Discussion about the Korg PolySix synthesizer

Awesome

> On Dec 28, 2013, at 7:02 AM, Terje Winther <terje.winther@wintherstormer.no> wrote:
> 
> 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
> 
> 
> 
> 
> 
> 
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