No LED backlight on laptop after unplugging and re-plugging display cable

[philipjfry]

Dear everyone!

I recently made a double-doodoo and I once again turn to this great community in the hopes that you may be able to help me out on this one: I recently received a Lenovo IdeaPad 5-15-ARE05 with a broken hinge for repair. The good news is, that the repair of the hinge went very nicely and I was able to fix it with some epoxy. It is possibly connected more securely now than before and nothing is visible from the outside, good as new. 

However, it seems that I made a crucial mistake by neglecting to disconnect the laptop's internal battery prior to unplugging and re-plugging the display cable (even though the laptop was of course completely turned off during the procedure, no standby/hibernation or the like). As a result, the backlight doesn't turn on now. To make matters worse, I then immediately tried to narrow down the issue by cross-connecting the display of this laptop to an identical model (backlight worked) and the display of the other laptop to this system board (backlight did not work). However, upon attaching the display and system board to each other on the good laptop, that backlight also ceased to function, leaving me with two IdeaPad 5-15-ARE05s with broken backlights. 

It was only after that that I did the necessary research, discovering that I should have disconnected the batteries first (something I didn't even consider because they were powered down and the laptops that I worked on in the past had removeable batteries which had to be disconnected anyway). Ideally, I just blew a fuse in the process, but the one fuse that I identified (F11) appears to still be OK (continuity and around 3.3v to ground on both sides when powered on). There are no visible issues on the board that I could identify and all the other components I was able to check (resistors, capacitors) appear OK to me as well (I just crudely checked for resistance and I'm not a trained electrical engineer though, so I may be wrong on that). I also checked the backside of the PCB, there doesn't appear to be anything there related to the display port. The switch that detects if the lid is open or closed also still works fine, because windows still changes display scaling on an external monitor when the lid is closed/opened.

Since the display lit up when I first connected it to the other laptop, the issue should be located on the system board, but this is where my diagnostics skills end and I'm hoping one of you guys can help point me in the right direction. ​I already tried to have a closer look at the pinout of that (supposedly) eDP connector (I've seen a video where it helped to short the brightness signal and it worked again at the cost of loss of adjustability), but I wasn't able to find anything that even remotely matches what (I think that) I'm seeing on this board.

I included some pictures of the area around the display connector, maybe somebody sees something that I missed? If necessary and helpful, I do have access to quite a bit of equipment like a (good) iron, a (cheap) SMD soldering kit, a scope and a FLIR camera.

Thanks in advance to anyone even bothering to read my wall of text!

[gabiz_ro]

That F11 (FI1 marked on board ) I think is for LCD power not backlight power.
But maybe I'm wrong.
If is LCD power then 3.3V seems fine
If is backlight power then something is wrong (defective) since there must be some higher voltage.

Usually for backlight there is another power line from main system power (DC input or battery)
If you cant find board schematics search datasheet for LCD display,you may find pinout and look for backlight Vcc.
If datasheet not available you may look at little PCB on LCD,there must be some step-up LED driver,even some testpoint mark on PCB,to identify Vcc backlight,then trace that line to see on which pin is on mainboard connector.
Then look on for damaged component,could be some fuse somewhere on board or even some MOS-FET acting like a switch, or some coil filter etc.

Looking on photos
pins 1,2,3 could be GND or some power lines
pins 7,8 could be some power lines since is going via a on-off switch (UG5)
pins 28,29,30 since are via fuse FI1 are some power lines

[Labrat101]

The inverter for the backlight i believe is inside the screen casing and you may have well crushed the cable that passes over and though the hinge
assembly the wire has properly got damaged . The wire goes though a channel that allows it to move . when the lid is opened and closed .
If the cable due to your repair got crimped and pulled tight it could have ether shorted or popped the small plug out of the inverter panel taped to
the bottom of the LCD .
Make sure the internal battery is disconnected and press the power on button for 5 seconds to make sure every thing has fully discharged .
also the inverter panel can be damaged by static . These cables are very easily damaged and the protection shielding on the cable were it passes though
the hinge assembly must be in the correct position and not torn

[philipjfry]

Thanks for your input, gabiz!

Alas, I haven't been able to find board schematics yet (paid or otherwise), but I'll keep looking. I was hoping that that eDP connector would be standardized on the system board side as well, but so far I haven't been able to identify an eDP pinout description that matches what I'm seeing here (pin groupings, etc.).

I think your assessment of the pins is accurate so far, I'm getting 0V on 1,2,3 and 3.3V on 7/8 as well as on 28/29/30. Too bad that the pins are so small and so close together, I dare not measure them directly at the connector as it would be super easy to short some of them in the process.

Regarding the approach of identifying and following the backlight power line from the PCB below the LCD, it is unfortunately taped up with EMI shielding tape and since that part should be ok (as it worked when I attached the display to the second system board), I'd rather not touch (i.e., visibly manipulate) that for now. I'll probably resort to it at a later time though, if no other viable options arise. Same goes for identifying the LCD panel itself: Unfortunately, it is stuck to the top cover with those (single use) removable strips and I don't have a replacement for them at the moment.

I'll keep you posted on my progress!

[philipjfry]

Thanks to you too, labrat!

Yes, the inverter is located at the bottom of the display, as usual. Good points regarding the cable, but in that regard I can say with probability bordering on certainty that the display cable is undamaged and that the connector has not loosened on the display side. The former is due to the fact that no significant force (pinching or otherwise) has been applied to the cable whatsoever, not even as a result of the broken hinge (as the issue has immediately been identified, the laptop hasn't been opened/closed in the broken state, the backlight was still operational when it arrived and the hinge repair was performed with great - mechanical - care). The latter is due to the strain-relieved construction, no force can be applied to the display-side connector by wiggling or pulling the cable without ripping off something else first.

Besides, the backlight worked normally when the display was attached to the second laptop (and it didn't appear to be a shaky connection like one would expect from a cable with a broken wire inside). Of course I made sure that the cable is routed correctly through the designated channel at all times. Furthermore, it would be one hell of a coincidence if the exact same thing broke on the cable of the second machine...

I therefore still strongly believe that it is an electrical issue on the system board rather than a mechanical one on the cable.

[Labrat101]

I assume when you removed the cable . You lifted the small Black release tab and did not just pull
it out, the cable is locked into that socket by that small clip . that can be lifted up with your finger nail.
removing the cable in the locked position can damage the edge pins or short them on removing
at an angle . as you mentioned the pins are very closely spaced  .
good luck if you have damaged or shorted any of the smd chips or transistors out they are a pig
to replace if you don't have a really good soldering iron and a microscope or a zoom camera setup.
I really would recommend finding a local friendly tech to do the job . For you .

[philipjfry]

Yes, I opened and closed the mechanism properly and both the socket and the cable are in pristine condition. If only it were such an obvious issue. To elaborate, I do have more than 2 decades of experience (successfully) repairing hundreds if not thousands of desktops, laptops, tablets and smartphones and this one of very few cases of me actually breaking something in the process. Certainly the first case of me embarrassingly breaking something noteworthy that I can't easily/cheaply replace or fix myself. Never crossed my mind that unplugging and re-plugging this cable on a completely powered off system could cause an issue. I'm not an EE but rather an IT professional / software developer by trade and hardware repair is more of an occasional (if quite regular) side gig, so I usually just swap defective components (hence the immediate impulse to cross-check with a 2nd identical laptop I had on hand). When pinpointing the specific issue on that component and things get dicey and/or microscopic, my skills are still limited (but I am taking every opportunity to expand them, out of personal interest). Obvious visible issues like bulging caps and a few things that can be identified by poking around with a multimeter or via thermal imaging (which suffices in almost all cases I am confronted with that are economically viable to resolve) are fine, but this is one of the few exceptions.

I do have an excellent iron that I know how to handle (as well as cheap SMD soldering equipment that I have yet to gather experience with prior to using it for actual repairs) and my phone can act as a semi-decent microscope in a pinch, but yeah, I'd happily leave this one and similar issues to someone doing that kind of stuff on a regular basis. I would even have another potentially quite profitable case in form of a literal metric ton of out of warranty "HP RP2 Retail System 2000"s that failed/are failing in exactly the same manner (not the backlight, but they all develop an identical intermittent issue on their system board; possibly a case for another post) for that person, but I haven't been able to find someone suitable near me so far. Over here, it's mostly about tourism, we don't really have that kind of industry (and by extension, educational opportunity). Broken in-warranty laptops and smartphones are exclusively shipped hundreds of miles or more to other countries for repair (I know because I handle the RMA for many of them) and it's not just because labour is slightly cheaper there.

Whether I want to or  not, I'll probably have to dig myself out of that hole with my own two arms, hence my call for help on this forum. The last (which was also the first) time I posted was about a laptop system board issue as well and with the help and experience of some lovely members of this great community (can't praise them enough), we were able to narrow it down to a defective MLCC with a thermal camera. I'm still hopeful that we can pull off something like that again! 

[Labrat101]

OK . We have all had something that just doesn't  go right . I have also worked on electronics for 50yrs and had many a thing that just seemed impossible and spent a week banging my head on the desk .. And just put it to one side
and came back to it after a week  or so and found the smallest resistor tucked in a corner of the board burnt 
If you can fix phones then you know Murphy can be really cruel .  If from what you have said You are experienced enough . I have dismantled also many laptops . but finding a damaged component is a pain . you will have to trace
the supply voltage back, there are some FET chips 5 pin maybe on the other side of the board may have burnt. 
Also try searching for the schematic by the panel numbers and not the model . The circuit diagrams can be found
for Lenovo try different search pattens . I do have quite a few Lenovo panel layouts but not your one .
  Put up a photo of the complete board and all the number on the panel. etc every thing .
being retired I have time to do long searches and dig in IBM/Lenovo archive   
and the service/serial/ number also helpful
 

[gabiz_ro]

What voltage you have on UG5 pin 5?
But I don't think they put backlight Vcc inside ,most of time is on first or last pins.
So must be on 1,2,3 or 28,29,30
28-30 have 0 volts but are they GND pins?
On capacitor near JEDP marking which side is GND and what voltage you have on?

[philipjfry]

Hi again, labrat and gabiz,

and sorry for the delayed response, things are very busy at the moment and my camera wasn't very cooperative when it came to making high quality photos of the PCB, even after I waited for good lighting conditions. Sorry for the somewhat potatoey quality, I'll send additional closeups if desired. There are no visibly damaged components though, at least not to my eye.

gabiz, I double checked my measurements and I am (still) getting 3.3V to ground on pins 7/8/28/29/30. Pins 1/2/3 measure 0V to ground, both before and after the capacitor near the JEDP marking. Well, to be precise, I am getting 0.01-0.02V across the capacitor (and -0.01-0.02V when swapping my multimeter probes, but I consider that within measurement tolerance of my cheap multimeter?

UG5 measures as follows:
1: 3.3V
2: 0V
3: 0V
4: 1.8V
5: 3.3V

Thanks again to both of you for your help and patience!

[gabiz_ro]

So,no voltage across capacitor.
Check fuse FG2 seems located near that cap but on other side of PCB

[Labrat101]

I was looking at your photos
when I spotted this Is this a crack from the power socket to the black strip plug .
or is it a trick of lighting or something else .
I looked on another Lenovo that we had around not the same model but has the same screen and same year that cap on yours with no
power on the one I tested had 19v  . and I was trying to follow the tracks on your board when I notice this suspicious crack.
Please check and confirm  . if yes this maybe were the volts are going or not going . These power sockets do take a pounding
 form the plug being pulled out sharply

[philipjfry]

You guys are the BEST! 

I checked FG2 just now and it was the culprit! So my initial hope of just having blown a fuse like I had seen somewhere else on a different model was "reasonable". I was just too blind to see that fuse on the back. FG1 and F11 I saw and checked, but FG2 somehow eluded me although it's right there on the other side.

Thanks for yout time and effort gabiz and labrat, I wouldn't have been able to do it without you! <3 (Well, maybe after putting it aside and having another look at it one week later, like you mentioned, labrat, but that's by no means a certainty.  )

[Labrat101]

Really please that the problem has been found