MacBook Air a1466 no Backlight

[ACS-Repair]

Hello All,
I've been working on an 820-00165, currently with no backlight. There is a picture on the screen. At an earlier date, the backlight was working but it would drop out regularly, and sometimes not come on at all. I replaced U7701 thinking it might be a corrosion issue, but it wasn't. I haven't been able to get the backlight to come on since even though the screen does activate and I can see a flashing folder when lit from the back.
I have confirmed after the U7701 replacement that I have the following:

PPVOUT_SW_LCDBKLT_FB 8.5v (at U7701 pin A5)
BKL_EN 2.73v
BKL_PWM 1.07v

The LCD connector was replaced earlier due to corrosion but the backlight was working (unstable) after that had been completed.
Very strange. I have no boost but it's not clear to me why. I would appreciate anyone's insight.

[mon2]

Re: MacBook Air a1466 no Backlight

Hi.

Remove the display.

Then check the voltage at fuse F7700.

Is fuse F7700 blown?

[piernov]

Re: MacBook Air a1466 no Backlight

Xw7720?

[ACS-Repair]

Re: MacBook Air a1466 no Backlight

I pulled the display and got 8.58v on both sides of the fuse F7700. The fuse is good. This is the case whether the display is connected or not. I'm using a test display that is know working and the display is active.
There is no corrosion at Xw7720 but I ran a wire from there to U7701 to confirm that the FB was present at U7701.
I'm going to look closer at the LCD connector to make sure that nothing is amiss there. I'm getting 0.6v on the 6 LED return lines. Not sure if that is normal at this low backlight voltage.

[mon2]

Re: MacBook Air a1466 no Backlight

Also have a good close inspection of the LVDS blade contacts (alcohol / compressed air cleaning) - especially the LVDS cable (although you are using a good known display so may not be a factor). Although it is a pain, we did a repair where the cable contacts were damaged and split with a crack. The customer had beer spills across the board.

The replacement cable was available from mobilesentrix in Toronto. The teardown of the LCD assembly was required but was well worth the effort in the end. For us, this poor cable was causing the LCD fuse to blow and tanked the backlight voltage.

[ACS-Repair]

Re: MacBook Air a1466 no Backlight

I did a clean of the LCD cable and socket connectors and they they look very good. I checked a couple more test points...
BKL_FAULT = 0V
BKL_SEL = 3.33V
I did a quick inspection and found that the IC I installed for U7701 is a 52e4 rather than an LP8550. Some sites show that it is compatible but it was not what I ordered. The writing on this IC is so small that I can barely see the pin-1 indicator. I hope that isn't what's causing the problem. This should be simple and straight forward... I don't understand.

[mon2]

Re: MacBook Air a1466 no Backlight

If you need the LP8550 - ping me a PM. Can have it drop shipped to you for Tuesday (most likely). It is purchased from Arrow and we have free shipping with them. We have many of these here in Windsor but the shipping fee will not be practical - lower cost to ship out of Arrow to you.

BTW - Mobilesentrix have these if you are nearby.

https://www.mobilesentrix.ca/lcd-bac...66-a1278-a1286

[mon2]

Re: MacBook Air a1466 no Backlight

What is the voltage @ BKL_EN ? (or is that the label you meant with BKL_SEL ?) If yes, then the voltage is ok.

Also, the FAULT pin @ 0v does not necessarily mean it is an issue as this pin is OD (open drain). It appears that this FAULT pin is going to a test point (TP7701) and do not see a local pull-up to define the 'soft high level'.

It is possible that the pin is tri-stated and only if you apply a pull-up resistor like 10k or could be higher to +3v3 - then you can measure this logic level to know if it is indicating a fault or not.

With a pull-up resistor to +3v3 on TP7701 - does the FAULT line remain LOW (0v) ? Then there indeed is a FAULT. Otherwise this line will float to the HIGH (+3v3) = no fault.

[ACS-Repair]

Re: MacBook Air a1466 no Backlight

On the LP5880, Tim Herrman mentioned that he thought these might be in low supply one day but could be replaced with an LP8552. Neither of these appear to be available, yet substituted with the 52e4. I will take you up on this if I determine that the 52e4 is the problem, thank you for the offer. I should look up a datasheet to confirm. Wasn't sure if anyone has successfully used the 52e4.
The BKL_SEL should have been BKL_SCL. BKL_EN is 2.73v.

[mon2]

Re: MacBook Air a1466 no Backlight

See attached.

The '52e4' markings are from Texas Instruments for their p/n LP8552.

Need to study the LP8552 to see if it is a valid cross for the LP8550 but the short answer is to use the LP8550 since it was deployed in the original design.

Update - see attached.

1) the LP8552 appears to be able to support a lower Vin value than the LP8550 to boost the LED backlight output voltage. That is nice but not required for this logic board.

2) the pinout of both devices appears to be the same

3) the EEprom defaults are not the same between the 2 devices (LP8550 & LP8552). Not sure if these different eeprom values will halt the use of the LP8552 but you can always test and report your results.

I would suggest to use flux and try to resolder the LP8552 you currently have onboard. 'Make it dance' and see if that improves your results.

[piernov]

Re: MacBook Air a1466 no Backlight

LP8550 is available on Mouser.

[ACS-Repair]

Re: MacBook Air a1466 no Backlight

Thank you for the comments guys. Good to know that the part I have is an LP8552, Which Tim suggested was a possible replacement. Interesting about EEprom differences (thank you for the extensive research on this). While it is suggested as a direct replacement, has anyone actually used it successfully? I did reflow it, but it didn't dance the way it should. I'll give it another try.
I need to be careful with Mouser. Last week I ordered FDMF3030 FETs for an 820-3662 board I'm working on and they shipped me FDMF6823A without letting me know there was going to be a substitution. What would not be good is for me to order the LP8550 and have them ship LP8552, something I have already. I would need to know that I'm getting the part that I ordered. It seems that all the signals required are present but the chip is not turning on.

[mon2]

Re: MacBook Air a1466 no Backlight

The offer is open to ship to you the LP8550 from Arrow:

https://www.arrow.com/en/products/lp...as-instruments

At this time, they have stock. Really want to test the free shipping perk we appear to have for international locations.

[ACS-Repair]

Re: MacBook Air a1466 no Backlight

Well, the dance was all it needed. I now have 24v at the LCD connector and the backlight is working. I appreciate the help from you and piernov in helping me fix this board.
I'm going to put it back together to make sure all is good but I believe it will be.
Thanks again!

[mon2]

Re: MacBook Air a1466 no Backlight

Good job!! Learned the technique from watching Louis' videos. I also believe this method helps to align the balls that we cannot see.

I still need to scout for a good microscope...my eyes cannot handle this tiny stuff these days. See some very tempting models on Aliexpress (AliExpress
Insein Li Fung Microscope Store). It is on my Christmas wish list Even the ring light is critical for a nice view of the target specimens.

Watch the Northridge Fix video where he has a ring light with leds at various heights / steps to prevent shadows, etc. It is cool video indeed.

[piernov]

Re: MacBook Air a1466 no Backlight

I need to be careful with Mouser. Last week I ordered FDMF3030 FETs for an 820-3662 board I'm working on and they shipped me FDMF6823A without letting me know there was going to be a substitution. What would not be good is for me to order the LP8550 and have them ship LP8552, something I have already. I would need to know that I'm getting the part that I ordered.

It could be an mistake on their end but I have never heard of such issues with Mouser, they ship what you order unlike random eBay/Aliexpress sellers. And of course all parts are genuine and new, unlike most stuff on eBay/Aliexpress.

The only time I ordered an LP8550 from eBay because I wanted to save on shipping fees it was bad, brightness adjustment not working. Worked perfectly fine after replacing with one from Mouser (and of course programming slope).

[mon2]

Re: MacBook Air a1466 no Backlight

@Piernov - the offer certainly is extended to you. If Arrow has your parts, just send a PM and we should be able to drop ship it to you with free shipping. We have this perk with our account so may as well use it. It makes sense for us to prototype daily and we often source SMD resistors / caps for repairs. They carry the original TI # LP8550 device. If it makes sense, we may start some Amazon store for electronic parts as a sideline to support the same.