Question My computer only turns on if my CPU power supply is not connected.

Mar 8, 2020
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I’ve had issues with my new build, brand new ryzen cpu and new motherboard. All parts have been comparability checked. After setting up everything I try to power on and I only get a flash of light on my gpu and nothing turns on. After troubleshooting I find that all my fans, gpu, cpu cooler, and ram (led) turn on and act normally only if my cpu power is unplugged (2, 2x2 coords). Power is supplied to my keyboard (but not my mouse oddly, doesn’t light up when plugged in) but of course without cpu power I don’t get any visuals on my monitor. I know it isn’t an issue with the cooler placing. My psu is older, but it worked perfectly well the day before I moved it to the new case, same attachments except it’s “providing” power to a better cpu. My psu is a supernova 850 g2 so I don’t think power is the problem, I don’t see anything visually wrong with the mobo or the cpu power coord. Both the cpu and mobo are brand new so I’m doubtful they’re at fault, yet the the psu working perfectly less than 24 hours ago makes me think that isn’t the problem either. Does anyone have any suggestions
 
Mar 8, 2020
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What is your full system specs? Including brand and model.
Cpu Ryzen 7 3800
Gpu zotac gaming 2070
Psu evga supernova 850 g2
Mobo MSI x570-a pro
Ssd Samsung Evo 870 500gb
Hdd seagate 1000gb
Ram 2x8gb Oloy 3200mhz
Cpu cooler Amd wraith prism
Tower Cylon Aerocool mid tower
 
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Mar 8, 2020
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RAM? What flash of light on your GPU? You probably mean a light that stays lit?
The light is the LED of the gpu, meaning it got power for an instant but couldn’t maintain it and shut down I think, this was fixed once I unplugged my cpu power coord as everything worked fine once it was unplugged, (except the cpu)
The ram is oloy 3200 mhz (2x8gb)
 

Ravz

Great
Jan 28, 2020
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Make sure you've connected the appropriate CPU(8+4) and GPU(8+6) cables coming off from the PSU.
 
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Mar 8, 2020
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Unseat the cooler, remove the CPU from the socket and check for bent pins.

Also check all motherboard standoffs are in place, with screws tightened.

Lastly: double check you're using the correct EPS power leads from the PSU.
Pins are checked and mobo is set, I’m using the 2 4pin plugs for the cpu, I’d swap ends but they don’t fit into my psu because of the release levers
 
Mar 8, 2020
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Make sure you've connected the appropriate CPU(8+4) and GPU(8+6) cables coming off from the PSU.
My current coords are 6+2 for gpu and 2, 4pin connectors for cpu, there’s a cpu 2 slot that can hold one 4 pin, would that be the difference maker
 
Mar 8, 2020
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Update: I turned on the pc with the cpu power plugged into the mobo and not the psu, I tried to plug it in the psu as it was running and the moment I do it all powers off, is there a fail safe in the psu that stops its output? May need to recheck compatibility
 
Update: I turned on the pc with the cpu power plugged into the mobo and not the psu, I tried to plug it in the psu as it was running and the moment I do it all powers off, is there a fail safe in the psu that stops its output? May need to recheck compatibility
The PSU has protection that will protect it from a fault. If it turns on with the 4+4 pin EPS cable connected to the PSU, but not connected to the motherboard, then we can eliminate both the PSU and the cable as having a fault. I believe you've already demonstrated that.

Now, connect the PSU to the motherboard with the 4+4 EPS (and other connectors of course) but remove the CPU itself, you can leave the CPU cooler fan connected. Turn the system on, it should attempt to start but fail at CPU in the POST process with lights on and fans operating.

If it does nothing then the motherboard's CPU VRM most likely has a fault, so RMA the motherboard.

If it does nothing ONLY with the CPU installed (as you describe it does now) then the CPU would most likely have a fault and should be RMA'd since you've checked for bent pins, there are none, and as well checked for badly mounted motherboard. You will not successfully RMA the CPU if it has any bent or broken pins.

There is, of course, another possibility: one of the EPS connectors on the cable might be miswired such that 12V is being shorted to ground but ONLY when attached to the motherboard. With a hope they didn't miswire both connectors you can try one only of the 4 pin connectors at a time to see if it will start.
 
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My current coords are 6+2 for gpu and 2, 4pin connectors for cpu, there’s a cpu 2 slot that can hold one 4 pin, would that be the difference maker
I missed this before!

Yes it could... try moving the two 4pin EPS connectors around all three sockets to find a combination where it will start. If that motherboard requires power connectors in all 3 sockets would be really strange... I'm not sure even a 3950X has the capability to draw enough power to max out an 8 pin EPS socket.

Maybe if pushing 5Ghz on LN2. Maybe.

If the board does need it, then you'll need at least one EPS splitter if your PSU can't provide three. Unless you really ARE attempting 5Ghz on LN2...in which case then you will need a PSU with that many EPS power connections.
 
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