Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

[gecce1978]

Hi guys, The problem of lenovo laptop is faint screen(backlight) lenovo logo font looks too dark..
my question is : I have attached the pictures of the motherboard,Which areas do I need to check..which component is controlling the backlight...
Note:screen and cable absolutely solid...

[mon2]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

see attached and check the +20V feeds for the LCD backlight - believe these will be of interest:

a) PQ1007 which you can read @ PJ1002. Use your meter in volts mode, with red lead on PJ1002. Black lead on a good ground on the logic board. Post your reading.

b) PU902 appears to be the boost regulator that takes in +5volts and boosts it to +20V for the backlight LEDs.

Pins 1 & 2 on the LCD connector = LEDVDD. Tracing LEDVDD rail you can see it is connected to +20V.

Confirm the voltage of LEDVDD rail. If this rail is not +20V then this could be the issue that is causing the backlight to not behave normally.

Start with the above..

c) just added the boardviews of the board

UMA version

DIS version

[gecce1978]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

thank you so much 🙏

Sir,How can I test it with a multimeter?

[gecce1978]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

Check Pq1005 mosfet ?

[mon2]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

Carefully check the voltage on pin 1 of the lcd connector and post the voltage your meter reports.

Use DC volts setting, red lead on pin #1, black on ground.

Best to do this with the screen attached but as noted, be very careful to not slip on the pins.

[Sametbey]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

You should see + 20V on 1 and 2 pins of the Panel socket on the motherboard, + 3.3V on pin 14 and pin 16.

[gecce1978]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

Monday I will look and information,thank you very much for helping me.. 🙏

[gecce1978]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

No 3.3v on 14th and 16th legs

[gecce1978]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

1st and 2nd legs come in 20 v

[mon2]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

What is the exact voltage on lcd connector, pin 14?

This is an active low signal. If logic low then display will remain OFF.

[gecce1978]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

no signal

[gecce1978]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

lenovo text looks faint

[Sametbey]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

LVDS 14pin controlling 119pin of BL_ON IO chip. 16pin INV_BL is controlling processor via R19. R18 must be + 3.3V on one leg and 2.6V on the other. R19 can be removed. does not control luminosity.

[gecce1978]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

Samet bey,what is r19 ?

[Sametbey]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

Check out the boardview file. It coincides with the back of the LVDS socket.

[stsoukis92]

Re: Please help ! Lenovo dg421 dg521 dg721 nm-b242 rev 1.0

Hello!I have the same motheboard!When i plug the charger, the laptop is taking around 0.23A for few seconds and next 0.0!I have already checked with multimeter,i have 20v on the input and to the capacitors aroung the power supplies.I have also checked the voltage around the sio chip and nowhere exist 3.3v!