Yamaha CS30 SN 4145

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via this Vemia listing

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Pic of the inside.

"Released in 1977, the Yamaha CS30 is the flagship of their CS Monophonic range. It is as close to a modular synthesiser that Yamaha ever made and its modulation and routing options are extensive.
This one plays and sounds great. The synthesiser responds to all the pots and sliders as it should and it's ready to use and create weird and wonderful noises and textures. However, to get it to 100 percent functionality the synth requires a service to fix a few issues. Neither the combined 1+2 audio jack nor headphone output work, though the individual 1 and 2 jacks are fine. The sequencer works when switched to Manual and steps through by pushing the red button. However, the 'Clock' function only works when the synth has been powered on for a while and has 'warmed-up'.

Cosmetically the synth is in pretty good shape with nothing broken or missing. There are lots of very small dots of blistered paint, some with a hint of surface rust. The most noticeable ones are at the top of the control panel but these have not increased in size or number over the past decade-or-so. They don't look too bad, in my opinion. Inside is clean and dry with no signs of damp. One of the C keys has a cigarette burn (WHY??!!!) which has melted the plastic slightly. However, all keys are secure and function correctly. The case has a few marks here and there, but overall it looks good.

The case was designed to make servicing very easy. The front control panel is hinged at the back and can be opened by removing just two screws from each side panel. This then reveals a very neat collection of boards and wiring.

The CS30 can be easily integrated into a modern MIDI studio setup. By using an appropriate converter you can interface various features of the synthesiser via its numerous control inputs and outputs:

Foot Controller (CV) for volume in.

Sequencer CV and Trigger (gate) out.

Key Voltage (pitch CV) in and out.

Trigger (gate) in and out.

External (audio to the filter) in.

It features two voltage controlled oscillators (VCOs), two voltage controlled filters (VCFs - both featuring low-pass, band-pass and high-pass options), two voltage controlled amplifiers (VCAs) and three envelope generators. It also sports a ring modulator and a voltage controlled low-frequency oscillator (LFO).

It is fairly unusual for a hard-wired synthesizer in that it features a variety of switches which allow the user to re-route the signal flow from the usual VCO-VCF-VCA convention. For example, VCO 1 can output a square wave through VCF 1 whilst also outputting a high-pass filtered sawtooth wave through VCO 2. Another example is that the envelope generators can be assigned to any of the VCO, VCF and VCA modules and can also be inverted. Because the LFO is voltage controlled, the oscillating speed can be governed by an envelope generator and different waveforms can be applied to the other modules all at the same time. The CS30 also features an external signal input for filtering other musical instruments and audio sources, which can also be used as a modulation source. It includes an onboard eight-step analogue sequencer."