Editing IBM 5120 Maintenance Guide (section)

== PSU Voltage Checks ==

The 5120 PSU is larger than the 5100 / 5110 supply because it must drive 2 × 8-inch floppy drives plus the 9-inch CRT. Probe each rail at the planar power connector.

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''IBM 5120 PSU rail tolerances (typical linear PSU; verify against MIM SY34-0192)'''
! Rail !! Approximate target !! Notes
|-
| +5 V (logic) || +4.75 V to +5.25 V || PALM, ROS, RWS
|-
| +12 V || +11.4 V to +12.6 V || Floppy drive logic, deflection
|-
| +24 V (typical) || ±5% || 8-inch floppy spindle motor and head load (may be 12 V on some drives)
|-
| &minus;5 V / &minus;12 V || ±5% || Bias rails
|-
| CRT high-voltage anode || ~12–15 kV (9-inch CRT) || Measure with HV probe only
|}

The 9-inch CRT anode voltage is higher than the 5-inch CRT on the 5100 / 5110 — handle with extra care.