JRC JMR-7230-S3 Quick Operation Manual page 7

2.3.2 When "DISP Brightness" is not being
displayed in the Key Assignment Indication
Area (key allocation display area)
1
Press the "MULTI" dial for a long time.
The Display Brightness function will be assigned to the "MULTI" dial.
2
Adjust the screen brightness by turning the "MULTI" dial to the left or right.
The entire screen becomes brighter when the dial is rotated to the right.
The entire screen becomes darker when the dial is rotated to the left.
5