When I turn the power on in the PSU, the MB standby LED comes on, but no fans go on, and nothing else happens. The PSU is able to power on an old Dell system without problems. The weird thing is that when I hook up the 24-pin ATX from AX850 to the old Dell and keep the 8-pin power from the AX850 on the new Asus MB, the Asus MB's fans come alive. In this hybrid configuration, if I connect the 24-pin power from the old Dell to the new MB, then the whole thing comes alive. I can power on to the BIOS config stage. So, why isn't the new MB not powering on with the AX850?
What happens when only 24pin power from AX850 is connected to MB (everything else disconnected: 8/4pin EPS, graphics card, ssd/hdd/optical drives etc) and you try to turn the system on?
Does your old dell system use 8/4pin + 24pin power connectors? Have you tried running it entirely on AX850?
With just the 24-pin connected to the AX850, the MB LED comes on, but nothing else happens. No fans, and no power to the power switch.
I can boot the old Dell with the AX 850. It only has a 4-pin socket and a 24-pin socket. The old Dell PSU only has a 4-pin 12V connector, and it won't power on the new MB. But as mentioned above, 8-pin from AX 850 and 24-pin power from the Dell PSU does get the new MB to POST and get to the BIOS setup.
I've decided that it must be the motherboard. I got a Thermaltake PSU that is working in another computer to see if it will power up the new MB. It doesn't. Just the LED glows. I don't know why the old Dell 24-pin works. That one still has pin 20 wired whereas the AX850 and Thermaltake don't. Do you think that has something to do with the problem?
Pin 20 is no longer used in modern PSUs. In older ATX specification it was -5 V, which now is not required, because most of the power comes from 12V rail.
This whole situation is weird, normally I would agree that the mainboard is faulty, but your hybrid connection test shows that it works. Next I would suspect a PSU, but it works too. What is even more weird is that the ASUS manual on page 42 (2-24) shows ATX power connector with -5V on pin 20, but lower, in hints section, it says they recommend PSU compliant with ATX 2.0 or newer, which doesn't use pin 20. Maybe they just used a old 24-pin ATX connector picture.
After your another test with Thermaltake PSU, I agree that it looks like the mainboard is faulty. I remain, however, very confused. RMA your board and let us know if that helped.
I returned the Sabertooth X79. Got an Asus P9X79 Pro, hooked it up to AX850. The motherboard had problems. The fans did not go on until I pressed the power button. Then everything ran fine, except that 3 of the 8 RAM sockets were bad. I returned that one and got a different, new Sabertooth X79. Same exact problem as before. MB LED lights up but nothing else happens. Same thing with Thermaltake 750W. I know everything works, because I'm able to run the MB by doing the hybrid PSU trick. I was able to install Windows 8 onto the SSD using the hybrid power trick. I went ahead and updated the MB BIOS to 3309. No change.
Any thoughts? 850W should be plenty, but does the system need even more power?
I went back and forth and back and forth in my mind. Should I try another PSU for the Sabertooth or just go with the AX850 and P9X79 Pro. I actually went to the store and bought an AX1200 and a P9X79 Pro. Came home still debating what I should do first. In the end I decided that I was just tired of this and went ahead with what I knew worked -- AX850 with the P9X79 Pro, albeit the previous P9X79 Pro was faulty. Changed out the MB and hooked up the AX850 to the new P9X79 Pro. Voila! It sings! I didn't have the heart to open up the AX1200 just to test it out on the Sabertooth, so I just returned the AX1200 unopened along with the Sabertooth. We will never know why the AX850 didn't work with Sabertooth X79.