I may have an answer to what's causing the uneven illumination problem in the Powerbook G4 Aluminum 15", and any other laptop with a similar problem.
I fix Macs (since 1985), and I've opened practically every model. However, until recently, I hadn't had anyone bring me any Powerbook G4 model for a lamp (backlight) replacement (I like to call it the "backlight", but the official term is "lamp", so I'll use that--it's the long, thin fluorescent tube that runs the length of the bottom of the display), so I couldn't get a look inside one of these displays to see what's going on. So, I'd been speculating on what's causing this uneven illumination problem (you can see my earlier posts in this discussion), but came up only with unproven and unlikely theories, such as a lamp that was darker on one side than the other, or inverter boards that were putting RF noise into one side of the lamp. I finally got a chance to directly investigate when a client brought a Titanium Powerbook G4 to me about a month ago--he had dropped his and it had broken the lamp. I took the display apart, and installed a replacement lamp, and what do you know--the uneven illumination problem showed itself. I've seen this with Titaniums, but not as bad as the Aluminum PBG4. I later asked the client if he had seen this problem before I had taken his Powerbook apart, and he confirmed that he had, so I didn't feel too bad about whether I had done something wrong. I fiddled with things, and found that the position of the reflector that the lamp sits in, is far more critical than I had realized--even a slight bending or uneven positioning of the reflector can affect illumination along the bottom of the display. This reflector is a highly polished, long rectangular piece, or channel, with one end open where it faces the clear plastic layer; the lamp sits in this reflector. It sits along the bottom edge of the display, reflecting the lamp's light up into the clear plastic sheet that transmits the light throughout the display, which then reflects off a white sheet behind the actual liquid crystal display panel. I found I could slightly twist the reflector, and cause the bright areas to dim, and vice versa. By applying hand pressure along the entire length of the reflector, thus better aligning its whole length, I was able to even out the illumination to an almost satisfactory degree. The only trick is how to maintain that amount of pressure when I reassemble the display, since I won't be following the client around to apply helpful pressure to his display.
So, the most likely theory is that the reflector that the lamp is housed within, isn't positioned exactly properly, and may be getting bent slightly over time--miniscule changes in the position of the reflector can make a big difference in illumination at the bottom of the display. Letting the lamp warm up will sometimes help, both because the lamp gets brighter, and possibly also because as the reflector warms up, it may sometimes twist back into a better position and shape. All this could explain why some people see the problem come and go, and also see the problem develop only over time, even with a replacement display. Opening and closing the display may be torquing the reflector over time.
Some people who send their Powerbook in for repair, say the repair paperwork says the inverter board was replaced, but not the LCD, but they say the illlumination has improved. I wouldn't be surprised if some Apple techs are manipulating the reflector into a better position, and replacing the inverter either with one that puts out more voltage to increase the bulb's brightness, or just to be on the safe side, or to say they replaced a part, or all three possibilities.
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