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If the problem is only on the internal screen, then it's either a failing screen or failing video cable.
 
Yes, possibly a new LVDS cable. A bad inverter won't do that. A bad inverter would cause part of the screen or the whole screen to not light up, but still show a good image.
 
Imac 20 screen problem

Thanks for posting this with images!
Those images helped me diagnose my issue.. usually folks talk about vertical stripes and darker at bottom when compared to top and I have searched high and low and finally had someone who was having the same issue as me and pictures to back it up.😉

Much appreciated and now to get a replacement LCD ...😱
 
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