Yamaha CS01 II Monophonic Synthesizer SN 2341

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via this eBay listing

Pics of the inside below.

"See video [in the listing while up] for working features. The feet switch works but you sometimes have to fiddle with it. I bought this in the late 1980’s so it is firsthand. I have kept it in a dry healthy environment. It was bought in Europe and I checked it in the US using a European plug and a proper power transformer. The internal loudspeaker doesn’t seem to work. In the video I connected it to an external speaker. Batteries didn’t seem to power up the synth. Not sure why, the connexions are clean. Maybe modern alkaline batteries are weaker than the older ones. This looks like a toy but is actually a very powerful and sought after synth.

TECHNICAL FEATURES
Improved filter: The CS01 II features a 24dB resonant low-pass filter with an adjustable slider for resonance, a significant upgrade from the 12dB filter with a simple on/off resonance switch on the original model.

Sound generation: It includes a voltage-controlled oscillator with selectable waveforms (triangle, saw, square), pulse width modulation, and a white noise option.

Envelopes and LFO: A single ADSR envelope generator controls both the amplifier and filter depth, and a basic LFO can modulate the VCO pitch or filter cutoff.

Portability: Designed for portability, it has a mini-keyboard, a built-in speaker, and can be powered by batteries.

Controls: It includes pitch and modulation wheels, a glissando (portamento) function, and an input for a breath controller.

The CS01 II is a monophonic synthesizer, meaning it can only play one note at a time.

Sound: It is highly regarded by electronic musicians today for its ability to produce "bubbly and growling" bass sounds and other fat analog tones.

Target audience: While its small size and simplicity made it a good beginner's synth when released, its excellent sound quality has made it a valuable instrument for experienced musicians."