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820-00281 - No internal display

overshoot

overshoot

Member
Hi,

I have that board that I did purchase from eBay for a no internal display issue.
The board was not reworked, I can tell.
The only visual sign of damage that I've found was a slightly corroded Cap C7290.
The rest of the board is looking great.

The external display works fine and I can boot the system to macOS.

I thought when purchasing that board that it would be another dead U9850 but I did replace that chip already 4 times, from purchased chip online and even with one from a board that had display prior to me taking the chip (working board but with bad SSD) so I can now confirm that's not the fix.

The SMC_LID signal is 3.3V
I am getting 5V on PP5V_S0SW_LCD and 3V on PP3V3_S0SW_LCD so I am assuming that the LCD is told to turn on but no backlight.
Though it's kind of impossible to tell if there is an image on those screens but I feel like there isn't.

EDP_BKLT_EN is 0V and there is no short on that line.
The SCL/SDA lines have activity.

What else am I missing here potentially?

EDIT: I can actually already confirm it's a no image as the screen is not detected even when plugged in and the signal goes directly to the external one.
In system profiler, there is only the mention of the externalone.
I have tried 2 different known good displays with no difference.
 
Last edited:
overshoot

overshoot

Member
I do have it stable at 3.3V as well as EDP_PANEL_PWR_EN
Though from the time I get 20V at the charger and I get the signal to go to 3.3V, it takes almost 20 seconds... that's with the board alone + screen.

FYI, I have tried that board with 2 different topcases and screens but same thing.
Touchbar lights up and works.
 
Last edited:
2informaticos

2informaticos

Administrator
Staff member
If DP_INT_HPD stays high, PCH detects the screen correctly.
Compare diode mode readings for EDP data lines of J8500 with good board.
 
overshoot

overshoot

Member
I already did and I'm getting 0.37 on each EDP_INT_ML lines and 0.42 on the EDP_AUXCH lines which compares fine with a known good board.
Overall the screen connector has the same values compared to a known good board.
 
overshoot

overshoot

Member
Both do the same between faulty board and working board.
LCD_MUX_EN goes to 3.3V, goes to 0V when amperage goes to 0 and then back at 3.3V
LCD_MUX_SEL goes to 2.5V, goes to 0V when amperage goes to 0 and then back to 2.5V and finally 3.3V (at that point working board has the screen turning on)
 
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