Xbox One won't power on. Missing 5v

[Technomancer]

Hello folks,

I have an Xbox one that I am trying to get working for a friend that won't power on. After doing some probing around the board, and with the help of this nice standby voltage diagram I found, I think I narrowed the problem down to the MOSFET marked as U4B4 on the board. I've attached a picture of the MOSFET that someone pulled from a data sheet. Its an Si4501BDY-T1-GE3(data sheet hyperlinked). From the measurements I have taken, S2 and G2 are both getting 5V, G1 is getting 0.3V, S1 is getting 0V, and drain is <0.02V. On the working unit I was able to compare it against, the measurements for the S1/2 and G1/2 voltages were the same but the drain was fluctuating from about 1.3V to 1.8V. I tried replacing U4B4 in case there was an issue with the MOSFET itself but it didn't fix the issue.

I took a picture of both boards and marked what I could as far as voltages go an attached the pictures. At this point I am at a loss as to what is happening here. Is it the case that there is supposed to be voltage supplied to the drain of this MOSFET (which I measured as being 1.3-1.8V on a working board) and that is what is missing? Or is there an issue with one of the other inputs to the MOSFET that is supposed to allow it to output 1.3-1.8V on drain?

I am rather new at this type of work so any help would be appreciated.

*Edit* That "working" Xbox One that I got the measurements from apparently isn't really working either I just reassembled it and can't get it to boot but at least the mosfet in question has the voltages on the drain side, assuming they are supposed to be there of course.

*Edit2* That "working" Xbox One is in fact working. I seem to be dealing with a bad power supply at a minimum from the broken Xbox one. When I used the power supply from the broken Xbox one, the working Xbox One wouldn't power on. When I switch to the power supply that came with the working unit, it powered on. However, when trying the power supply from the working Xbox One on the broken board, it didn't seem to make any difference.

[stj]

Re: Xbox One won't power on. Missing 5v

is the output of the mosfet shorted to ground by a bad capacitor?

[Technomancer]

Re: Xbox One won't power on. Missing 5v

is the output of the mosfet shorted to ground by a bad capacitor?

None of the pins on mosfet seem to be connected directly to ground. S1 seems to be connected to ground via those 2 capacitors just above it and slightly to the right (C4B12 and C4B13). The resistance to ground on all the pins on the mosfet read in the kilo or mega ohms though so it doesn't seem to have a short.

[stj]

Re: Xbox One won't power on. Missing 5v

that still sounds wrong.
a 5v circuit should have a resistance in the ohms or hundreds of ohms.
do a diode-check between the fet output and ground in both directions - maybe something is open circuit after the fet!

[wwamann]

Re: Xbox One won't power on. Missing 5v

Do you have a orange or white light on the power brick. if orange then 5v power enable signal on pin 9 of power connector (J9A1) is probably not present. Pin assignments 1-4 ground, 5-8 12vdc (when pin 9 enabled), 9 power enable (5Vdc from board), 10 5v standby from power supply, 11-12 ground...