820-00840 external image only

[Atomrepair]

This one came in with liquid damage that was repaired by someone else. U2890 and some components around it were replaced, flux was still on there.

This board powers up fine and shows an image on external display, but not on the built-in screen. No corrosion around J8500 or the board to board cable. I've tried a known working display, but nothing.

Backlight is not coming on, and there is no image visible when viewing with a flashlight.

I compared J8500 pins to those on a working board in diode mode; they're the same.

I suspected U8400 as its somewhat close to U2890 and replaced it, but no change.

Stuff measured:

BUF_EDP_PANEL_PWR_EN	3,4V
PPVOUT_S0_LCDBKLT	0V

U8400 input check
BKLT_EN_R	3,3V
PP5V_S0_BKLT_A	5,1V
BKLT_SD	12,8V

As I understand it, BKLT_SD needs to be lower than 12,8V for U8400 to open up. I'm guessing that U8400 simply isnt 'told' to activate. Any help be welcome!

 

[2informaticos]

Image must come on before backlight.
So you don't have a backlight issue.

Do you get both 3V3/5V_S0SW_LCD voltages?
DP_INT_HPD should go high at same time.

Liquid ingress on backlight area is not good at all.
Data lines from internal copper layers could be affected.

 

[Atomrepair]

Ah, image before BL. Didnt know that, thanks.

Measured (with display attached):

PP3V3_S0SW_LCD	3,3V
PP5V_S0SW_LCD	5,1V
DP_INT_HPD	3,3V

 

[2informaticos]

Look s like CPU issue then.
Its internal LCD output is bad, if no image for sure.

 

[Atomrepair]

Well, that sucks. Can you think of a way to make it think the internal display is actually the external one?

 

[2informaticos]

PCH detects intrernal LCD, as voltages and hot plug signal stay high.
eDP data bus is porobably damaged...