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As shown in this video, the screen is dark, except at the few first steps when the brightness is moved just out of zero.

The notebook works otherwise perfectly fine. The image on the external monitor is fine.

Have you seen this before or can guess what the culprit is?

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Yes. A loose cable, loose cable to LCD panel connection, bad inverter, failing backlight, power supply/voltage regulation issues inside the Mac all can cause what you observe as a symptom for brightness jumping and not working.

If you gently and slowly open and close the lid, most loose connections can be made better or worse. The powering it off and then seeing a little moment of light when you initially turn on the backlight circuit seems to indicate a power / inverter issue, but that could still be the bulb or bad luck/timing with a cable jostling, too.

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So the backlight bulb could need to be changed, even though it flashes on occasion? – Calaf Jan 30 '13 at 23:21
It's embedded inside the LCD panel, so changing it us a factory operation (or more likely a scrap of that part), but yes - it could simply be the bulb. You'd need a multimeter to snoop the current / voltage on the wires to know for sure. – bmike Jan 30 '13 at 23:41
After taking it apart, confirming with an Ohm-meter that the inverter cable is fine, and blowing air while reseating the connectors, the problem remains. Can I exclude that the backlight is to blame, if it (the backlight) consists in fact of multiple bulbs? – Calaf Feb 1 '13 at 21:51
@calaf It seems highly unlikely that all backlight sources would dim/wake in unison if one of them were failing. Occam's razor says that if they all dim/brighten togehter one of them isn't causing the inverter to overload / fail, but that the inverter or control circuit itself is to blame (logic board / cable / inverter). – bmike Feb 1 '13 at 21:56

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