I’m taking a look at an Xbox one for a friend that won’t power on. The power supply brick is outputting 5v and, after coming across the standby voltages image, I started probing around to see what may be missing. I noticed that, at a minimum, I am missing 5v to some parts of the board. I think I have narrowed it down to something being bad in with U4B4. There are 4 pins on one side of the component that are supposed to be getting 5v (I marked U4B4 in the link) which aren’t, and after that the 4 resistors above it are supposed to be getting 5v. However, on the other side of U4B4, the second pin from the right is supposed to be getting 5v, which it is, but so is the pin on the far right which is not marked as something that is supposed to have 5v in the diagram (to be fair it isn’t marked at all).
What exactly is U4B4 and should that pin on bottom right also be getting 5v?
Link to the part of the standby voltages page with U4B4 marked:
Link to the standby voltages page:
Thanks a bunch for finding this! I am still learning so bear with me here but in this particular configuration, is voltage going from source to drain (I assume so since there is no voltage on drain but there is on source 2)? With 5v measured on gate 2, is that gate expecting a negative voltage to open (I thought I read somewhere that some MOSFETs use negative voltage) or does that indicate a problem with the MOSFET that 5v on gate 2 isn’t allowing voltage to go to drain?
Any info you could provide would both help my understanding of these components and perhaps the problem as a whole.
Hi Greg, I can’t add very much more to the problem due to lack of information all round, perhaps regard U4 as a block with outputs controlled by inputs and don’t get bogged down by the how.
I think U4 may be controlled by the small U3 next to it but without an overview of the whole standby mechanism it makes little sense. If you suspect one of these i/c to be faulty, replacement is the only sure way and even then only if you can prove the original to be ok by substitution. Good luck!
Ok, I tried replacing U4B4 but no dice. The voltage coming out of the drain of that chip is still very low, like <0.02v. I think I may be in a bit over my head with this one as I really don’t know what would prevent that from happening with the input voltage measurements on the source side being identical to a working unit that I borrowed from someone. The voltage on the drain on the working unit fluctuates from about 1.3v to 1.8v, or so my multimeter suggests. Below are the voltage readings I got from probing around that area on both units:
Working Xbox One (slightly different board): http s://imagizer.imageshack.com/img923/2958/kuHYlc.jpg
Broken Xbox One: http s://imagizer.imageshack.com/img923/9092/5JLH8x.jpg
Any additional thoughts on the issue or suggestions on what to try and probe next would be greatly appreciated.