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820-3023 macbook air no backlight

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MaxFix

Member
After cleaning this board no backlight, after some digging around decided to replace the U9701 backlight driver. On plugging in the charger i get 50V on the backlight driver, but this drops down to ~20V after a split second. Without a screen plugged in i get 0 volts on the drive line. I've tried with a known good lcd. Am i just bad at soldering, or could there be something else going on?
 
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MaxFix

Member
It's taken a while for me to get back to this one, there's a lot of weird stuff happening with this machine.
PPBUS_S0_LCDBKLT measures a steady 8.05 V without a screen, but as soon as i connect a screen it drops to about 4.8 V

i checked for presence of BLK_EN, it seems that the signal is coming through to R9731, but it stops there. Resistance on this measures in the high Mohms. I replaced the resistor, and also U9701, just in case. Resistances seem back to normal, but after i turn on the machine, with a screen connected, BKL_EN does not reach U9701. after disconnect it turns out the resistor is burned out again.

Also BKL_PWM measures 0.45 V at U9701 with a screen connected.

It seems to me there could be trace damage under the U9701 area that is causing this, but i'm a little at a loss since i've never seen this before. Is there something i can still try?

edit: how do i see if the connector is burned up?
 
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dukefawks

Administrator
R9731 is 200K. Now please tell me how much voltage you need to put through that to burn it up......yeah impossible indeed.
There should be no 8V present without an LCD connected as Q9706 will not be told to open. I really have no clue what you are doing.....
 
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MaxFix

Member
Well neither do i, since i replaced the capacitor with a good one, measure the path, everything seems ok, but after trying it the resistor is no longer conducting. I know this is unlikely, but it does seem to be what is happening. I've seen the sticky about these backlight circuits, and i will redo the entire thing soonish and come back to this again to let you know if i'm making some stupid mistake with this.
 
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dukefawks

Administrator
It is impossible to burn up a 200K resistor with the voltages present on the board. Resistor was killed by external forces, bad soldering etc.
 
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