The PSU and mobo are a couple of years old, but previously tested good.
They've been in storage, and I just put them in a new case.

5v standby is good. Power switch works and is in the right place. But when
I close the power switch, the power-on pin (of the ATX power connector)
stays high (4.4v).

The 5v standby line is 5v disconnected and 4.4v plugged in. That sounds
like too much of a load.

Hey! The fans just spun for an instant when I wasn't even touching it.

Now the fans spin for a second when I press power.

Maybe something is loose or shorted.

Will try without the video card and other connectors (like reset)
disconnected.


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