Strange issues with turning on computer

[Good Grief]

Hello, 

I'm an experienced PC builder and I just got a new motherboard, CPU, and GPU for a new build. I am using the same PSU and case that I got 5 years ago. I am using a Ryzen 3600, MSI Tomahawk B450 mobo, ASUS RTX 2060, Corsair RM750x (2015) and an NZXT S340 case. I set everything up with a previous mobo (ASRock Legend B450) and nothing worked. I figured it was an DOA board so I returned it. The issue was specifically pertaining to the POWER SW pins not working, the motherboard and computer would not turn on at all even though it was receiving power. I even tried the trick with the screwdriver to turn short the power button pins on the mobo but it didn't work. So I returned the ASRock and I set up my old PC components and the case button switches worked on my old motherboard. So I got this new MSI, and THE SAME THING HAPPENED. I have no clue what to do. I even tried a really weird setup where I plugged in my brother's power supply and case button cables and it still didn't work. Any suggestions would be appreciated

[WhitetailAni]

Bad case or power supply.

[Radium_Angel]

29 minutes ago, Good Grief said:

Hello, 

I'm an experienced PC builder and I just got a new motherboard, CPU, and GPU for a new build. I am using the same PSU and case that I got 5 years ago. I am using a Ryzen 3600, MSI Tomahawk B450 mobo, ASUS RTX 2060, Corsair RM750x (2015) and an NZXT S340 case. I set everything up with a previous mobo (ASRock Legend B450) and nothing worked. I figured it was an DOA board so I returned it. The issue was specifically pertaining to the POWER SW pins not working, the motherboard and computer would not turn on at all even though it was receiving power. I even tried the trick with the screwdriver to turn short the power button pins on the mobo but it didn't work. So I returned the ASRock and I set up my old PC components and the case button switches worked on my old motherboard. So I got this new MSI, and THE SAME THING HAPPENED. I have no clue what to do. I even tried a really weird setup where I plugged in my brother's power supply and case button cables and it still didn't work. Any suggestions would be appreciated

Pull the system out of the case, place on a phone book, attach to the mobo *only* the things needed to power up (not to boot, so dump the GFX card) and see what happens.

If it powers up, then power down, add video card, and see what happens.

Report back.