Yamaha AvantGrand N3X Owner's Manual page 45

Connections
Local Control on/off
Normally, playing the keyboard will control the internal tone generator. This situation is called "Local Control ON."
If you want to use the keyboard only for controlling an external MIDI instrument, turn off Local Control. With this
setting, this instrument will produce no sound even if you play the keyboard.
 To turn the Local Control ON or OFF:
Default setting: ON
While holding down [FUNCTION], press the C6 key repeatedly (as necessary).
C6 Highest key (C7)
N3X • 45
Owner's Manual