$150 worth of smoke

$150 worth of smoke in the lab. See attached picture, note the bubbling and pitting of all the chips on both boards.

I just spent about 4 hours finishing the build on my i3, including installing a heated print bed. Since I have a heated bed now, I need a bigger power supply. I grabbed an old PC power supply out of the bin and hooked it up.
Bad idea. There is now a pervasive smell of burning electronics in the lab.

I am now out of the i3 business until I get an entirely new set of electronics. I verified that I had everything hooked up properly, and the power supply appears to be putting out 12 volts, and the first two times I powered it up it seemed fine except the X motor driver appeared to have died for some reason.

The 3rd time I powered it up, the thing just flipped out and must have been generating a ridiculous amount of voltage. In about 5 seconds after turning it on, all of the Pololu driver chips CAUGHT FIRE. The Arduino is toast as well. The RAMPS board is POSSIBLY OK since there's not much on it to go wrong but I'll order up another just in case.

RepRapDiscount, here's $145. Lesson learned, a good power supply is cheaper than replacing all the electronics. Though I DID test the supply and it seemed to be reliably putting out 12 volts.

I'm probably going to forget about PC power supplies and just buy a proper 12V/20A supply.
I've seen this before in some of our robot builds -- some PC power supplies just can't handle off-spec loads. Things like not having a 5v load will cause the 12 volts to go unstable, or downstream voltage regulators can cause the PC regulator to oscillate.
Yeah, someone over on G+ just pointed out the actual problem. Note that the motor driver on the left is plugged in one pin off. This sends VMOT (12v) power back through the 5v bus. boom.

$150 stupid tax, do not pass go.