Casio CTK-4200 Service Manual page 31

F. LCD CHECK
F-1. Press the "2" button to select the "LCD CHECK".
F-2. Press the "2" button and check to see if all LCDs are lit.
F-3. Press the "2" button and check to see if all LCDs are turned off.
F-4. Press the "0" button to return to the main screen.
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CTK-4200