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

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No backlight. Should this part be 0V?

The problem is as usual:

  • no backlight
  • external light source reveals screen content

One tan part near the backlight chip has 0V (see photo, right column of SMD parts, next to last part). Is that the expected value?

The bigger tan part (bottom right corner) has fluctuating voltage between -0.012 and 0.013V. That's not what it's supposed to be, right?

Any comments, suggestions or thoughts on this are appreciated.

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FYI - Just because the SMT device is tan in color doesn't mean it's a cap.

In fact I suspect your issue is a blown fuse link which is the device in the far left with the letter P printed on it. Carefully short across it with a pair of tweezers for a second resting the system on its side turned on does the back light turn on at that point?

But before you replace it with a new one you should make sure you don't have another other problem which caused the link to blow.

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8 Comments:

Thanks for the hint about the color. I replaced "cap" with "tan part". Shortening with tweezers or jumper cable didn't help... Dosn't 0V across the fuse mean it's OK?

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Testing devices isn't always as straightforward testing a cross them with a volt meter. Often it has to do with what you are using as the reference (chassis ground). Then you should see the voltage on both sides of the fuse in this case.

If you are testing a fuse out of circuit an Ohm meter would be the way.

Even then you can't assume the resistor, capacitor or other discreet SMT device is whats damaged just by testing with a volt meter. You really need to have a good schematic diagram of the circuit and do a bit of studying to understand what the designer of the circuit was thinking. The given leg of the circuit may not be active until a condition occurs.

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I measured between the two sides of the fuse.

Do you think replacing the tan part (which I think is a cap) makes sense?

Page 71 in the PDF below has several caps with different capacities. I didn't figure out which part on schematics corresponds to which part on PCB.

https://www.scribd.com/doc/229746094/App...

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If you have downloaded the full schematic set for your logic board - 820-2936 'Flying Cloud', you should be able to find the area by searching for LCD backlight to get to the correct drawing.

I suspect the LED driver is blown LP8550. It has a tendency to fail.

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Thanks. But it's BGA...

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