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You're seeing the very top of the raster, which also means you're seeing a part of the picture that's normally out of view. As I understand it, the first few lines of the video signal were repurposed to add text or other info. It shows up as dancing white lines. To get rid of it, you need to play with vertical height, centering and linearity. First center the raster in the middle of the screen, then use the height pot to overscan the CRT face slightly. Lastly, adjust the linearity pot so the raster is uniform top to bottom. This is best done with a crosshatch pattern on the screen, but you can eyeball it with off air images.
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Evolution...
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