Editing IBM PC XT Maintenance Guide (section)

== Power Supply Checks ==

The XT PSU is a 130 W steel-cased unit. Both US-only (120 V) and international (switchable 120/230 V) variants exist; the international version was introduced after the initial release. Both are rated 130 W.

=== Voltage check ===
With the PSU connected to the motherboard and the machine powered on, measure across the P8/P9 motherboard connectors:

{| class="wikitable styled-table" style="width:80%;"
! Rail !! Healthy range
|-
| +5 V || 4.85 – 5.20 V
|-
| +12 V || 11.4 – 12.6 V
|-
| &minus;5 V || &minus;4.75 – &minus;5.25 V
|-
| &minus;12 V || &minus;11.4 – &minus;12.6 V
|-
| Power Good || rises to ~+5 V about 100-500 ms after +5 V is stable
|}

The XT's +12 V rail also drives the hard drive. A weak +12 V on a freshly-restored XT can manifest as a 1701 hard disk error rather than as a power-up failure.