Whoops, CPU Power != PCIe Power

Gerowen · September 15, 2019

Story time. My brother called me up today and asked me to help him upgrade his home server. He has a home server that he uses for his family and himself, but when it comes to technical details, I help him with certain things, so once and a while he'll need help and I don't care a bit to run over and help him out. He had ordered a used Xeon processor, board and some ECC memory to upgrade his home server. We unpack everything, apply thermal paste, attach the CPU cooler, pull the old board out, stick the new board in, hook everything up, plug it in and bam, no life. Not even fan spin.

During the installation of the new parts, my brother wanted to help so he sat across from me and when I asked for a cable, he handed it to me. I "did" notice when he handed me the CPU power connector that it was 6 pins instead of 8, but I've seen adapters for such a scenario, and seen boards, even in factory PCs, boot with a smaller adapter connected to it. I simply pointed out that he might want to consider getting an adapter so that his CPU can get the proper amount of power it wants. It never occurred to me to actually check the label on the connector. His kitchen had minimal overhead light, which meant I was working off ambient and a cell phone light, and he had told me it was CPU power, so I just assumed he was right.

So after we realize it's not doing anything, we start taking things back apart and double checking things. Move the RAM sticks over into the other channel, try shorting the pins on the JF1 header with my pocket knife to make sure I hadn't misinterpreted the labeling, etc. And when I unplug the CPU power connector I just happened to glance down at it and see that it reads PCIe. A slight panic hit me. The plug FIT, there was no unusual force applied, it was keyed the same as the right 6 pins on the connector and it plugged right in as if that's where it was intended to be, albeit minus two pins, which like I said, I've seen before even in factory built PCs. I quickly fish out the correct CPU power plug and hold it up next to the PCIe power plug and sure enough, the +12V and ground pins are flipped, and the reality of what we just did hits me. We straight up just dead shorted his CPU.

I tried plugging in the correct CPU power plug, and while the machine did power on and give us fan spin, it never did POST. No beeps, no video output, nothing, just fan spin and that was it. So, lesson learned, double check the labels coming from the power supply when plugging things in. Apparently just because something fits, doesn't mean that's where it's supposed to go.

Windows7ge · September 15, 2019

For any standard non-proprietary computer that uses standard 20+4, 4+4, 6, 6+2 connectors the CPU will ALWAYS be either 4 or 4+4 (sometimes 8+4 or 8+8) but never 6.

I do know what you're talking about though. Some pre-built machines with proprietary form factors may have CPU power with varying pin numbers (such as 6) it's important to distinguish these from the open standard platforms used when building your own computer as the connectors may be different but look similar.

Gerowen · September 15, 2019

18 minutes ago, Windows7ge said:

For any standard non-proprietary computer that uses standard 20+4, 4+4, 6, 6+2 connectors the CPU will ALWAYS be either 4 or 4+4 (sometimes 8+4 or 8+8) but never 6.

I do know what you're talking about though. Some pre-built machines with proprietary form factors may have CPU power with varying pin numbers (such as 6) it's important to distinguish these from the open standard platforms used when building your own computer as the connectors may be different but look similar.

I never even thought about the fact that it was 6 pins instead of 4 or 8, and the fact that it fit like that's where it was supposed to go just reinforced the thought that maybe it was an odd connector on that non-standard power supply he's got. I mean he only had like $50 in the board and CPU together, but still, that's $50 wasted because neither of us took 3 seconds to actually double check the label on the connector.

He asked whether he should replace just the CPU or the board and CPU together and I told him since he got them so cheap to just replace them both, that way it doesn't matter if it's the CPU, VRM or both that got cooked. He's only home on weekends and it would be a pain to only order one part and realize the other or both were dead and have to wait another week for the other one to arrive.