Socket the replacement. Use a good socket that grips the chip tightly - thermal excursions can walk an IC out of its socket. Techniques for repair Check the fuse. Dust the damn thing, with kleenex and Qtips. Of course you've never changed filters on the fan.
Dust kills power supplies.
After this, you'll change them. Look at a working version of the supply, if any are available. Get a schematic if you can. Apple and Sun are lousy for schematics, HP is good. Clones are impossible.