Xbox turns on and right back off immediately. my question is, how can it be the power...

A couple months ago my Xbox started turning off right after I tried turning it on. Everything online I read said it had to be the power supply. So I swapped it out for one I found on Amazon and boy that was a mistake. As soon as I tried to turn it on after I'd installed the new power supply I heard a little "pop" I quickly unplugged it and then disassembled the Xbox again. Once I had the power supply out I cracked it open and the transformer had oil residue all of the place. Icing on the cake. I put it all back together and placed the old power supply back in the Xbox and plugged it back in. USB port still works but it still will only turn on and then right back off. Did I just get a dud with the power supply replacement? Was it ever the power supply or is it something else I've overlooked? Please help!

:)