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I'm pretty confident that the PSU is bad in this computer, but I want to understand what my multimeter is telling me. With the PSU plugged into the wall only, the multimeter says that pin 9 is a little over 5 volts. When I plug the P1 connector into the motherboard and test pin 9 again, the voltage is now between 3.70 and 3.90 volts.

The computer will not power on at all. I put in a different PSU that I knew worked and it powered on just fine.

My question is why would the voltage be lower on pin 9 when the P1 connector is plugged into the motherboard? Is that normal (or a good sign that the PSU is bad)?

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You're measuring 5V when there is no load on the PSU, and presumably this is supposed to be a +5V tap. With a load, the voltage sags down to 3.8V. Power supplies do tend to sag (a little) under load (as current is drawn), unless they are designed (and built) to be "stiff". But your supply has dropped past the 5% or 10% regulation specification. –  sawdust May 3 at 6:12
    
I don't know if I was clear but the computer was not powered on at any time, but I don't know if that's what you meant by "no load on the PSU" –  Matt May 7 at 20:07

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