MB no turn on, apparent damage but voltage, but none from voltage reg IC - ThinkPad P17 MB NM-C651 - what next?

[wraster]

Hi, I have a ThinkPad P17 motherboard with intel chipset, NM-C651 that appears to not power on.

- No light with DC plugged in, no signal to external monitor connected directly to HDMI on motherboard (no GPU module attached).
- There appears to be damage on a ferrite bead chip (pic here) on the main power rail (20 V), but there is a 20V reading to the in-side, and 15V reading on the out-side. After this chip, the power goes to an IC that I think regulates voltage (NCP81210MNTXG_QFN40, PU1301 in the boardview screenshot here), but I read no voltage on pin 7, nor 11-13.
- After this IC in the circuit is the inductor just physically below it (down in the boardview screenshot here - PL1302) which also reads no voltage on either side.

My questions are:

Does this suggest that this IC is bunk? Or could the problem be with how the ferrite bead chip is changing the power signal?
What do you recommend next to diagnose, or move on to try replacing one or both of these components?

Much thanks

[mcplslg123]

First step is to remove the coiland then measure resistance to GND on both ends. One end is very likely shorted. If indeed shorted, then remove components on that line one by one to remove shorting or u can inject 1V/5A on the shorted side of coil.

[wraster]

Thanks -The coil you mean is the burned-looking chip, right? PL1301? or PL1302, the bigger one?

[mcplslg123]

PL1301

[wraster]

First step is to remove the coiland then measure resistance to GND on both ends. One end is very likely shorted. If indeed shorted, then remove components on that line one by one to remove shorting or u can inject 1V/5A on the shorted side of coil.

mcplslg123 can you explain a little about the logic behind your proposal? I am very new to this but understand the basics of electronic circuits and components. What physically could likely be causing a short here? Malformed tracks within the plastic board? Or are you thinking something internal to some component?

[mon2]

Most likely it will be a shorted component. What is known at this time is that the inductor that is in series of this load is the inductor @ PL1301. It is now defective / burnt from the excessive current draw. Now, this part must be fluxed and removed with tweezers. Then with no power to the board, test the resistance to ground on each side of the PCB pads. Need to know if the low resistance that caused to burn out this inductor is on the producer side (regulator side) or the consumer side (load / downstream). Post the exact, measurements in ohms for your tests.

[wraster]

I understand, thank you!

My summary is attached.

I removed the part and measured resistance to ground on both pads:

1. Upstream pad: 1.5M Ω
2. Downstream pad (connects to IC): more Ω than my multimeter can measure


I discovered that one of the nearest capacitors, closer to the power source, was completely missing. I used hot air to remove the inductor and while this missing capacitor should have been on the other side of the board, it's not right underneath and I wasn't blasting that area with hot air directly, so while it's possible that it fell off while I was desoldering the other component, I don't think it was. I measured resistance to ground of both pads where this capacitor would be and:

1. 170k Ω
2. 10 Ω

That side of the capacitor connects to ground in the figure so that makes sense, right?

So is the 1.5M Ω to ground from the inductor less than expected?

[wraster]

Hi mon2 or mcplslg123 can you share your insights on the measurements from my board? Does it indicate to you whether there is a short on one side or another? Thank you!

Any thoughts on the apparent missing capacitor and it's possible contribution to the source of the problem?

[mon2]

Is your meter manual or automatic scaling for the resistance mode? If manual, switch to a higher scale to take a measurement on the pad that is reporting 'OL' = over limit on most meters.

What is the resistance across the coil PL1301 that was removed? Place the meter probes on each end of this removed component. The burn marks indicate that high current has passed through this part or someone with a high temp soldering iron has touched each of the ends of this part. If no one has worked on this board, then recommend to replace this part. Technically the part is not required but is acting like an EMI noise filter. In a pinch, the pads could just be shorted to allow for the 20V to travel to the IC. Otherwise, Digikey / Mouser should offer a replacement for the component.

https://www.digikey.com/en/products/...KgHIoAiIAugL5A
* order a few extras

Share the meter face pic of your meter for a quick review for the setting that is reporting a high resistance to ground on the IC side. That is, perhaps you will need to order other parts so wait till more is known about this board.