3 6950's in a MSI 870-G45. Help Please!

[Mr.Bitcoin]

Any help from people with this MB would be great. I've got the cable to hook up the 3rd card to the 1x PCI slot (the other 2 are in the 16x slots). When I take out on of the cards in the 16x slot, the card in the 1x slot works fine. So I know that each PCI slot works, they just won't all work at once.

In other words (somewhat redundant, I know, but) this is also what my brother tried:
I connected the card to the PCI express slot, and tried it with both no additional power and with additional power from GPU power adapters on the card. In linux I could only see 2 cards. I verified that the adapter works, by removing one of the PCI 2.0 cards, and the PCI express card worked at normal capacity.

In Windows I had dummy plugs, and I set up monitors for the cards. I could never get it to work right either.

Thanks!
Mr.BitCoin

[Desolator]

Is your power supply sufficient for powering 3 cards at once?  You'll need at least 3 individual 12V channels and usually a 700W or more decent PSU to run most setups.

[Mr.Bitcoin]

Yup, I'm all set there. using a Rosewill Xtreme Series RX750-S-B 750W.

[BrianH]

Long term, I would say that power supply is not going to cut it. 3 x 6950s draw 600w on the 12V rail at stock. Short term, it should probably at least boot.

However, it probably is related to the board deactivating one of the PCI-E ports. You can pinmod it using this method: http://forum.bitcoin.org/index.php?topic=30831.msg387449#msg387449

Keep in mind, there is a reason the manufacturer disabled that port, because it may have insufficient traces to support the power from 3 ports at once.

[Mr.Bitcoin]

As an update, I got it working. Thanks for all the advice btw.

What got it stable was stopping unnecessary services in Wondowz, turning off the fancy graphics settings for the monitor, splitting the workload so 1 card is running Phoenix and the other 2 are mining with OpenCl, and cranking the fan speed up. Turns out the PSU can can handle all that, but its most likely at its upper limits.

-Mr.Btc