Macbook A2289 820-01987A no backlight

[AlexVG]

Hi everybody,
I have Macbook A2289 820-01987A.
It have had 1.8V short circuit problem that I fixed.
But after 20 minutes working the backlight went out.
I found out missing of EDP_BKLT_EN going to U8400 from U0500.
Ground down level goes from U0500. (I tread to remove R8442).

The machine starts perfect. I can watch image on the screen with a flashlight, and it works with the external screen.
Do you have any suggestions what kind of problem I came across?

Thank you in advance.

[mon2]

Check the open lid sensor signal line. The trackpad is linked to the open lid sensor so review all interconnected cables.

[AlexVG]

Hi Mon2.
Really thank you for your help.
Recently I have checked Lid lines.
I am sure that LID_ OPEN_RIGHT and LID_ OPEN_LEFT have reaction (Hi) (I checked them on U6650)
Moreover, machine starts when I open the lid.
But it's quite complicated to check IPD_LID_OPEN on U4802
and on U8400 with all connectors connected because they are on a back side of the main.

___________________________________

I Have managed to check IPD_LID_OPEN. Not directly, but throw J5100 connector.
IPD_LID_OPEN is present.
And now to check EDP_BKLT_EN signal, I need to connect wire.

[mon2]

Does the backlight now remain always dark?

[AlexVG]

Yes.
But keyboard lights is present (so as it were before)

[mon2]

As a test, flux and remove R8442 (0R resistor). Then with a piece of wire, mate PP3V3_G3S to BKLT_EN_R. This is the EN pin on U8400 and you are applying a logic '1' onto this enable pin.

Does this turn on the LCD display backlight? Perform this test only if you are comfortable and with a steady hand. Alternatively, you can solder a piece of wire to do the same.

[AlexVG]

Thank you.
Yes, I thought about it.
Will try tomorrow.
It would be good luck if only U8400 damaged.

[AlexVG]

I guess I have good luck! It doesn't work!
(I connected enable signal to 3.3V)
It most likely means that I have U8400 problem.
It's not a big problem.
I have ordered it and waiting for delivery.

[mon2]

Strange because keyboard light is from the same part. Not clear on why the lcd backlight enable signal is not on. If lid is open, the enable should activate.

Are you sure the trace to the chip was used to enable?

[AlexVG]

I meant that I tested it without connected all connectors. Only power supply.
Keyboard light presents if the main is in the chassis with all connections.
I checked EDP_BKLT_EN on the table. I did't know that it need lid open signal.

I am waiting for new 48B1A04. Hope that it is a problem.

[AlexVG]

I am really stuck.
I replaced U8400 and Q8400.
I got EDP_BKLT_EN signal from processor.

Q8400 gets voltage drop on resistor R8402, However, voltage on gate of Q8400 does not change (always 12.6V). This voltage should drop to 5V, but it does not.
Please help me

[mon2]

Still working on my beauty sleep but check the resistance of R8402. It is important. Be sure that the backlight pin does pull to 0v, then the gate voltage will drop.

As a test, short the gate pin to ground with a piece of wire. Does the backlight turn on with this manual bypass? The mosfet is a p-channel type so gate voltage must be lower to enable.

[mon2]

Also, test the mosfet as well. It could be defective.

Back to counting sheeps..

1 Apple customer
2 Apple customer…

[AlexVG]

Hi Montwo.
So sorry for the yesterday.

Yes, I have replaced Q8400
And checked R8402
Moreover I tested U8400 in diod test mode and compare with another board.

What do you mean "the backlight pin does pull to 0v" ?
Where?

[mon2]

R8402 pin #2 needs to be at 0 v to pull the gate voltage low. Be sure r8402 is of the proper resistance.

Can you test the gate pin at ground with a piece of wire?

[AlexVG]

Hi, Montwo.
It were problem in missing BKLT_SCL, and BKLT_SDA going from U8549
I have managed to solve it.


It's a very interesting A2289 model.
Looks like the A1708, but without connected J5100 and J5110 it's not available to start the screen.
It makes this board complicated to check.

Thank you.

[AlexVG]

I am sure you know it, but I found out this article very useful.

https://logi.wiki/index.php/Backlight_Troubleshooting