problem with power on

hello guys, this 2nd different PC have same problem, first one gigabyte g31 with pantium D CPU and the other one also gigabyte AMD 770 with phenom ii x3 720 CPU.
problem is motherboard not turn on, only thing i found it is when i remove the CPU 4 pin power PC turn on but sure without signal, but when i put it pc only do some kind a short Circuit, CPU fan just shake without run and PC not turn on.
1- i replace the PSU = same thing
2- replace CPU = same thing
3- remove and unpagled everything just turn mobo+cpu even remove the ram's i expect run with peeps but same thing nothing happen
4-i expect both of them dead motherborad"s or give me an idea
thanx a lot guys