Yamaha PS-20 Keyboard - Specs / DIY Modification Help

[cartesia]

Hi guys,

First I've not been on VSE for a good five years.. so hi again!

Now..
So I've done a bit of circuit bending and synth mods before, but always in the case where someone else had done things previously that I could copy either through pictures or guides. My most successful was probably a Boss DR-110 analog drum machine modded for individual drum parameter controls with a breakout box for the knobs.

Now I'm trying to do something which I don't have a guide for and am hoping someone here could perhaps take a look and help me out. I picked up a Yamaha PS-20 keyboard and quite like the sound of it, but it only has preset sounds with buttons to change between them.

I have the service manual for the keyboard here:
https://www.dropbox.com/s/wln7arv4uerel ... m.pdf?dl=0


It looks to my eyes like everything is mostly in easily moddable format, ie there are only a few ICs and lots of space between components. The manual is quite thorough although i'm not capable of comprehending it at a high level. I understand the abolute basics of electronics and audio processing but by no means know enough.

Basically I guess the thing I'm wondering if people can help for me is to describe where certain "parts" of the sound generating are on the circuits. I get the basics, like one board is even labelled "filter & amp", but I don't actually know how the sounds are generated in this keyboard (eg, it has an organ1, organ2, trumpet, etc.. are these just different resistor/parameter sets for the same voice engine/circuit? Or would they be 8 really basic fixed voices that arent generated from a common 'engine'? Does it have a proper VCF or similar?

I'm not hoping to turn it into a complete versatile synth, but it'd be cool if I could get like a few voice controls to vary it a bit.