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I just got an old AT PSU from a guy. I want to test whether it works. I plugged a hard drive on it and plugged the PSU in the outlet, but it won't turn on. I know that ATX PSUs need to have two wires twisted up for them to power on, but I thought it wasn't needed for AT PSUs? Is there something I need to do for it to power on or should I declare it dead?
Picture of the PSU : 
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The female connectors at the end of the black cable must have been connected to a double switch. You need to connect them together.
I can´t tell you which should connect to which, but my guess is brown - black and white - blue.
But don't take this for granted, if you have a multimeter check which are the wires that connect to mains plug and make sure you don't connect them together.
Bruno Ferreira
Posted 2012-07-18T00:13:27.567
Reputation: 158
Someone else told me to do the same thing after seeing the label on the top of the PSU. You were right! I connected the black and the brown wire together and the blue and the white wire together. I plugged the PSU and it powered up! Right now they are well taped together and isolated using electrical tape. – None – 2012-07-18T01:00:29.147
Wow, be super careful, the insulation has slipped off some of the wires Bruno mentions and that is at mains voltage! – gnibbler – 2012-07-18T00:29:28.687
Yes you are right, I put the insulation back on right after I noticed it on the picture as I was posting it. – None – 2012-07-18T00:48:55.930