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)

[Sofian75]

JEDP1

[lotas]

INVT_PWM = 0.126V - The problem is that the power supply to the inverter is too low.

[Sofian75]

INVT_PWM = 0.126V - The problem is that the power supply to the inverter is too low.

It comes from UC4.
Is there a way to check if the chip is dead?

Should I try to bridge pin 4 and pin 5 of UC4?

[lotas]

The matrix loop (cable) itself is fine, did you use a multimeter to test this line from the chip to the matrix?
Does the signal from the CPU go to 2 pin SN74LV1T125?
And what is the resistance of this INVT_PWM line relative to ground?

[Sofian75]

The matrix loop (cable) itself is fine, did you use a multimeter to test this line from the chip to the matrix?
Does the signal from the CPU go to 2 pin SN74LV1T125?
And what is the resistance of this INVT_PWM line relative to ground?

What is the matrix?

I have 0.126v at pin 2, same thing than pin 4.

I checked the resistance to the ground of UC4:

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

Shorted?

[lotas]

What is the matrix?



Shorted?

Panel (screen).
Yes, low resistance, disconnect the cable and check which side is short (chip side or cable/panel side)
Maybe the cable itself is broken, the insulation at the bend is damaged and has a short to ground...

[Sofian75]

Panel (screen).
Yes, low resistance, disconnect the cable and check which side is short (chip side or cable/panel side)
Maybe the cable itself is broken, the insulation at the bend is damaged and has a short to ground...

Same resistance on both side with screen connected or disconnected.

The only other thing on the INVT_PWM line is RV6.
Should I desolder it and check again the resistance to the ground on the line?

Is there another way to check if either UC4 or RV6 is the one shorted?
Voltage injection on this line?

[lotas]

This is definitely a resistor?, maybe it's a ceramic capacitor, if so, then it can short to ground, remove it from the beginning, if the short does not go away, then remove the chip.


Is this GND?

[lotas]

del

[Sofian75]

This is definitely a resistor?, maybe it's a ceramic capacitor, if so, then it can short to ground, remove it from the beginning, if the short does not go away, then remove the chip.


Is this GND?

Yes according to the boardview it's a 100k resistance, I removed it, check the resistance and it was fine, 100K, and of course the short was still there.
Next I removed UC4 and the short on the line was gone.

With the chip removed I was not worried of damaging the cpu anymore so I bridged pin 4 and pin 5, now the screen is working but as expected it's at 100% brightness and moving the slider in windows doesn't do anything, I ll have to order the chip.

Thanks for your help 👍

[lotas]

Ok!