YAMAHA CS-15 SN 5409

via this auction

"This is a very versatile 37 key 3 octave monophonic synthesiser with higher pitch priority. It has 2 independent tuneable VCO’s producing Sine, Square or Triangle waveforms with each VCO having their own Mixer and VCF-VCA channels. There is also an in built noise generator and inputs & outputs for control volts & triggers for both VCO’s. There are controls for portamento, pitch bend & brilliance and master volume, a headphone output and high & low level audio outputs. It is a U.K. model and runs on mains voltage which is selectable for 110,130,220 or 240 volts at 50/60 Hz. (see photo No. 8)...

The non-standard silver “D-Type” connector on the back panel was fitted by me as part of a project whilst studying electronics at college. I fitted an interface to the synthesiser to emulate key presses which was driven by my own sequencer software running on a Sinclair ZX Spectrum. This modification did not involve any changes to the original circuitry and I subsequently removed this interface. I left the “D-Type” connector on the back panel as I didn’t have a suitable blanking plate but there are now no internal connections to this socket. The four mounting pillars for the interface board (you might just about see them on 9th photo) were also left in place for same reason.

I have the original owners manual for the synth and an original official Yamaha service manual which contains 28 pages of full circuit diagrams, parts list, servicing and calibration information and a separate enlarged (approx 3ft. X 2ft) foldout full circuit diagram. This manual is probably more rare than the synthesiser and may be worth a fair bit on its own, however it seems like it makes sense to bundle it with the synth."