Lenovo Ideapad 100-15IBD turns on for a second and stops

Hello everyone!
I have already seen several posts with this same symptom, but all in the air and without a solution.
The laptop is a Lenovo Ideapad 100-15IBD with NM-A681 motherboard, which after powering on and pressing the power button, tries to start up but after 1-2 seconds it turns off reaching a maximum consumption in the power supply of about 80mA.
After several checks I found that in the Pq40 mosfet, there is a short between S-G.
After opening PR365, the short disappeared, so I proceeded to replace PU908.
After soldering the new chip and putting PR365 back, before powering on there is no short between S-G.
After powering on and pressing the power button, the symptom is the same again, it turns on for a second and turns off, and after checking, there is again a short between S-G in the PQ40.
I have desoldered PU908 again and the short has disappeared between PQ40.
Any ideas? PU908 is currently removed from the motherboard

Zemog ha scritto:Pq40 mosfet, there is a short between S-G.

Presuming that there is no physical short to PQ40, it seems that PU908 goes into protection mode, shorting HG output to SW pin and preventing PQ40 from activating. What is the resistance to ground on PL9?

Vogelek23 ha scritto:Suponiendo que no hay un cortocircuito físico con PQ40,

PQ40 is not shorted and is working fine

Vogelek23 ha scritto:¿Cuál es la resistencia a tierra en PL9?

30,5 Ohm

Please measure the voltages to ground on all power coils when the logic board is switched on (you may need to switch the board on several times to take all measurements). After that, please disconnect all the power, wait 1-2 minutes and measure the resistances to ground on the same coils.

I have soldered a new PU908 chip, because it was removed, and I have made the indicated measurements

PL303 (+3VALW) ... 3,37V .... 24kΩ
PL304 (+5VALW) ... 5,1V ... 80kΩ
PL501 (+1,35V) .... 1,33V ... 212Ω
PL701 (+1,05VS) ... 1,04V ... 160Ω
PL705 / PL706 (+VGA_CORE)... They are not present.
PL9 (CPU_CORE) ... 1,69V ... 30Ω

Zemog ha scritto:PL9 (CPU_CORE) ... 1,69V ... 30Ω

So it seems that all the voltages, including CPU_CORE, start properly. Therefore, the current consumption should be higher than 80mA. Please remeasure the current consumption after the board has been switched on.

Vogelek23 ha scritto:Vuelva a medir el consumo actual después de encender la placa.

Consumption remains at 80mA.
The 80mA consumption coincides with the fan's rotational speed.

Do you have the battery connected? If so, please disconnect it and remeasure the current consumption. What kind of power supply do you use? Is it a bench power supply with a dedicated Lenovo adapter? If so, what is the resistance between ground pin and central (ID) pin on that adapter (measured on bare adapter)? With all SMPS-es turned on, including CPU core supply, there should definitely be a significantly higher current consumption.

Vogelek23 ha scritto:Do you have the battery connected? If so, please disconnect it and remeasure the current consumption

The battery is disconnected. Only the motherboard with heatsink and CPU fan is there.

What kind of power supply do you use? Is it a bench power supply with a dedicated Lenovo adapter?
I connect the power supply to the motherboard via a cable taken from an original Lenovo power supply.
Anyway, using an original Lenovo power supply, the symptom is the same, with the difference that I cannot see the consumption. I could insert an ammeter between the power supply and the motherboard, to see the consumption with the original power supply.

Now I'm thinking about a history of other repairs with the same symptom. I've had a similar problem before, and the solution was to replace one of the input mosfet, the one that gives way to the B+ line, which, on other occasions, although the mosfet was working, for some reason it didn't let all the current through and the equipment had the same symptom. After replacing the equipment they worked fine.
In this case they would be PQ301 and PQ302. I'm going to look to see if I have them or can rescue them from another motherboard, and I'll try to replace them.
The power supply is an ATTEN APS3005S-3D adjustable power supply

Zemog ha scritto:cable taken from an original Lenovo power supply.

What is the resistance between central (ID) pin and GND wire on bare cable (not connected anywhere on both sides)?

Zemog ha scritto:although the mosfet was working, for some reason it didn't let all the current through

Measure the voltages on all pins of PQ301 & PQ302 while the motherboard is switched on and takes 80mA.

Vogelek23 ha scritto:What is the resistance between central (ID) pin and GND wire on bare cable (not connected anywhere on both sides)?

The connector is 4.0mm x 1.7mm type.
In a bare cable with no connection anywhere, the resistance between the central in and GND is either open or infinite.

Vogelek23 ha scritto:Measure the voltages on all pins of PQ301 & PQ302 while the motherboard is switched on and takes 80mA.

PQ301: S... 20,1V G....5V D... 20,1V
PQ302: S...20,1V G... 5V D... 20,1V

When I connect the power and after turning the fan on for a second, before pressing the power button, there is already 20V on Drain of PQ302. Before pressing the power button the voltage here should be 0V, right?
Moreover, before pressing the power button, all voltages on PQ301 and PQ302 are the same as after pressing the button.

Zemog ha scritto:Before pressing the power button the voltage here should be 0V, right?

No. There should be 20V as long as the charger is connected.

Zemog ha scritto:Moreover, before pressing the power button, all voltages on PQ301 and PQ302 are the same as after pressing the button.

This is correct with the charger connected.

Zemog ha scritto:The connector is 4.0mm x 1.7mm type.

So it is just the classic 2-pin barrel DC jack. All of that leads to the conclusion that the CPU is very likely faulty.

But shouldn't there be a little more consumption than 80mA?