I'd been using my XClio Greatpower 550w PSU since getting it 16 months ago in an old Athlon XP system, which doesn't require use of the auxiliary 4-pin CPU power connection (though the PSU has one).
When I got the new motherboard & CPU (which does require the auxiliary 4-pin CPU power connection) I built the system using this PSU. On power on, fans started spinning, but no beeps and no display (note using onboard graphics so no graphics card in the equation).
Tried all the tricks - disconnected/reseated everything, took board out of case to rule out shorting, removed all connections to board besides CPU & PSU etc. Same result, so grudgingly I RMA'd it.
In the interim the very old PSU I'd put in the Athlon XP system to replace the XClio was playing up, so I bought a cheapish new PSU to replace it (Thermaltake 500w). When the new board & CPU were returned from Scan having tested ok, tried them with the new PSU and of course the b'stard's working fine. Connected the XClio again, fans running but no one's home..