Start by stripping your console down, removing the power supply, DVD drive, hard drive so you are only dealing with the motherboard and fan.

Use a bench to inject 12V into your 12V pin header where the original power board usually connects.

On first power being applied, but no buttons being pressed, you should see the current go from around 20mA up to multiple amps (probably your bench current limit) then after a while it will fail boot and go back down to 20mA. This is the console booting into standby.

Once it settles to a few milliamp, measure the voltage regulators.

To give a general overview of the power flow to the console I will list below in a kind of order of bring up.

On top of the actual power rails, there are inline resistors, fuses, inductors (filters) that could be bad. It is worth checking any of those if you find a failed regulator.

The first obvious check is after applying 12V to the connector, make sure it makes it to some key locations.

Next check you have the following 1.1V and 1.8V standby voltages that should always be present.

Both are powered by the 5V / 3.3V rails depending on state.

The 1.1V is always enabled, and then the output of power good from the 1.1V regulator enables the 1.8V regulator.

Notice the 5th pin on the regulators are sent low by the SMC (U2).

Notice the outputs of the U42 / U33 for the 1.1V and 1.8V outputs respectively do not come on until the KIC enables them (from pressing the power button).

If you want to test these output rails without the KIC involvement, simply short pins 4 and 5 together to enable them.

You should see the 1.1V and 1.8V output voltages appear on the test pads V_SB1P1 and V_SB1P8, as well as a rise of around 20mA on the 1.8V rail and

Next check you have the following 3.3V and 5V standby voltages that should always be present, and come off the 12V rail.

The 5V rail is always enabled, unless the KIC sends EN low.

The 3.3V rail is enabled by the 1.8V standby regulator output.