Lenovo Legion Y540-15IRH, PQ201?

[mist79]

I am still new to this, so I might sound brainless/ignorant in parts of my description/questions, since I have a lot of gaps in my knowledge, but I will do my best.

The laptop is Lenovo Legion Y540-15IRH, and the motherboard model is NM-C221 Rev 2.0 (Schematics and BoardView can be easily found). To save time I have the exact problem in the laptop as the one shown in this video:
https://www.youtube.com/watch?v=tfucZ_WKpz4

I was checking the voltages I found that the main power rail V20B+ is good, and same for the +3VALW circut, but when I tried the +5VALW, the circuit was shorted and I think the problem came from one of these 2 MOSFETS PQ603 (AON7380_DFN8-5) and PQ604 (AON6380_DFN8-5), and while checking voltage with the multimeter on DC mode I made a mistake of touching two pins of the drain of PQ603 with the red probe, which caused more problems. And now I think there is a short even in the main power rail, since when checking for voltages I could see that there is voltage coming in to PQ201(AONS32314_DFN8-5) 20.5V, but no voltage coming out from that MOSFET or anything else afterward. My questions are:

-Will replacing PQ201 with the right component, fix the main power rail problem and at least revert the motherboard to the initial problem?
-Am I on the right track or should I look for the short somewhere else entirely?

I would really appreciate any help, thanks in advance. And if you need clarification or have any questions, I would be happy to reply.

[mon2]

Carefully, on PQ201 measure the voltages on pins 5-6-7-8 (any of them is ok) = DRAIN cluster of shorted pins to allow for heavy currents to pass through. This is where you are measuring the 20V from the power adapter.

(PQ201) Carefully measure the voltage to ground of pin 4 (GATE). Then finally the source pin cluster 1-2-3 (any of them is ok).

Post each measurement.

[mist79]

I re-measured the voltage to ground of PQ201 GATE (pin4) and it's 0V. And the voltage to ground of PQ201 Source (pin1) is also 0V

[mon2]

Remove all power. Meter in resistance mode.

Measure the resistance across the DCin mosfets @ PQ201 and also PQ202 as follows:

source (1-2-3) & drain (5-6-7-8) ; pick any of the pins in the group - place the meter probes across 1 & 8 (as an example)
source (1-2-3) & gate (4)
gate (4) & drain (5-6-7-8)

Repeat for PQ202. Post each measurement. We are checking if either of these mosfets is leaky (most likely yes).

[mcplslg123]

Wiser to measure resistance to GND on CLR just after 2 dc-in mosfets. To me,it seems like a shorted main rail.

[mist79]

For PQ201 resistance Between:
Source (Red Probe) & Drain (Black Probe)= 2.02
Source (Red Probe) & Gate (Black Probe) = 3.62
Gate (Black Probe) & Drain (Red Probe) = 5.41

For PQ202 resistance Between:
Source (Red Probe) & Drain (Black Probe)= 2.06
Source (Red Probe) & Gate (Black Probe) = 3.63
Gate (Black Probe) & Drain (Red Probe) = 5.33

Wiser to measure resistance to GND on CLR just after 2 dc-in mosfets. To me,it seems like a shorted main rail.

First CLR (Current Load Resistor) after the 2 DC-IN mosfets is PR201, and it resistance to Ground = 0.01

[mon2]

That is a hard short. Would you have an adjustable power supply? Use it at 1 volt (not higher) to inject onto this line and see who heats up.

[mist79]

I am still new to this,I don't have an adjustable power supply or a thermal camera.(I do have a soldering station with a heat gun tho)

What else can I do to identify this short otherwise?

[mon2]

Next suggestion is to logically view where this short may be present. Hope it is not the CPU but time to pull the band-aid off.

Meter in resistance mode (use lowest scale if it is a manual type) -> Measure the resistance to ground at the inductor group that is near the CPU. What is the resistance measurement? There should be multiple inductors in this area as the multiple power rails take turns supporting the power to the CPU so that not one is over worked. The CPU current demands are very high so multiphase power supplies are used.

[mist79]

I really appreciate you explaining what happening to me, and why we are doing what we are doing; I need that since I am just a beginner. So I am not sure if this is what you mean, but here is the measurements for the group of inductors that are close to the CPU (The boardview that is provided is not the exact same as the model I have, but close enough). Resistance to Ground, on the lowest scale, for:
PL3001 = 1.3
PL3002 = 1.2

PL 3003 & PL3004= 1.3
PL3021= 10.5
PL901 = 32.4


If these are not the components that I need to measure, plz take a look at schematic or the boardview and let me know which ones I am supposed to measure.

[mist79]

Next suggestion is to logically view where this short may be present. Hope it is not the CPU but time to pull the band-aid off.

Meter in resistance mode (use lowest scale if it is a manual type) -> Measure the resistance to ground at the inductor group that is near the CPU. What is the resistance measurement? There should be multiple inductors in this area as the multiple power rails take turns supporting the power to the CPU so that not one is over worked. The CPU current demands are very high so multiphase power supplies are used.

So does this mean I have a faulty CPU now?

[mon2]

No. Continue to check each and every inductor for the same resistance to ground as the one of the main power rail.

[mist79]

No. Continue to check each and every inductor for the same resistance to ground as the one of the main power rail.

Can you clarify what you said there? what do you mean by checking every inductor in the main power rail, can you specify which ones? (parts number from the schematic [password: Y540] so I can check it). and what do you mean by "for the same resistance to ground" what value are we looking for?

[mon2]

On every board, there are numerous power rails. These are often switching power supply rails. A switching power supply rail requires the use of an inductor (often grey in color and 2 leads). We are wanting to check each inductor's resistance to ground to see if there is a similar resistance to ground measurement. If there is not, then the fault is not on a high side mosfet. So far, I do not believe that your CPU is shorted. Ideally, you will find that it is a capacitor onboard that is defective and we can smoke it out with a voltage injection.

For now, continue to test each and every inductor in resistance mode to ground. If you find a similar low resistance reading - post the details - otherwise continue to test other inductors. This is only a cursory review of the power rails for other shorts.

[mist79]

Resistance to Ground for all the capacitors in the motherboard (this took a while):

PL101=44.1kΩ
PL102=44.1kΩ
PL103=44.1kΩ
PL104=61.2kΩ
PL105=61.2kΩ
PL201=2.3MΩ (pin1)
PL601= ~0.7Ω (can't get a clear reading)
PL602=16.3 kΩ

PL901= 32.1Ω
PL1401=160Ω
PL3001= 1.4Ω
PL3002= 1.4Ω
PL3003= 1.4Ω
PL3004= 1.4Ω
PL3102= removed, but 2.5Ω from one pin
PL3201= 10.3Ω

PL7301= 0.3Ω
PL7302= 0.3Ω
PL7401= 28.3kΩ (pin2)
PL7402= 12.7Ω
PL7501= 0.3Ω
PL7502= 0.3Ω
PL7503= 0.3Ω
PL2101= 42.8Ω

[mon2]

PL601= ~0.7Ω (can't get a clear reading)

This one is in trouble. Double check this resistance to ground again.

[mist79]

I just did, it starts at 0.7Ω and goes down to 0.3-0.2Ω every time. Do you think replacing this capacitor will fix the problem we have with the main power rail?

[mon2]

The shorted part could be anywhere on the board that makes use of the (+5VALW) power rail created by this circuit.

Flux and carefully lift up one leg or both on PL601. Then you can test the resistance again to determine if the shorted part is on the producer side (PU601) or the consumer side of this power rail.

Do inspect the caps nearby PL601 to see if they look defective or burnt out.

[mist79]

Then you can test the resistance again to determine if the shorted part is on the producer side (PU601) or the consumer side of this power rail.

Before I remove PL601 can you explain to me what you mean by that? test the resistance how? of PL601 to Ground?

And nothing seems burnt out to me near PL601 from both sides of the board.

[mon2]

Flux and carefully lift up one leg of the electrolytic cap at PC616 that is to the left of this inductor.

Then measure the resistance to ground again of this power rail.