Lenovo Legion NM-F771 No turn on

[judamex]

Hi, I have a legion nm-f771 that does not turn on.

-when injecting voltage with the power supply it only consumes 0.1A, but it is very fast as if it were turning on and off very quickly
-the 20V rail has 3.6V and goes up and down very quickly from 3.6 to 4.5
-I changed the charging IC without success
-I checked ACDRV, CSMSRC and the voltages are correct
-I removed the main input mosfet and the BQ24780S behaves correctly sending the 20V correctly
-the main input mosfet sends good measurements outside the motherboard

I am a little lost in this, I would appreciate the support, there are no schematics or boardview available and I am relying on a very similar nm-D601 mother board

[mon2]

Solder back all the parts. No bypass wires.

Meter in DC voltage mode. Attach the power adapter. Carefully measure the voltage to ground of each pin on the charger ic. Post each measurement with their pin number and function in a list for a review. Need exact values.

[judamex]

Solder back all the parts. No bypass wires.

Meter in DC voltage mode. Attach the power adapter. Carefully measure the voltage to ground of each pin on the charger ic. Post each measurement with their pin number and function in a list for a review. Need exact values.

The measurements are as follows
I put pins 1 and 2 like this, because they change voltage very quickly from 3 to 4

[mon2]

Forgot to include the pin functions in the table.

Is this list with or without the battery?

[judamex]

Forgot to include the pin functions in the table.

Is this list with or without the battery?

Corrected list, measurements are without battery connected, no peripherals and corresponds to a BQ24780S

[mon2]

Thanks. Remove all power. Meter in resistance mode.

Measure the resistance to ground on pin 1 of the charger ic. Checking if the main power rail is shorted.

Measure the resistance on pins 1 & 2 on the charger ic. Checking if the current sense loop of the main power rail is ok or not. Measure directly on the charger ic pins.

[mcplslg123]

Remove the 2nd dc-in mosfet and apply power .Does pin4 of 1st mosfet is now 25V?

[judamex]

Thanks. Remove all power. Meter in resistance mode.

Measure the resistance to ground on pin 1 of the charger ic. Checking if the main power rail is shorted.

Measure the resistance on pins 1 & 2 on the charger ic. Checking if the current sense loop of the main power rail is ok or not. Measure directly on the charger ic pins.

measurements of pin 1 and 2 of charger ic

black terminal to ground and red to pin 1 and 2 gives 30k and going up

red terminal to ground and black to pin 1 and 2 gives 30k and going up

[judamex]

Remove the 2nd dc-in mosfet and apply power .Does pin4 of 1st mosfet is now 25V?

i remove the sence resistor and give me a 20V

[judamex]

bump

[mon2]

20v on pin 4 of first mosfet?

[mcplslg123]

What you have written is just half the info reqd. So if u remove CLR,20V is reaching CLR but what's the voltage on pin4 of dc-in fet?
Whats the resistance to GND on CLR pin 3-4(where u have zero volt)?

[judamex]

20v on pin 4 of first mosfet?

The first p-channel mosfet has a voltage of 8.9 and the second n-channel has 25v, both on pin 4. In the following image I explain it better

[judamex]

What you have written is just half the info reqd. So if u remove CLR,20V is reaching CLR but what's the voltage on pin4 of dc-in fet?
Whats the resistance to GND on CLR pin 3-4(where u have zero volt)?

These are the measurements on the entry line

[mon2]

These look fine.

Now remove all power. Meter in resistance mode. Measure the resistance to ground on pin # 1 of the charger IC. Then repeat to measure pin # 2 to ground. The current sense resistor is between pins # 1 and # 2 on this charger IC. We need to know the resistance to understand why there is a large voltage drop in the power path.

[judamex]

These look fine.

Now remove all power. Meter in resistance mode. Measure the resistance to ground on pin # 1 of the charger IC. Then repeat to measure pin # 2 to ground. The current sense resistor is between pins # 1 and # 2 on this charger IC. We need to know the resistance to understand why there is a large voltage drop in the power path.

The measurements look good, I took them with the resistance outside the board like the previous image.

[mcplslg123]

No power attached,resistance between pin1-2 of charging IC? Voltage on all pins of PD5401?

[judamex]

No power attached,resistance between pin1-2 of charging IC? Voltage on all pins of PD5401?

The measurement between pin 1 and 2 is 4.2Mohm, Voltage on PD5401

[mcplslg123]

I beleive you mean 4.2 milli ohms and not mega ohms. Correct? If thats correct,then nothing wrong here.

[judamex]

I beleive you mean 4.2 milli ohms and not mega ohms. Correct? If thats correct,then nothing wrong here.

Yes, they are Megaohms.
I put the pr5401 resistor back and the problem returned, without it installed the entire charging system seems to work normally.