Question New build turning on and off, RMA'd multiple components without change, running out of ideas

[skons321]

Hi all,
I've had an issue with my build where it will turn on for around 20 seconds and then turn off (without ever booting). I've RMA'd the MOBO, CPU and PSU tried running it with just these components (both before and after RMA'ing) and still have the same issue. Interestingly, If I don't plug in the 8 pin 12v connector it turns on and stays on (again, both before and after RMA'ing) and yes I am using the proper CPU 8 pin cable and not a PICE 6+2 pin connector. I've tried taking the MOBO out of the case in case contact with the metal was causing shorts. I've tried clearing the CMOS. I feel like this must be my fault somehow at this point since I'm having such a specific issue across multiple components, but I'm not sure what it could be. I'm about to just take it into Microcenter and pay them troubleshoot/build it for me. Please let me know though if anyone has any ideas what the issue could be.
Specs:
Ryzen 2700
B450 Pro4 ATX
Sapphire Rx470
Team T force vulcan DDR4 3000 16gb
Cooler master 750W Gold
Inland premium 500gb nvme
NZXT H500

 

[boju]

I suppose you have tried shorting the power on pins to rule out if case's power button is somehow not disengaging?

Ram placed in 2nd and 4th slot from cpu?

The cpu heatsink fan plugged properly to the cpu_fan header? Not chassis/system fan?

 

[skons321]

I've tried all of the above, unfortunately. I've also tried reseating the CPU and redoing the thermal paste several times.

 

[boju]

Probably best take it to Microcenter, they'll have variety of parts and test methods. Could be the graphics card causing it since that's the only part not changed?

What happens if you remove the gpu? No display obviously but just to see if the system remains on.

 

[skons321]

I've tried removing the GPU. Running it as only MOBO, CPU, PSU, and have tried it both with and without RAM. No matter what the problem remains the same. The only change is when I don't plug in the 12v 8pin and then everything stays on.

 

[boju]

Puzzling. With the things you've done and tried it doesn't make sense.

Since the system behaves differently with the cpu 12v power, if this cable is modular check it's direction. Not sure whatll happen if this cable is arse about having ground wires opposite side in the plugs, probably notched different so not sure if this is possible?. Only thing i can think of sorry.

 

[skons321]

I def have it plugged in the right way now, but I'll try flipping it for shits hahah. The only thing I can think of is I must have gotten a replacement component with the exact same problem as the previous out of pure bad luck

 

[boju]

Can and has happend people receiving back rma'd parts. Remember to take photos of any serial number printed on pcb or sticker to be sure.

Think your plan to see Microcenter is your best course of action.