LCD Backlight not working

Hey,

I have an iMac Intel 21.5” EMC 2308 whose LCD backlight isn’t working. Thus the screen is black all the time.

The LCD itself however works. If I turn it on in a dark room I can see that everything is being displayed correctly.

The three diagnostics LEDs (not four, because this iMac doesn’t have an external graphics card) are all on, indicating that the iMac is running and communicating with the LCD.

Since an image is displayed on the LCD the problem is not the LCD data cable. The LCD temperature sensor + cable are also working, the fan is running with low RPM when the LCD is connected.

So now the problem lies either in the LCD panel, the backlight board, or one of the two cables going from the backlight board to the LCD.

I disconnected the VSYNC cable from the backlight board and LCD and measured it with a multimeter, it is functional, even when it doesn’t look like it is. Also no connector/pins of the ribbon cable have been bent while plugging it in or out.

Next the backlight LED power cable. I didn’t measure it because I didn’t want to disconnect it from the LCD, but it looks functional and I doubt that at least three wires fail at exactly the same time (since it supplies three groups of LEDs with power).

So the issue is most likely either the backlight board or the LCD panel. That’s why I did a few measurements on the test pins:

P1 is ground, that’s the pin I measured the following voltages against:

P4 is on 12V as it’s supposed to be (in standby and when on)

P11 is on 12V when on

P13 is on 3.3V when on (backlight enable pin)

Next I measured the LED power supply pins on the backlight board:

When the LCD is connected each + pin is on ~12V, each - pin on ~0V. When no LCD is connected the + pins are not on 12V.

Afterwards I measured the ENABLE pin of the VSYNC ribbon cable connector:

The pin was on 0V when the LCD was connected and when the LCD wasn’t connected.

So I thought that this enable pin could be the problem, that’s why I used a 50kOhm resistor to pull it up to 3.3V, but it didn’t work, the LCD stayed black (could be because I failed to create a good connection between the resistor and the pin).

Then I measured the three pads marked in the following picture, as I suspected one of them may be responsible for the power supply of the IC APP003SN.

However, all of them had 0V on them, with an LCD connected.

So, my question now is what is more likely to cause this issue, the LCD panel or the backlight board? Or what can I do to determine where the issue resides?

Thanks in advance for your help.

Best Regards,

Felix

EDIT: Oh, and I also did several PRAM resets.

EDIT2: Also the system was opened before. I was trying to replace the HDD with an SSD. After the replacement the iMac worked just fine when lying on the back without the front glass. So I switched it off, mounted the front glass and placed it on its foot. I turned it on, everything working fine, but then the display turned black and it didn't change since then. That was four months ago.

EDIT3:

Four pictures of the iMacs components:

Logic board LCD connector:

Logic board LCD connector cable:

Other side + temperature sensor cable:

VSYNC ribbon cable:

Other side:

LCD backlight power cable:

The logic board LCD connector cable is a bit broken (see the upper spot I marked in the following picture):

And also the VSYNC ribbon cable doesn’t look so good, but I did a continuity check and it was successful (it’s still possible that it has a loose connection that works in some positions).

Aside from those two cables everything looks functional to me.