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Casio PT-20

(photo from eBay, showing my specimen)

This tiny instrument seems to be just a downsized Casio PT-30 and sounds very similar. (I haven't examined the hardware yet.) Unlike the latter it is missing the LCD, the cartridge slot, some of the sequencer buttons, the transpose buttons and 2 high note keys (thus also the "arp. 6" rhythm). Instead of separate rhythm and chord volume sliders it has only a combined slider. Its small case resembles the Casio PT-1. The original German retail price in a German Conrad catalogue from 1986 was 199DM (about 100€).

The likely most advanced member of this hardware family was the ultra- rare keyboard boombox Casio KX-101 (37 mini keys, chord button pad, complex sequencer that saves data on audio cassettes), which was even 4 note polyphonic.