Casio AP-20 Service Manual page 15

4. Certain keys do not function
5. A certain key does not function
Replace the main PCB.
Do the keys function?
No
Replace keyboard PCBs.
Clean the contact.
Does the key function?
No
Replace the key contact rubber.
Does the key function?
No
Replace keyboard PCBs.
Does the key function?
No
Replace the main PCB.
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Yes
End
Yes
Yes
Yes
End