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.)
I worked on another one that needed the works. The board got cracked in front due to a collision with the metal frame behind the keys somehow. watch that. One key worked as there were bad joints and cracked pins on the stressed keyboard i/c AND most of the keys didn't work also because they needed my contact disks installed to restore them to like new function. Perfect now. Except it needed a good tuning since people had fooled with controls and this brings up some IMPORTANT THINGS that should have been more obvious in the tuning instructions I have anyway.
First of all before making changes set all the knobs as in the picture given above the instructions. That's not just a reminder of where things are ;-). SECONDLY when you do the first part of the stretch tuning in part 3., hit the high C. Why they left that out is beyond me. And the color codes given there are SILLY because you can't get to the wires anyway unless you have a hook probe usually. So instead follow THIS method with sharp probes:
(Preface: Or actually I think this way works better: Alternate Tuning instructions
1) ON KLM-398 with knobs set as in diagram and high C triggered, adjust vco1's offset to +-.1mv (this is a joke I see... you actually can't reliably even adjust them to that level and it wouldn't matter anyway since a slight bump of any pitch knob and they're wildly off :-) What idiot wrote the instruction in the Korg manual for this one??) of 0 by adjusting VR2 while measuring IC1 pin 7.
2) repeat 1 for vco2 adjusting VR1 while measuring IC1 Pin 1.
3) repeat for vco3 adjusting VR4 while measuring IC2 Pin 7.
4) repeat for VCO4 adjusting VR3 while measuring IC2 Pin 1.
That should save a lot of pain. ANOTHER thing I did was simply NUMBER from 1 to 16 the potentiometers on the diagram that shows their physical location! Obviously with 1 being the first pot we trim on vco1 and 16 being the last pot we trim on vco4 according to the procedure listed in the manual. Above the diagram I summarize the 4 steps that are done on each VCO. Like this:
step 1- 16' A#4 (meaning hold the high A# while the vco being tuned is on 16' setting)
step 2- 16' A#1
step 3- 2' A#1
step 4- 2' A#4
Hopefully that will speed up the process. It's very confusing the way they laid things out/numbered things really.
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.. Contact