Yamaha CS10 monophonic synthesizer 

The CS10 is a small monosynth that was commercialized by Yamaha in 1978. I purchased it back in august 1978 for FF 3,100  (roughly US$ 620 of 1978). It has a V/Hz VCO with saw and square signals, PWM, a state variable multimode VCF, 2 ADSR envelope generators, a VCA, a LFO and a noise generator. It also provides an audio input with a schmitt trigger for processing external signals through the VCF.
The schematics and the service manual can be found at E-Music DIY archive
Mods : addition of an octave divider and of a V/Hz to V/octave converter. Increased resonance range.

Technical specifications : click on this link

The new board

Schematics of the mods
The PCB matrix
Component layout