Yamaha made a lot of sound chips and that is what the "YM" designation on IC's made by Yamaha is. My guess is early 70's as all IC's from Yamaha that I have seen are four digit numbers after the YM. These chips were and are used in almost everything that makes sounds or noise electronically. Like a organ, synthesizer, PC sound cards, video games, arcade games and also found in vehicles, machines and appliances to make various warning alarms and announcements. Purple ceramic IC chips mostly from early 70's for sure.
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