Ok, I've got some old computers sitting around and a motherboard I'm not sure works. I'm sure the NEW powersupply I bought isn't working, but can I take one of the old power supplies and connect it to the new motherboard (minus the missing pins) just to check if the motherboard even beeps. These PSUs are old with 20 pin main connectors whereas the NEW motherboard is 24 pin and the old PSUs only have 4 pins for the 12v rail, whereas the new motherboard has 8. If I plug in the old PSU with the extra pins missing will the motherboard at least show some sign of life so I know it's not dead?
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Probably won't work or is a bad idea.
Get your defective new PSU replaced, and wait to test it with the new PSU.