820-00426 Backlight on off

[No-Clean]

Clean board, backlight comes on and off, already tried 2 known good screens and same problem, the weird part is that when backlight is off I still get 40+V on pin 1 of J8300 or PPVOUT_S0_LCDBKLT.

Diode mode measurements without lcd connected.

Pin 1 -1.22
Pin 2 - NC
Pin 3 - 0.53
Pin 4 - 0.529
Pin 5 - 0.636
Pin 6 - 1.361
Pin 7 - 1.365
Pin 8 - 0.614
Pin 9 - 0.597
Pin 10 - 0.62

 

[2informaticos]

What is the exact output backlight voltage in both cases?

 

[No-Clean]

When there is screen brightness 44V otherwise 43V, I am afraid this is a GPU issue, this morning we got some weird lines on screen, just for kicks I replaced the backlight driver but same thing.

 

[2informaticos]

Backlight circuit looks good.
I bet you always have backlight on the screen, but not easy to detect if image disappear.
There should be image problem.
Check for USB activity and external video at same moment.

 

[No-Clean]

Correct, backlight circuit seems fine, image stays on for 1-2 minutes after backlight is gone then image is gone from internal screen, but image on external monitor remains fine, I guess this rules out GPU, thanks!

Now what?

 

[2informaticos]

Clean/replace video connector.
Check all traces and caps from its data lines.

 

[No-Clean]

LCD connector is perfect, but I cleaned it and blew compressed air into it, checked and compared diode mode on connector and same thing. When turning on Mac shows image for about 1-2 minutes then brightness is gone and I use a flash light to check for image I can see the image and some lines start to come across screen, moving the connector makes no difference.

 

[No-Clean]

Okay I replaced Q7701 and Q7706 and board has now steady image and backlight!!

I replaced those because I just remembered a similar issue in the past and got solved that way, or maybe I just got lucky.

Thanks for help.

 

[No-Clean]

I am pretty sure @2informaticos knew that my last repair attempt replacing Q7701 and Q7706 was going to fail, because again the backlight circuit was fine, at the end U8310 was the problem, that chip had very little corrosion under it and when the computer runs for a little bit the connection will get lost, problem fixed!

Thanks again!