Circuit Overview:
A subtractive analog. 4 SSM2033 VCO's, each with it's own footage, waveform, tune and volume knob. And unison or poly mode of course. Nice arpeggiator and fx section which allows a flexible set of modulations. Two LFO's assignable and hard routed in various ways. The 8049 ROM mask CPU is compatible with the Polysix's; a machine which adds the 8048 for memory control. MP-4 has no memory. Basic adsr for the SSM2044 vcf and vca.
Service Tips:
The one I had major problems with here, had suspected a bad 8049. This due to not seeing any movement on the lines going to the 4042 chips and no response on certain panel led's. Verifying all traces however revealed some eaten through as with the polysix. Also oscillator one takes a longer time to warm up. I haven't figured this one out yet. Just tuned it up for stability when warm. I've met people who have a programmer to do the 8749 if anyone ever needs a replacement cpu. (They have a window to erase the EPROM where the 8049 version is hard wired basically.)
Parts:
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..