Casio AP-10 - Troubleshooting Distorted Sound; Troubleshooting Certain Keys Non-functional; Troubleshooting Single Key Malfunction

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Replace PS2M PCB.
Is a sound ok?
Yes
No
Replace MA1M PCB.
End
Replace MA1M PCB.
Do the keys function?
Yes
No
Replace keyboard PCBs.
End
3. Distorted sound
4. Certain keys do not function
5. A certain key does not function
Clean the contact.
Does the key function?
Does the key function?
Yes
No
No
Replace key contact rubber.
Replace keyboard PCBs.
End
Yes