IBM 5150 Hardware Maintenance Manual page 108

Remove your diskette. Position the System Unit Power switch
to Off, wait five seconds, then power on. Observe the spindle
during the POST.
Spindle·
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Front View Diskette Drive
DID THE SPINDLE ROTATE ON DRIVE "A" BEFORE THE
"BEEP" AT THE END OF THE POST?
NO
Go to page 600-20.
YES
600-13