Question Mobo AB350 Pro4 caught on fire when turned on new pc build

Dec 26, 2019
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Hi guys,

So my mobo just caught on fire and I wonder if my other components are still good.

This is my first pc build. I got two used MOBOs from my friend. Both are AB350 Pro 4. I installed everything on one of the MOBOs but there’s no HDMI signal, so I decided to test with the other one. As soon as I turned the pc on the CPU side of the MOBO caught on fire. I could see visible flame and smoke. The speaker was beeping but I turned the PSU off immediately so I lost track of how many times it has beeped.
Can anyone tell me what could be the reason? I had exactly the same set up on both of the MOBOs. The first MOBO did not catch on fire but the second one did. Could there be any other causes other than a bad MOBO? I am scared to try again and I want to make sure my next try doesn’t start another fire.
The other thing I’m not exactly sure about is the GPU. My GPU has two ports, one is a 6-pin the other is an 8-pin. My PSU has one PCIE cable that has a 6+2 pin and a jumper 6-pin. I plugged both into the GPU. I’m wondering if I shouldn’t plug in the jumper 6-pin. I’ll list my built below.

PSU: evga 750W bronze power supply (used)
MOBO: ab350 pro 4 (used)
GPU: red Devil RX 580 8 GB (used)
CPU: Ryzen 5 2600 (new)
I built my pc on a testing case which has acrylic panels with metal frames. I’m positive that all standoffs are installed and tighten.
I’d appreciate if anyone could let me know what went wrong and if my CPU is good. Thanks!