A simple 2 vco monophonic synth with ring mods. Apparently the filter design is an unpotted version of those used in the earlier 700(S) and 800DV units according to Colin Fraser. He notes "Only difference is that the 770 uses diode arrays instead of discrete devices and doesn't have the cutoff range shifted by the octave selector switch as on the 700(S)."
There are lots of discrete transistors and 2902 op amps in these. I've never worked on but at this age look for failing electrolytic capacitors. AN esr meter is always nice.
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