Gough Lui  I tried removing the bigger MOSFETs and they seem fine, also removed the capacitors. Still a short to ground.

I wonder if it's the reeeeeeeally tiny IC in that power rail?

See video: you can skip to the end for testing.

https://youtu.be/GzWI6J4IZn4

The more I think about it, the more I think that TIIIIINY IC with a resistor on it is the first stage of the gate controlling the on/off state of that power rail.

Look at that original XBOX One image (big image with standby voltages).

That tiny IC is connected to a smaller trace that comes from that big black thing at the bottom of section 9E of the motherboard. I bet that's the control turning it on/off.

I bet those MOSFETs were fine, but I'll still replace them. I may remove U9E2, U9E1, U9D2, and then test continuity to ground again on the voltage rails.

I keep trying to remove parts to sort out where the short is.

The more I think about it, the more I think that TIIIIINY IC with a resistor on it is the first stage of the gate controlling the on/off state of that power rail.

Look at that original XBOX One image (big image with standby voltages).

That tiny IC is connected to a smaller trace that comes from that big black thing at the bottom of section 9E of the motherboard. I bet that's the control turning it on/off.

I bet those MOSFETs were fine, but I'll still replace them. I may remove U9E2, U9E1, U9D2, and then test continuity to ground again on the voltage rails.

I keep trying to remove parts to sort out where the short is.

Hi,

Across precisely what points was 1 ohm measured? Were the probes across the through-hole pads of the capacitor? Or were the probes placed elsewhere? I'm wondering if you're just measuring across the same plane inadvertently (e.g. the ground plane or some different rail) - but it's a guess, because I don't know where the probes were positioned. To a multimeter, there is no difference between a zero-ohm measurement across the same power plane, and a measurement of 1 ohm. The margin of error is large with such a measurement, when the resistance is really low.

Had the MOSFETs been in a faulty shorted condition, then that could be confirmed by testing them removed, but you've done that and the short is still present. Whatever tiny IC you're referring to, could not have been gating the MOSFETs to a shorted state, because you tested for resistance with the power off. They could not be gating the MOSFETs with no voltage applied.

Also, I couldn't tell from the video (the first one) but it seems that power was applied with one of the MOSFETs unsoldered. That's not good, because if they are all different power rails, then it could damage things if one of the rails is deliberately left unpowered while the others come up. Hopefully the system quickly powers off to try to protect from that, but to deliberately power it up with that removed is still a big risk, for no benefit. There will be no power supply rail redundancy or anything like that in the xbox - it's a consumer item, they don't do that because it saves a cost in the tens of dollars, off the end price to the customer. (And even the expensive industrial equipment wouldn't implement such redundancy at the point-of-load as far as I'm aware - I could be wrong).

That's a good call. Anyway, it measures short (yes 0.6-1.2 ohms all over the power rails).

That's why I'm thinking the tiny IC is involved because even if I removed all of the MOSFETs and the capacitors it still registers a short all over the power rails except for anywhere it previously registered 12V. Those are OL (over limit).

https://www.youtube.com/watch?v=GzWI6J4IZn4

At the end of this video you can see me testing all over the board. In 5 minutes I'll post a screenshot with indications where it is short.

shabaz

The red dots represent a short. When I ground, I ground the black probe to the rings, or to one of the test points on the motherboard marked ground.

Testing the MOSFETs themselves free from the board, they register in the megaohms of resistance. I think they were fine. I could test all of them to be sure.

[edit] TO clarify... both when the MOSFETs were installed and now when they are no longer there, it still registers a short. (0.6-1.2 ohms, very low resistance)

If you get a chance, could you check the measurement directly across the capacitor holes (i.e. by flipping the PCB)?

Since each red location on the photo appears to have via stitching (I can't be sure, but it appears that way), it is possible to all be ground plane connections. Some circuit boards will do that (they will add more layers to be the ground plane, and some may use the top side partially for this too).

I tested from the big heat sink hole rings to the positive and the negative on the capacitor holes and got short on both. I assumed that would likely be a good ground point.

I can try measuring across tonight after work.

I see. In any case the red points are possibly connected to the ground plane I suspect - especially if this is the case across all three power sections. These are likely three separate rails, and it would be too much of a coincidence for all of them to have failed in this manner.

X86 (at least Intel) have pretty complex supply requirements (assuming this is x86 - I know nothing about xbox).

I don't know what the reliability of an xbox is, but the mechanical stuff (spinning drive) may have a very low reliability in comparison to the rest of the xbox - but I'm presuming that has been eliminated or confirmed working in some way (I didn't watch the entire videos).

shabaz Of course the hard drive was the first thing I tested. It works great and the partitions show fine in a Windows machine. (5 NTFS partitions I believe)

Yes XBOX is X86. It's literally just an AMD APU running Windows 10 specialized for a game console.

I just measured across the capacitor holes. (Caps are not present at the moment, tested them separately. They are fine. Around 820uF which is their marking.)

The capacitor holes from negative to positive result in:
C8E3: 1.7 ohms

C8E2: 1.7 ohms

C8E1: 0.8 ohms

C8D3: 0.8 ohms

C8D2: 0.8 ohms

C8D1: 0.8 ohms

Further testing:

R9E5: 2.4 ohms

R9E4: 2.3 ohms

R9E3: 1.2 ohms

RT9E1: 8000 ohms (8kohms)

TP9G1 GND --> TP9F1 CPUCORE: 1.7 ohms

TP9G1 GND --> TP9D2 GFXCORE: 0.8 ohms

The small chip may well be a MOSFET driver, the chances of them going bad are pretty slim.

As for your readings ... I'm not sure what that could mean - if not the MOSFETs, maybe that's actually current flowing through the actual CPU/GPU chips? I tend to avoid mindless probing across pads that aren't just across "one" component because there's always a chance of interference or current being driven into things which shouldn't be driven in such a way.

Does the resistance hold stable or does it change after a few seconds? Are there no non-obvious solder-splashes, or board getting pinched/crushed at locations (especially important for multi-layer)?

- Gough