LG K10 2016 no backlight fix

[DEXj98]

Hi everyone, I'm new here and I've got my first question for you: yesterday I've received an LG K10 (4G/2016 model) with broken backlight. Everything else works fine, I can even see the screen and all the icons in the dashboard but without any backlight. At first I thought that replacing the screen was the only way to solve the problem, but before doing so I wondered if there was some component on the mainboard to check (eg. ICs, diodes, smd ferrite beads, ecc...) or something similar to do. Do you know what could cause this kind of problem in smartphones? By googling I managed to find some informations but only about iPhones. Help would be appreciated. Thank you all!


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[Rasz]

do you have microscope? diagram/boardview of the phone?

[james_s]

The first thing I would do is see if I could identify the connection for the backlight, often it's brought out on separate pins of the ribbon cable from the display. Phones can be a nightmare to work on though.

[DEXj98]

Sorry for being late, hadn't much time to answer you, I'll try to be quicker now! Thanks everyone for the responses, I don't have a microscope neither I think it's possible to find a diagram of the board, but I have some very powerful magnifying lenses. You can find a photo of the display connector and nearby components attached to this message. I also forgot to mention that the phone got damaged right after falling.




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[james_s]

It's a long shot but the soldering on that diode looks a bit sketchy, that may well be part of the boost converter that drives the backlight.

[wraper]

It's a long shot but the soldering on that diode looks a bit sketchy, that may well be part of the boost converter that drives the backlight.

Soldering is not sketchy and there is no backlight boost converter on the photo to begin with.

[wraper]

Boost converter may also be in LCD itself, look for relatively big inductor on the flex cable surrounded with other components. As I've seen on other smartphones in the past, it usually was either faulty backlight in LCD or faulty inductor in boost converter.
It seems that backlight is easily accessible on the LCD where backlight FPC is soldered onto main FPC. You could try measuring voltage there.

[james_s]

It's a long shot but the soldering on that diode looks a bit sketchy, that may well be part of the boost converter that drives the backlight.

Soldering is not sketchy and there is no backlight boost converter on the photo to begin with.

I'm not sure how you can tell with any certainty that the solder joint is not sketchy, it's blurry and out of focus and looking from the top down but it was only a suggestion to take a closer look, not a definitive diagnosis. I also don't know if there's a converter there, do you have a schematic? I don't, but there's a diode, an IC, capacitors, there could be an inductor hiding there somewhere, and certainly plenty of TFT based devices do have tiny boost converters to drive the backlight. The phone was dropped, the backlight stopped working, regardless of where it is located I would start by looking for anything that might be involved in powering the backlight, especially any bulky or fragile parts that may have suffered mechanical damage.

[wraper]

I also don't know if there's a converter there, do you have a schematic? I don't, but there's a diode, an IC, capacitors, there could be an inductor hiding there somewhere, and certainly plenty of TFT based devices do have tiny boost converters to drive the backlight.

It does not look like boost converter. Inductor won't be "there somewhere" but placed right next to IC. And it will be relatively large inductor, sort of 0805 size is bare minimum, more likely twice as large.

[Rasz]

or faulty inductor in boost converter.

its never the coil

[wraper]

or faulty inductor in boost converter.

its never the coil

Except I replaced quiet a lot of inductors to fix the backlight  . Failing inductor is a plague for some models.

[Rasz]

its a meeeem

[wraper]

its a meeeem

I know, but it's an iphone meme.

[james_s]

The times I've had an inductor be the problem it had physically broken or come off the board. At least they're easy to test, if it's not open circuit and it's not missing, the inductor is probably fine.

[wraper]

if it's not open circuit and it's not missing, the inductor is probably fine.

In some phones they fail because of short in the winding. Though as phone was dropped, inductor could break mechanically as it's quiet heavy component with relatively weak terminals.

[DEXj98]

Today I'm gonna check the cable to see if I can find those joints to check the voltage on, but I don't remember seeing any before. The inductor also seemed to be missing at all on the board, though I looked for it mainly nearby the LCD connector. I'll also send a whole picture of the board so you can know if all the components are in place


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[Nusa]

its a meeeem

I know, but it's an iphone meme.

You don't have to watch it, but to prove that even they have to fix coils sometimes:

[wraper]

Today I'm gonna check the cable to see if I can find those joints to check the voltage on, but I don't remember seeing any before. The inductor also seemed to be missing at all on the board, though I looked for it mainly nearby the LCD connector. I'll also send a whole picture of the board so you can know if all the components are in place


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It almost never will be nearby LCD connector. Also it might be under some soldered shield.

[DEXj98]

Ok I've made some photos of the board. You can find all of them attached to this message. I also measured the voltage on the wire, as hinted by wraper, and here are the results (I will refer to the solder tracers as shown in the last photo):

1-3: 32V
1-2: 32V
2-3: about 0V (may they be both ground? They don't look like...)

I really appreciate your help, thank you all!




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[james_s]

32V is an unusually high voltage to find in a mobile phone, without knowing anything specific about the unit, that sounds like it could be the output of an LED driver boost converter running open circuit. Is that with the display connected that you're measuring that?

[DEXj98]

32V is an unusually high voltage to find in a mobile phone, without knowing anything specific about the unit, that sounds like it could be the output of an LED driver boost converter running open circuit. Is that with the display connected that you're measuring that?

Yes I powered the phone and the display was turned on (without backlight).


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[james_s]

That certainly suggests that the driver is operating and there is an open circuit somewhere in the LED string or the wiring to it. It may be a fault within the display assembly, those LED backlights are usually pretty tightly integrated and difficult to get to without breaking the display.

[DEXj98]

That certainly suggests that the driver is operating and there is an open circuit somewhere in the LED string or the wiring to it. It may be a fault within the display assembly, those LED backlights are usually pretty tightly integrated and difficult to get to without breaking the display.

That's exactly what I thought from the beginning. Looks like I'll need to test the phone with another LCD sooner or later...


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[james_s]

You might inspect *very* closely for a tear in the cable or connector, or see if you can gain access to the backlight without destroying the display. It's already not working so if you do break it further it's not a huge loss.

[DEXj98]

You might inspect *very* closely for a tear in the cable or connector, or see if you can gain access to the backlight without destroying the display. It's already not working so if you do break it further it's not a huge loss.

Makes quite a good point. I'll keep you up for news!


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