Editing IBM PC AT Maintenance Guide (section)

== PSU Voltage Checks ==

The 5170 PSU is a 192 W switcher with two motherboard connectors, P8 and P9, similar in pinout to the XT but with different rail capacities. To verify rails under load, use a digital multimeter with one probe in a chassis ground point.

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''IBM 5170 PSU rails (nominal)'''
! Rail !! Acceptable range !! Where to measure
|-
| +5 V || +4.75 V to +5.25 V || P8/P9 red wire, any disk drive power connector red wire
|-
| +12 V || +11.40 V to +12.60 V || Disk drive power connector yellow wire
|-
| &minus;5 V || &minus;4.75 V to &minus;5.25 V || P8/P9 white wire
|-
| &minus;12 V || &minus;11.40 V to &minus;12.60 V || P8/P9 blue wire
|}

The 5170 PSU requires a '''minimum load''' of approximately 7.0 A on +5 V and 2.5 A on +12 V to start. Submodels shipped without a hard drive include a '''5 Ω 50 W power resistor''' on the +12 V hard drive power connector to satisfy the +12 V minimum. The resistor draws about 2.4 A in normal operation and is normally hot to the touch &mdash; that is by design, not a fault. If the resistor is missing, the PSU may fail to start until a hard drive (or equivalent load) is connected.