With the known good PSU installed and its fan doesn't spin up it looks like the motherboard. The PSU requires the motherboard to make contact between the green power on wire and ground in order to turn on the PSU.
Another thought here, try jumping out the on/off header and see if that works, it could just be a bad momentary switch.
dcc
Posting Virtuoso
1,534 posts since Mar 2005
Reputation Points: 138
Solved Threads: 36