PC won't boot with two pci-e cables connected but does otherwise

[bluescorpion33]

Hello,

This my first post here at tom's hardware. (sorry for the long post but I just want to provide as much detail as I can)

The specs for my pc that I built back in 2008 are:
PSU - Zalman ZM750-HP 750W
MB - Asus Maximus II Formula
CPU - Intel Q6600 OCed at 3 GHz
RAM - Corsair TWIN2X4096-6400C4DHX (2x2GB)
GPU - Gigabyte GTX 460
OS - Windows 7 64bit

My problem is that my PC won''t boot if a graphic cards that needs two pci-e powers cable is connect to my pc. More precisely my pc only boots with either no graphics card or a graphics card with only one pci-e (6 pin) power cable connected to it even with cards that need two 6 pin cables (but then I have no display).

For example I have been using a GTX 460 for more than a year now with no problems even until last night and a gtx 260 for about three years before that.

But now my pc won't boot with either my gtx 460 or my gtx 260 if they have both of the 6 pin pci-e power cables connected but will boot up if only one 6-pin pci-e power is connected but the monitor has no display (goes on standby). So then I dug up my old geforce 8800 gts card that requires only one 6 pin pci-e power cable and with that my pc boots up and turns on just fine with windows working properly.

Now my question to you guys is do you think that this problem with the graphics pci-e power problem is a problem due to a fault with my motherboard or is it a problem with my power supply? Or do any of think that something else might be the problem?

 

[Herr_Koos]

Powers supply would be my first guess. One of the 12V rails has likely failed. If you can borrow a PSU from somewhere you should be able to verify this.

 

[bluescorpion33]

So I got a new psu today, a Corsair TX750 v2 and now my pc boots just fine with the gtx 460 and everything is running well again.

However the funny thing is that I gave the old Zalman ZM750-HP psu to my brother to see if it works on his machine and it does work even when connected to the gtx 260 with both pci-e cables. His pc is booting just fine with my old psu. Maybe the fact that system uses less power overall than my system, e.g. my q6000 is OCed to 3ghz at 2.1v whereas his core i5 is powered at 1.7v, so maybe thats why my old psu can boot his system but not mine?