MacBook A1708 820-00840-A backlight issue

[Riananyaina]

Hello Everyone ,

today I'm coming to you regarding my issue which is no Backlight .

I have image on my screen but no light .

The macbook was one that I bought few month ago that I used with an external screen . Now I started to do some analysis on the MB which my limited knowledge .
First thing is that it suffered from liquid damage . The fuse was not ok when I openned it the first time . But I already changed it after testing that there is no short to ground on this line .

Please find below few informations which can help for the analysis :
- I attached all ok component with few measurements
-There is no short to ground on the output line with & without the screen connected .
-I know the screen backlight is good because I tested it with another MB .
-I tried to manually ground the SD line and it gave me 12v on the backlight output PPVOUT_S0_LCDBKLT.So the Q8400 is working fine .
-I measured EDP BKLT EN : 3.3v (HIGH ?)
-I measured BKLT SCL & SDA : around 3.3 v ( so communication is working ? )
- I changed the U8400 but still no change.


I read a post that the BKLT_PWM could be the issue but I don't really know how it works . One other backlight IC , I saw PWM input . On LP8548 , it's not the case . I only see that BKLT PWM KEYB but I assume it's for the keyboard . And this one is working .
On rossman video , he talked about bypassing the PWM control which will result to a maximum brightness all the time but it's ok for me if I can get that .
He did it by injecting the expedted voltage on the PWM input of the backlight IC . But since there is no PWM input on the A1708 I don't know how can I do it .
I saw that the PWM is generated by the U0500 whic is the CPU ?
And then go to the J8500 on the TCON board . Can I send 3.3v on the pin 16 of the J8500 with a jump wire ?


I hope that with all those information you can help me .

Thanks in advance .

[mon2]

Re: MacBook A1708 820-00840-A backlight issue

https://macmyths.com/how-to-fix-flexgate-macbook-pro/

-I know the screen backlight is good because I tested it with another MB .

Same set of cables? If yes then this is not the fault.

Check lid open signal status - linked to the trackpad.

Update:

1) check the voltage to ground of LID_OPEN. This signal may be ok since you have EDP_BKLT_EN = '1' (high).

2) check the voltage to ground of BKLT_EN_R to be sure this same enable status is reaching the IC. Do check the backlight IC for any possible corrosion.

3) What is the voltage to ground @ pin #16 of J8500 ?

[Riananyaina]

Re: MacBook A1708 820-00840-A backlight issue

https://macmyths.com/how-to-fix-flexgate-macbook-pro/


Same set of cables? If yes then this is not the fault.

Check lid open signal status - linked to the trackpad.

Update:

1) check the voltage to ground of LID_OPEN. This signal may be ok since you have EDP_BKLT_EN = '1' (high).

2) check the voltage to ground of BKLT_EN_R to be sure this same enable status is reaching the IC. Do check the backlight IC for any possible corrosion.

3) What is the voltage to ground @ pin #16 of J8500 ?

Hello ,

Yes I used the same cable for the test .

1) check the voltage to ground of LID_OPEN. This signal may be ok since you have EDP_BKLT_EN = '1' (high).
For the LID_OPEN , I will check but I think it's good . When the macbook is connected to an external screen when I close the LID , the screen goes off . Until I move the mouse .
When I open , it comes back . I'll measure it anyway .

2) check the voltage to ground of BKLT_EN_R to be sure this same enable status is reaching the IC. Do check the backlight IC for any possible corrosion.
Will do it and update here

3) What is the voltage to ground @ pin #16 of J8500 ?
I'll do it too and I will update here .

[mon2]

Re: MacBook A1708 820-00840-A backlight issue

Based on the feedback, the lid_open status is fine.

[Riananyaina]

Re: MacBook A1708 820-00840-A backlight issue

If the EDP_BKLT_PWM is the issue , do you think forcing this signal to be at 3.3v or 5v will enable the backlight circuit ?
As on this video :
https://youtu.be/gC_mixwqLHI

[mon2]

Re: MacBook A1708 820-00840-A backlight issue

In theory, yes but concern is that the model of your logic board does not appear to have a current limit resistor between the source and target (display).

That is, your original source for the PWM signal is driving a logic low = 0 volts.

But for the fix, you plan to assert a hard wired 3v3 from a local power rail.

That is 2 outputs fighting out the signal levels.

The hack from the YT / Louis' fix, had a 33 ohm resistor in series which limits the current by the original driver.

If you really wish to attempt this fix, trace the PCB trace that is linked to the PWM line. Then with a sharp cutter, cut the trace. Plan to do it in a location where the same PCB trace can be repaired if required.

After the trace is cut, then apply the 3v3 onto the LCD side of the (PWM) cut trace for testing.

[Riananyaina]

Re: MacBook A1708 820-00840-A backlight issue

In theory, yes but concern is that the model of your logic board does not appear to have a current limit resistor between the source and target (display).

That is, your original source for the PWM signal is driving a logic low = 0 volts.

But for the fix, you plan to assert a hard wired 3v3 from a local power rail.

That is 2 outputs fighting out the signal levels.

The hack from the YT / Louis' fix, had a 33 ohm resistor in series which limits the current by the original driver.

If you really wish to attempt this fix, trace the PCB trace that is linked to the PWM line. Then with a sharp cutter, cut the trace. Plan to do it in a location where the same PCB trace can be repaired if required.

After the trace is cut, then apply the 3v3 onto the LCD side of the (PWM) cut trace for testing.

So I finally measured the voltage that you asked .

1) check the voltage to ground of LID_OPEN. This signal may be ok since you have EDP_BKLT_EN = '1' (high)
LID OPEN = 3.4 v
And EDP_BKLT_EN = 3.3v

2) check the voltage to ground of BKLT_EN_R to be sure this same enable status is reaching the IC. Do check the backlight IC for any possible corrosion.
BKLT_EN_R = 3.3v
No corrosion seen .

3) What is the voltage to ground @ pin #16 of J8500 ?
I wasn't able to measure it with the macbook turned on because it's on the other side but MB out with screen connected , and charger connected I read
EDP_BKLT_PWM = 2V
TCON_BKLT_PWM = 2V


This macbook is strange ... I all input seems to be good ..
I hope we'll find the issue .

[mon2]

Re: MacBook A1708 820-00840-A backlight issue

Does the pwm voltage change with the keyboard use of the backlight intensity keys?

[Riananyaina]

Re: MacBook A1708 820-00840-A backlight issue

Does the pwm voltage change with the keyboard use of the backlight intensity keys?

I don't know how I can do it because of the connector which is on the other side of the board . The same for the available test point .
Do you have an idea ?

[Riananyaina]

Re: MacBook A1708 820-00840-A backlight issue

Does the pwm voltage change with the keyboard use of the backlight intensity keys?

I finally succed to measure the PWM while changing the intensity with the keyboard .
So yes it change from 0(min) to 2v(max) .

[mon2]

Re: MacBook A1708 820-00840-A backlight issue

So the pwm signal is not the fault.

Check the steering diode in the backlight circuit and do this in diode mode. Remove all power. Measure the diode with both probes.

What is the reading?

Then reverse the meter probes.

What is the reading?

[Riananyaina]

Re: MacBook A1708 820-00840-A backlight issue

So the pwm signal is not the fault.

Check the steering diode in the backlight circuit and do this in diode mode. Remove all power. Measure the diode with both probes.

What is the reading?

Then reverse the meter probes.

What is the reading?

OL and 0,33v for the diode.

But right now what I cannot undestand is that why I have 12v on source and gate of the Q8400. The votage divider isn't working because the IC doesn't short it(SD) to ground. Why ? I don't know ...
What is sure is that Q8409 is good because when I manually shorted the SD line, It allow the 12v to go from source to drain and on the gate I measured 5.7v

[mon2]

Re: MacBook A1708 820-00840-A backlight issue

I think you have replaced this backlight IC ?

Consider to flux and reflow this part but protect the tiny parts in the region with kapton tape. Use low air pressure.

Keep heat away from the LCD!!!!

Any possibility of missing parts in this area?

Post pics if you can. Be sure the enable pin is reaching the backlight IC as well as the I2C lines.

[Riananyaina]

Re: MacBook A1708 820-00840-A backlight issue

I think you have replaced this backlight IC ?

Consider to flux and reflow this part but protect the tiny parts in the region with kapton tape. Use low air pressure.

Keep heat away from the LCD!!!!

Any possibility of missing parts in this area?

Post pics if you can. Be sure the enable pin is reaching the backlight IC as well as the I2C lines.

Yes indeed I replaced the backlight IC.
Do you think , I'll need to replace it again?I'll need to find a new one.

I checked everything around but I didn't found any missing parts.
Please find attached some pictures. I hope you can see well.

[mon2]

Re: MacBook A1708 820-00840-A backlight issue

The 3rd pic shows the component to be at an angle.

Recommend to apply flux and low heat on the top surface in circles.

Keep up the heat around the perimeter of the device till the solder melts. When it does the ic will self align onto the pcb pads. If required, increase the air pressure but be sure the other parts do not go flying. Topside air will melt the solder pads and underside metal belly to align the mount.

Then test again.

[Riananyaina]

Re: MacBook A1708 820-00840-A backlight issue

The 3rd pic shows the component to be at an angle.

Recommend to apply flux and low heat on the top surface in circles.

Keep up the heat around the perimeter of the device till the solder melts. When it does the ic will self align onto the pcb pads. If required, increase the air pressure but be sure the other parts do not go flying. Topside air will melt the solder pads and underside metal belly to align the mount.

Then test again.

Hello , so I did solder it back and as you said it aligned itself onto the pad now .
Unfortunately still no backlight .
I'm thinking maybe the IC that i bought might be bad ?
I'll find another one .
After measuring almost all the board , I couldn't find any bad area . All input seems to be good . So the remaining component which could be bad is the IC .... .

[Riananyaina]

Re: MacBook A1708 820-00840-A backlight issue

The 3rd pic shows the component to be at an angle.

Recommend to apply flux and low heat on the top surface in circles.

Keep up the heat around the perimeter of the device till the solder melts. When it does the ic will self align onto the pcb pads. If required, increase the air pressure but be sure the other parts do not go flying. Topside air will melt the solder pads and underside metal belly to align the mount.

Then test again.

Hello @Mon2 ,

I tried to find for broken trace or something like that and still nothing .
But When I looked around J8500 I found a shorted line which is the PPVCCGT_S0G .

I don't know what is that and if there is a link with the backlight .

What is strange is that the macbook work fine with an external screen .

Do you have an idea of what could be the problem ?

Thanks in advance ;.

[mon2]

Re: MacBook A1708 820-00840-A backlight issue

This is a GPU power rail so it is a low voltage rail. By design, low voltage rails are with a low resistance to ground. Just like the CPU power rail.

If you can run the external monitor in normal mode which should kick in the GPU power rail then there is nothing wrong here.

To give you an example, we are developing with a relatively new micro where the VCC power rail is 0v3. Ideally hoping the widget can be battery powered for a while before it needs to be recharged.

Lower voltage = will run cooler.

[Riananyaina]

Re: MacBook A1708 820-00840-A backlight issue

This is a GPU power rail so it is a low voltage rail. By design, low voltage rails are with a low resistance to ground. Just like the CPU power rail.

If you can run the external monitor in normal mode which should kick in the GPU power rail then there is nothing wrong here.

To give you an example, we are developing with a relatively new micro where the VCC power rail is 0v3. Ideally hoping the widget can be battery powered for a while before it needs to be recharged.

Lower voltage = will run cooler.

Hello mon2 ,

Thank you for the explanation above .

I was a bit despoerate for me macbook case but yesterday I discover that on Logowiki there was diode measurement value of good board . And after doing this I found 2 strange points . I hope you can highlight me .

I strarted from there :
https://logi.wiki/index.php/MacBook_...500_Edited.jpg

On the pin 24 I found 1100 instead of 512 which is almost the double . Is this value telling us something ?
On the pin 43 which is the backlight circuit output normally , I found OL . If we have open line do youthink that the backlight circuit can detect it and decide to not send voltage ?

Many thanks in advance for your help .