Power Rails (Sega NAOMI 2)
The power rails on the Sega NAOMI 2 are 12V, 5V and 3.3V.
The 12V rail pulls typically 0.2A.
The 3.3V rail pulls around 0.8A.
The 5V rail pulls between 3 to 4.5A.
The 3.3V rail must be present first.
If the 3.3V rail comes up after the 5V or 12V rails then the system will not boot until completely powering off all rails.
After that the 5V and 12V rails must be present, and the 2 fans on the GPUs present and working for the system to boot.
The 5V rail is the most critical.
If the power supply cannot deliver 4.5 Amps on initial boot it will fail to boot at all.
Once it has booted the power draw will drop to around 3.5A.
Even if your bench power or PC ATX power supply measures 3.3V, 5V and 12V, due to the high currents, it is important to measure the voltage at source.
Use a multimeter and verify the voltages (especially 5V) are correct at the power input pins of the NAOMI 2 board.
Typically 5V will droop to as low as 4.5V under the volts drop of the cables. If so, use the bench power supply to increase the voltage at the bench up to compensate for the voltage drop, until the voltage at the NAOMI input measures 5V for example.
You can remove and re-apply the 5V and 12V rails at any time, or remove and re-add the fans while the system is running, so long as on initial power up the 3.3V rail is the first to arrive. Removing the 5V/12V or fans will instantly turn off the consoles booted screen and it will go no screen until they are all restored, and then the boot sequence will re-initiate.
As mentioned, the two fans on the GPUs are required to be connected, fully working and sending the speed pulse back to the processor in order for it to start boot.
The case fan is not required to be connected to boot.