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The PS-ON terminal is not an input; it's a signal that the power supply is on. If you don't have that, then the power supply needs to be triggered.
Most of those power supplies use a bridge rectifier off the power line that goes to a switching circuit, a MOSFET on a heat sink with an IC controller and a flyback diode and an inductor to generate a signal to go to the little high frequency power transformer.
So first see if you are getting AC to the bridge, DC out to the MOSFET. You probably do, and thus the problem is in the control circuitry that tells the IC to start up.
Any more detail probably requires a diagram. One thing you can do, with power off and all capacitors discharged, measure every diode, resistor, and transistor to see if any are open or shorted. Also check for fusible resistors; they are usually gray or light blue dull finish, about a 1 Watt size or smaller. If one has opened, there is the problem. Of course, there may be an important reason it has opened.
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