Apple A1286 Mid 2012 MacBook Pro 820-3330 LCD Backlight issue

[barry101]

Hi everyone.

I hope someone has some suggestions on how to proceed with this issue.

I have an Apple A1286 2012 MacBook Pro with 820-3330 Logic board. My customer purchased this from me in full working condition. Unfortunately, it overheated on him due to being left running on his bed.

Investigation showed the LCD Backlight was not working (had a latent image and external monitor worked), the battery was no longer functioning (system couldn't read it or charge it) and the Samsung 850 EVO SSD was dead (not seen by finder or disk utility). Quite the trifecta.

Investigating the backlight problem showed the Fuse was goo, the boost voltage was present and all enable and control signals and voltages to U9701 were fine. The LVDS connector appears fine. However, there were no pulses on the Backlight return lines.

Changed U9701 (it was the first time I needed to change this chip and it appeared to go well) but the problem remained. Trying to save time, I installed another 820-3330 board but the backlight still didn't work (latent image present). Thinking it must be the display, I installed a different A1286 2012 display assembly/LVDS cable with the replacement board and still just a latent image. Tried the replacement display with the original board with the replaced U9701 and same thing. The replacement display and logic board were purchased on eBay as working parts.

Is it possible that the original display failed and killed the U9701 and then killed the replacement U9701 and then went on to kill the replacement board U9701?

I tried the original 2012 display on a test bed setup with an 820-2850 board (the LVDS cable connector fit - left the locking bale up to avoid any shorts). The board booted but did not get a latent image or backlight. The test setup then worked fine with a different display assembly. So this test seems inconclusive.

Any suggestions are extremely welcomed.

Thanks

[mon2]

Re: Apple A1286 Mid 2012 MacBook Pro 820-3330 LCD Backlight issue

See here on LCD compatibility:

https://logi.wiki/index.php/MacBook_..._Compatibility

[barry101]

Re: Apple A1286 Mid 2012 MacBook Pro 820-3330 LCD Backlight issue

Thank you for your reply and the link. I was aware of most of that information as it related to A1286 displays but there was additional info I did not know about so thank you. Greatly appreciated!

Question: Do you think a hard failure in an A1286 display can cause three U9701 chips to fail such that there is no pulsing activity on the backlight return lines? It's frustrating that changing the U9701, then the entire logic board and then the entire display/LVDS cable could not correct the issue. Am I overlooking something here?

[mon2]

Re: Apple A1286 Mid 2012 MacBook Pro 820-3330 LCD Backlight issue

No but will review the schematic after a late dinner. Also our buds from down under will be starting the midnight shift soon

You should have more feedback soon.

Update:

Investigation showed the LCD Backlight was not working (had a latent image and external monitor worked), the battery was no longer functioning (system couldn't read it or charge it) and the Samsung 850 EVO SSD was dead (not seen by finder or disk utility).

OMG. Macbook needs an exorcism like yesterday.

Q1: Do you have USB activity with a pendrive ?

Q2: With the original LCD display, do you have a backlight voltage ? What is the value of the backlight ?

Q3: Any heat being generated by this unit ? From where ?

Remove the LCD display.

Remove the battery.

Attach the original power adapter.

Check the voltage of:

BKL_SCL
BKL_SDA

Post the measurements.

Do the fans & thermal sensors work on this unit ? They could not cool down the laptop ?

[barry101]

Re: Apple A1286 Mid 2012 MacBook Pro 820-3330 LCD Backlight issue

Thank you.

[reformatt]

Re: Apple A1286 Mid 2012 MacBook Pro 820-3330 LCD Backlight issue

Post voltage for PPVOUT_SW_LCDBKLT, usually 25V or more. Then check LCD_BKLT_PWM duty cycle, and see if that varies with brightness up/down adjust on keyboard.

[mon2]

Re: Apple A1286 Mid 2012 MacBook Pro 820-3330 LCD Backlight issue

Just updated the thread above. The fresh eyes are here

[barry101]

Re: Apple A1286 Mid 2012 MacBook Pro 820-3330 LCD Backlight issue

Hi. Thanks for the replies!

When I went back in to get the requested measurements, I was surprised to find there was now no backlight voltage. Also, the fuse was now blown. Checked for short to ground at fuse and found none. Replaced fuse and it blew again. Found pin 40 on the replacement display LVDS cable connector was mangled and shorted to ground (should have checked for short at the connector instead of the fuse). Checked the original board where I had also plugged in that display and its fuse is now also blown. Cable on original display is fine. Checked eBay listing photos of the replacement display and cable connector was fine so somehow it got mangled/shorted when I installed it (done this many times in the past, never had this happen before - always proceed with great care). The damage wasn't from probing this morning since the blown fuse on the original board must have happened yesterday.

So the theory that a hard display failure in the original board blew 3 U9701 chips is definitely looking very shaky. Anyway, I will report back after repairing the fuses on both boards and reinstalling the original display which has a good cable. Again, thanks to all!

Barry

[barry101]

Re: Apple A1286 Mid 2012 MacBook Pro 820-3330 LCD Backlight issue

Update: Replaced blown backlight fuses on both the original and replacement boards. Here are the requested measurements:

Original board with original display AND replacement board with original display:
PPVOUT_SW_LCDBKLT: 38.4 VDC.
LCD_BKLT_PWM: varies with keyboard brightness control up/down

Original board and replacement board without display connected, without battery, and connected to original power adapter:
BLK_SCL 3.3 VDC.
BKL_SDA 3.3 VDC.

The laptop does not produce any abnormal heat.

All laptop functions and interfaces work fine, just no LCD backlight. So regarding the pen drive, it works fine and shows up on the desktop on the external monitor.

Spoke with customer. He said he had it on the carpet and it got hot. He shut it down or at least he believes he was able to shut it down (I think it may have been still running with the lid closed) and the next day it wouldn't work. He doesn't use it a lot and literally took several months to bring it to me. Therefore, I can't be sure which specific failure or failures occurred do it running on the carpet and getting hot since any of the failures would look like it wasn't working.

Since the replacement display needs a new cable, I will have to order one in order to use it for troubleshooting. I have some A1286 pre 2012 displays that have either broken glass or bad LCDs that still have good backlight. I will try trimming one of those LVDS cables to fit the 2012 socket and see what happens.

In the meantime, any other suggestions are welcome.

Thanks again everyone.

[mon2]

Re: Apple A1286 Mid 2012 MacBook Pro 820-3330 LCD Backlight issue

I will try trimming one of those LVDS cables to fit the 2012 socket and see what happens.

No, do not recommend this approach. The cables (Aliexpress; Mobile Sentrix) nor used displays (Ebay?) should not be expensive. Perhaps pickup a donor box like the same for further testing?

Trying to trim the cable contacts will open up another pandora's box of issues.

The backlight voltage appears to be ok.

Can you confirm the each of the OUT1..OUT6 lines on (BKL_ISENSx) are present ?

The 0 ohm resistors on each of the lines must be present to allow for the backlight to function.

[mon2]

Re: Apple A1286 Mid 2012 MacBook Pro 820-3330 LCD Backlight issue

Confirm the LCD is being powered @ pins 2 & 3 of the LCD connector (J9000).

Also that the LVDS inductors are present as they are required for all of the LVDS signals to reach the display panel.

These are L9010 & L9011.

Voltage of:

LVDS_DDC_CLK ?
LVDS_DDC_DATA ?

[barry101]

Re: Apple A1286 Mid 2012 MacBook Pro 820-3330 LCD Backlight issue

Update: I took a 2010 A1286 display assembly that was missing pin 40 but otherwise worked, trimmed the connector to fit the 2012 connector and connected it to one of the boards. After ensuring no pins were shorted to ground, I powered it up on the bench and the backlight came on.

Conclusion: The original display assembly was the cause of the backlight problem and nothing was wrong with any of the U9701 chips.

The replacement display assembly pin 40 was apparently mangled/shorted to ground during installation wiping out the fuses on both boards.

I will order a new 2012 LVDS cable, repair the replacement display and install it.

The original display assembly with the backlight fault will go to the scrap parts pile unless there is some way to repair its backlight circuit.

Thanks to all for your time and valuable suggestions! Greatly appreciated!

Barry

[barry101]

Re: Apple A1286 Mid 2012 MacBook Pro 820-3330 LCD Backlight issue

To Mon2

I agree that trimming the LVDS connector is a risky move. Only did so just to test the logic board since I didn't have a cable in stock and removing the glass is such a pain. I used a 2010 display assembly that had a missing pin 40 on the LVDS connector. Very carefully installed and verified every pin wasn't shorted before applying power. As soon as the backlight came on, immediately shut everything down and disconnected power and the display. Display went back into bad parts inventory.

Regarding the other clock and data signals, since there was latent info on the display that could be seen by shining a light through the display and the display was being recognized by the processor, I was pretty confident that those signals were okay.

Thanks again for your help, much appreciated!