Mac Mini not booting, white led blinks, fan spins briefly, no chime

I'm having an issue with this model (intel i5, 4GB DDR3 ram), where it starts up, white led turns on for a while, fan spins up briefly but the mac mini does not turn on. This process repeats forever.

I've removed the power supply and I'm measuring the voltage. With no load, they all read 11.88V (any of the right pins with any of the left pins). Then I press the switch on button, all pins go down a little (like 11.82V) but the rightmost left pin + any of the right pins read ~9.22 for one second, they go up to 11.88V, then down again, then up and down, etc.

In a picture I've found on the internet, this "rightmost left pin" is marked as ground.

The point is: is this behavior right? Should the ground pin have 12V, 0V, or what?

Ps.: I'm from Brazil and English is not my first language.

Red: negative pins
yellow: neutral (? maybe not) pin
blue: positive

PSU details

Model details

Voltage reading between any one positive and one negative pin

Voltage between neutral and any positive pin when computer is trying to boot up. The reading fluctuates between 11.8V and 9.2V consistently - i.e., when trying to boot up voltage goes down, fan spins briefly, stop, voltage goes up, rinse and repeat

Reading between any negative pin and neutral pin when the process described above is happening. Voltage goes from zero to 2.5V at the same time the above process is going from 11.8V to 9.2V

Demonstration of how I'm measuring things. In this example, one negative pin (black probe, left), one positive pin (red probe, right)

mobo for reference (side 1)

mobo for reference (side 2)

Extra information: all tests were done with RAM installed and with no RAM installed, same results. Also, sometimes, for no clear reason (at least to me), after probing the mobo randomly with my multimeter (hoping some fairy could tell me what's wrong :-D ), the mini boots up - I hear the chime. But after turning it off and then on again, the miracle stops working.