About the yamaha cs-15d..
This is a partially preset, partially variable monophonic analogue synth which was released in 1979. It was part of a synthesizer range that included the CS-20M, the CS-40M, and the CS-70M. It has a very solid warm sound for a single VCO synth. The preset section is divided into two channels. Channel 1 has 15 sounds with the usual array of unrealistic instrument imitations and Channel 2 has 14 synth sounds plus a button to activate the 'manual' section of the synth. There is a slider to mix between the two channels and channel 2 can be detuned.
The 'manual' section comprises 1 VCO with a variable control between pulse and saw waves. There are also controls for pulse width modulation, noise and footings (32' - 2'). The 12dB filter has cut-off and resonance plus a dedicated ADSR envelope with a control for envelope amount. It has a nice wet and warm sound. There is a separate ADSR for the amplifier.
Most controls to the left of the preset section affect the presets as well as the manual section. These include a modulation section with 1 LFO (sine and square), S/H (random) and repeat. Modulation can be routed to either the VCO, VCF or VCA. There is also a global tuning pot and a performance panel that includes portamento (which isn't very precise) modulation and pitch wheels as well as a 'brilliance' slider. Why a slider for VCF modulation instead of a wheel? God only knows. One feature of the pitch wheel that has synthesists divided is that it doesn't have a spring although a soft 'click' is felt when moved back to the center-point. It can be handy to reach the lowest possible notes without having to hold the wheel in place.