Electronics > Repair
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. |O
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.
Navigation
[0] Message Index
[#] Next page
Go to full version