[SOLVED] No POST, tried replacing all components

[chinchilla122]

I have two computers right now, and realized last week that one of them wouldn't POST and would restart by itself every ~30 seconds.

Specs:
Ryzen 7 3700X
Powercolor RX 5700XT
Corsair 850w
G.Skill Ripjaws 16gb ddr4
ASRock Steel Legend b450
TPLink Wireless Card
2x 1tb HDD
1x 250gb SSD

I've tried replacing basically every component with a working analog including

Ryzen 7 3700X > Ryzen 7 2700X
G.Skill Ripjaws 16gb > Corsair Vengeance 16gb
RX 5700XT > MSI GTX 960
Corsair 850W > Seasonic M12II EVO 500w
Steel Legend b450 > MSI Tomahawk b450

All of the replacement parts are from a working PC, I put the replacement parts back into the other working PC and it booted up perfectly. In addition, I took out the CPU, RAM and PSU from the broken computer and put it in my working computer and it started fine. Afterwards, I took the working PSU, RAM and CPU and put it back into a brand new motherboard and got a new but similar issue. Everything seems to start fine (all fans spin, all lights start) but there is no display.

So basically: I've replaced the CPU, RAM, MOBO, GPU, PSU, and tried disconnecting all HDDs and wireless card, but nothing has changed. I've also plugged the CPU, RAM and PSU from the faulty computer into a different one and it's started fine. I put the working CPU, GPU and the brand new mobo into the broken computer and it still doesn't start. This has been seriously bugging me for about a week now, so any help at all would be appreciated

 

[Ralston18]

Have you tried a new/known working CMOS battery?

Broken computer: what make and model case? Check the power switch and reset button. Take a look at the I/O panel. Maybe a short somewhere.

 

[kurdtnz]

Do you know what BIOS version the motherboard/s were shipped with? Some of the B450's still need a BIOS update to run the 3rd gen CPU's.