Mac Pro 1,1 got fried by turning it on

Hi,

I have a 2006 Mac Pro, 3ghz Model, and I used it as a file server without a graphics Card.

When I turned it on today - as soon as I pressed the button - I heard a poof (even felt it a little), and the Mac was done…

I opened it up and took a few sniffs, but the mainboard didn‘t seem to smell fried, neither the power supply.

The guy who gave me the Mac also gave me a spare Mainboard with it - the one that got replaced - and so I think that maybe somethings wrong with the power supply. It maybe likes to give out too much, so the mainboards get cooked - but I can’t even say if the spare mainboard is fried - just an assumption.

What do you think, what exactly poofed at that moment and killed the device?

Thank you everyone for your help!