Text to accompany photos :

Oscillator Module

Oscillator 1, range H/M/L, pulse width, waveform (ramp,square,triangle), tuning switch.
Oscillator 2, range H/M/L, pulse width, waveform, level.

Filter/Amplifier Module

Attack, decay, filter cutoff frequency, master volume, filter voltage source, resonance on/off.


PIC board Photo

Shows 18 pin PIC microcontroller and keyboard decoder.
The PIC produces three outputs: two are oscillator outputs, either in-tune or detuned.
 The other is a trigger output for triggering envelope. The PIC scans the keyboard via 74HC154 decoder using top key priority. 
The PIC is programmed in assembler for speed and accuracy necessary for producing two oscillators in tune - 
the chip needs to run at 20 Mhz. The PDF file contains detailed of how I wired up salvaged Hohner keyboard to the 74HC154.

Back of oscillator panel photo

Back of oscillator panel showing waveshape circuit. The dual waveshape circuit divides the PIC square wave into lower octaves (selectable range)
and at the same time converts the square wave into an approximation of sawtooth (stairstep). The stairstep is fed into a comparator to produce a pulse width adjustable square wave. The square wave is also integrated to produce triangle.
All waveshapes are selectable for each oscillator.
Interesting effects can be obtained by mixing different waveforms produced by the two oscillators.
If you listen to detunedsaw.wav you should be able to hear the beat of the two oscillators in detune 1 position.

Voltage controlled filter, VCA, Envelope and Output Amp photo

Uses a single LM324 Quad op-amp and 4 transistors. The voltage controlled filter cutoff frequency is controlled either manually or follows AD envelope shape. The AD envelope feeds VCA. Finally a x2 output amp boosts the signal enough to drive headphones or line output.
This is just about the most simple VCF/VCA I could build. Other VCF's can be used. 
I have left 1 position free on the filter voltage source switch for input from a LFO or other modulation source.