Question PC POSTs/boots only very rarely and randomly ?

[matstersolo]

So my PC only boots sometimes (that's an understatement) but when it does it runs perfectly fine.
Most of the time it has no video output and no usb function and hangs, with no indication it's running apart from fans and LEDS, it turns off instantly from this state upon pressing power button.

I have tried probably every troubleshooting option, that i've seen from any similar issues i've found on forums.
One of note is that I did buy another PSU already, however, it is the same model that I already had. It didn't change anything.

I bought the same PSU because my PC ran with all the exact same components fine for a long time before this issue started happening, it was a much cheaper, and of course I don't know if PSU is even the problem so didn't want to spend much on it

I've seen other's seemingly having the same issue where a better & higher-wattage PSU has fixed the problem, but also lot that a new PSU didn't fix.

Was buying the same wattage, same model PSU a mistake?

PC Specs
Mobo: MSI Z790-S WiFi
CPU: i5-14700Kf
GPU: Asus TUF 3080 Gaming OC
RAM: 16GB DDR5
SSD: 1TB NVMe
PSU: Corsair CX750

 

[Misgar]

PSU: Corsair CX750

The CX series are what I term "low end" PSUs. I rarely use CX and only in unimportant builds that aren't used very often.

For a 3080 GPU, I would have used an RM850 instead, possibly even an RM1000. Better design, more power headroom for the GPU, longer warranty.

 

[Misgar]

but also lot that a new PSU didn't fix.

Obviously, if there is some other (as yet undiagnosed) fault in the system, fitting a "better" PSU won't fix your PC. It's 50:50 as to which is at fault, but you won't find out until you fit a different PSU.

I'd half expect your system to boot up from a 750W PSU, but I don't know what peak (instantaneous) current spikes are drawn by the GPU and CPU during booting into Windows.

I was running a power test on a PC yesterday using a wattmeter (average reading, not peak) and when the Windows Desktop appeared, the power consumption increased from 170W to 250W for a few seconds. 7950X, RTX 3060, RM850 PSU. I'm running a video transcode at the moment and the wattmeter is showing 399W continuous. At idle, the PC draws 135W.

High end GPUs like the 3080/90, 4080/90 and 5080/90 demand high transient currents (lasting milliseconds) on top of any average power readings you can measure.
https://forums.tomshardware.com/thr...rue-power-pulls-does-750w-psu-cut-it.3697122/

This review mentions the 3080 can draw 320W maximum and they recommend a 700W+ PSU.
https://www.techpowerup.com/gpu-specs/geforce-rtx-3080.c3621

I don't know if the 320W figure includes any optional overclock with MSI Afterburner, but with a low-end 750W PSU like the CX750, it might not be capable of providing short-duration, high-current spikes, demanded by the GPU and CPU. That's why you'll see people recommending more powerful PSUs with additional current capacity headroom.

Do you have the CX750, CX750M or CX750F? I think the F and M versions have modular cables.

 

[Bob.B]

So my PC only boots sometimes (that's an understatement) but when it does it runs perfectly fine.
Most of the time it has no video output and no usb function and hangs, with no indication it's running apart from fans and LEDS, it turns off instantly from this state upon pressing power button.

I have tried probably every troubleshooting option, that i've seen from any similar issues i've found on forums.
One of note is that I did buy another PSU already, however, it is the same model that I already had. It didn't change anything.

I bought the same PSU because my PC ran with all the exact same components fine for a long time before this issue started happening, it was a much cheaper, and of course I don't know if PSU is even the problem so didn't want to spend much on it

I've seen other's seemingly having the same issue where a better & higher-wattage PSU has fixed the problem, but also lot that a new PSU didn't fix.

Was buying the same wattage, same model PSU a mistake?

PC Specs
Mobo: MSI Z790-S WiFi
CPU: i5-14700K
GPU: Asus TUF 3080 Gaming OC
RAM: 16GB DDR5
SSD: 1TB NVMe
PSU: Corsair CX750

Remove the 3080 GPU and connect to the Intel iGPU....test.

 

[matstersolo]

Obviously, if there is some other (as yet undiagnosed) fault in the system, fitting a "better" PSU won't fix your PC. It's 50:50 as to which is at fault, but you won't find out until you fit a different PSU.

I'd half expect your system to boot up from a 750W PSU, but I don't know what peak (instantaneous) current spikes are drawn by the GPU and CPU during booting into Windows.

I was running a power test on a PC yesterday using a wattmeter (average reading, not peak) and when the Windows Desktop appeared, the power consumption increased from 170W to 250W for a few seconds. 7950X, RTX 3060, RM850 PSU. I'm running a video transcode at the moment and the wattmeter is showing 399W continuous. At idle, the PC draws 135W.

High end GPUs like the 3080/90, 4080/90 and 5080/90 demand high transient currents (lasting milliseconds) on top of any average power readings you can measure.
https://forums.tomshardware.com/thr...rue-power-pulls-does-750w-psu-cut-it.3697122/

This review mentions the 3080 can draw 320W maximum and they recommend a 700W+ PSU.
https://www.techpowerup.com/gpu-specs/geforce-rtx-3080.c3621

I don't know if the 320W figure includes any optional overclock with MSI Afterburner, but with a low-end 750W PSU like the CX750, it might not be capable of providing short-duration, high-current spikes, demanded by the GPU and CPU. That's why you'll see people recommending more powerful PSUs with additional current capacity headroom.

Do you have the CX750, CX750M or CX750F? I think the F and M versions have modular cables.

The old one I replaced was CX750M, which is modular. I replaced it with the CX750.
I had ran the card for so long on the cx750m without issue even with OC. I did upgrade motherboard/cpu and ram recently but still I think over a month or more running fine.

 

[matstersolo]

Remove the 3080 GPU and connect to the Intel iGPU....test.

Unfortunately it is the model without integrated graphics. Interestingly my keyboard LEDs light up as they would when it would boot when I don't have the 3080 in, which does make me think it's an issue with the GPU but when it very randomly eventually boots with the 3080 in, I can't believe there is something wrong with it when it runs perfectly fine after getting past that boot stage.
I might be able to try my brothers low end AMD gpu tomorrow and see how that goes.