Righty-ho, grab the Moog Taurus circuit from retrosound.de and make a cup of tea.
Got tea? Got circuit? Good.
Start at the top left. Notice how everything is labelled with what it is? I'm going to attempt to explain how it does it.
The "Oct Divide Chain" is a set of very precisely-matched resistors wired from 15V to ground - 12 in total (they are probably multi-pinned "resistor packs" like you used to get for terminating disk drive chains. Anyone who has never used Windows 3/Linux kernel 2.0 will be too young to remember them ;-)
Okay, so 15V at the top, 0V at the bottom, first tap is 2/3 of the way in (four up, eight down) - this gives us 10V. The next one is tapped 1/3 of the way up giving 5V, one at 1/6 up giving 2.5V and one at 1/12 up giving 1.25V. Don't take my word for it, use Ohm's Law and work it out for yourself. Step away from the computer, pick up a notepad and pencil, and go do some arithmetic. Oh, and make another cup of tea. Mine's black, no sugar.
Right, we've got our precise reference voltage to be fed to the resistor ladder. The 4016 is a quad analogue switch, which selects which reference voltage we choose. The opamp forms a little buffer amplifier so that the reference voltage isn't "pulled" by the load when you press keys. What's the oscillator thing below it? Well, that injects a tone onto the control voltage, which is detected by the amplifier and filter just to the right of the keyboard assembly. It's far too high a frequency to affect the control voltage for the oscillator (and indeed it's easy to filter off) but it's possible to detect it at a low level and use that to generate the note on/off trigger.
I can't find any information on what the values of the keyboard voltage dividers are.