Quote from: MrTeal on October 17, 2014, 01:59:27 AM

Quote from: lightfoot on October 17, 2014, 01:27:55 AM

Ok. Well I got the new FETs in and decided to take the old ones off this board. Symptom was a .3 ohm resistance on the 12 volt line instead of the normal 300 or so ohms.

Started pulling FETs, then found they were not coming off. Fuck these things are small. Too small for my normal picker, I'm going to need a special nozzle. Then I realized my pre-heater was broken.

*grumble* Took pre-heater apart, wire had broken at the phenolic junction between normal wires and the nichrome type wire in the heater. Fixed it, back in business.

Here's a little thought if you think you can burn these FETs off with just hot air: Forget it. In order to remove them without damaging the board, you have to take the board to 380F pre-heat, then sit on them at 450c air for *30* seconds each. Say what you want about the board, but man does it pull HEAT away from these FETs.

Started pulling, did the left 6 and no change to the resistance. 7-10 same thing, started feeling really grumpt because I have to PUT THESE BACK! Then I did #11. Instantly resistance went to 300 ohms. And the underside of the FET was bad. Looks like I found the bad one.

Now to let things cool, then put on new fets tomorrow. I'm going to need flux for this one, these are going to be hell to solder back on. But do-able, and I have now proven that FET shorts are what shut down power supplies.

On ward. We're getting there. By the way if anyone else wants to follow along give it a go!

C

If you want to invest a little in equipment, I'd recommend a thermal imager. They're incredible tools for debugging these kinds of problems since they can detect <1C temperature differences. Solder on a couple wires and use a current limited power supply, and you can see quite quickly which fet is the one that's shorted.

Along these lines, if you have a constant current power supply and apply ~2-3v to the PCIe pins you can see the shorted FET glow under IR, saving the time and trouble of removing good FETs.