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820-3115 No backlight - missing LCD_BKLT_EN

ytesfay80

ytesfay80

Member
This board had a blown F9700 fuse and 0v on LCD_BKLT_EN. I replaced F9700 but I am still getting 0v on LCD_BKLT_EN. BKLT_PLT_RST_L measures 3.3v.

I googled around and saw that LCD_BKLT_EN comes from LVDS_IG_BKL_ON so I measured R1981 and that measured a proper 100k.

There was some water damage around the U1800 and Q9707 area. I have given the area a 99% alcohol and toothbrush scrub since I dont have an ultrasonic yet.
I have done the testing with the display connected. The LVDS connector and cable both look fine. When I disconnect the display and use my thunderbolt to hdmi cable on my tv it works fine.
Would this probably be a problem with the U1800 chip since that is where LVDS_IG_BKL_ON is produced?
 
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ytesfay80

ytesfay80

Member
I checked Q9707 again and I see that Pin 3(LCDBKLT_EN_L) is showing 12v in voltage mode and 0.99v in diode mode.
There is still 0v on LCD_BKLT_EN.
I don't see how LCDBKLT_EN_L is getting any voltage. Shouldn't the mosfet only open when there is a higher voltage on the gate then the source?
 
larossmann

larossmann

Administrator
Staff member
Q9706 is a P type MOSFET that will only open when the voltage on the gate is much lower than the voltage on the source. R9788 & R9789 are a voltage divider whose purpose it is to send 5v to the gate of Q9706. A voltage divider needs a path to a lower voltage/ground on the "bottom" for it to work though, which means R9789 needs a path to ground.

R9789 doesn't get a path to ground unless Q9707 opens. LCD_BKLT_EN and BKLT_PLT_RST_L need to be present in order for Q9707 to open, allow R9788/R9789 voltage divider to work, and allow Q9706 to pass power.

This usually happens when the LCD is recognized. If LCD_BKLT_EN isn't present, either SMC_LID is stuck at a low voltage because of corrosion by BIL connector or the machine doesn't recognize the LCD.

What voltage is present on pin 2/3, and 4 of the LCD connector?
 
ytesfay80

ytesfay80

Member
Thanks for the explanation.
Pins 2 and 3 of the LCD Connector show 0v and pin 4 shows 3.32v
Also I checked pin 5(SMC_LID) of J6955 and R6961 and got 3.45v. That number isn't low for SMC_LID is it?
 
larossmann

larossmann

Administrator
Staff member
ok, so we end troubleshooting of no backlight, and start troubleshooting of screen detection. If LCD_PWR_EN is 0v on U9000(check that for me), the machine is not seeing the LCD. Do you have a known good LCD & cable lying around for testing? Have you reset PRAM? Do the resistors for LVDS_DDC_DATA and LVDS_DDC_CLK check out good?
 
D

dukefawks

Administrator
Measure on diode mode pin 6 and 7 on LCD connector and carefully inspect pin 4 for corrosion and 3V present.
The 2012 models have a unique LCD connector so no other models are compatible.
 
ytesfay80

ytesfay80

Member
dukefawks said:
Measure on diode mode pin 6 and 7 on LCD connector and carefully inspect pin 4 for corrosion and 3V present.
The 2012 models have a unique LCD connector so no other models are compatible.
Click to expand...

In diode mode
pin 6 - 0.514
pin 7 - 0.543

There doesn't look to be any corrosion on pin 4 and there is 3.32v on it.
Pins 6 and 7 also have 3.32v on them.
 
ytesfay80

ytesfay80

Member
Yes. that is with the cable plugged in. When I remove the cable pin 6 measures 0.605v and pin 7 measures 0.609v in diode mode.
 
D

dukefawks

Administrator
At this point I would first try another LCD assembly to rule that out. Don't rip the original apart it might be fine. Get another machine to test or order a cable+LCD to test.
 
ytesfay80

ytesfay80

Member
I just ordered a used lcd and cable from powerbook medic. Is that a good company to purchase from? I think louis might have some experience with them.
 
D

dukefawks

Administrator
You'll get your stuff, but in my opinion they are too expensive. But if you want it quick then sure. Make sure the cable you ordered is for 2012+ models!
 
ytesfay80

ytesfay80

Member
My lcd and cable I ordered came in today and I tested it and the lcd is still not being recognized as there is nothing on Pin 1(LCD_IG_PWR_EN) of U9000.
I checked R1984 and I also got no voltage there and 91kohms on the resistance.
Here are some measurements on the lcd connector with the cable plugged in
diode mode | voltage
Pin 2 - 0.575 0v
Pin 3 - 0.575 0v
Pin 4 - 0.331 3.31v
Pin 6 - 0.604 3.31v
Pin 7 - 0.599 3.31v

Is this an issue with U1800 chip?
 
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