Crashing problem when running 2 x 7950's on Corsair CX750 in mining rig

Question

I currently have 2 PC's with a Corsair CX750 PSU in both of them. I bought 4 x Sapphire Radeon 7950's with the intention of having two in each PC. The cases are open and the GPU's have been mounted via PCI-e riser cables so they run a bit cooler away from the motherboard.

From everything I read before purchasing the components, the CX750 would have plenty of power to run 2 x 7950's at the same time, but I'm having problems running them together.

Currently I can only get a cgminer stable system (Windows 7) when running the two GPU's at intensity 16 together. Anything higher than that and the sytem crashes. However I can run individual cards at intensity 19/20 for a long time without crashing.

I've tried running GP0 at intensity 16 and GPU1 at 19, but the crashing still occurs.

I've tried swapping the cards but they don't seem to be the problem. And I've tried different combinations in each of the PC's (they use different motherboards), but the crashing still occurs.

So this IMO rules out the GPU's, the motherboards, the CPU and memory from causing any problems. Which leave the PSU and the riser cards as the possible cause.

The PC's are using Zalman cases and the wiring is nice and tidy, and the temperature are fine.

So what could be causing the problems here? I had BAMT running before I tried Windows, so I thought that might the problem. But alas not.

Surely the CX750 PSU's are up to the job? Any ideas?

PS. I'm running the mining rigs inside cases instead of an open frame because I want to use the components for actual work at a later date. As they only have 2 GPU's in each there shouldn't be a heat issue, and the cards run at a steady 70c.

Answer 1

I eventually figured out this problem, and it was solved by lowering the voltage on the cards to 1100 or 1150.

Stock Sapphire 7950 boost cards are volted at 1250, and when running both cards concurrently it was making the system unstable and thus it would crash. Even so, I would have expected a Corsair CX750 to still be able to handle them both, but it seems not.

Lowering the voltage through Sapphire Trixx makes the whole rig stable, and I can now run at 18/19 intensity without any problems. Heck I might even try 20 soon ;)

Hope this helps anybody out in the same situation.