I'm refurbishing a Pentium 120 PC with an AT power supply. It seems to work, at least nothing pops or smokes, but there's a slight problem with voltages, particularly on the motherboard connectors. Here's what has to come out of the pins, and what actually does:


Pin/wire Nominal voltage Measured voltage
1 (orange, PG) +5V +5.15 V
2 (red) +5V +5.15 V
3 (yellow) +12V +10.44 V
4 (blue) −12V −9.11 V
5 (black) GND
6 (black) GND


Pin/wire Nominal voltage Measured voltage
1 (black) GND
2 (black) GND
3 (white) −5V −4.01 V
4 (red) +5V +5.1 V
5 (red) +5V +5.1 V
6 (red) +5V +5.1 V

I guess 5.1 V instead of 5 V isn't much of a problem, but −9.11 V definitely isn't −12 V. Measurements were taken without any load on the PSU.

What do you think might be wrong with this PSU? I was planning to recap it, would that help?

  • 3
    When you measured those values, was it connected to any load (motherboard, hard drive, etc?) or did it have no load at all?
    – Justme
    Commented Jun 9, 2021 at 4:42
  • 1
    There was no load. Commented Jun 9, 2021 at 5:09
  • 9
    So re-test with load attached. They don't run so well without any load. It is possible there is nothing wrong with it. Chanses are capacitors are fine, and if you recap it, can you trust your capacitors are properly selected and replaced so that the PSU does not get damaged or does not become a shock or fire hazard?
    – Justme
    Commented Jun 9, 2021 at 5:39
  • 2
    It is normal, those PSU-s do not have separate power transformer for each voltage source. They are monitoring only +5V which should be stable all the time, other voltages are not required to be so stable for normal operation anyway and they will fluctuate with current on 5V rail.
    – ziza
    Commented Jun 9, 2021 at 6:45
  • 2
    Yep switching PSU require load... they do not work properly without it and some might get even damaged while on "longtherm" without load.
    – Spektre
    Commented Jun 9, 2021 at 9:19


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