MacBook Pro A1706 - 820-00239 - No backlight

[ryogadan78]

Hi guys, I have a MacBook Pro (MB 820-00239) with LED panel backlight problems. I haven't seen any oxide damage on the mainboard or other evident problems.

The problem suddenly occurred after turning it off regularly and putting it in the bag. When I turned it back on, the backlight no longer worked.

Where can I start testing and measurements to identify the problem?

I have board view of the mainboard.

Thanks!

[mon2]

1) could be flexgate issue

2) does the caps key led toggle after a normal boot time? This means the logic board has booted and you are at the login screen.

3) check if the display is working with a flashlight or phone torch light across the front lcd panel. Can you see any of the usual graphics?

4) If not, check the voltage to ground of ppbus_g3h. If it is low, remove all power and check the resistance to ground at the same ppbus_g3h fuse.

Post each measurement .

[ryogadan78]

Hi mon2, thanks again.
The MacBook comes to the login screen regularly, I can see the usual graphics using a flashlight sideways.
The flex cable does not appear to be bent or broken, but I could be wrong.

Can I make any specific measurements on the motherboard related to the voltages that handle the backlight?

[mon2]

Measure the voltage to ground of the backlight output voltage with the screen connected. This is the same voltage you can measure at the lcd connector on the board or nearby. This should be a suitable boosted voltage to power the backlight led array.

also, measure voltage of:

BKLT_EN_R
BKLT_SD

[ryogadan78]

Hi mon2,
I could not find a place to measure backlight output voltage. Can you help me (a net point alternative)?
The other measurements, with LCD connector plugged in, are:

BKLT_EN_R 0V
BKLT_SD 0V

[mon2]

Measure the voltage to ground of:

I2c_bklt_scl
i2c_bklt_sda

with and without the lcd connected to the logic board.

confirm the pp5v_s0 is present.

[ryogadan78]

Here the measurements with LCD connected to mainboard

BKLT_EN_R 0V
BKLT_SD 0V
I2c_bklt_scl 0V
i2c_bklt_sda 0V

Here the measurements with LCD NOT connected to mainboard

BKLT_EN_R 0V
BKLT_SD 5.76V
I2c_bklt_scl 3.68V
i2c_bklt_sda 3.68V

[mon2]

You are losing key voltages when the lcd is attached.

Remove all power. Meter in resistance mode. Measure the resistance to ground of PPVOUT_S0_LCDBKLT with and without the lcd attached. You can measure this resistance at the output inductor/coil or any of the output capacitors that are also on this rail. One meter probe here and other meter probe to ground (metal shield is ok).

[ryogadan78]

No power connected, Meter in resistance mode (200k)

WITH AND WITHOUT LCD attached same measure: PPVOUT_S0_LCDBKLT resistance to ground is over 100k ohm (keeps growing as I leave the tip in measurement)

I also measured the voltage at these points and it is 12.8V (with and without LCD attached)

[mon2]

Ok. Measure the voltage to ground of pp5v_s0 with and without the lcd. Check the resistance to ground of this rail as well under the same conditions.

[ryogadan78]

Forgive me for the delay but I had health problems.

WITH AND WITHOUT LCD attached same measure: pp5v_s0 5,08V - 200ohm (resistance to ground)

[kevingill]

Couple more things to check. Is F8400 OK? Do you have a voltage on PPVIN_S0SW_LCDBKLT?
Usually it's the backlight driver chip that goes U8400.

[ryogadan78]

Hi,
F8400 is OK.
PPVIN_S0SW_LCDBKLT voltage 12.8V

[ryogadan78]

Hi mon2, I think you have found the problem once again.
Looking carefully under the microscope, I noticed that the U8400 chip has a burn hole.
Do you think this is just the problem? Is it easy to change it?

[ryogadan78]

1x P8548BOSQ-02 48BO-02 ... 48B0-02 ... LP8548B0SQ LP8548B1SQ LP8548 QFN-24 | eBay Is it the right component?

[mon2]

Good job. You guys have found the root fault. Yes, this chip is defective if it has a crack or hole in it. Flux it well and wait till the solder melts to replace it use a slight nudge of the tweezers to remove it. Best to learn the process on a donor board. Yes the part on eBay looks suitable. Be sure to solder back in the same orientation.

[ryogadan78]

Thank you very much. I will update you as soon as the component arrives.

[ryogadan78]

Hi mon2. The IC arrived today. I removed the old IC without too much trouble, and was about to solder the new one. I had a doubt about the layout because strangely the marker is aligned differently on the two chips (compared to the lettering). I'll send you two photos, do you think it is positioned correctly this way?


Original chip

New chip just in position (not soldered)

[mon2]

Wait. The location and part in the original pic is not the backlight controller. The TMP513 is a current sensor. This part is not defective. Place the original TMP513 back onto the board.

you will have to locate the defective backlight ic with the hole in it. Share new pics when ready.

https://www.ti.com/lit/ds/symlink/tm...lsrc%253Daw.ds

[ryogadan78]

You're right I got confused. I will provide