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I'm trying to diagnose an apparently random sudden power-off problem on a desktop, so I swapped out components until the power supply and mother board were the remaining suspects.

I removed power supply A and replaced it with B. Power button no longer turns it on. Uh oh. So I did the paper clip jump on B and the power supply fan came on, as well as some of the case fans that were still hooked up.

I plugged A's 24 pin and 8 pin into the motherboard, power button makes everything come on like before.

Switched back and plugged B's 24 pin and 8 pin into the motherboard, power button does nothing. Immediately after the paper clip test confirms it has power. Random probing of other wires shows power.

I switched them back and forth a few times to make sure I wasn't missing something. A makes fans spin, LEDs turn on the moment power button is pressed. B does nothing, indistinguishable from being unplugged.

I volt-metered the 24 and 8 pins of A and B and got close enough readings: the same ~3.3s, ~5s and ~12s in each pin position between the set of 24 and 8.

The widest discrepancy was A having a ~10V reading and B reading closer to 12. But A is the one that works (other than the initial problem), so that doesn't seem like it explains this.

A is 600W, B is 650 W.

How do I proceed? With the power supplies I mean

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    You are measuring only volts. The amps you can get from the battery are just as important.
    – Eduardo Trápani
    Commented Jun 7, 2020 at 3:57
  • You neglect to describe how you measured the voltages. Are the measurements while the PSU is powering the mobo, or are the PSU plugs dangling in the air?
    – sawdust
    Commented Jun 7, 2020 at 6:05
  • 1
    If your PSU caps are stuffed I posit that the voltage could be OK under no load, but not be able to rectify enough under load (or cause power thats not smooth enough)
    – davidgo
    Commented Jun 7, 2020 at 7:05
  • 1
    I'm not an elected engineer, but I postulate yes because (a) At 6 watts per fan that's a load of only 36 watts - so not much, (b) fans are very tolerant on low voltage and power drops - I'm almost certain fans could even be run on even bridge rectified AC which would - which would be what a simple ac-dc converter with no caps would look like - unlike CPUs (c) bad caps are very common - especially in PSU's and (d) your voltmeter tests say very little, because they are not reading distortion/noise - and 10 volts for a 12 volt rail is way out of spec.
    – davidgo
    Commented Jun 7, 2020 at 18:09
  • 1
    See evga.com/support/faq/FAQdetails.aspx?faqid=59025
    – davidgo
    Commented Jun 7, 2020 at 18:14

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