Editing IBM PC (5150) Maintenance Guide (section)

== Power Supply Checks ==

The 5150's PSU is a heavy steel-cased unit, '''63.5 W''' on the original Type 1 or '''130 W''' on the Type 2 fitted to machines with a hard drive. Both are over 40 years old.

=== Voltage check ===
The PSU does not regulate properly under no-load. 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 &ndash; 5.20 V
|-
| +12 V || 11.4 &ndash; 12.6 V
|-
| &minus;5 V || &minus;4.75 &ndash; &minus;5.25 V
|-
| &minus;12 V || &minus;11.4 &ndash; &minus;12.6 V
|-
| Power Good || rises to ~+5 V about 100-500 ms after +5 V is stable
|}

The '''&minus;5 V rail is essential''' on the 16KB-64KB board (the 4116 DRAM will not function without it) and on the 64KB-256KB board for chip U1 in the cassette circuit. A missing &minus;5 V triggers POST error 131 on the later board.

=== Modern PSU adapters ===
A modern ATX-to-P8/P9 adapter does '''not''' produce the &minus;5 V rail. On the 16KB-64KB board this produces a totally silent "dead" machine (the POST fails the first 16 KB RAM test and halts). On the 64KB-256KB board the machine displays a '''131 error''' on power-up. Retain the original IBM PSU and recap it rather than swap to ATX.