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Old 08-27-2015, 09:06 AM   #1
raider988
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Default Asus k52jr no backlight

Hello everyone,

I am not an expert on hardware, but I do my best and I'm here to learn so please be patient, every little step is my first step in this.

I have trouble with my laptop, motherboard model k52jr, LCD led screen model lp156wh2. I have isolated the problem to the backlight, computer works perfectly with external monitor plugged in, the native screen works except for the backlight and I can see the desktop if I shine light on screen.
I did some research and find out about multimeters, how they works, what they do, so I bought one at a fair price and started doing some tests.

I downloaded the specs for my screen, but I was unable to take measurements on the 40 pins on the back of the screen, everything is simply too small, so I had no idea which pin receives what. I checked the screen plug on the motherboard instead, but I counted only 30 pins on it. Anyway, a couple of them reported 19.4 volts (I connected one end of the multimeter to the ground), so I guess the motherboard is doing its job.
I checked voltages on the back of the screen then, i found an F1 and an F2 fuse. They passed continuity but when the screen was plugged in they measured 19.4 volts instead of the 32 written on the board. I check VLED and VLED_OUT and they both get 19.1-19.3v.
At this point I don't know how to isolate the problem a little more and get closer to the solution, I am aware that i probably made it simpler than it is, but that's precisely why I'm asking for your help.

I suspect it might be the led strip itself, since everything seems powered on, but i have no way to test them directly.

Any kind of insight is very welcome, I attached some pictures if you wanna take a look.
Thanks a lot guys

f1_lcd.jpgf2_lcd.jpglcd_back.jpglcd_closeup_center.jpglcd_closeup_left.jpglcd_specs.pdf
mother_back.jpgmother_front.jpg
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Old 08-27-2015, 05:11 PM   #2
Th3_uN1Qu3
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Default Re: Asus k52jr no backlight

19 volts on back of the screen is ok. For the LEDs to turn on you need a couple more pins to be 3.3v, specifically LED_EN and PWM. Pins 36 and 35 on the back of the LCD connector. See here for reference: http://s599.photobucket.com/user/Chr...l?t=1332836621 You are most likely to find that you are missing LED_EN.

A bad screen or a bad LCD cable can cause this. I have a similar Asus laptop in for servicing where the screen shorted out LED_EN and melted the wire in the LVDS cable, so this one it's going to need both a new screen and a new cable. Yours might be luckier.
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Old 08-28-2015, 06:40 AM   #3
raider988
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Default Re: Asus k52jr no backlight

First, thank you so much for the suggestion!

So, I got a thinner probe and took those measuments. I don't know how can you do it for a living guys, I had a hell of a time to measure those tiny pins and I couldn't have done with some help to take the readings and write them down. I double checked them just to be sure.

This is what I got:

35 ==> 3.3
36 ==> 3.2
37 ==> 0
38 ==> 19.2
39 ==> 19.2
40 ==> 19.2

So i got luckier than the Asus you repaired and I think I can assume that whatever is not doing its job lies on the screen, but correct me me if I'm wrong, I might not being considering things I don't know.

A big part of the problem is that i know nothing about the circumstances that lead to the backlight failure, a friend took this laptop to a technician for a keyboard issue and it came back with the backlight problem. So she came to me to recover her datas and left the laptop as a present. I don't really need it to get fixed imediately, I think this is a great chance to start experimenting and learning without paying any real consequences if I fuck up big time, however it goes I want to learn a big deal in the end.

Any other suggestion is greatly appreciated, I'd like to concentrate in finding a way to test those leds since they seem to be the next best candidate for backlight failure.
I found this page which describes an reletively approchable way to test the led circuitry, which gets close to what I want to do, but it's not really that.

I'd like to feed energy directly into the led strip itself, so first I need to know which are the points where i should stick my pos and neg, which voltage and amperage I need to get even a glimpse of light from them.
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Old 08-28-2015, 07:37 AM   #4
raider988
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Default Re: Asus k52jr no backlight

UPDATE:

I took it one step further and measure voltages directly on the led strip socket and I got those values (image attached):

led_strip.jpg

1==> 19.1
2==> 19.1
3 to 9==> 0

Can I safely assume that the led strip itself is not working anymore?
If yes, what may have caused the problem in the first place and how can I be sure I won't have the same problem once I replace them?
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Old 08-28-2015, 01:38 PM   #5
raider988
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Default Re: Asus k52jr no backlight

Update 2

I took out the led strip trying to check voltages on the leds themselves and something strange happened.

I connected the back probe on the ground as usual, when i put my red probe on the side of one led it lit up, and other leds every 5 or 6 too. The more I moved away from the plug, the more bright they became, but never as bright as they should be. When i moved to the last led i got the highest value of 16.4v, while the first was 0.8. I check
every led and they all work.

At this point I'm lost.
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Old 10-12-2017, 08:53 PM   #6
raider988
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Default Re: Asus k52jr no backlight

After couple years a lots of learning I took back my old screen and gave it a look.
The ribbon cable was damaged. I have no idea where because it's not visible, but that's it.
Thank you all for the help!
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