820-00875 No backlight

[No-Clean]

Clean board, no history, I do have USB activity and image.

With known good LCD connected and new U8400:
PPVOUT_S0_LCDBKLT 0V not shorted
PP5V_S0_BKLT_D 5V
PP5V_S0_BKLT_A 5V
ISNS_LCDBKLT_N 13V
ISNS_LCDBKLT_P 13V
BKLT_EN_R 0V R8442 OK
EDP_BKLT_EN 0V I think this comes from CPU

 

[2informaticos]

As per schematic note, enable comes from PCH.
If image is present, PCH detected internal LCD correctly and should enable backlight too.
Try PRAM reset 2-3 times.

 

[No-Clean]

Right, never mind, there is no image, but I do have USB activity for sure, already tried clean ME bios and same thing.

 

[2informaticos]

Missing image explains backlight problem then; you should get image first.

Do you get steady 3V3/5V_S0SW_LCD?
If yes, DP_INT_HPD must go high too.

 

[No-Clean]

Both are missing

 

[2informaticos]

EDP_PANEL_PWR_EN should appear at least in the first few seconds (when chimes).

 

[No-Clean]

Yes EDP_PANEL_PWR_EN does come up for about 1sec at 3.3V when Mac starts up.

 

[2informaticos]

Did you check 3V3/5V_S0SW_LCD at same time?

 

[No-Clean]

3V3/5V_S0SW_LCD comes up for about 1 sec when booting.

 

[2informaticos]

No pulse at DP_INT_HPD at same time?
This denotes a problem with LCD assembly, or video connector.

 

[No-Clean]

DP_INT_HPD also pulses 3.3V at boot up, flex and lcd are known good ones.

 

[No-Clean]

But still no image

 

[No-Clean]

@2informaticos Should I check anything else?

 

[2informaticos]

Looks like PCH doesn't get DP_INT_HPD signal.
May have bad trace/solder, or internal damage.