Apple's discussion boards and the MacFixIt inboxes are filled with a surprisingly high number of reports indicating backlight burnout (though other factors may be at play) on Apple Cinema Displays.
Most readers report that the top half, bottom half, or other portions of the screen become suddenly dimmer than the rest of the screen. Generally, this symptom would mean a failed backlighting mechanism, requiring hardware replacement or repair. Some readers report being quoted prices over US$ 500 for authorized repair service.
However, MacFixIt reader Joe Streno found that zapping the PRAM - which would do nothing to resuscitate a dead backlight - resolved the dimming problem on his 20" Cinema display:
"There is a very strange problem that I've encountered with my 20" Apple Cinema Display and Power Mac G4 tower. Upon turning on my G4 1.4 GHz DP one morning, the top third of my display was noticeably dimmer than the rest of the screen. The power on light was slowly blinking three times, pausing, then blinking three time again and so on.
"However, I discovered that zapping the PRAM fixes the issue. No more dim screen. No more blinking display power light. Everything is back to normal."
Zapping the PRAM can be accomplished by holding down the keys Command-Option-P-R in unison at startup.
Apple Knowledge Base article #34593 states "If you see a repeating sequence of three short flashes, the monitor is detecting input in the wrong video format. Check that you have the correct graphics card installed and that you've installed the correct monitor software," and "If you see a sequence of two short flashes and a long flash, the monitor is detecting a problem with a backlighting lamp. Contact an Apple-authorized dealer or service provider for additional information about resolving the problem." However, the article makes no reference to resetting PRAM.
Readers who have sent their displays in for repair under AppleCare or standard warranty are being told that the part "P30A inverter DC-AC" is being replaced to fix the dead backlight.Update: Voltage issues In some cases, voltage interference from external PCI-connected devices can cause backlight failure on Cinema Displays. MacFixIt reader Peter Zieseniss writes: "I had this problem as well. It turned out that a peripheral attached to a proprietary PCI card in the Power Mac was being turned on and off while the computer was on, this, in turn, would de-regulate the amount of voltage carried by the video card to the display (this is an ADC display and an NVIDIA card). Hitting the CUDA switch (Editors Note: The CUDA switch is a small silver button located on the motherboard near the middle of a small square box on most Power Macs) fixed it."
You can avoid this type of interference by making sure that you turn off your Power Macintosh G4 (or any other Mac with an external PCI chassis) before powering up proprietary external PCI-connected devices.
If you are having a dim Apple Cinema display issue, and have tried resetting the PRAM, please let us know your results.