Bob_C on the Basement BBS asked about ways to improve the video quality of the Atari 800XL. I knew there is a resistor that could be clipped, but wasn't sure which one so I asked my friend, Paul Westphal, of EightBitFix.com . He provided the following information:
One simple fix to the Atari 800XL's video is to clip video limiting resistor R57. If you have it, you can remove it from your Atari 800XL motherboard. Later revisions of the motherboard already had it removed. But if you you have R57, you can just clip it and your video will appear crisper on either a TV or monitor.