I'll describe the modification of a professional arranger keyboard, model Hohner PK-250, in detail. This home keyboard from 1985 offers many possibilities like poly and solo sections, drum machine, portamento and chorus effects, Hohner chords, etc. It produces stunningly high-class sounds with a warm, analog character.
While studying this instrument more specifically, I discovered two Curtis CEM 3391 chips (24 dB low-pass filter with resonance up to selfoscillation) and several SSM 2024 chips (a VCA module which was also built into the legendary "Waldorf Wave"). I got hold of the chips' data sheets and found out that the very flexible CEM 3391 chips in this machine were hardly taken advantage of, and that many important functions were blocked by fixed, unchangeable control voltages.
I built a simple board with 7 adjustable control voltages, using standard CV model 7805. I used the +12 V direct from the device's power supply. Thus, 7 independent CVs were at hand I could now adjust with external potentiometers.
After replacing the device's fixed CVs for the preset sounds with my own, the machine had 7 important synthesizer functions. Despite of the preset sound settings, I can now manually adjust and drastically change the cutoff, resonance and ADSR envelope values.
The result is astonishing and, through the built-in MIDI interface, this device has become a full analog synthesizer with assertive sound.