Replacement power supply for Apple Cinema Display
NB: Photos copyright Eroni De Oliveira
A reader sent me some information on replacing the Apple power supply for the Cinema Display. He realised that a 24V 'LED Strip' power supply should meet the power requirements for the display.
The power supplies sell for around $20 which is a considerable saving compared to the Apple version around $100 to $200 (depending on usage). He was in the process of testing 2 'dead' displays and was able to exchange parts between them to diagnose the problem for each display…
I ended up fixing one of the monitors by switching CCFLs between them, which confirms my belief that the most common problem among these older units, is related to the backlight bulbs. The other monitor has a cracked LCD display.
Like any fluorescent lamp, as they get old, and darkens it's extremities, their current consumption changes, triggering protection circuits in the ballast board and/or the logic board, as well as in the external power supply itself. Remember that these bulbs work with high voltage (around 420 Volts) and any small current variation at that level causes a much higher one at lower level, hence the need for protection circuits despite having a 3 Amp fuse at the ballast board. That explains why, sometimes, a more powerful power supply is able to correct it (temporarily nevertheless).
Another issue has to do with the high cost of the original power supplies been, most of times, twice the price of a defective monitor. I decided to use an alternative power supply that cost around $ 20.00 and are very stable and ripple free. They are designed to power LED strips, with an output of 24 Volts, 5 Amps (about 120 Watts). They have others with different specs also. The output voltage can be "fine tuned" to precisely 24.5 Volts; there is a control for that purpose.
The connection is simple: the shield (from the monitor) is grounded along with the ground from the AC line (green wire). The red, from the monitor, goes to the positive and the black to the negative. The "control/protection" from the monitor (center thin wire) I left disconnected. I trust that the 3 Amps. (on the ballast board) should blow, in case the bulbs get really bad.
– Eroni De Oliveira.