Circuit Overview:
A synth 'improving on' the PolySix with digital oscillators using 8253 timer chips and LM13600 ota based filter, but lacking the diverse effects section (no chorus I see as vse claims), adds joystick with dco and vcf amounts for some useful real time control. Otherwise in parameter mode you type in parameter number and up/down button edit. And it maintains the arpeggiator in similar form with polysix. Instead of 1 VCO, it's a 2 dco Typical subtractive analog architecture with memory. The Midi version is Korg's first Midi synth, though a retro for polysix was released about the same time. I had a non-M poly 61 version that has MIDI ports also.
Service Tips:
Problems with the keyboards are common. The 'conductive impregnated rubber' key contacts tend to get 'dulled'. I and others have had some success rubbing them with acetone. This apprently can cause the surface of the rubber to break down some and peel off with abrasion and reveal a fresh surface that has conductive material exposed. This is my theory after many operations on these and many successful ventures. I have seen cases where it just won't work though. Probably carbon thins out as you get below surfaces. SO GOOD NEWS! I did a lot of research and am having produced contact disks that adhese on nicely once you clean w/acetone. So think they don't noticeably alter performance. But on these gold contacts use 1500 grit paper and swirly about 10X per contact to get rid of old material deposits I believe... Also here is the Poly-61M MIDI board checklist and troubleshooting guide (KLM-605)
I've seen some battery spills that damage ic12, IC8, IC9, IC18, IC21,etc. and sometimes 8255 ppi chips and destroy nearby interconnects (A couple pins broke underneath CN12 i had a scrap lambda fortunately and shortened 16 pin one). Anything around..best just heat gun them out and clean up/verify everything first. One ate a trace completely and required extensive cleanup with toothbrush/ baking soda solution to get the board back in best shape. All digital stuff seemed to work well from the panel but I can't get a sound out of them. Very possibly I just hadn't edited release eg...which..on another was the final fix until i put battery in. Then it died for some reason. I studed a bit and see IC1 is functioning normal. It scans notes and i hear them in the jumbled random 'patch' in the memory's jumbled bits. The buttons on the right work to light those 3 led's. But all gates are on all the time from boot...and no programmer stuff works. Pin 21,22,23 are active since they are attached to IC1's pins. Perhaps instructions are handed between cpu's that way. INT never moves from high on ic30 also. And it's own pin would have to reset ic38 for that to happen. I think this is one that I replaced ic8 on to get signal bck to pin 6 (INT) noticing crumpled out. TURNED OUT though main issue was ground went open on it and nearby chips. Verify though hole off underside trace from battery - terminal!
(ALSO NOTE my old Xeltec eprom programmer was reading cpu's intermittently! new 8749's I burned would verify always. however both function fine on working poly61 board.) I've made a lot of notes. The number for pins on gate out pre 4503 buffers are wrong. 12 should be on top, 10, 8, 6, 4 and 2 going down. Instead of 1, 3 etc. And on gate LED's 4.7k is grounded not LED as shown. Many annoying errors.
I have some spares - $5ea
Semiconductors:
I have most chips save ssm2056, and I can burn MPU's from 8749 to replace 8049's for IC1 and IC30. Also in Europe those chips, i/c pins and Syntronics replacement board available when battery damage is too bad!
or Contact me.
An insightful post to SDIY from Karl Dalen: ---- VCF: 12db LP LM13600 using a MS20 VCA with limiting diodes in resonance feedback but before VCA also the feedback not only goes to the input of the filter but to first cap like half way S/K and no local OTA gain loops but one fixed over both. I have checked trough all other analog Korg products and this VCF are unique to the P61.It should have a peculiar Bw hump because of this. VCA: MS20 final VCA. One SSM2056 EG split use VCA and VCF. DCO1: It's a complete saw VCO core, a regular Op amp integrator/comparator design. The ramp reset comparator also feeds another comparator that sends this signal back to *both* 1/3 of DCO1 and DCO2 8253 timers gate input, then one of the 8253 timers sends a pulse back to the integrator NPN ramp reset transistor in the VCO!! Odd! Could this perhaps mean that half of timers are actually used to measure the frequency of the VCO and if wrong in realtime restart it on a per cycle basis?! It could work if the value are compared to a predefined LUT. Or is it just a bogus thing to distract anyone viewing the schematic? The output of the ramp integrator are sent to a comparator to create PW wave. DCO2: DCO2 are made out of 1/3 8253 driving a 4520 divider who's output's are selected by a switches and summed by a weighted resistor net acting like a DAC to create saw. A redundant signal goes to divider reset to actually turn off the chip.. Apparently resolution of wave are likely dependent of octave selected otherwise only one switch would be needed. VCO ramp integrator note CV are a 7bit R2R generated by the same CPU that scan's KBD that then feed a common expo who are muxed out to each VCO. (Polysix/Trident) The driver for the timer DCO's are a VC RF multi vibrator who in turn are driven by bend and vib voltages, the CLK signal are then driven into a prog divider to create semitone offsets for DCO2, finer detuning presumably by 8253 timer itself. There are no regular muxed DAC for the parameter CV's instead each has its own DAC trough a bag of 8255 driving a even larger bag of weighted resistor net's. DAC for Cf of VCF are only 6 bits. However PWM, VCF, Vibrato modulations are full resolution from the analog triangle MG. In other words an awful sounding poly but a great, peculiar and funny VCO/DCO design. Disclaimer: I might be utterly wrong in all that said here. Thanks for your attention....G'day... KD ------
