[SOLVED] Motherboard flashes repeatedly when turned on

[steveRoll]

I recently got a bunch of components for a new PC. I built all the parts together (except for the GPU), turned it on, but the monitor didn't display anything. In fact, it didn't even get into POST. So I started removing components one by one, and eventually got to breadboarding just the motherboard, PSU, CPU and cooler. Here's what happens:
When I turn it on, the motherboard doesn't beep, its LEDs flash 7 times, it waits a little, then shuts down.
After a while, if the PSU is left on, it may turn itself back on again, and the same thing happens.
What could be at fault here? It's could be the motherboard, CPU or PSU.
These are the components:

• Motherboard - Gigabyte B365M DS3H
• CPU - Intel i5-9400F
• PSU - Corsair TX650M
• Cooler - Arctic Alpine 12 CO

 

[Calvin7]

Looks like no graphics solution. "F" type CPU means no integrated graphics.

 

[steveRoll]

Looks like no graphics solution. "F" type CPU means no integrated graphics.

That doesn't seem to be it, unfortunately. I plugged in the GPU, its fans started spinning, but the exact same thing happened to the motherboard.
Besides, I don't see how the lack of integrated graphics could cause this anyway.

 

[steveRoll]

Nevermind, you were correct! Last time I forgot to connect the GPU's power to the PSU. Indeed, the motherboard itself would not produce any output, and I had to connect HDMI to the GPU to see anything.
And I was also testing all of this without RAM, which prevented it from passing POST.
Also, when I thought the PC shut down and resumed the cycle of powering on and then off, everything was actually working correctly, and it didn't shut down after that.
Thanks!