IBM 5150 Hardware Maintenance Manual page 112

Position the System Unit Power switch to Off for five seconds.
Then position the System Unit Power switch to On. Check the
voltage at Pin 18 on the diskette signal connector for a
decrease from 2.0-5.5 Vdc to 0-0.8 Vdc at some time before
the beep at the end of the POST.
•
DID THE VOLTAGES AT PIN 18 ON THE DISKETTE
SIGNAL CABLE CONNECTOR DECREASE FROM
2.0-5.5 Vdc TO 0-0.8 Vdc?
NO
Replace the Diskette Drive Adapter. See Section 3,
"Removal/Replacement. "
YES
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