[SOLVED]820-2915-B with no backlight

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DannyRinc

New member
Hey all,

Have a MBPRO 15" with no backlight. No liquid damage, supposedly just stopped working.

What' I've measured:

Fuse: 12.6v on each side
BKLT_EN: 3.2v
LVDS Pins 389,39,40; all at 29.6v

Figured it was a bad LCD right? Nope, tested an LCD that works fine and no backlight still.

The current sink resistors all measure 0 ohms, getting between 3.2 and 3.5v on them. I figure the LED Driver is boosting fine since I'm getting 29v at the LCD connector.

Bad LVDS connector maybe?
 

DannyRinc

New member
same as connector side. The GMUX is acting up, removed resistor for PWM and ran pp3vs0 straight to the pad going to BKL_PWM and the backlight is good now. I'll check caps and voltage going into it to see what's up.

Yeah I wouldn't know where to begin with this... Not sure what enables that signal.
 
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dukefawks

Administrator
You could replace the LP8550, it can pull down the PWM line. Or just live with the fact that the BKL is always on 100%
 

DannyRinc

New member
I'm thinking about it... But if I remove R9704 I have 0v at pin 1, so the GMUX still isn't putting out 3.3v, so it wouldn't be the driver pulling it down right?
 

larossmann

Administrator
Staff member
U9600 sometimes causes no brightness on 3330/2915 boards and it is much easier to sell someone on permanent full brightness than it is to replace the mux. It's up to you at this point. If you feel your skills are up to it you can replace it, but if they aren't you killed a board that came to you with no backlight. Selling that to the customer is far more difficult than "we can only give you max brightness all the time."

The best would be if something but U9600 is causing the light to always be low, you replace it and it still doesn't work.. that would suck. :)
 

dukefawks

Administrator
I have also had 2 of these boards where PRAM did not bring back the BKL. I booted it in the OS without BKL and adjusted the brightness with the keyboard and it started working again. No idea what voodoo that was....try it. Also once I flashed the EFI ROM and that brought back the BKL.
 

DannyRinc

New member
Thanks for all the help guys, talked to the customer and we mutually agreed it was better to just have full brightness permanently than to attempt a U9600 swap. Even if it was a reflow I still would prefer full brightness. You guys are great! Btw I got the idea and hard wired the PWM signal from another forum where Louis posted that solution. You're a lifesaver!
 
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