24" Cinema Display Backlight problem

Hi, I have a 24" Apple Cinema Display model A1267. A couple of weeks ago it started flashing black and then eventually went completely black.

It still charges the Macbook Pro, audio works and screen is detected by the Macbook.

After reading quite a bit on the internet, I found that it seems to be a generic backlight issue with these displays.

One of the debugging steps was to disconnect all power consumers from the display, i.e. any devices plugged into the usb port. the Macbook pro charger, and also the USB that goes into the Mac. Then plugging the mini display port plug in, flashed the expected screen image on the display for a split second, possible two flashes.

This tells me that the backlight LED's are sort of working, but that the backlight controller and/or the CPU on the logic board thinks there is a problem. Now from what I could gather out of the data sheet for the HV9982, the fault conditions could be either:

1. Over voltage protection

2. Short circuit protection

3. Open circuit protection (basically OVP)

I checked the power output signals of the backlight on a oscilloscope and found that the voltage rises to about 74V (after 24V level for short time), then decays down, jumps again to 74V and then decays down to 24V. At 24V the LEDS are not visible. I can post the images of the scope if needed.

The voltage ties in with the panel (LG240) data sheet that indicates a voltage of 73.2V (off the top of my head) for the 3 LED strings.

I also monitored the TP81 with the scope to see if there is a dip, but it remains constant at +24.5V

All components on the board were visually checked and they look fine, no burnouts etc.

So my questions are:

1. Where can I obtain a schematic diagram for the logic board

2. Could this be a CPU firmware issue

I have all the equipment required to test and repair this logic board. I have my own electronic design house designing many different products, including LCD backlights.

There seems to be quite a few "dead" cinema displays out there that could benefit from a repair guideline that does not involve swapping out expensive logic and PSU boards, only to have them die a few months down the line with a similar problem.

I am offering to find the root cause of this problem, which seems to have something to do with the tolerance on some components, causing the problem to increase with age.

Best regards

Mikkie