@EvilMike there are 3 holes below where the wires stick out. They are (left to right) 3.3v, 5v, 12v. Use a small screwdriver to get in there and adjust. The decal on the top of the psu should tell you the pins to use when checking the voltage with your multimeter. You have a multimeter right?
If something doesn’t boot in my experience it’s been the 3.3v, but it’s worth checking both 3.3 and 5v. The 12 doesn’t usually need to be spot on.