The Centris 650 DOES have a battery:
This was probably the reason it would not turn on.
If you've noticed, your ATX supply will not turn on if you just plug it in to the wall. Try it if you haven't yet.
ATX supplies require the PS-ON wire to be connected to a black wire (ground) in order to turn on for a project like this. This wire is usually green. See this site:
http://4volt.com/Blog/archive/2009/03/12/howto-hotwire-a-pc-power-supply.aspx
Good luck. Double-check all you work before you turn it on!!!