Ironically i was just fiddling with one of my old AT power supplies.
To get proper +/- 12V, you often need to adequately load the 5V supply. To my surprise however, this loading is to about 5% of the rated supply power.
For me, i had a 200W power supply, and at 5V i had to load it to about 10W. Do the math and you see i need a 10W, 2.5 Ohm resistor! (( I didnt happen to have one lying around )).
With some make-shift temporary testing resistances, I was drawing nearly 2A though the 5V line before i got it up to 12V regulation!
Hope it helps...
:NaN: