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TL;DR

My PC won't turn on after I connected it to my smartphone via USB 3. Something smells burned on my phone's end, but otherwise it appears to be fine. How can I tell what part of my PC is broken?

Forgive me for writing such a long question, but I tried to describe everything that happened with as much detail as possible, so that you can more easily and accurately help me with my problem.

I had recently built a PC, and now it won't turn on.

It all started when my PC was transcoding videos for my phone to play. Since the transcoding process did not use hardware acceleration, my CPU was under almost 100% load then.

After it had finished transcoding a couple of videos, I decided to transfer some of them to my phone. I used a brand new USB 3.1 cable I hadn't used much before (if at all) and plugged it to my PC first, then my phone. Naturally, I used the USB 3 port on my case's front side. I could've used the USB 3.1 port on my mainboard, but I didn't feel like looking around behind my PC.

The phone immediately turned off, followed by a burning smell; there was no sound, though. One of my 2 monitors turned some shade of dark magenta. The monitor in question was hooked directly to my mainboard (thus it must have been running on the Intel HD 580 belonging to my i7 6700K). The other monitor that kept showing me an image was hooked to my GPU instead.

My first concern at that point was my smartphone. The burning smell appeared to come from its USB-C port and it could no longer be turned on. Luckily, after removing the battery and putting it back in, the phone turned on as if everything that happened had been a bad nightmare. However, the nightmare had only begun.

I then turned towards my PC. I found it weird that my second (iGP-powered) monitor was only showing magenta when it should've been showing the current transcoding progress instead. That's when I noticed my (USB) keyboard and mouse wouldn't react at all. Maybe some sort of USB controller had been knocked out? I decided to power off my PC and power it back on.

My PC immediately launched its BIOS setup, something it had never done before, not even after I had finished putting it together. The keyboard still wasn't responding at all. I decided to plug it into a different USB port and reboot the PC. Still nothing. I then decided to plug the mouse in a different USB port instead. More specifically, I plugged it into the same front USB 3 port that I had previously plugged my phone to.

My mouse didn't seem to turn on, but I could hear all the fans on my PC roaring at full speed. It didn't do that with my keyboard in the very same USB port, though. Shortly afterwards, my PC turned off by itself, permanently this time. No matter what I tried, I couldn't get my PC to turn back on.

In an attempt to figure out what exactly broke, I chose to remove the PSU and use it to power my old (pre-built) PC. Everything seemed to work perfectly, so it likely wasn't my PSU. However, that was pretty much the only thing I could test, and now I'm at an impasse.

I'm not sure what caused the problem in the first place. Was it the cable? Was it the port? Or maybe I didn't wire the case's USB 3 port properly to the mainboard, even though I was perfectly capable of using it to install Windows 10 and had been repeatedly using it to recharge a wireless headset? I can pretty much rule out the smartphone, since I had been using it with my old PC before without any troubles.

None of that matters to me right now. What matters is to determine what broke. Luckily, since I had built that PC only 1 or 2 weeks ago, I still have a warranty, and I'd like to avoid having to replace all the components if I only need to replace one of them.

The question is, how can I tell what is broken exactly? Could it be that my CPU fried? Or was it my mainboard? Maybe it's just the case's power switch and USB ports that somehow broke?

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Nolonar
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  • Do you have a multimeter? I would check the voltages coming from the USB 3.0 port and see if there are any different from your other ports. – NetworkKingPin Jun 03 '16 at 23:02
  • @NetworkKingPin. No, I don't have one. Besides, I can't even turn the PC on anymore, not even with another PSU, so I'm not sure if a multimeter would help. – Nolonar Jun 03 '16 at 23:05
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    Sounds like you used a poor quality USB 3.1 Type C cable and you caused physical damage to the port, how much damage that extremely poor quality cable can do, depends on the design of the motherboard and the port's connection to the motherboard. Could be the motherboard or the CPU at this point, throw that cable away, or verify its reviewed by that google Engineer who has found hundreds of bad Type c 3.1 cables – Ramhound Jun 03 '16 at 23:13
  • Related to Ramhound's point. There's a good article out there of an engineer that had one of these cables destroy his test environment machines. http://hackaday.com/2016/02/04/the-usb-type-c-cable-that-will-break-your-computer/ – NotAdmin Dave Jun 09 '16 at 21:38
  • For those who are curious about how this story ends: The culprit was very likely the USB cable (which is indeed Type C). After a few days, the smell on my phone's USB port has faded significantly, but the C end of the cable still smells as strongly as it did back then. As for what broke on my PC, it was indeed the mainboard. After replacing it, my PC works again. Since it survived a 1 hour stress test, I only need to determine if the port sustained any damage; the mouse that I had plugged in before my PC died has also died. Thank you all for your help, I appreciate it. – Nolonar Jun 09 '16 at 22:25

2 Answers2

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Sounds like ultimately you may need a new motherboard. As for how to diagnose it, a multimeter may help in determining if/how many USB ports simply don't work anymore. If they don't read 5v, they won't work. I suppose it's also possible they are delivering 5v but the data is down (governed by a chip on the motherboard).

Your CPU should be fine but until you swap out of motherboard it will be hard to tell. The iffy/weird graphics could still be related to the motherboard itself.

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First and foremost: I wouldn't recommend trying a suspicious PSU in a computer. Try the rest of the components. Think of what could have happened if the PSU was the faulty component and was supplying overvoltage.

To answer your question, USB faults shouldn't be able to fry a whole motherboard, but it can happen.

Now, considering that your computer doesn't turn on anymore, the most likely problems are that your motherboard is dead (as pointed out by Arg0nawn) or there is a short somewhere. Since you used the front panel ports you can try disconnecting them from the motherboard. If there's a short in the front panel USB ports it will isolate it, allowing your computer to run as normal again.

Now, if it works for you, test the rest of the USB ports, just to make sure your USB controller isn't dead.

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