Most ATX PSUs have a small gauge brown wire that is a "sense" wire, it must be connected to the 3.3 volt (orange) rail. Look at the 24-pin connector that goes to the motherboard, you'll see that it is wired together with one of the orange wires. If you have removed that connector, you may have to solder the brown wire to the spot on the PCB where the orange wires are/were.

Also, some chargers have large capacitors in them that charge up when the charger is turned on, so you have to turn the charger on BEFORE you power up the PSU, or it will trigger the over-current sensors in the PSU and it will shut down.