A demo of my modified CS15D dual channel synth.
The original features two channels, where for channel 1 only presets can be selected. Channel 2 has a manual switch besides the presets where you can set the sound via the control section. Since the presets are nothing but a set of resistors, selected by the respective switch, it was obvious that they could be replaced by controls. The service manual made it easy to adapt the channel 2 manual circuitry for channel 1.
A more detailed explanation of the mod can be found here: anlage-e.de/cs15d-channel-1-modification/