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820-00239: Backlight Issue (Backlight goes out when adjusted)

M

macRepairAmb

Member
I have this unit that is having a backlight issue. It boots up completely to OS and runs, but when i attempt to adjust the backlight intensity, i then loose backlight and can't get it back. I still have image on the screen (can see the image when i shine a light on the screen). On other occasions, if i don't mess with the backlight intensity, it will eventually start to flicker out and then go out completely.

I've checked for corrosion or liquid spill/damage and found none

I'm at a loss as to where to start with this one. Any suggestions?
 
2informaticos

2informaticos

Administrator
Staff member
Check if the machine can get screen replaced free by Apple first.
If not, then try known good compatible LCD assembly.
Display itself is involved in backlight (PWM) adjust...
 
M

macRepairAmb

Member
I replaced the display twice. At First when i got the machine in, it powered up but with no backlight. So i ordered a replacement display after I tested the unit with another display. The first display that arrived i thought was defective due to the flickering backlight. But had the same issue on the second display. So i reinstalled the original display and the backlight worked until I then attempted to adjust the intensity, then it dropped out. Upon power cycling the unit, the display worked again, until i reattempt intensity adjustment.
 
M

macRepairAmb

Member
Would you happen to know what chip sets the backlight intensity when we adjust it on the laptop? Whenever the backlight drops out,
PPVIN_S0SW_LCDBKLT drops from 13v to down to about 1v.
I've replaced U8400, and Q8400. with the same result.
 
2informaticos

2informaticos

Administrator
Staff member
CPU talks with LCD display and this one send final info to U8400.
SMC is somehow involved too.
Just search for BKLT_PWM in schematic...
 
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