Dell G7 7790 LCD backlight issue

[RepTech]

Does anyone have an experience with backlight issues on the Dell G7 7790 laptops?

I have one in my shop that was worked on by another shop after the screen was broken.

The other shop replaced the screen then told the customer the lcd cable was the issue and they couldn't repair it.

We have tried a second replacement and lcd cable just to be sure it's beyond the obvious issues. The laptop does work perfectly other than the backlight. It works great on external monitors and the lcd is working except for the backlight not being lit.

We are equipped for micro-soldering but I have been unable to locate a board diagram for this particular model.

I'd appreciate any help anyone may be able to give on this.

[mon2]

Re: Dell G7 7790 LCD backlight issue

How many pins on the lcd screen connector on the logic board?

It should industry standard and then can Google for the pinout.

Post the details here. Maybe 30 pins?

Post the lcd display markings as well.

[RepTech]

Re: Dell G7 7790 LCD backlight issue

It is 30 PIN.

I don’t see any markings on the LCD connector of the display. If you mean the model info it is as follows:

N173HCE-E3A Rev. C1
09C1BHY99B50001

HP H/W:C1
HP P/N:L43245-JG1
CT:CHRTK01K6CR04Y

This is off the replacement screen. Original is also N173HCE-E3A Rev. C1

Both are InnoLux displays.

Thank you!

[JaroTech]

Re: Dell G7 7790 LCD backlight issue

Hi, on the motherboard, does the connector for the screen have 40 pins? If so, are pins 37 38 39 40 connected together?

[mon2]

Re: Dell G7 7790 LCD backlight issue

Remove all power.

Meter in resistance mode.

Check if pins 26 to 29 are common = Backlight pins.

If they are connected together, then this is very likely your backlight rail.

Confirm this detail first. If yes, continue.

Still with no power, check the resistance to ground of the backlight cluster of pins.

What is the resistance?

[RepTech]

Re: Dell G7 7790 LCD backlight issue

I haven't had a chance to do anymore testing on this as of yet cause I've been out of the office. I really appreciate the help and will post more once I've tested.

I do have the schematic for this board that works with FlexBV. I managed to find it last night.

[RepTech]

Re: Dell G7 7790 LCD backlight issue

Hi, on the motherboard, does the connector for the screen have 40 pins? If so, are pins 37 38 39 40 connected together?

The board side connector is 40 pins and those are connected together.

[JaroTech]

Re: Dell G7 7790 LCD backlight issue

I'm using a different schematic than your laptop model, maybe it's not the same as yours. Near pin 40 you have a fuse, if you have a voltage on one side then the fuse is burnt.

[RepTech]

Re: Dell G7 7790 LCD backlight issue

Well, after the weekend of the laptop sitting it will no longer power up properly.

It sat all weekend, still in the laptop, with the internal battery disconnected. Now today it will power on but only lights the power button and caps lock. No post, no fan spin, nothing. I'm starting to think these G7 7790's are junk builds by Dell based on what I've read about them.

We've done the typical of removing and testing memory modules, etc, but same issue. Perhaps the board has more issues than originally thought.

Any ideas on this one now?

[mon2]

Re: Dell G7 7790 LCD backlight issue

Connect the battery and try to boot up again.

[RepTech]

Re: Dell G7 7790 LCD backlight issue

I'm sorry, should have been more specific. I have already re-connected the battery and attempted to boot. At this point it lights the power light and caps lock on solid, no response or video output of any kind. I can not turn caps lock off.

The CPU and GPU both heat up, but the fans do not power up at all. If I hold the power button down to try to power it off it will power off and does kick the fans on right before it powers off.

[RepTech]

Re: Dell G7 7790 LCD backlight issue

Your schematic seems to be better than the one I'm using. Everything in the one I'm using is exactly opposite of the board. It's accurate thus far as long as I remember its backwards.

The fuse is good.

As I stated in the last post the board will not even post now yet we have done nothing to it other than test without power connected prior to this new issue.

I'm using a different schematic than your laptop model, maybe it's not the same as yours. Near pin 40 you have a fuse, if you have a voltage on one side then the fuse is burnt.

[JaroTech]

Re: Dell G7 7790 LCD backlight issue

As I said, I don't have the schematics for your laptop, I'm using the Dell G5 5590 G7 7590 Pegatron Vulcan. And for the boardview, Dell G5 5590 Vulcan (cad). If I remember correctly the boardview was also opposite of the board, I just flipped it in BoardViewer.

[mon2]

Re: Dell G7 7790 LCD backlight issue

Start with the basics.

1) Is this the VULCAN logic board ? (as it is named on my schematic - designed by Mr. Spock ??)

2) Review the charger IC -> with the DC adapter connected, confirm the voltage of +AC_BAT_SYS - what is the voltage reading ?

We are checking to see if the mosfets are passing the voltage onto the logic board.

3) If not, remove all power -> meter in resistance mode (lowest scale - 2k is ok).

Check & post the resistance measurements for:

(each mosfet linked to the charger IC)

source (1-2-3) & drain (5-6-7-8)
source (1-2-3) & gate (4)
gate (4) & drain (5-6-7-8)

The first 2 mosfets are the most of interest but if unable to boot with a charged battery, then the other mosfets on this charger IC should also be investigated ( @ PU6900 ).

[RepTech]

Re: Dell G7 7790 LCD backlight issue

The board has the same result no matter if it's battery power only, charger only, or charger + battery.

We have found shorts in the following locations that appear as if they could be related to the GPU.

C5248, C5249, C5252, C5253, and C5250 are all short to ground. So it would seem there is a short somewhere related to +1P0V_GPU based on our schematic.

It is the vulcan board.

We will test as you requested as well.

[rediii]

Re: Dell G7 7790 LCD backlight issue

did you do a cmos-reset (does it have a bios-bat?). remove all power including cmos-bat. hold pwr button for 1 min and release. reconnect cmos-bat, power cable and take a look. often dell notebooks do have a strange cmos behaviour. did you check the screen located cable as well? do you have the 3.3 v lines on screen board? caps on the low site near gpu are very low on ohms. could be normal. try to reflash the bios externally. don't know if it helps, give it a try