IBM 5150 Hardware Maintenance Manual page 110

Reinstall the Diskette Logic Printed Circuit Board. (See
Section 3, "Removal/Replacement.") Position the System
Unit Power switch to On. Check the voltage at P 11-1 for
0-0.8 Vdc before the LED is lit - at the end of the POST .
•
DID YOU HAVE 0-0.8 Vdc AT P11-1?
NO
Replace the Diskette Drive assembly. See Section
3, "Removal/Replacement."
YES
600-15