2009 MacPro Xeon tower will not start 12 days ago I was working on my MacPro when it made an audible 'pop' and shut down. 

Pressing the start button resulted in fan noise but no startup chime. After looking online I suspected the Power Supply  Unit. I managed to find a replacement PSU (took weeks) but I've just installed it and it hasn't worked the oracle. 

I've tried resetting the SMC (considering it hasn't been plugged in for over a week I believe this is simply a matter of starting the machine up again?). I can't use any utilities as the machine won't boot. Disk is OK, I can read files off it, etc -- the machine isn't even looking for a startup disk!

Anybody out there got any ideas? I'm getting desparate now -- this machine is my main workhorse!
The audible "pop" you heard was the magical bubble Apple keeps all the technological goodness in break.  Once it pops, the magic is let out and your Mac will no longer work.

Seriously though....

I have personally heard the "pop" on many different types of computers - it could be something in your power supply (which you already diagnosed it isn't) to something on the logic board.  

It could be a blown or bulging capacitor (left) or a burned out component  (right). (I know the pic on the right is of a PC, but it's an excellent example of both a blown and bulging capacitor in one picture; they go the same way in all electronics, not just Macs)




What you need to do is start removing components and visually inspecting them.  If you blew something, it would be quite visible.  You would see charring or a physically damaged component.  You are going to have to do some detective work to see what the cause will be and this means you will have to take the Mac down to bare metal.

Once you have figured out what it is, you can then figure out what you will need to do to repair it.