Question PC won't boot after replacing PSU - Tried everything to solve the issue

Sep 21, 2023
Dear community,

My PC seems to have a hardware problem.
Could you help me fix it or guide me on which component I should replace ?

- CPU (APU): AMD Ryzen 7 PRO 5750G
- RAM: G.Skill 32GB DDR4-RAM PC-3000 (2x 16GB)
- M.2: NVME M.2 SSD 500GB Kingston NV1
- PSU: be quiet! System Power B9 350 Watt (80+)

I want to upgrade my PC by replacing the original PSU (be quiet! System Power B9 350 Watt (80+)) for a Seasonic FOCUS GX-650. Later, I will install a GPU, but I didn't receive it yet.

Now, the PC cannot boot, neither with the new, nor with the original PSU...

What happens when I turn the PC on :
- CPU Fan starts spinning
- The MOBO EZ debug LEDs turn on and off in turn
- The screen wakes up from sleep
- Few things appear on the screen (eg: Bios menu) and 5 seconds later "no signal"
- Fan stop spinning for half a second and spin again. The EZ debug LEDs turn on again.
--> It is as if the PC starts to boot but cannot boot properly and try again and again.

Tests I have done :
- Remove the MOBO from the case and put it on a non-conductive surface.
- Unplug all unnecessary components from the MOBO and only let connected: CPU + CPU Fan, 1 RAM stick on 2nd slot from the left (tried with both RAM sticks I have), PSU 24 pin connector, PSU CPU connector.
- Verify that the RAM stick is well in place.
- If no RAM stick is connected, the CPU fan spins and the EZ Debug LED "RAM" is on.
- Try with a RAM stick from another PC.
- Clear CMOS by removing the small battery of the MOBO for hours or by shorting the corresponding MOBO pins.
- Flash newest BIOS adapted to B550M Pro Vdh MOBO using the BIOS flash button. The flash went well I guess (led flashes for a few seconds, then turns off, then the system turns on including the fans, and then turns off). Also tried with older versions.
- Make sure I didn't mix the cables between the new and old PSU.
- Remove the CPU and make sure no pin is bent (and put it back).
- Visual inspection to look for a damaged component or weld.
- Turn on the MOBO without using the case button, but by shorting the corresponding MOBO pins.
- Try with a new case button.
- Try with the old and new PSU.

Is there a test you think I should perform ?
Also, do you think I should replace the MOBO or another component ?

Thank you for reading,

Best regards,

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