XBox One S - No Power On - Missing 3.3V

[LiftedTrace]

U9F1
Powered on and off for each test as my GPU started to get hot when plugged in.
Everything is disconnected ( HD, CD, FAN & Heat Sink, WIFI card) only have front card plugged in for power button....which it seems I didnt need as when I plugged it in, that chip came to life.
Measured each pin to ground.
With board facing in the direction of the USB ports and HDMI and stuff towards the back, power button towards you. I dont know which pin is 1, I opted for the pin on the bottom by the dot on the chip and went CCW around it. This is what I found.

Pin:
1: .75V
2: 1.5V
3: .75V
4: 0V
5: .755V
6: 1.8V
7: 0V
8: 1.51V
9: 1.51V
10: 0
11: 1V (After about 5 seconds....was fluctuating above and below 1V, then stabilized)
12: 5V
13: 1.52V
14: 1.65V
15: 6.55V
16 & 17 : 3.26 (Pins Tied together )
18: 2.44V
19: 0V
20: 5V

Hope this helps.
Ill leave it apart if you need me to take any other quick measurements.

[Maxwell55]

Thank you so much, I read and reviewed this thread religiously after the Xbox went dead. The only dust was found around the bottom right corner near U9F1, this unit had no water damage, I inspected both sides of the board for damage and checked as many components as I possibly could and like I mentioned all other voltages are present except the ones I listed. I will get to check your findings hopefully tomorrow and report, I don't have many smd tools to work with so it will be a challenge, unless it's a cap or something easily removable, can you please verify if there should be continuity between TP9D2 & TP9F1? Wish me luck..

[LiftedTrace]

I measure 2.5 ohms between them two points.

[LiftedTrace]

For what's it's worth, I get 1.73 ohms from TP9F1 to ground,
And .86 ohms from TP9D2 to ground.

[Jason Henry]

It's possible the unidentified chip is causing the issue with the 3.3V output. Try checking for any visible damage or consider replacing the chip if you can identify it. Good luck with the Xbox repair!

[Maxwell55]

Hi, sorry for delay I was tied up with other stuff. Ok so I checked the resistance on those points
I got 2.9 Ohms, 1.9 Ohms and 1.1 Ohms respectively from those test points. I will get on to checking the U9F1, hoping not to short any pins.

I don't see any damage, corrosion, anywhere or any other visible suspects from both sides of the board
Forgot to mention I do have 3.3V at TP4D1, not sure where its bogging down

Edit: Impossible to measure any voltages on U9F1 as the board cuts out 1 sec after I turn it on

[LiftedTrace]

Are you sure the power supply is working as it should?

[Maxwell55]

Yes, it's definitely working and no shorts between the rails.
Measured voltages on PSU = 11.99V unloaded
then;
TP881 = 11.98V
TP4D2 = 5.0V
TP4D1 = 3.3V
TP4D4 = 1.8v
TP4D5 = 1.1v

when the pwr button is pressed, the white led comes on, it beeps (chirps) once and goes off.
When I opened it up, I only found dust on / near U9F1 (bottom right of the board), cleaned it off but found nothing else on either front or backside of the board
WHile checkingthe mosfets, I then noticed no voltage to MEMIO, GFXCORE & CPUCORE, all mosfets were checked and none were found bad

[LiftedTrace]

So the issue i had sounds similar but mine was caused my water damage and it took out some caps on the back side of the board.
For example, C1T1 and C1T2 and all similar on this side and the other that are connected to the main chips power supply.

I ended up finding some off a donor board and just replaced them all.
Honestly dont even know what value they were but they were similar insize lol. I know, not ideal, but it worked.

Perhaps you have a fault where there is one or more caps open instead of short.
 

[Maxwell55]

multiple blow caps has crossed my mind, as I am unable to find any shorts. The only other way would be to inject voltage but I don't have a thermal camera to spot any heat points
I will continue to check the backside but I'd sure like to know where the rail comes from to see if I can follow it along to where its not reaching the mosfets and ram area

[Maxwell55]

Still unable to find the problems, I've checked all usual suspects, caps, fets..etc
I don't where else to look

Would you be able to measure the U4C1 outputs voltages on pins, 1,2,3 & 4 ?

[LiftedTrace]

Would you be able to measure the U4C1 outputs voltages on pins, 1,2,3 & 4 ?

Sorry for the delay, I missed this post in my email.
Here are the voltages for that chip.
(Powered on means it beeped and fan started spinning.)
1: 0V Powered Off, 11.8V Powered On
2: 0V
3: 11.9 Powered Off & On
4: 0V
5: 0V
6: 1.58 Powered Off & On
7: 0V Powered Off, 3.27V Powered On
8: 1.58 Powered Off & On
9: 3.26 Powered Off & On
10: 1.58 Powered Off & On
11: 0V Powered Off, 3.26 Powered On
12: 0V
13: 0V
14: 11.8 Powered Off & On.

[Maxwell55]

Thanks, I checked and compared the voltages at LM339 which supplies the gated voltages and this is what I got.
Note: I did not have the HDD or optical drive installed or anything else.

1: 0v off & on
2: 0v off & on
3: 11.96 off & on
4: 0v off & on
5: 0v off & on
6: 1.58v off & on
7: 0v off & on
8: 1.58v off / 1.58 on
9: 0v off / 0.9v on
10: 1.58v off / 1.58v on
11: 0v off & on
12: 0v off & on
13: 0v off / 29mv on
14: 0v off / ~3v on

I suspect this IC could be fried,but I'm not 100% sure yet,  I will check the continuity between the gates

Edit: What is strange on the LM339 is pin9 input voltage, I'm not getting the 3.26V that supplies the FETs for the Gated voltages, Can you take a reading on R9D15 whenever you can? Thanks