The 120V appliance will take twice as much current as the 240V one. The frequency (50Hz or 60 Hz) is irrelevant unless a synchronous motor is involved; even then the difference is small. Note. a motor-driven clock, or anything similar like a time switch will tell you the wrong time, and the error will get progressively worse.
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No. The power supplies used within ALL Yamaha Tyros 3 are the same, so you do NOT even need a transformer - simply use the appropriate power cable. However depending on where the instrument is being shipped to, the voltage/frequency label will be written with respect to that region.