IBM 5150 Hardware Maintenance Manual page 117

Check the voltage between P20-1 and P20-2 at the servo-board
for 11.25-12.6 Vdc.
DO YOU HAVE 11.25-12.6 Vdc FROM P20-2 TO P20-l?
NO
Replace the Logic Printed Circuit Board. See Section
3, "Removal/Replacement."
YES
600-22