Atari 5200:Atari 5200 Model Differences
Some page content & researching collaboratively taken from io55.net's Atari 5200 page with permission.
There are two 5200 models.
Models
Models are arranged chronologically.
4-port model (1982-1983)
This version of the 5200 has four controller ports as its primary identifying feature and was the initial version of the console released in 1982. However, its major downside is that the console uses a single cable for combined audio, video and power; and requires an unusual switchbox. Thus, a large element of unreliability and risk comes along with this version because a broken switchbox means no power and no video output for a 5200 without an easy replacement. These can be modified to bypass the switchbox by adding a power jack and removing an inductor from the main board.
Most 4-port models also have smooth, smoked plastic controller storage lids and front bezels, which are easily scratched and attract dust. These were replaced at some point in 1983 with textured matte pieces. The VCS Cartridge Adaptor (CX55) is also not compatible with most 4-port models without modification, as the 4-port's cartridge port is not properly wired to pass through the composite video and audio from the CX55 and unregulated DC voltage to the adapter.
4-port variations
Some later 4-port 5200 consoles (mainly those manufactured in mid-1983) use a newer revision motherboard, and these are also the only 4-port consoles that are compatible with the CX55 out of the box. These can usually be identified by their serial number, which contains an asterisk.
In addition, when Atari Corporation began assembling and selling 5200 consoles in 1985 and 1986, they would often use whatever parts were at hand from the massive parts inventory they had inherited from Atari Inc., since the 5200 had already been officially discontinued. Because of this, some 4-port consoles exist which use the 2-port bottom case but have an old 4-port motherboard inside, along with a factory power jack modification to fill in the spare hole near the channel select switch. Because these odd Atari Corp. units use the original 4-port motherboard, they are not compatible with the CX55 without modification.
2-port model (1983-1984)
Despite removing two controller ports, this version is widely considered to be the superior model of the two, as it has a built-in power jack and does not rely on an external switchbox for power. Due to this, one can use any standard RF switchbox or a 75 ohm to F-type connector to connect the 5200 directly to a TV. All 2-port models support the CX55 adapter, but the 2-port does have some minor game compatibility issues due to possessing a different BIOS from the 4-port model. The BIOS chip can be swapped with one from a 4-port or a 2716 EPROM burned with the 4-port BIOS.
References
Wikipedia page on the Atari 5200
Atari 5200 information and specs (Game Trog)
Atari service bulletin on modifying 4-port models for the CX55 (mirror)