[SOLVED]820-2915 Weird backlight issue

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Sykulski

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This one came in with severe water damage. After ultrasonic cleaning and fixing CPU VCORE power issue it started up with no backlight. I knew it would because it had a lot of corrosion around backlight circuitry. I changed all corroded resistors and caps around LP8550 and also the LP8850 itself. Some traces needed wire. Also the pulldown resistor on BKLT_PWM was corroded so I have changed it and put a wire on the broken trace.

Now the backlight is working partially. When I start the computer backlight turns on. The brightness is 100% even if I turn it down. BKLT_PWM is pulsing - long pulses on high brightness and short pulses on low brightness - but still full brightness around 40V on LCD connector. But when computer goes to sleep backlight does not come back on every time. Sometimes it comes right away - sometimes after 30 seconds - and sometimes not at all before I shut down and turn the machine back on. When backlight is not working after sleep then I get 19-20V on LCD connector.

I tried giving BKLT_PVM 3V from PP3V3_S0 but still only 19V on backlight after sleep.

What am I missing ?
 
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dukefawks

Administrator
Seems like an issue on PWM pin. Remove R9704 and measure on diode mode on C9704. Could also be a bad C9704. Also measure diode mode on pin 1 of R9714/16 and pin 2 of R9741. Also check value of these resistors.
Oh and the LP8550 you used, have you used these of the same batch before? They do have an EEPROM with settings in them...
 

Sykulski

Member
Thanks Duke for pointing me in the direction of measuring diode mode on C9704 with R9704 removed. It showed up that I had a microscopic OL around the probe point for C9704. I couldn't even see it in the microscope on highest zoom setting. But the multimeter saw it :) Made a trace and brightness now is adjustable and backlight wakes from sleep.

Thanks again you can mark this one solved. Thumbs up!
 
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