This MS-20 is fully functional and recently serviced. Some rust is evident on the bottom but it is in otherwise great condition.
Korg's best known and most lauded monosynth. The MS-20 is a two oscillator, dual filter, semi-modular analog synthesizer with 6.3mm patchbay which allows re-routing of the internal synth architecture.
The MS-20 has two VCOs. The output of VCO1 can be set to triangle, sawtooth, adjustable-width pulse or white noise. The output of VCO2 can be set to sawtooth, square, rectangle or "ring mod" which is in fact an exclusive-OR between the pulse output of VCO1 and the square wave output of VCO2. Although the pulse width of VCO1 is adjustable, there is no voltage control for this parameter and as a result pulse width modulation is not possible. The VCOs have Hz/V voltage control inputs but note that oct/V scaling is available via judicious use of the "total" CV input socket.
There are two VCFs in series. The first is a 6 dB/oct   high-pass, the second a 12 dB/oct low-pass. Resonance ("peak" in Korg parlance) is adjustable but not voltage-controlled. Originally, the filters used Korg's proprietary [KORG-35] chip. Later, the filters were redesigned to use off-the-shelf LM13600 OTAs. MS-20 units fitted with LM13600 filters have a small daughter board; the front panel on such units often has a tell-tale mounting screw whose head is visible in the vicinity of the VCA block diagram.
The final component in the audio path is a VCA. It is a single-transistor design, based on a selected 2SC945 which is an NPN silicon device (equivalenced by 2N2222A according to the Towers' International Transistor Selector book, update 5 (ISBN 0-572-02121-6 90100)).
The MS-20 has a low frequency oscillator, labelled "modulation generator". It has two controls (rate and shape) and two outputs (pulse and sloped). The shape control was a rather unusual feature for the time. It affects the duty cycle of the pulse output and the shape of the sloped output. When the control is at the anticlockwise end, the pulse width is very high (pulse output stays high for most of the cycle) and the sloped output is an upward sawtooth wave. At the clockwise end, the pulse width is very low (pulse output stays low for most of the cycle) and the sloped output is a downward sawtooth wave. At midway, the pulse width is 50% (pulse output is a square wave) and the sloped output is a triangle wave. The pulse output is available through the patch panel. It is unipolar (0 or +5V) and suitable for triggering the S&H or the envelope generators. The sloped output is bipolar (-2.5V to +2.5V). Like the pulse output, it is available through a dedicated jack on the patch panel. It also feeds the top modulation bus and so can be used to modulate the frequency of the VCOs and the cutoff frequency of either or both VCF without patching. There are two envelope generators. EG1 has Delay, Attack, and Release parameters, whilst EG2 has Hold, Attack, Decay, Sustain and Release. The envelope generators have S-trig trigger inputs.
The MS-20 also includes a noise source (pink and white), a separate vactrol-based modulation VCA, a wheel controller, and a trigger button. It is possible to connect the MS-20 to a step sequencer like the SQ-10 (another product by Korg). This involves making use of the CV input and the trigger input which connects straight to the SQ-10, this means that the synthesiser and the step sequencer can work in synchronisation to play the notes when the next step of the sequencers pattern is triggered. In theory it is possible to connect any analogue sequencer to the MS-20 providing it has a trigger and CV output.
The MS-20 includes a frequency-voltage converter, envelope follower and gate extractor which can be used to drive it with an external signal. By connecting the output of the F-V converter to the CV input of the VCOs (VCO 1+2 CV IN) and the output of the gate extractor to the trigger input of the envelope generators (TRIG IN), the operator can play the MS-20 through his/her voice or another instrument. Instead of triggering the envelope generators, one can connect the output of the envelope follower to the main VCA's INITIAL GAIN input. In that mode of operation, the envelope generators are not used and the envelope of the MS-20's output is simply that of the external signal.