820-3115 weird no image issue

[No-Clean]

Board had corrosion in multiple parts, but nothing major.

Already tried reballing and replacing SMC

The weird problem is that there is USB activity with the display connected (tried 2 different ones) when I connect the display the USB thumb drive instead of getting activity it turns off then it tries again but it also turns off for a second or two, when this happens PPVCCSA_S0_CPU pulses to 0 then back again, this doesn't happen with LCD disconnected.

With LCD connected Pin 2 of J9000 is 0V, U9000 Pin 3 PP3V3_S5_LCD is 3.3V but Pin 1 LCD_IG_PWR_EN is 0V.

 

[2informaticos]

Please specify, missing internal and external image?
No internal image at all, w/ or w/o external monitor connected?

External monitor is driven through Thunderbolt.
Do you get correct Thunderbolt voltages?

 

[No-Clean]

Sorry, missing internal image only, external OK and when there is external image there is no internal image either.

I can see something trying to show up on screen but there is no backlight so it is hard to see what is showing up, but whatever it is it is not a clear image or anything normal.

Since I do have external image do you still need Thunderbolt voltages?

 

[2informaticos]

Thunderbolt is OK, if you get external video; wasn't clear from your first post.

You can't get internal image without 3V3_LCDVDD; for this reason no backlight also.
Post voltage and diode mode to ground at pins 4, 6, 7 of J9000.
Check L9008 and R9008/9.

 

[No-Clean]

L9080 is OK and its diode reading on both lines are ok too.
R9008/9 both OK

J9000
4 0.341 3.3V
6 0.590 3.3V
7 0.570 3.3V

 

[2informaticos]

Test with known good LCD assembly.
Be sure LCD connector has good solders, traces and internal terminals

 

[No-Clean]

Tried 2 known good LCD with board alone, LCD connector looks clean inside and out, solders look perfect, not sure what else to check.

 

[2informaticos]

PCH talks with LCD through LVDS_DDC_CLK/DATA lines, before enabling LCD_IG_PWR_EN.
You should check with o-scope if communication exists.