Voice of Music 920, ca. 1953
Approximately 11-3/4 inches / 30 cm wide.
"Tri-O-Matic" comes from the record changer's ability to automatically play three sizes of record—7, 10 and 12 inches—and set the tone arm down at the proper place for each size. There is a small button (barely visible in the picture above) that is actuated by the presence of a 10- or 12-inch record on the turntable, and a small arm or puck that is actuated only by a 12" record as it falls from the stack onto the turntable surface. So, if neither the button or the puck is actuated, the mechanism knows it has a 7" record to play; if only the button is actuated, it means a 10" record; if both the button and the puck are actuated, it means a 12" record. Ten- and 12-inch records may be intermixed; seven-inch records must be played separately.
When I was about 10 years old, my parents replaced their V-M record changer with a brand new Magnavox console. It contained a Magnavox Micromatic changer, made by Collaro, that used an entirely different sensing mechanism design. It used no buttons or pucks. Instead, before each record was released to drop to the turntable below, the tone arm would swing over and touch the stack of records waiting to play. The mechanism used the point at which the tonearm was stopped by the record stack to calculate the required set-down point for the needle when playing the next record. With this system, records of any diameter could be played together in one stack, as long as the diameters were ordered largest-to-smallest up the stack.