The service manual in PDF is here: SYNFO - THE 20th CENTURY SYNTHESIZER DATABASE
Select SERVICE MANUAL and CONTINUE, scroll down to find XP-50 PDF download.
There are procedures to test Memory, Pitch Bend, MIDI, Sound, etc.
Also check backup battery voltage with meter.
Full schematics are available in this PDF - in case you choose to troubleshoot the problem.
Best regards,
Mike