ASUS Q502LA LED Screen/Backlight Issues

[PJK65]

Hello Everyone.
Glad to be a member here! I am pretty new to working on laptops. I was hoping I was going to able to fix this one, but I am having some problems...
This laptop works perfectly with a HDMI monitor hooked up to it. Here in lies the problem. No backlight! So i figured, replace the screen/backlight assembly. Then I had very dim video (could be seen with a flashlight) but still no backlight. Found after doing some homework that it had a blown ferrite bead, which I replaced. Like an idiot, I forgot to unplug the battery, and when I was hooking back up the 30 pin cable, something sparked, and now I have no video or backlight! Ugh. I have done a little voltage testing on the J4502 connector. Pins 22,23,24, and 25 all have 19v on them. I have no 3v on any of the J4502 pins. There is 3v at the J4501 going to the touch panel, and 3v at other spots on the board. I'm just not sure where to start to troubleshoot? I have the boardview for the this laptop, but it's the first time i ever used the program. Does anyone have any ideas? I'm also concerned that there is no video on the screen now as well...Any help would be greatly appreciated!! Thanks

[PJK65]

Re: ASUS Q502LA LED Screen/Backlight Issues

Did a little more testing, and found +3v on pin 1 of U4505, but no voltage on pin 6. The boardview says +3 on pins 13, 14, and 19 of J4502 screen connector. No voltage on any of these. I can upload the boardview, but I don't want to violate any forum rules. This laptop is a very good friend of mines, and I sure would like to give it back to her working! Any ideas on whether or not the screen itself might have damaged the motherboard circuit, and maybe I need to order another one?? Thanks!!

[JaroTech]

Re: ASUS Q502LA LED Screen/Backlight Issues

Hi, what voltage do you have on pin 4 of the U4505? If you have a voltage, the U4505 is faulty. This chip will give the 3.3V on pins 13 and 14 of the J4502 connector. You can also check the L4501 coil. The U4505 chip is a G5244A31U.

[PJK65]

Re: ASUS Q502LA LED Screen/Backlight Issues

Hello!

Thanks so much for helping me out! There is no voltage on pin 4 of U4505. There is no voltage on any pins of U4505 except pin 1. There is no voltage on either side of L4501 coil either. Checked resistance on R4581 and it is right at 100K. I see that pin 1 of R4581 traces to EDP_VDD_EN on the CPU. I'm guessing since there is no voltage present, that this could be the problem? What voltage should be there? Is something else preventing the CPU from applying voltage here?
Thanks!!

[JaroTech]

Re: ASUS Q502LA LED Screen/Backlight Issues

U4505 may be shorted to ground, if you remove this chip, you can check if you have a voltage on pin 4 of U4505 from the CPU.

[PJK65]

Re: ASUS Q502LA LED Screen/Backlight Issues

Thanks for the tip! Just removed U4505, and there is still no voltage on pin 4. Does this mean the CPU is damaged, or is some other underlying issue preventing voltage on EDP_VDD_EN?

[JaroTech]

Re: ASUS Q502LA LED Screen/Backlight Issues

It is difficult without the schematic to find if all the voltages are good. I know that there is a repair guide on a Russian site, it will be useful to have it.

[reformatt]

Re: ASUS Q502LA LED Screen/Backlight Issues

Usually this kind of SNAFU results in damage to the CPU if the connections are direct. However, they all seem to be isolated so you are in luck. If the laptop was off, but the battery was connected, the only thing active would have been the AC_BAT_SYS so L4510 would be the first suspect.

The 3V supply you have to check when the laptop first powers on. If there is no EDP communications from the display, then the CPU will remove the enable some time after power up. The display could also be damaged or your 19V is missing.

[PJK65]

Re: ASUS Q502LA LED Screen/Backlight Issues

Thanks for the help everyone! So I followed reformatt's idea, and checked the voltage coming from EDP_VDD_EN right at power up. It happened very quickly, but there indeed was voltage present for about 1 second, and then it went back to 0. This tells me that there is no EDP communication present from the display. Since I have 19v present, I have decided to purchase another display. Just have to get U4505 soldered back on. This looks like it might be a challenge, due to the fact that the pins are on the bottom of the chip. These SMD components sure are tiny compared to the thru hole ones on amplifiers!

[reformatt]

Re: ASUS Q502LA LED Screen/Backlight Issues

Forgot to mention also check your LCD cable for damage or bent/burnt pins.

[PJK65]

Re: ASUS Q502LA LED Screen/Backlight Issues

Hello Forum Members...
Wanted to post that I got the laptop fixed, thanks to some kind folks on this site! The screen was in fact bad. Replaced it AND the cable with new ones, and it is up and running great again. I wound up also purchasing another motherboard, as I could not find G5244A31U high side switch, or an equivalent in the US. I destroyed the one I un-soldered on the original board. I should have checked availability before I messed with it. I'm new to these tiny components, and obviously need to be more careful when working with them! Lesson learned. At least I have a board to practice on now. Thanks again for the help!