IBM 3270 Maintenance Manual page 212

4. Check the voltage at pin 12 on the signal
cable connector for approximately 5.0 V dc at
the start of ' POST.
WAS THE VOLTAGE APPROXIMATELY 5.0
Vdc AT THE START OF POST?
NO
Replace the diskette drive (see Section 5,
"Removal/Replacement and Adjustments").
YES
October 1984 3-600-13