A DIY KLM-475/509 replacement for the Korg Poly-61 Synthesizer
Some years ago I started a project with the goal of increasing the resolution of control parameters in the Korg Poly-61. My initial approach was a daughterboard which plugged into the sockets of the two MCU chips on the digital board and generated control voltages for the voice board. While this approach may have worked out, I abandoned it as being too much effort for the final result. The main shortcoming of this approach was that it did not address existing damage from battery leakage on the digital board, which is fairly common. Recently I decided to make another attempt at a Poly-61 project, but this time the plan was to replace the entire KLM-475/509 with a new design which would increase the resolution of the parameters and also add MIDI. It seemed that a single PIC microcontroller would have enough processing power to replace both of the old MCU's. I chose a 12-bit DAC to generate all of the control voltages, with a CD4051B de-multiplexer feeding eight sample and hold circuits. I decided to implement the LFO in firmware so that additional waveforms could be provided. Initially, I had hoped to use PIC chips to replace the 8253 timers, but ended up scrapping that idea. Luckily, it is still possible to buy 8253 chips, and using them simplified the project. Below is a photo of the first version of the pcb. (the MIDI cable is not attached but you can see where the 10-pin connector goes. The flex connector was replaced with a single row male header, as the original flex cable was ruined by battery leakage corrosion and had to be replaced with a different type) The design is simpler than the original and takes up less pcb space, since there are no 40-pin DIP IC's. The cassette in and out circuitry has been removed, since MIDI sysex can be used instead. I placed the JST connectors in the same positions on the pcb to ease installation. I am currently using the v2 pcb for firmware development, which has a few small changes.
Here is a photo showing the new pcb and the Korg KLM-475B. (The KLM-509 is very similar to the 475B)
I am currently working on finishing and testing the new firmware. Here are some of the features which have already been implemented:
1) 0-99 settings for cutoff,resonance,attack,decay,sustain, and release
2) 0-63 settings for PW/PWM
3) 0-15 settings for LFO1 to DCO and VCF amount
4) 0-41 settings for LFO1 frequency
5) LFO1 waveforms: triangle,ramp up,ramp dn,square,random
6) LFO1 delay settings 0-7
7) DCO2 Interval settings of 0-12
8) LFO2 (for PWM) freq setting 0-41
9) LFO2 waveforms: triangle,ramp up,ramp dn,square
10) Random arpeggiator mode
11) MIDI In supports Note On/Off, program change, CC parm control
12) MIDI channel can be changed, and is saved in EEPROM when power is off
13) Patch save and load via MIDI Sysex (all patches at once, not individual ones)
14) 80 patches are now implemented, saved in EEPROM (no battery needed)
15) Rotate or "always start from voice 1" key assign modes
16) DCO1 can now be disabled
17) DCO2 "extra waveforms" now usable
In general, the range of the control voltages is the same as before, although the
LFO's can be set to a much lower freqeuency now.
A few more testers needed:
I am currently working with a musician friend to help fine-tune the firmware and test the different features. I would like to find one or two more people to help with the testing. Must be located in the continental USA. The catch is that I will only be supplying a bare pcb and a parts list, so they would need to purchase the rest of the components and assemble and install the board themselves. Most of the parts are available at Digikey and Mouser. The 8253 chips and the 18-pin flex connector are not.
Once it's tested and "released", then what?
I will not be supplying assembled boards. This is a DIY project, for people who are comfortable with buying parts and soldering. I will supply the bare pcb, a parts list, and a PIC chip programmed with the new firmware. I have created sysex versions of the factory and service patches, which will be available here for download after they have been fully tested.