The Sigma is I believe one of the great monophonics of all time. Matched transistor pairs under a rubber wrap to keep them from drifting. 12 of the Korg 35 filters used to give individual filtering to various voice components. For example Tuba, Trumpet, and another brass all have their own filter control. And can all be mixed. My biggest complaint? They should have put a volume knob under each rocker switch :-). Some of the voices are a little obnoxious to mix well and I'm thinking of doing a mod of some kind on them. But what a great synth with the dual joysticks. The xy axis of the right one control HPF and LPF and it's just an awesome control to have over the sound. AT is done by opto sensor and a paddle which varies it's obstruction of it when pressure is applie to a metal rail under the keys. Keys are same as MS and PS series even. This is a culmination of the success of those instruments; creating a compact super performance monophonic.
It's all discrete except the filter modules. THey're fairly simple and well designed and I haven't heard of a lot of failures. Mine just had a lot of dirt/spill related issues it seems. Maybe some bad interconnects. Gave it the once over and it worked. EXCEPT...intermittently I and increasingly worse as it would warm up a problem appeared. High notes would register ok but some lower notes would not. They would gate but not open the s+H somehow. This turned out to be a leaky input on the dual op amp 4558 designated "IC1" in the bottom left of schematic page for KLM166. (There's a photo at least since the left that board out of the manual!) The 100K resistor could be reduced to compensate but I just replaced the amp because it could degrade further and cause future problems. Below a certain threshold it failed to detect that a note had been hit so as to open the S+H CV cell! Usually in the first octave with some marginal triggers that gave funny pitches in between.
By the way if you have the typical problem with shafts breaking I'm going to try to stock some machined copies that can be easily replaced as follows:
1) take a dremel tool and small curved end grinder. Create a pit instead of the broken off screw in the center.
2) Drill a fairly precise center hole with 1/16" bit so that 14guage wire will fit it.
3) Get the machined part and stick wire in it's hole leaving about 1/4" sticking out. Then a tag of super glue and together they go! If they get bashed in the future it will be a very minor straightening/regluing job. Enjoy! I have also designed a shorter knob that is more optimal for single handed play I feel. Let me know if you'd like to optimize your machine with this mod!
I believe you'll draw mostly blank looks from Korg on these units. I have some parts that are the same so feel free to ask what the current stock is on a given thing. Some of the less common parts are getting quite hard to find though. THings pop up on ebay now and then.. Contact