Display brightness mechanisms failing (#5)

We continue coverage a problem where the brightening mechanism (fluorescent tube) fails on various Macs, particularly iBooks.

When the problem manifests, the screen suddenly becomes extremely dark, but normal system functions continue. Users find that they are still able to view display content if a bright light source is held near the darkened screen.

Yesterday we noted two distinct hardware issues -- a faulty inverter board or faulty backlight cabling -- that are causing dim displays in a number of cases. However, as noted previously and by continued reader feedback, a purely software issue also seems to be at play in some situations.

MacFixIt reader Anthony Burokas, for instance, writes:

"I've read the continuing saga of the iBook screen going dark and while it seems to be centering on backlight failure, or wires, this really fails to assess what many people have said, and I can also corroborate- rebooting fixes it.

"If the backlights were truly at issue, a component failure, or failing wires, touching three keys at once (Command-Control-Power) would have absolutely no effect. But it does.

"I use my iBook as my office machine. It's on all day, every day. It barely moves. It will dim and then shut off the backlight properly. When I move the mouse it comes back dozens and dozens of times. Then, without any rhyme or reason, it won't come back. The mouse is still lit, the HDD is still running, but the backlights don't come back on. Touching three keys has always solved it. Tilting the display never has."

Meanwhile, reader John Sawyer offers a number of potential fixes, some of which have appeared in previous MacFixIt reports, for systems that are indeed affected by a software issue:

• Remove any connected peripherals.
• Make sure F1 key isn't stuck down.
• Press the F2 key (with the fn key pressed and not pressed) to increase the screen brightness.
• Make sure you're using the proper PowerBook power adapter--the Powerbook G4 DVI and later models use a 65 watt adapter, and earlier models use a 45 watt adapter.
• Press the logic board's reset button if it has one; if it doesn't, you can reset the logic board of many iBooks and Powerbook G4s from the keyboard, by entering Shift-Control-Option-Power (you need to do this with either a good power adapter or a good charged battery installed).
• Reset the NVRAM chip and the PRAM chip: power off the computer, then power it back up and quickly hold down Command-Option-P-R until the Mac chimes twice. You need to do this from a powerup instead of a restart, since doing it from a restart resets just the PRAM chip.
• Reset Open Firmware: restart the Mac, and quickly hold down Command-Option-O-F until the Open Firmware screen appears. Then enter:

set-defaults [then press the return key]
reset-nvram [then press the return key]
reset-all [then press the return key]

• Restart from a known-good CD or external hard drive; if the display is normal, then the problem may be something in your OS X installation, the quickest fix for which is usually to reinstall OS X.

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