KenO wrote:

Could not find any schematic for ilssan model ISP 120SI

Reepair.net/en or ElektroTanya.com may have something, but more likely
you'll have
to find the controller chips and rely on data sheets and applications
notes from the makers of those chips.
But generally, to fix an ATX power supply you just look for physical
damage, bad solder joints, and check
diodes and transistors with a meter.