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Mid 2012 model, A1278 / 2.5 GHz i5 or 2.9 GHz i7 processor.

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Backlight is not on! How to fix?

Just to make this short and sweet, I already replaced the screen, and the LVDS cable. I also tested the back light fuse and it does work. I bought the mac off Craigslist a few months ago and when I bought it the screen flickered every now and then. It was the back light dimming and shutting off for a second or two. The logic board was replaced by apple years ago after a water spill but it didn't fix the flickering issue. The previous owner says its been happening for a while. After taking the whole thing apart I cleaned out everything and it all looks good. I currently am using an external monitor which works fine and doesn't flicker. The computer runs perfectly also. When I tried to replace the LVDS cable the screen didn't seem to flicker anymore, but I shattered my glass screen in the process and had to wait for a new one to come in. In the meantime I kept my computer shut and plugged into an external monitor so the LCD wouldn't get damaged. Once the glass screen came in i put it on and found out the back light doesn't work at all (for the first time) I replaced the LCD and that didn't do anything either. Ive been searching everywhere for an answer but cant seem to find one besides fuse problems which is not the case when I test it with a multi-meter. Please help! I need to do my HW for class lol.

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Turn the laptop on, and with your multimeter, measure the backlight fuse (google 820-3115 backlight fuse location) by putting the red probe on the fuse and the black probe on a motherboard screw hole and report back. It should be around 40-50 volts. If it's 0v, then there is likely a short circuit somewhere along the circuit, and if it's around 12.6V then the backlight boost circuit isn't working. These is a common logic board issue and is usually caused by water damage - you can get this fixed for around 200-400 dollars. Unfortunately, unless you have some soldering equipment you won't be able to fix it as it requires you to replace SMD components.

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3 Comments:

Backlight fuse should be 12.6V, not 40V-50V, since this is before the voltage is boosted by the IC. You will see over 25V on C9020 if backlight boost circuit is working, 12.6V if not boosting and 0V if there is a short.

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Jeez I need to get my facts straight, thank you for correcting.

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Tested the fuse, its reading 12.4V on both sides

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Corey Tarter will be eternally grateful.
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