Effectively, there is a "logic sequence" that takes place. When you swapped out p/s units, you should have noticed a small wire harness that connects to the frt. power-ON/OFF sw.. That isn't a true electrical sw. as most people understand though still a switch. Upon hitting that sw., since connected the "mtrbd." drives a logical request that in turn uses a signal that trips the demand for power-ON/OFF to the ATX power supply. Also, on newer mtrbds. a small LED(lit LED) is present to show some voltage present in order to accept such requests is active. For whatever reason the logic behind the new power supply isn't working or the bios setting was changed or some OS power mgnmt. is ashew.
I suggest you locate that frt. panel ON/OFF harness connection and reseat it several times. I'm looking at possible h/w causes. but since you've installed new h/w plus related s/w and drivers, these too should be looked at. Further, do a shutrdown and just wait longer and see if it does power down. The latest drivers is a must for any new h/w to eliminate that.
OK, first let me state the problem I'm having. Around the time Doom 3 hit the streets I wanted to do some upgrading, stick of mem, vcard. Everything went extremely well until I decided I wanted a new power supply. I bought a bad one from some local dealer, not a big chain like BBUy, CUSA. It didn't do much damage except from then on I have to resort to shutting my PC down by the current Power Supply button on the back. It really hasn't bothered me, but the last few days my father and have been pondering over what turns the ************** off in the first place.
When I installed it all it did was (tickle) a bit then shut down. Man, let me tell you, I have tried everything (I) know. My father hasn't really dug deep into it, of course if he did he could probably fix it but it's not that big of deal. What do some of you guys think it could be? Which is where my question is, what actually powers the PSupply down?

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