PSU needs a little help from it's friend

Redmond_1

Commendable
Jan 27, 2017
1
0
1,510
I suspect this is a problem with the PSU. About 18 months ago I bought a 1200w Dark Power Pro from BeQuiet to power my Z68 i5-2500K rig from 2011. There's a GTX770 and a couple of SSDs in there too. (PSU was bought for some serious Maxwell SLIage which I've not done yet.) Everything fine until 2 days ago.

Now my computer only turns on after I've used another PSU, my old 750 Watter, to connect into the motherboard ATX 24-pin connector. A quick press of the on-button spins the case fans for 1/2 second, then plug back in to the Dark Power Pro for and we're good to go.
Checked for loose screws in the case, CMOS battery, faulty
Anyone got any ideas as to what's causing this? BeQuiet are offering to test, but I'd like to see if I can fix it myself - as it's clearly not dead - before shipping a heavy PSU to Germany with insurance.