820-00426 no internal display boots fine on external, does not detect internal

[MUUK]

No liquid that can be seen. Case is in good condition and dosen't suggest a drop or physical incident.

No backlight and no image on internal LCD.
External monitor connected, machine boots with external as main screen. (showing Apple logo etc externally so not detecting internal)
Tried new internal DP cable to LCD, no change.
Tried known good 2015 LCD assembly no change.

PPVOUT_S0_LCDBKLT a flat 0v diode mode (LCD attached) 1.19v drop to ground.
PP5VR3V3_SW_LCD 5v

Working off a boardview for 820-3787 the area round the LVDS/eDP is different.

Using the 820-3787 board view all the LED_return lines (pins 3 through 8) are (diode mode) 0.49 - 0.52v drop to ground.
Checked other pins and compared to board view no unexpected shorts.
regretfully don't have any donors for these at this time.

Any help or thoughts appreciated!

 

[2informaticos]

First of all, forget "v" sign when you post diode mode.
That generates confusion.

"PPVOUT_S0_LCDBKLT a flat 0v diode mode"
Becauise in this case you have a short...
Unless you've interpreted "OL" shown in diode mode as 0V; wrong assumption.

Always post diode mode readings in 3 digit after dot point.
For example 543, 0.543, or .543 and we know what that mean; if diode mode is mentioned.

Clarify now what you get in diode mode (red probe to ground) at PPVOUT_S0_LCDBKLT w/ and w/o LCD connected.

 

[MUUK]

My apologies that sentence was not clear.

PPVOUT_S0_LCDBKLT is 0v when machine running with LCD attached.

diode mode (LCD attached) 1.195
diode mode (LCD NOT attached) same 1.195

 

[2informaticos]

Confirm if eDP cvonnector (LCD) is same as 820-3787 MLB.
If yes, check voltages at pins 9 and 28-30.
Be sure R/L8300 are OK.

 

[MUUK]

Seems like this has a much similar eDP/LVDS area to the 820-00163 which i do have boardview / donor for.

With LCD attached and machine powered i do get
(pin 5) PP3V3_S0_EDP_SW = 3.3v
(pin 8 ) LCD_BKLT_PWM_R = 2.4v
(pin 9) LCD_HPD_CONN = 3.3v
(pins 28-30) PP5VR3V3_SW_LCD = 5v.

comparing all pins to 820-00163 (donor) not seeing anything with more than 5-10% difference in diode mode.

R8300 reads as 0ohm as per schematic.
L8300 seems to be doing its job as 5v is present on pin 2

Real head scratcher.

 

[JohnB8812]

With all those voltage lines there I would think you should have image....tried known good 2015 15" screen? Can't be 2013/14 has to be 15.

 

[MUUK]

Yes, 2015 15" known good screen testing with. Totally don't get it at all.

 

[2informaticos]

LCD seems to be correctly detected.
Did you test complete LCD assembly?
May be bad video cable, or even LCD connector.
Are you sure no image at all?
Check with a flashlight (from the back) for the quiestion mark folder (no SSD connected).

"(pin 8 ) LCD_BKLT_PWM_R = 2.4v"
Any change pressing backlight control keys?
Also check pins 6 & 7 at same time; better an o-scope there.

 

[MUUK]

Yes whole new assembly. Also tried new LVDS/edp cable on the old screen.

Tried with phone torch and mircoscope LED ring. Cannot see anything.

PWM does not change with brightness keys.

If both internal and external displays attached i get the Apple logo / ALT key screens on the external (as if the internal was disconnected).

Not sure if this goes anywhere but...
LCD_BKLT_EN is 0v
I would still expect an image however even if backlight is 0v.

Will have to have a look into pins 6 & 7 and get back,

 

[2informaticos]

May be GMUX issue.
Reflow and change it, if still no result.

 

[MUUK]

Made it back to the top of the pile, went through everything again. Noticed that FL8303 was chipped replaced it and it now works. Thanks for your help.