polysex mod kit, I had the idea  
that I could also route the new LFO to the mod wheel, which I did.  
That way I could use the internal LFO (MG) to VCF and VCA, and the new  
LFO to VCO with another setting for added variation. However, because  
the new Kiwisix processor needed access to the modwheel in order to  
handle MIDI I/O, that didn´t work. I still implemented it, though, but  
have to be careful not to overload the outgoing MIDI signal. The  
result is a bit different to what you think, but it does give me some  
alternatives for performance.

- The bass mod. It does works, but either my ears aren´t good enough,  
or my speaker system (with sub-woofer) isn´t good enough, because I  
really can´t hear that much difference. I needed to measure it to  
confirm that it worked properly.

- For the treble mod I first tried with a resistor similar to the one  
in my synth, which was a 4.7k resistor, and not a 33k resistor as in  
the service manual and on the web mod pages (...and the 1k resistor  
that is supposed to be there is not present on my synth). That really  
didn´t work. It was suggested I used a 33k resistor over the 22k  
resistor, and I had a hard time understanding how that could work, but  
in the end I found an 30.9k resistor that worked very good.

- I bought the wrong versions of the front panel switches with  
internal LEDs in them: They had the right colour (green), but I bought  
the versions where the LED on/off are synchronous with the switch. I  
wanted the voice LED to work all the time. Now they will only work  
when the "VCO - VCF" mod is on. On the other hand, then it will be  
clearly visible when the mod is on, and if I want the lightshow I can  
always turn them on, but keep the amount pot at zero.

What is missing:
- I wanted the plexiglass pitch and modwheels with LEDs, that somebody  
have offered in the past. It seems that they are unobtainable now.
- I did consider one of those lovely new all-wooden cabinets, that you  
can have in several types of solid wood. I sent out some inquiries,  
but didn´t get any response, and my original cabinet is not that bad- 
looking after all (a little nick in front under the keys and ever so  
slight damage at the rear, lower corners), so I am keeping the  
original cabinet for now.
- New bushings? The keyboard was quite clean and trigger fine, so no  
need for service there, but I might need to take it all apart, clean  
the contacts, take off all the keys and clean them, and install new  
bushings. Some day…

All-in-all, it was a lot of work to do it all. The drilling, cable  
stripping and pre-soldering, mounting, soldering and debugging was a  
bit of work, and the planning and thinking during the project also  
took a lot of time. However, it was all highly enjoyable work, and I  
now have
- a much better understanding of the Polysix in general
- a much better understanding of all the mods and changes involved
- a great-sounding and great-working polysynth!

Photos of the results are in the Yahoo group photo section, under  
"Polysix Modifications".


All the best,

Terje