I'm using an old ATX power supply to power a PlayStation 3. I've built a small circuit to make the power supply only fully turn on when the PS3 is turned on, by using an optocoupler that connects the PS_ON wire to GND. The thing is that it works only to turn the power supply on, when I turn the PS3 off the power supply keeps working. Even if I disconnect the PS_ON wire from the circuit and hold it in the air, it won't turn off, only does so if I connect the PS_ON wire to 5V. Now I must say that I desoldered all the wires coming from the power supply except for a 12V wire, a GND wire, the PS_ON wire and the 5VSB wire. I don't know if this could be the reason behind the problem. Here is a pic:
I found the problem, it was a grounding issue. The power supply PCB is supposed to touch the case in the bolting points, but only does so when it is bolted on, making the GND (0v DC) connected to the power plug ground. Since I had been soldering and testing, I had removed the PCB and it happens that I still hadn't put it back on. Still, a big thank you to those who tried to help!