ARP 2600 semi-modualr synth
A synthesizer which is built as an integrated unit, but can have its signal routing altered by the use of patch cords (or, as an alternative, the pin matrix), in the manner of a modular synthesizer. Usually, a semi-modular will have a normalled default signal routing which is in effect if no patch cords are used; inserting patch cords into input jacks overrides the default routing. Well-known examples of semi-modular synthesizers are the ARP 2600, the EMS VCS3 (which used a pin matrix), and the EML Electrocomp 101.
Semi-modular synths appeared in the early 1970s, in response to the needs of performers (particularly touring musicians) who wanted to achieve some of the flexibility of a modular, but could not afford the bulk, expense, or relative fragility. Some, such as the ARP 2600 pictured above, came from the factory built into a sturdy road case. Some techs made a business of customizing the normalled routing of semi-modulars so that the performer could achieve their most-often used patches using few or no patch cords.