Motherboard LA-B972P it has no backlight

[resistor83]

hi everyone, I have this hp laptop, motherboard LA-B972P that hasn't had the backlight from a few days. However, you can see the image through the flashlight but without backlight.
I also tried to change the monitor but the backlight is not present. It can only be seen through a flashlight. So I guess there is a problem on the motherboard.
What measurements could I start taking?

Thank you

[mon2]

Re: Motherboard LA-B972P it has no backlight

What is the voltage readings of the following ?

INVPWR_B+

[resistor83]

Re: Motherboard LA-B972P it has no backlight

INVPWR_B+ 19,60v

[ahmarzo]

Re: Motherboard LA-B972P it has no backlight

Check 3v and 19 v on led circuit

[mcplslg123]

Re: Motherboard LA-B972P it has no backlight

UG1-pin1 Voltage??

[resistor83]

Re: Motherboard LA-B972P it has no backlight

UG1-pin1 Voltage??

UG1-pin1 3,31V

[JaroTech]

Re: Motherboard LA-B972P it has no backlight

Hi, what voltage do you have on pin 23 of your LCD connector?

[mcplslg123]

Re: Motherboard LA-B972P it has no backlight

Pin24 also

[resistor83]

Re: Motherboard LA-B972P it has no backlight

pin 24 3,35V
pin 23 0,07V

[JaroTech]

Re: Motherboard LA-B972P it has no backlight

What voltages on pins 1, 2, 4 and 5 of UT3.

[resistor83]

Re: Motherboard LA-B972P it has no backlight

What voltages on pins 1, 2, 4 and 5 of UT3.

pin1 3,35V
pin2 3,30V
pin4 0,07V
pin5 3,37V

[mon2]

Re: Motherboard LA-B972P it has no backlight

pin1 3,35V
pin2 3,30V
pin4 0,07V
pin5 3,37V

Pin #4 is the wrong output value. This is a push-pull, 2-input AND logic gate.

It is being powered (pin # 5).
Both inputs are 3v3 = '1'.

So pin # 4 should be also 3v3 = '1' unless something is pulling it down.

Remove all power. Meter in resistance mode.

Check the resistance to GROUND of pin # 4 - what is the measurement ?

If the resistance is not very low, then this component is defective and should be replaced.

[mcplslg123]

Re: Motherboard LA-B972P it has no backlight

Yes, agree with @Mon2's observation. However, you can test if display comes on screen by bridging between pin23-24 . This is just a quick fix and not solution in perfect sense.

[resistor83]

Re: Motherboard LA-B972P it has no backlight

pin4 2,4ohm

making the bridge between pin 23 and pin 24 the display turns on!

[mcplslg123]

Re: Motherboard LA-B972P it has no backlight

pin4 2,4ohm

making the bridge between pin 23 and pin 24 the display turns on!

Glad to know that.

[resistor83]

Re: Motherboard LA-B972P it has no backlight

so can i keep it with the bridge?
UT3 needs to be replaced or can I keep it?

[mcplslg123]

Re: Motherboard LA-B972P it has no backlight

You can keep it this way. It wont harm you.

[resistor83]

Re: Motherboard LA-B972P it has no backlight

thank you @mon2 and @mcplslg123

just another question:
I realized that if I only use the battery I always have the backlight off.
The video drivers are already up to date and the brightness is at maximum

What could it be?

[mcplslg123]

Re: Motherboard LA-B972P it has no backlight

Brigtness will be at maximum and you cant adjust it with this fix. It should have worked with battery alone also. You need to measure those same pins with battery power only(laptop turned on obviously).

[resistor83]

Re: Motherboard LA-B972P it has no backlight

sorry for the delay, i had a very busy week at work.

I made the measurements with battery only:

INVPWR_B + 15.85v ***

UG1: 3.31V

pin23 24 LVDS: 2.85V ***

UT3:

pin1 3.35V
pin2 3.30V
pin4 0.14V ***
pin5 3.37V


I noticed that the brightness only disappears when I am inside windows. If I stay in the bios the brightness remains.
I mean with only the battery connected