This is a Yamaha CS-40M, the duophonic flagship of Yamaha’s CS line from 1979, made in Japan. It’s a true analog synth with four VCOs, used as two voices with two oscillators each, or stacked as one huge four-oscillator mono voice in unison. It also has 20 patch memories, which was rare for an analog synth at the time, so you can save your sounds and recall them.

It sounds unmistakably Yamaha CS: thick, warm, creamy leads and basses with a smooth multimode filter. A ring modulator adds metallic, bell-like tones, and the LFO reaches into audio rate (about 500Hz) for FM-style growl. Sample & hold handles random modulation. The CS-40M and its CS siblings were used by Vangelis, Ultravox and Duran Duran.

Specs:

• 44-key keyboard (3½ octaves)
• Duophonic: 2 voices × 2 VCOs, or 1 voice × 4 VCOs (unison)
• 4 VCOs with sine, sawtooth and pulse waves, pulse width modulation, octave range 64' to 2'
• White noise
• Ring modulator
• Multimode VCF: low-pass, band-pass and high-pass, with cutoff and resonance
• Envelopes: ADSR for the filter, ADSR for the amp, plus a separate modulation envelope (+/–)
• LFO: sine, saw up, saw down, square, and sample & hold, running up to audio rate (about 500Hz)
• Performance controls: pitch, modulation, brilliance, portamento, sustain
• 20 patch memories (2 banks of 10), with a cassette interface for backing up patches
• CV/trigger outputs; external control inputs
• No MIDI
• Voltage: [check the rear label / voltage selector]
• Dimensions: 38.9" × 18.5" × 7" (988 × 470.5 × 177 mm)
• Weight: about 46 lbs (21 kg)

I purchased this fully serviced about 6 months ago - beautiful monster of a synth, but (as you would see from my other vintage synth listings) this beast is just not conducive with my setup.