A2141 820-01700 No LCD Backlight; 55 ohms between PPVOUT_S0_LCDBKLT and ground

vinaypundith

New member
Hello,
I have a 2019 16" MacBook Pro (A2141) with no LCD backlight (the laptop does power on, boot to macOS and display stuff on the screen, but the backlight is off so you have to shine a light at the screen to see anything).
I tested the PPVOUT_S0_LCDBKLT line at the terminals of L8452 and CC786 while the laptop was running, got 0 volts (while the corresponding points on a working A1707 unit I tested were at 50-ish)
With the motherboard removed, I'm getting 55 to 56 ohms resistance between any point on PPVOUT_S0_LCDBKLT and ground (a screw hole).

So what I'd like to know is:
- Does that resistance indicate a "short to ground" on the PPVOUT_S0_LCDBKLT line? A working A1707 board I tested is an open circuit between that line and ground.
- If it is a short to ground, how would I find which component is causing it?
- Where is this power line generated, what chip/component, and from what source power line? I can't seem to find it on my schematic or boardview.
- If it is not a short to ground, what do I test?

Thanks!
 

2informaticos

Administrator
Staff member
First of all, welcome to the forum!

"Does that resistance indicate a "short to ground" on the PPVOUT_S0_LCDBKLT line?"
Not pure short, but low enough resistance to block the backlight output rail.

I bet for one of 2.2uF/100V capacitors, C8460-73.
Removing L8452, you can discard an internal leakage into J8500.

Voltage injection method can help you to find the culprit.
Start with low voltage and slowly increase, if needed.
I recommend first a visual inspection of mentioned capacitors.
If any looks strange, remove it and check again resistance to ground.
 

vinaypundith

New member
Alright, I'll try that. What's a good voltage to start with, to inject after removing L8452?
Also, when I desolder the capacitors, what temperature do I need to keep the hot air machine at, and what distance away from the board is best?
 

2informaticos

Administrator
Staff member
Each hot air station has its settings; practice on scrap boards.
I personally use JBC hot tweezer, to remove such components.

55 ohm is not very low resistance, but you have nothing to lose starting with just 1V.
Thermal camera can help you a lot in this case.
If no available, carefully touch corresponding board area and use alcohol evaporation method.

Just in case, check resistance to ground at L8410 too.
 
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