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Today I woke up with one of my rigs with 2 7950's and 2 7970's (had been running fine for two months or so) off (I noticed a drop in hash power overnight)

I quickly checked around for any burnt cables or connectors and didn't see any, so I pressed the power button and nothing happened.. I then shut the PSU (EVGA G2 1300W) off, waited for any caps to drain, then switched it back on and tried to boot.

The Gigabyte WF3 2.0 7950 I had my monitor plugged into (in the first 16x slot with a 16x-16x riser) experienced a small cap explosion (not one of the larger round ones, one of the little rectangle ones near the VRAM heatsink) and the machine failed to post, with card fans all spinning.

At this point I would assume Motherboard or PSU, however, I'm not sure which. I took the 7950 out and tried to boot with the remaining three cards (just in case it was only the 7950) and the machine failed to even spin up anything (made a slight whine and the fans turned maybe 1/4 a rotation).  Luckily no other cards appear to be damaged, however, I don't want to replace the PSU if this is a MB issue.

I would have taken pics, however, the cap is quite small and the pop/sparks only lasted a second or two... no crazy smoke/flames luckily

Interesting to note is this DID happen on both this machine and another, similar config (ASROCK MB but same model PSU, 4 7950's on that one) (complete with losing one cap on one video card), however, I didn't really think anything of it, as the machines both started right back up and continued hashing with no issues apparent. (other than the pop)

The MB is a GIGABYTE|GA-970A-DS3P R
The PSU is an EVGA G2 1300W
4GB DDR3 RAM
AMD 145 CPU

I swapped the PSU with a Dell one I knew worked, and I didn't get any video out (using a different GPU I also knew worked)

I then took the 1300W PSU and connected it to a Dell 745 MB (with the same test graphics card) and everything worked fine.

At this point I assume it's an issue with the MB, however, I don't want a replacement to get here only to damage that if it's the PSU instead.

Also, does anyone know what the cap that blew on my card did? If it was just a ripple cap or something not too essential then I'd rather not RMA it (It's under warranty till 2016 luckily) as that means losing at least 3 weeks or so of mining time :-( 

Had the same thing happen today on my Gigabyte 7950, would love to hear if anyone has any experience.   Luckily, plenty of warranty time left but an annoyance none the less.

After I posted this I researched a bit more and found a similar thread 3 pages long that details caps on these cards blowing.

What happens is the rig shuts down unexpectedly, you try to reboot, however, nothing happens, so you turn off the PSU, turn it back on... and boom!!!! Right?

What PSU are you using?

The two rigs that experienced this for me were running EVGA G2 1300's, which are under recall for an overheat/shutoff problem. This is what makes troubleshooting for me so difficult... hard to tell if it was the PSU (with known problem) or something on the card (perhaps VRAM overheating and blowing a ripple cap upon startup)

At this point I had to RMA 2 motherboards (one physically exploded haha, the other just doesn't POST), and I'm getting the 2 EVGA G2 1300's replaced too. I have a smaller 600W supply, plus a Dell MB with 1 16x slot. Tomorrow I plan to test the cards one at a time with CGminer for an hour or so to ensure none are damaged (other than the one missing a cap). I have another WF3 still running fine for the past week or so without a cap... I'll probably RMA both of those too eventually...

By next week I have a new MB/PSU/CPU/RAM/USB coming to put the (hopefully) good cards in... I'm not taking any chances as I want a rig that runs, not more dead junk to RMA

What also makes troubleshooting so difficult is you make a mistake (with one unknown bad component) and you're left with a dead MB / who knows what else damaged. I'm used to the worst thing that happens is you freeze the system/crash the drivers and need to reboot.