Hi
Got an iMac 4,1 to use as a donor screen for my G5 iMac which was the last and fastest G5 iMac so worth preserving. But my donor screen is dim in the corners, very noticably on white boot up screens where it looks greyed. I am guessing its related to LED lighting at edges but wondering if there any fixes for this as I have a 2nd iMac 4,1 which is in really nice condition and I've kinda decided I'd like to keep it that way and use the worse example as donor.
You’re part-way there, but it’s not an LED issue.
What you’re seeing is the cold-cathode fluorescent lighting — CCFL — ageing from years of prior use. Typically, on displays in the 20-inch-or-greater range, the displays utilize a pair of CCFL tubes: one latitudinally along the top edge and another along the bottom.
If you can remember when fluorescent tubes for interior lighting in offices and retailers began to reach the end of their lives, the ends would grow visibly dark when not powered on. This was the sign of the bulb needing a replacement soon. That’s basically what’s happening here.
You have a few options for improving your donor screen:
1) Find appropriate CCFL tubes for that display and replace them. This is going to be very, very difficult. The tubes are about 3–4mm in diameter and can snap open very easily. Also, because mercury vapour is in each tube, there’s health risk to inhaling those fumes. Proper disposal — not the trash — is also vital here. The other issue with this is even tracking down the replacement CCFL tubes for the size and specs you need. It might be either difficult or infeasible at this point in the mid–2020s.
2) Find a CCFL-to-LED-strip conversion kit and replace them. This is slightly less difficult, in that you’re not trying to install new CCFL tubes, but you’ll still have to deal with the challenge of taking out the old ones before putting in the LED strips. Moreover, the LED strips have to go into the lighting rail sleeves in the correct direction. Having the LED strip illumination elements abutting the LCD glass will create an undesirable “stage-lighting effect”. Another quirk is because you won’t be using the inline inverter needed for CCFL lighting, the mini logic board for LED, by default, has the rheostat/brightness run in the opposite direction. That is: if pressing the “increase brightness” button on your keyboard, the display may dim (and even flicker), leaving one with either going “max dim” to make the screen its brightest. It’s counter-intuitive, but it works.
As with the first suggestion, all of this means gingerly and carefully dismantling the LCD assembly without breaking key parts. It
can be done, but it isn’t intended to be user-serviceable in the slightest. I have done maybe five CCFL-to-LED swaps on a 12-inch laptop LCD. I have destroyed at least one of the LCD assemblies, with a second being kind of a bodge in my reassembly effort. The last one I did went well overall, but by then, I had practice (and 12-inch CCFL LCDs were and, relatively speaking, still are more common than a 20-inch display).
3) Buy a NOS CCFL display from a supplier. This may be eBay or PChub or via Panelook or elsewhere. You will spend money, but the opportunity cost is well worth it, as you only need to swap out the LCD assemblies and that’s the end of it. To do this, you’ll need to know which LCD maker and model your donor iMac4,1 uses (probably LG, but could also be Samsung or even a third maker).
Good luck.
