Korg Poly-61M
The Poly-61 with a MIDI board bolted in - a factory retrofit kit that turned a mid-eighties analog poly into something a sequencer could finally talk to.
Overview
The best evidence of what the Poly-61M is comes from the paperwork that shipped with it. Korg's KLM-605 MIDI retrofit kit - the "POLY-61 M POLY-61 MID1 kit" - is a Poly-61 with a factory MIDI board added, plus a service manual written to help technicians install and troubleshoot it. The kit documentation lists a structural diagram, an interconnection chart between the Poly-61 and the MIDI board, a retrofit procedure, block and circuit diagrams, a checklist and troubleshooting guide, a MIDI implementation table, and a parts list. It's not a marketing brochure. It's what came with a hardware upgrade.
The retrofit was not a plug-in module. The procedure has you solder a harness to the KLM-509 board, drill a hole in the rear control panel - the instructions call for a 6.5mm pilot enlarged to 11mm and then to 30mm - and mount a MIDI plate. That's why a Poly-61M can have a slightly awkward rear-panel connector. This was a field or factory install, not a clean redesign.
The MIDI implementation is more limited than the "M" suffix might suggest. The recognized receive data table covers note off, note on, release off, release on, all notes off, omni mode on and off, program change, and system reset. Velocity is explicitly ignored - the table says so twice. Note range is restricted to notes 36 through 96, and anything outside that window gets folded back into range by dividing by 12 and adding an offset. Program change is similarly boxed in: programs 0 through 63 only, with anything higher reduced by subtracting 64. Transmitted data is even narrower - note off, note on, release off, release on, and program change, and nothing else. No pitch bend, no modulation, no aftertouch over MIDI. From a sequencer you get notes, program changes, and a release on/off switch.
The release control is the one piece of MIDI data here that isn't just note plumbing. The service manual's troubleshooting section gives a concrete voltage test: confirm 2.0V or more at CN31 and CN-C pin-6 when the release switch is off, and 0.5V or less when it's on. A malfunction there usually traces to IC10 or IC13, both 40H374 chips. The kit documentation also notes a quirk. If you test a Poly-61M against a Poly-800 for MIDI verification, the Poly-800 ignores release switch data because it has no release on/off function.
Internally, the MIDI board runs on a 63B03RP microcontroller clocked from an 8MHz oscillator, divided down to a 2MHz square wave at IC3 pin-12 and a 500kHz square wave at IC14 pin-4. The troubleshooting guide walks through checking reset and clock waveforms with an oscilloscope, isolating IC4, IC15, and IC3 one at a time to find a bad chip, and checking the +5V rail to ±0.25V tolerance. Power comes in through connector CN-J on the KLM-605, fed from the KLM-480 supply board. If +5V is wrong with CN-J disconnected, the problem is upstream on the main board - KLM-475 or KLM-509 depending on which revision you have. The MIDI board itself sits on a wooden case, per the parts list, with a joystick assembly listed among the control components.
Mix (November 1983, p.90) described the Poly-61 — the base instrument this retrofit was built for — as a "versatile and user-friendly six-voice polyphonic synth" offering "sophisticated performance features and a range of warm and useful voicings," and noted a list price that the OCR cuts off mid-figure. Six voices, warm voicings, and a list price low enough that the magazine led with it. That's the machine the M variant was built on top of.
The Poly-61M is an analog poly from the early-to-mid eighties that got MIDI bolted on after the fact, while digital synths were arriving with MIDI built in. The retrofit checklist makes the point: you service this machine with an oscilloscope and a soldering iron, not a firmware update. The parts list includes standard logic chips (74LS04 hex inverters, 74LS138 decoders, 74LS244 buffers, 74LS20 gates), a 2764 EPROM, a PST-518 reset IC, and a PC-900 photo coupler. Nothing exotic. Nothing proprietary beyond the 63B03.
The Poly-61M is a Poly-61 with a specific, limited MIDI implementation. If you need velocity, pitch bend, or anything beyond note and program data over MIDI, this isn't the machine. If you want a six-voice analog poly with basic sequencing capability, it does what the kit says it does. The MIDI board is a discrete piece of hardware you may eventually have to troubleshoot with a scope. The service documentation is unusually complete for a retrofit of this era, so you're not guessing at the pinouts.
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