First, have you looked inside the psu housing for internal pots?
My FSP 530 has 5 of them.
Second, almost 99% of power supplies have a sense wire for the 3.3v power rail. It is easily seen if you look at the connector that plugs into the motherboard....the big one not the small one. Look at the corner of the connector where a thick orange wire is going. There should also be either an orange or brown thin wire running into the same slot as the thick orange 3.3v power rail. Snip that and cut away a good 1/2-1" section out and add a 40-70ohm fixed resistor between the 2 leads. Then solder to the top leg of the fixed resistor (top leg is the one that is attached to the lead that runs into the power supply and not intpo the plug) a 10K VR (attach to the middle leg of the VR). Then attach the top leg gof the VR to a ground point. Be sure you set the VR to max resistance first by measuring its resistance between the middle and top leg....to make things easier so you dont make a mistake, cut the bottom leg of the VR off....top leg is the one by the screw of course.
Now, fire up the psu and take a voltage reading....should get abit over 3.3v, then turn the screw clockwise slowly and watch the voltage increase. It may not move much at first, but be patient and dont turn it fast as it will start to increase after a few turns.
Let me know if this helps.
-Fire