Circuit Overview:
8 DCO's with one Korg Filter...so a mix of polyphonic and paraphonic. I have not studied the architecture in detail on this one...some day. The II which I have adds digital delay though in place of BBD ensemble effect.
Service Tips:
The machine I have was a lot of bad solder and cleaning issues. Generally fairly reliable I think. Mine had a cool problem when it midi-looped. A cyclic runaway repeating midi stream took it down to the same bizarre sound on every patch. I had to reload patches. This, being right before i saved the arrangement of course. I was just moving it into my midi setup after playing it for months and getting used to those sounds...gotta love it. I have to hold the 'most patches lost' record. At least the II has sysex loading and some of the factory sounds were pretty identical to some I liked.
UPDATE! I got back into these and figured out a few things. First the MIDI destruction was happening whenever I sequenced to it. The problem, believe it or not, was corrupt data in my MK2. The mk1 may be immune to this though corrupt data can also exist in a MK1 I see.
Another common problem appears to be Q1 blowing open. You'll measure between Base and collector a forward drop but not base and emitter. I replaced with bigger NTE378 and it works fine (reverse the leads). Anyway this problem will cause the unit not to power up at all. Seen this twice now so it's probably fairly common. Save you a ton of tracing time knowing that :-). Send donations to make up for the first time it happened to me and I had to figure out where everything went. :-)
Strangely also, I just worked on a couple of these in a row. Neither appeared..to have ever had a battery installed! Bizarre. Well they are set up for Lithium. Diode blocks them from receiving charge. Here is a link for detailed replacement instructions.
Parts:
I don't believe Korg has any support for these now. I have some parts including filters, so feel free to ask what the current stock is on a given thing. These are common and cheap on ebay.