Why black screen, gradated screen when working and screen freeze?

I have been having issue of intermittent black screen on my iMac.

The screen worked for sometime then started showing gradated screen. The top section being lighter with the bottom being darker.

I replaced the LCD display thinking that would solve the problem, but it still has the gradated screen and it freezes up.

I opened it back up and checked connections and cleared out the dust on the fans.

Now, it's back to a black screen. I took a picture of the power source while I had the computer open because it looked odd.

If someone could tell me if they think the power source is bad or if maybe the inverter is causing the issues, I sure would appreciate it!

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The whitish film covering the unit is moisture protectant

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A screen which is lighter on one edge than the other implies the backlight circuit is having a problem. Double check your invert cable connections. If that doesn't fix things then the inverter would be the next thing I would test or replace. This model uses CCFL tubes within the display they can be worn out.

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I replaced the display and the inverter and still black screen remains. Before I was getting intermittent black screen, but now always. Everything can be seen in T-mode from another computer, so that's good. I ended up buying a new iMac but would still like to solve this one's problem. I have a feeling it may be getting overheated because the fans run contantly and very loudly. The newer operating system must be too much for the older computer to handle. At least, that's my guess.

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@perna "gradated screen and it freezes up." is most likely not directly related to the CCFL or the backlight. The issue sounds like it is more related to the power board and video processor which may cause the issues with the backlight. Since it works after you cleaned the fan I'd suggest cleaning all of it and possibly replacing the thermal paste on all the heatsinks etc. Post some images your boards from both sides and of what your screen shows when it shows this again.

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