Dell Latitude 5285 no backlight

[redoxy]

Motherboard is LA-F371P Rev. 1.0
Schematics and Boardview attached. Boardview is in Allegro format.

MOD Edit: Schematic & Boardview copied and moved to new thread in "Schematic requests ONLY!" sub forum - https://www.badcaps.net/forum/showthread.php?t=122332
Reminder , rules and organization of this section - https://www.badcaps.net/forum/showthread.php?t=117483

Dropped laptop and shattered screen. Purchased and installed a replacement screen. Display is visible under flashlight but backlight isn't turning on.

Measured eDP Backlight Power B+= 7.95V at R502 with battery and power adapter connected.

Any help on what to measure or how to address this?

Thanks in advance.

[Techtiger]

Re: Dell Latitude 5285 no backlight

Supply on +LED_AN Only with adapter connected ? +INV_PWR_SRC supply? Supply on EN_INVPWR as you press PWBTWN?

[redoxy]

Re: Dell Latitude 5285 no backlight

Supply on +LED_AN Only with adapter connected ? +INV_PWR_SRC supply? Supply on EN_INVPWR as you press PWBTWN?

Thanks, Techtiger for the quick response.

+LED_AN = 0.380V with only power adapter connected (measured at PR410)

+INV_PWR_SRC = 8.84V with only power adapter connected and 8.00V with power adapter and battery connected (measured at PC400)

EN_INVPWR = 3.32V while pressing PWBTWN (measured at Q32 pin 2)

[Techtiger]

Re: Dell Latitude 5285 no backlight

+LED_AN = 0.380V voltage is low, what is the resistance to ground pin 19 20 21 any one JEDP1 ? voltage on PD400 both side ? +B voltage PR310 with power adapter

[redoxy]

Re: Dell Latitude 5285 no backlight

My apologies to the MODs for my ignorance in attaching the schematics and boardview in this thread instead of the dedicated subforum. I will ensure not to repeat this.

Thanks again Techtiger.

Resistance to ground pin 19 JEDP1 = 0.989M Ohms.

Voltage on PD400 with power adapter: pin 1=8.82V and pin 2=8.85V

+B voltage on PR310 with power adapter = 8.85V

[Boardsurfer]

Re: Dell Latitude 5285 no backlight

Hello. This model is one of the few Dell boards that does backlight boost circuit on the main board as opposed to another PCB normally attached to LCD itself. Look at PU400 which is backlight driver IC. this video demonstrates a good method for troubleshooting this circuit despite that it's for a macbook. Hope this helps.

https://www.youtube.com/watch?v=bJOXv_nAGgc

[redoxy]

Re: Dell Latitude 5285 no backlight

Thanks, Boardsurfer for the reply.
This s very helpful. I have been watching a few youtube channels to on no LED backlight most ended up being a blown fuse.
The link you provided has some great information.

So far I'm thinking PU400 is bad. A search of digikey mouser doesn't show the part in stock and no alternative parts are provided. Digikey describes it s prototype.

[Boardsurfer]

Re: Dell Latitude 5285 no backlight

Please tell me the Voltage at +LED_AN again. I noticed you said +LED_AN was .380V in the second post but then later you said PD400 was 8.8 both sides?? They should be the same reading so not sure what happened there. If you have nothing at all at PD400 then it's probably an issue with enable which is page 22 component Q33 (EN_INVPWR). If you have 8.8V at PD400 it's likely to be the driver IC assuming PWM is present.

[redoxy]

Re: Dell Latitude 5285 no backlight

Thanks again Boardsurfer.
I took the measurements again and
+LED_AN = 0.35V measured at PU400 PIN 22
LED_PWN = 3.04V measured at PU400 PIN 1

Also measured voltages on Q33:
Pins 1,2,5,6 all = 8.04V
Pin 3 = 0.74V
Pin 4 = 8.06V

All measurements were taken with battery installed and power adapter connected.

[Boardsurfer]

Re: Dell Latitude 5285 no backlight

LED_AN should be measured at PC403 , PC404 , PC405 , PC418 , PC406 or pin 1 of PD400. You could be getting an ambiguous reading at pin22 of PU400 due to issue with chip and the fact that there's a 1M resistor in-between LED_AN and pin22. We know enable is working because you have 8.8 on both source and drain on Q33. Make sure screen is open while taking readings because Hal or angle sensor could de-trigger enable signal. If you have 8.8 at LED_AN at pin 1 of PD400 then it's highly probable PU400 needs to be replaced.

[redoxy]

Re: Dell Latitude 5285 no backlight

Thanks!
LED_AN = 8.10V when measure at PC403.

I will try to see if I can find the chip. Since Mouser and Digikey do not stock, I'll have to get if from eBay or Aliexpress and trust that the parts are original.

Is there a way to find an alternative part?

I appreciate the help.

[Boardsurfer]

Re: Dell Latitude 5285 no backlight

Okay great I had a feeling that was the issue. I'm not sure of any alternate parts but I'm pretty sure I have one of these on a donor board at my work and if you'd like I can mail it to you. Won't be back in the office till Monday however.

[redoxy]

Re: Dell Latitude 5285 no backlight

Okay great I had a feeling that was the issue. I'm not sure of any alternate parts but I'm pretty sure I have one of these on a donor board at my work and if you'd like I can mail it to you. Won't be back in the office till Monday however.

Yes, that would be greatly appreciated.

I'll PM if that is okay with you.

Thanks!

[Boardsurfer]

Re: Dell Latitude 5285 no backlight

yup that works!