CPS-2 InfiniKey
I posted about replacing the batteries on Capcom CPS2 boards in the past. New developments allow for completely removing the battery while maintaining the original encrypted code. The InfiniKey is a little board you solder onto the game board (or B-board), which at power on will inject the right decryption keys for that game. You can then remove the battery and not have to worry about changing batteries before they die and/or leak acid all over your precious precious game. If you want to read all about how the process of injecting the keys was reverse engineered, these articles tell the story. Really cool stuff!
I did these games in order of what I think they’re worth, so I did Giga Wing first, then Mars Matrix, then finally Super Turbo. The process was super easy and I had zero problem or issues installing things. Real basic soldering. All my B-boards were Rev.6 so the install was the same across all three games.
I got my InfiniKeys from Paradise Arcade Shop. I’ve ordered stuff from them in the past and have never had any problems.