Power Rails v1.0 (Xbox Original)
If you have a version 1.1 these are the main power rail locations and what you should expect when powering up.
I recommend checking all pins and voltages with an oscilloscope, as you really want to be checking for quick dips or voltage oscillations when there are faults.
These all come from the PSU input on the power connector.
At various points the motherboard has bulk capacitors for these rails, such as the front left has 3 large 1500uF capacitors typically for the 5V rail, and an inductor for noise filtering.
The 9V rail is generated from an LDO at the rear middle of the board.
Left is 12V in from PSU, middle is ground, right is 9V out.
The 1.3V rail is likely for the Memory. It is generated via the SC1110CS sink/source DC controller.
The input is the 5V rail, and the output is the 1.3V rail.
The 1.5V rail is generated from the 3.3V of the PSU via a MTD3055VL N-MOSET.
There are two 1.7V rails. The right 1.7V rail is on the front right of the console by the power connector.
The MTD3302 N-MOSFET is setup in a high-side configuration so the output is on the source and the gate is simply driven higher than the drain.
There is a MOSFET on both the top and bottom of the PCB.
The output of the 1.7V is on the top side of the large red inductor.
There are two 1.7V rails. The left 1.7V rail is at the front left of the console by the Microsoft text.
The MTD3302 N-MOSFET is setup in a high-side configuration so the output is on the source and the gate is simply driven higher than the drain.
There is a MOSFET on both the top and bottom of the PCB.
The output of the 1.7V is on the top side of the large red inductor.
The IC just above this is involved in the power rails but I've not figured out exactly how.
Either way, the useful voltages are presented for comparison with a working console.
The bottom right pin (12V PWM) is the Gate pin of the MOSFET so this chip definitely drives and generates the 1.7V rail but might also generate the 1.3V rail.
The 2.65V rail is near the front left of the console just off center.
The gate of the 21N06LTA N-MOSFET that generates the 2.65V rail from the 3.3V rail generally sits around 4.75V DC but can oscillate above or below that by around 0.5V under boot up and loads.
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