Ok, so here's a concept: take the most sinusoidal waveform of your bent keyboard and filter it a little to remove harmonics. Step 2: convert it to a square wave with a Schmitt trigger - here's your first waveform. Step 3: use a 4040 to get one and two octaves below. Step 3: either route the received square wave of chosen octave straight to the output or feed it into this:
http://electro-music.com/forum/post-244968.html to get a sawtooth. Build 3 of such units and voila! you have 3 "VCO" on your cheap Casio keyboard! Ok, does that even make sense, I've never seen anyone do it, maybe there's a reason? And what would be the right Schmitt setup to get it to work? I used a TL072 set up as a non-inverting trigger from wikipedia with two 25k trimpots, but no matter how i set the trimpots, it doesn't output any square. Power: +/-5V, I'm viewing the signal on a scope. The signal from the Casio is around 1Vpp.