Lenovo z50-75 (NM-A291 REV 1.0) turns on for one second then shuts down

Hello everyone
I am repairing a lenovo z50-75 with a mother (nm-a291 rev 1.0).
The problem is when i press the power button it turns on for one second and the screen flash very quick then shut down.
I had opened the laptop and disconnected the lcd connector then pressed the power button and it turned on. Connect the motherboard to external tv with hdmi cable and i got picture. After a few minutes it shutdown again.
With visual inspection i had noticed that capacitor (pc81) is damaged, i had replaced it but nothing changed. With further more inspection i found near the capacitor that(pu5) is very hot even if the board is not on. I do not know what is pu5 and what is its function. Therefore i am asking you what to do next.

I do not know what is pu5 and what is its function

Measure the resistance to ground on pins 1,2,5,7,10 PU5

Lesio23 napisał(a):

I do not know what is pu5 and what is its function

Measure the resistance to ground on pins 1,2,5,7,10 PU5

Can you give me the part number so i can see the datasheet to know the pins?
I tried to Google the number written on it (Nd4ef) but no results

Chip PU5 is SY8206BQNC. Pinout is the following:




Measure the voltage to ground on coil PL6, when the laptop is turned on.

SY8206B

hussam napisał(a):

Lesio23 napisał(a):

I do not know what is pu5 and what is its function

Measure the resistance to ground on pins 1,2,5,7,10 PU5

Can you give me the part number so i can see the datasheet to know the pins?
I tried to Google the number written on it (Nd4ef) but no results

With visual inspection i had noticed that capacitor (pc81) is damaged

Also check if it is damaged RT8231AGQW

Lesio23 napisał(a):

I do not know what is pu5 and what is its function

Measure the resistance to ground on pins 1,2,5,7,10 PU5

Pu5 Pin #
1=355kohm
2=o.L
5=380kohm
7=3.7Mohm
10=25kohm

Voltage to ground on pl6 =1.35 v

Measure the voltage to ground on pin 5 of PU5, when the laptop is turned on.
Measure the voltage to ground on coil PL4, when the laptop is turned on.

matic napisał(a):Measure the voltage to ground on pin 5 of PU5, when the laptop is turned on.
Measure the voltage to ground on coil PL4, when the laptop is turned on.

Voltage to ground on pin 5 =3.3v
Voltage to ground on pl4=3.3v

Measure the voltages on all the coils.

All voltages seems ok.

Are you sure that the hot component is chip PU5 itself and not some other component near/under (on the bottom side of the board) it?

matic napisał(a):All voltages seems ok.

Are you sure that the hot component is chip PU5 itself and not some other component near/under (on the bottom side of the board) it?

Yes I am pretty sure that pu5 is heating up. My fingers still hurt me because it was very hot. Yesterday i let the board till today morning i had double checked all the readings then I fired it up. Guess what, it is working and the lcd also worked. I touched pu5 it was very cool no more heat. I will test the laptop few more hours and see if the problem will come back

Do you use for measure always the same power supply?.

Lesio23 napisał(a):Do you use for measure always the same power supply?.

Yes

The laptop now works fine with a small problem.
When the battery is mounted and the charger is plugged in the fan spin at high speeds and the charging indicator is always on(white) even if i disconnect the charger the charging light still on and white also when the battery is empty the light is white (but the battery chargs as normal). once i remove the battery and power it up with the charger, everything work fine and the fan back to normal operating.

Measure the voltages on all pins of the battery connector with and without the battery, connected power supply.

Machine napisał(a):Measure the voltages on all pins of the battery connector with and without the battery, connected power supply.

Without battery
1,98
1,98
3.3
3.3
3.3

With battery
15.7
15.7
3.3
3.3
0.3 ???