IBM 5150 Hardware Maintenance Manual page 98

Remove your diskette. Check the voltage from PI 0-2 to
ground while inserting a diskette in Diskette Drive "A." The
voltage should drop from 0.5-1.0 Vdc to 0-0.4 Vdc as the
diskette is inserted.
DID THE VOLTAGE AT PIO-2 DROP FROM 0.5-1.0 Vdc TO
0-0.4 Vdc WHILE INSERTING THE DISKETTE?
NO
Replace the Diskette Drive assembly. See Section 3,
"Removal/Replacement. "
YES
600-3