Yamaha SY99 Owner's Manual page 54

Re-record the specified
measures
Move the cursor to Measure and select a point a few measures before 015. This
will give you a chance to get the feel of the section you are going to re-record.
To begin recording press RUN. The RUN LED will blink on the beats. Play
along with the original. When measure 15 is reached, the original recording on
track 2 will disappear and your new playing will be recorded. When measure 16
ends, the original recording of track 2 will reappear, but playback will continue.
Press STOP and you will return to the song play display.
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