820-3437 with no backlight

Puevig reven

New member
I have a 820-3437 water damaged. Ran it through ultrasonic and replaced nasty looking C7796, but still no backlight. Checked bklt fuse its fine, C7796, C7797, C7799 all measuring correctly. I can not seem to find anything else wrong. Please help.
 

larossmann

Administrator
Staff member
.411 is too low. .400 to .475 signify bad feedback line to lp8550. 0.511-0.545 is what you want after the diode. What does ball A5 for feedback on lp8550 look like?

Also, what is your backlight voltage? 0v, 8.4v, 19v, 27v, or 49v?

0v means break in line or being told to stay off altogether.

8.4v means no boosting, but continuity all the way through on the top of the line from coil to diode, diode to output, etc.

19v means missing pwm or bad lp8550

27v means we're blind :)

49v means bad lcd/lcd backlight cable inside assembly is not plugged in/bad cable.
 

aprendiz

Moderator
.411 is too low. .400 to .475 signify bad feedback line to lp8550. 0.511-0.545 is what you want after the diode. What does ball A5 for feedback on lp8550 look like?

Also, what is your backlight voltage? 0v, 8.4v, 19v, 27v, or 49v?

0v means break in line or being told to stay off altogether.

8.4v means no boosting, but continuity all the way through on the top of the line from coil to diode, diode to output, etc.

19v means missing pwm or bad lp8550

27v means we're blind :)

49v means bad lcd/lcd backlight cable inside assembly is not plugged in/bad cable.
8.4v means no boosting
 

Puevig reven

New member
I am using boardview, and schematic I do not know how to verify that line is good, when i said the line looks good, I was speaking of the components look ok
 

aprendiz

Moderator
continuity test in diode mode, if beeps is ok if not is open, if you press "N" key on landrex and give the name of the net you are looking for, it will tell you all the places where this net should be...
 

dukefawks

Administrator
One way to find out for sure is to pull the 8550 and measure feedback lines before and after diode to the 8550 pads. But before you do that you can check BKL_EN and BKL_PWM. I still think this is going to be a bad 8550.
 
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