Lenovo Ideapad 3 17ALC6 - No Backlight

[Sofian75]

I need help please.
I purchased a new screen for this laptop, I do see a very dim picture but the backlight isn't working.
I tested the screen with another laptop and it's working fine, so I am trying to figure out what's wrong with this motherboard.

These are the measurement I ve taken:

+LEDVDD = 19V
DISPOFF_N = 3.3V
INVT_PWM = 0.126V
CPU_EDP_HPD = 2.42V
+LCDVDD_CON = 3.3V


Any idea what should I check next?
Could the problem be with the bios?

(Bios, boardview and schematic attached)

[SMDFlea]

Post links to the schematic/boardview. Reminder: rules and organization of this section - https://www.badcaps.net/forum/troubl...f-this-section

If you want someone to check your bios please post your request in the "Bios requests ONLY!" section - https://www.badcaps.net/forum/troubl...-requests-only

[Sofian75]

I thought I had included the boardview and schematic.

LINK to schematic/boardview ------> https://www.badcaps.net/forum/troubl...-hs761-nm-d521

[reformatt]

Connect to external monitor via HDMI. Check brightness isn't turned all the way down to minimum.

[Sofian75]

Connect to external monitor via HDMI. Check brightness isn't turned all the way down to minimum.

Nope, it was already at 100% brightness.
Moving the slider either way doesn't have any impact.

[reformatt]

DISPOFF_N is 3.3V. I'm thinking this needs to be pulled low by EC_BKOFF_N which comes from the EC. If you pull this line low at RV3, does the backlight come on? May still be dim, as brightness is controlled by the INVT_PWM (should be a square wave of sorts, pulse width varies depending on brightness).

Only thing that comes to mind that might cause that is the lid switch. Is that high or low?

[Sofian75]

DISPOFF_N is 3.3V. I'm thinking this needs to be pulled low by EC_BKOFF_N which comes from the EC. If you pull this line low at RV3, does the backlight come on? May still be dim, as brightness is controlled by the INVT_PWM (should be a square wave of sorts, pulse width varies depending on brightness).

Only thing that comes to mind that might cause that is the lid switch. Is that high or low?

LID_OD_SW_N is at 0v when power in on. (3.3 when off)

I took measurement on another working Lenovo laptop (530s-14arr) and the difference is that INVT_PWM is 2.24V instead of 0.126v.

INVT_PWN comes from UC4 pin 4, the 0.126V output at pin 4 is the same than the input at pin 2 from the CPU, shouldn't it be closer to the 3.3V of pin 5?

[reformatt]

It’s a PWM waveform so you need a scope. Duty cycle should vary as you adjust brightness up and down.

[Sofian75]

I don't have that.

But I connect it to a monitor using the hmdi, booted windows and changed the brightness to 50%

INVT_PWM went from 0.126 to 0.024V.

Tell me if I am wrong but it seems to me like UC4 is supposed to use the signal at pin 2 from the cpu to modulate the 3.3V at pin 5.
Instead of that I have the same voltage at pin 2 and pin 4. This is why I was suspecting that UC4 is dead:

Resistance to the ground of UC4:

Pin 2 = 75K ohm
Pin 5 = 3.6K ohm
Pin 4 = 2.8 ohm

Is there any risk to damage something if I bridge pin 4 and pin 5?

[reformatt]

UC4 is a level shifter and a buffer. Tying the output high, not sure but it would only affect the screen.

[Sofian75]

UC4 is a level shifter and a buffer. Tying the output high, not sure but it would only affect the screen.

UC4 was indeed shorted, after removing it and bridging pin 4 and 5 the screen is working, stuck at full brightness until I receive a new chip.
Thanks for your help.