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[SOLVED]820-2914-A no backlight issue

J

jgraffis

New member
I have this 17" 820-2914-A that has no backlight.
what i've done so far

backlight fuse test good with 12.3v on both sides
checked for voltage a lvds connector pins 38,39,40 i get 38.2v
checked pin 4 i get 3.2v
checked pin 2-3 i get 3.1v
i have replaced both lcd and cable with known tested components

it is posting video just no backlight. not really sure what I'm missing. any ideas?
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D

dukefawks

Administrator
38V means there is no load on the bus. Check connector/cable/LCD. You can connect a 15" assembly for testing if available.
 
J

jgraffis

New member
thanks duke. i did test with anther 17" and 15" using tested cables with same result. the only think i have left to try is change the connector. i will try and report back.
 
J

jgraffis

New member
voltage on BKLT_EN ON R9715 is 3.11v
voltage on BKLT_EN ON R9731 IS 0.0V
voltage on BKLT_PWM ON R9704 IS 0.0V

i find it interesting that there is voltage on r9715 but not on r9731 so i checked for continuity and everything seems ok.
 
D

dukefawks

Administrator
If it is on R9715 it must be on R9731, it is a resistor divider and one cannot work without the other. It does mean that I cannot rely on your measurements though cause they are obviously wrong.
But BKLT_PWM is not there if you measured correctly. Do a PRAM reset and see if it comes on. May need to do it a few times and power off and boot into OSX. Then try to turn up the brightness on the keyboard.
 
J

jgraffis

New member
i've measured them a couple times because thats what i thought too. and still same results. but in the mean time i decided to check for short to ground on bklt_pwm and sure enough it was grounded. i traced it back to R9693 removed the resistor and sure enough short to ground on pin1 of R9693. I'm guessing this means U9600 has an issue.
 
J

jgraffis

New member
i did remove the r9704 and the short to ground is on the pin 2 LVDS_BLK_PWM_RC. Which if I'm correct means i have to change U9701. ill report back soon as i get that changed.
 
J

jgraffis

New member
ok. i got the U9701 changed with no more short to ground on LVDS_BLK_PWM_RC, but still no voltage on it and no voltage on either side of R9704 which i believe comes from R9693 which has no voltage either. so would that be an smc issue?
 
D

dukefawks

Administrator
Remove R9704 and connect pin 2 to C9711. This puts 3V on the PWM input and LCD will always be on full brightness. Else you need to change the MUX which is not worth the hassle.
 
J

jgraffis

New member
i did as asked with running a wire and it worked. So what I'm wondering is could i run a wire from a 3.3v sleep line and have the screen turn off during sleep mode?
 
D

dukefawks

Administrator
Uhm backlight should go off in sleep, does it stay on? Does it even really go to sleep, light on front pulsing? I bet J4800 not connected.
 
J

jgraffis

New member
i just miss read what u said, thought the backlight would always be on when the machine is on or in sleep. but i see now its just the screen brightness will always be at highest.
right now I'm just testing for backlight out of the system, i will put the board back in the case and test then report back.
 
J

jgraffis

New member
It does go to sleep but screen brightness is not adjustable which I'm ok with. Hopefully it doesn't kill the battery too fast lol.
 
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