IBM 5150 Hardware Maintenance Manual page 99

Remove your diskette. Check the voltage from TP-7 to ground
while inserting a diskette. The voltage should decrease from
2.0-5.5 Vdc to 0-0.8 Vdc as the diskette is inserted.
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DID THE VOLTAGE AT TP-7 DECREASE FROM 2.0-5.5
Vdc TO 0-0.8 Vdc WHILE INSERTING THE DISKETTE?
NO
Replace the Diskette Logic Printed Circuit Board.
See Section 3, "Removal/Replacement.
YES
600-4