Atari 5200 talk:Atari 5200 Mods Wiki

Disassembly
Remove outer case screws. Metal shield has 15 metal tabs all around that need to be bent almost perfectly straight. May need to use very carefully use an xacto knife to separate metal due to slight rust. Once you get under it, use a plastic spudger to gently pop the shield off of a tab. Work your way around all the tabs. Unless you have very good flush cutters, cutting a tab will likely make it harder to remove the shield. Any tabs that were mangled or you had trouble with can be cut off once you remove the bottom shield.

Resources
http://www.atarihq.com/5200/5200faq/index.html