The pause button on the 7800 controls the same signal as the 2600 Color/BW switch, but it's momentary, not steady state. You can install one yourself to help with 2600 games but IMHO it's not really necessary. For Starmaster, use a difficulty switch to toggle the map on (undocumented feature), for Space Shuttle you can just use the backup engines, for Ghost Manor play as the girl only. CroSBow at AtariAge was the one who found that Starmaster trick and he was generous enough to confirm the others for me. Please email if I missed one.
If the 2600 chroma (color) signal is up too high, it causes interference in 7800 games, so 7800s were actually made sort of "crippled". I converted my 7800 to s-video and got around the problem by installing a switch that connects either the 2600 or 7800 chroma signal, not both.
Tigervision never goes in and some Activision/Imagic ones are too tight for some 7800 models. Some people use a high-score cart as an extender, but I made my own by gutting an asteroids game and adding a cart connector (picture at bottom of page). This was a fun project because it only took one night and didn't drag on like most of them do. Someone else said they shaved the Tigervision prongs down with nail clippers.
Some Activision games (Robot Tank, Decathlon, Space Shuttle) are not compatible with SOME 7800 systems. I published a fix on RGVC (which is copied below) and Chad Schell and others followed up on it followed up on it (search book starting p228): Hacking Book
The circuit change mentioned above was put in place because a few of the original 7800s (not many) did not work with 2600 Dark Chambers. If yours doesn't you can play it on the Cuttle Cart which seems to work, or just play the 7800 version. Trying to install the circuit by hand if definitely not worth it.
There's a suspicion that some 7800 models have destroyed superchargers in the past. Nothing has been determined for sure and people have gotten into flamewars over it. I had an SC go bad in my 7800, but think it wasn't making a good connection and it was driving against the 7800 or its various logic banks were driving against another. I'd say try it at your own risk (and post about it at your own risk also). People say it should work if your 7800 works with Robot Tank.
Saw this posted once, but my four 7800s all work fine (can anyone confirm?)
ACTIVISION COMPATIBLITY FIX An expansion port 7800 has the following circuit: _____ 11 | | 10 10 |} D2-----|D Q|-----------| } (6502) | | | } 8 | | 9 | }------A12 (cart) |> | A12----| } |_____| (6502) | } |} 74174 7408 Flip Flop AND A later model 7800 has an extra flip-flopish circuit between the two pin 10's above: ,-------)|--------<-----------, | | | _____ | | | | | P10 ----+-| | | 74174 | | |---, | |_| | | | | | | ______ | |_____| | | | | | | | | `-->--| | 4 | _____ | |-->-+--->---P10 | | | | 7408 Phi2 -----| | ,-->--| | (out) | | | | | | |---' |______| D1 via ---| | flop | | |_____| 7402 NOR gate (ALL THREE) The fix I did was as follows: 1. Cut trace coming from 7408 P10 with an exacto-knife 2. Verify with an ohmmeter that trace was cut 3. Wire up a SPDT switch so that 7408 P10 can be connected to 74174 P10 (expansion circuit) or 7402 P4 (later circuit) 4. All games (Space Shuttle, Robot Tank, Decathlon, Dark Chambers) should work with the switch set in one position or the other