820-00165 Backlight not working properly on one side

This machine came in for no backlight. I did not find any water damage and the fuse was good. So I replaced the driver IC U7701 and that fixed the problem "almost" The machine isn't quite right. Along the bottom left of the screen you can tell that some of the back light LEDs are not properly functioning and this makes that side a little darker. See the picture.

I have encountered this before and ended up replacing the driver ic to fix it so I tried another one just incase and it did not solve the problem.
I checked R7717-R7722 and they were all good. I also checked all the resistors around U7701 as well as replaced it a second time. This issue shows up on both the original screen and my test screen so it is definitely the LB.

R7721 and R7722 (BKL ISEN5 & BLK ISEN6) are both reading 4-8v which is not right. There is not muchere except for the plug and U770. I really don't have a solid understanding on exactly how this circuit works. I have looked at J8300 very closely under the microscope and can't find anything wrong with it.
 

Attachments

Last edited:

SMMRepair

Member
Bad LP8550, or bad LCD connector. Probably just a bad LP8550. Are you installing new chips, or re-balling used ones? If re-balling, you gotta make sure they're all perfect and the same size. Check all the LED_RETURN lines of course (just check diode mode at the connector-side resistors). I bet one will read .000. Consider that someone might have damaged the LCD connector (look inside of it), which is what damaged the original LP8550. You might have a good LP8550 now, but a bad connector still. If connector is fine, just replace LP8550 again.
 

2informaticos

Administrator
Staff member
Compare diode mode readings for all return resistors.
You should get less than 1% difference.
Check w/ and w/o LCD connected.
 
Diode checked LED_RETURN lines. All .633. Replaced J8300. Same issue. I will harvest a LP8550 from a known working board. Hard to believe as I have replaced the LP8550 3 times already but there is nothing left.

That fixed it! Screwed by the Chinese again!
Where should I get these chips! This is not my first rodeo. I know these were balled and flowed perfectly!

I even split balled the thing this last time and presto!
 
Last edited:

SMMRepair

Member
Yeah not much to the backlight circuit. Gotta make sure your pads are perfectly clean and chip is re-balled well. Honestly, most times I see this issue, it's caused by uneven solder and not "necessarily" a bad chip. But I re-ball them all myself usually, pulled from parts boards (tired of getting the choppy backlight LP8550's). If you bought these "new", then yes, avoid that seller.
 
I had another bad LP8550 on another macbook air today. Replaced the IC from the same batch and same problem. I am going to reball it and try the same one again.
 
Last edited:
Another update. The reballing did not help. Grabbed another and another and every one is this way. I got a whole bad batch. To the bone yard
was purchased on Aliexpress " SZ Xcls Store "
 
Last edited:

piernov

Moderator
Staff member
Get them from Mouser (or another reputable supplier, Digikey should be ok too) and program the correct slope. (
)
I've got garbage LP8550 from eBay where brightness adjustment wasn't working at all, Mouser one fixed it.
 

SMMRepair

Member
Thanks, Piernov! Will give that a try. Any instructions on how to create the Linux LiveUSB (mainly, where to obtain Linux, etc)? I've never messed with Linux before, but assume the process is the same as creating a MacOS USB drive.

Will be a bit of a process to do this for every single backlight repair with the new backlight drivers, but at least it will prevent customers from asking about why it's choppy.
 

piernov

Moderator
Staff member
This one is Ubuntu 20.04. I create the LiveUSB from another Linux system with `dd`, but you should be able to follow these tutorials if you're on macOS — https://ubuntu.com/tutorials/create-a-usb-stick-on-macos#1-overview — or Windows — https://ubuntu.com/tutorials/create-a-usb-stick-on-windows#1-overview —.

It should be possible to create a minimal LiveUSB that will run the script automatically and have it done in less than a minute without typing anything, but I haven't tried to do that.

Unfortunately it can't be run from macOS itself as I didn't find a way to access the PCH SMBUS under macOS. Linux thankfully has a kernel driver and a userspace library for that.
 

SMMRepair

Member
Thanks so much! If you're interested, and ever got the hankering to whip up a quicker script/method to accomplish that, I'd be happy to compensate you. Just shoot me a message if you ever do!
 
Top