In standard form, the Korg Poly-800 is a pretty decent synth. Widely considered to be Korg's answer to Roland's affordable Juno series, the Poly-800 employed similar digitally controlled oscillators to offer early 80s consumers an alternative to Roland's budget model.
>>> More about Korg Poly-800 (Vintage Synth Explorer)
There are two most popular and relatively straightforward mods for this synth. The first adds knobs for low-pass filter cutoff and resonance to the front panel and also switch between 12 / 24 dB modes. This is the so-called Moog Slayer mod. The name probably should be taken with a pinch of salt, but there is no doubt that the filter really comes to life with additional controls.
The second is the so-called FM800 mod, which adds frequency modulation of the filter cutoff by the output signal from DCO.
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Cutoff & Resonance knobs: This mod can be found around the net under several different names and versions but essentially it adds knob controls for the low-pass filter cutoff and resonance parameters for realtime control. The resonance knob can take the resonance well into self oscillation and far beyond what is programmable from the front panel. This can produce the kind of screaming top end and subsonic bass that gives these machines a whole new range of sonic possibilities. The filter cutoff knob allows you to perform long filter sweeps for those squidgy burbling analogue lines that you knew your Korg was capable of but could never actually program. Both parameters are still also controlled by the normal programming methods which can set the range of the knobs control.
12 / 24 dB switch: Somewhat mysteriously the filter chip used in these machines has both 12 and 24 dB/Oct outputs but only the 24 dB one is actually used. This mod adds a switch to select either a 24 dB or less drastic 12 dB filter response.
>>> Korg Poly-800 Filter mod details
In addition to the instruction above you can also add 12 / 24 dB switch for the LPF. For this just add two-pole switch that will select signal output from pins 5 or 6 of IC1 to capacitor C103 on the main PCB. You can desolder C103 and add wire from these pins to the switch on front panel.
For any connections between PCBs and controls on the front panel I recommend use only shielded audio cables where each shield connected to the GND on PCB. This will significantly reduce unwanted interference.
Filter modulation: This mod consists of switch to select a source and the modulation amount knob. Essentially this mod allows you to modulate the LPF cutoff by the audio frequencies from DCO and create all kinds of clanging cross modulation and bizarre alien splatters & sweeps. This effect works best with the resonance cranked up into self oscillation which is where the new resonance knob mod comes into its own. The switch allows you to select DCO1 or DCO2 as source for the modulation. The source three pole switch also has an central position to off this mode.
>>> Korg Poly-800 Filter cutoff modulation details
In addition to the instruction above you can add three-pole switch which will be select the source for the LPF cutoff modulation. In the instruction above, the one pin of the modulation amount potentiometer connected to pin 7 of IC4 and resistor R74. Add three-pole switch that will be select output signal to the modulation potentiometer from DCO1 (IC4 pin 7 - R74) or DCO2 (IC5 pin 7 - R76). On the central position this switch will off the modulation mode. Also you can use for the modulation amount the linear taper potentiometer up to 10K.
The "Noise level" and "Delay feedback" knobs it is just potentiometers that were on the main PCB and on the delay PCB, desoldered and connected to the same value potentiometers on the front panel. Noise level - VR3 (1M) on the main PCB, Delay feedback - VR1 (4.7k, but can be use more, up to 10k, for more fun) on the delay PCB.
In case you want to replace a dead battery on the main board, keep in mind that you will lose all the patches stored in memory. Look at the instructions below how-to replace battery and download factory default presets.
>>> Korg Poly-800 II Battery Replacement
>>> Korg Poly-800 mkII factory default patches
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