Genesis:CD BIOS Mod

This is an advanced mod. If care is not taken while removing the original BIOS chip, multiple solder pads and traces can be lifted, which will result in an extremely difficult repair being necessary. The key to avoiding this is patience.

Unlike its competitor, the TurboGrafx-CD/PC Engine CD, games on the Sega/Mega CD are region locked. In addition, the vast majority of CD units will only function on Genesis/Mega Drive consoles that match their region (i.e. a PAL Mega CD will only work with a PAL Mega Drive). Fortunately, both these issues can be fixed by replacing the BIOS with a hacked region-free version, which ignores both the region flag in the game and the region setting of the attached console.

This modification can be performed on both Sega CD models as well as combination consoles such as the CDX and X'Eye. The latter will require a different installation as the BIOS chips on these consoles are SMD chips which do not have modern equivalents.

Please note that this guide assumes you have an otherwise functional unit.

Materials and Tools Required

• Components and materials
  • Leaded solder
  • Flux
  • 27C1024 EPROM (for standalone CD units)
  • 40-pin DIP socket (preferably dual wipe)
  • Isopropyl alcohol (91% or higher) or flux remover
• Tools
  • JIS/Phillips head screwdriver
  • Chip puller (for systems with a socketed BIOS)
  • Desoldering gun (not required but highly recommended)
  • Multimeter
  • EPROM programmer
  • Hex editor software

Installation (for standalone models)

BIOS Removal and Socket Installation

1. Flip over the Sega CD and remove the six screws on the bottom. On the model 1, you will also need to remove the two black screws near the expansion connector. With all the screws removed, lift the top cover off, minding the expansion connector as you go.
2. For North American and PAL units, remove the RF shielding that covers the motherboard.
3. Remove the motherboard from the case, then set the case aside.
4. On both models, the BIOS is a standard 40-pin DIP chip located close by the expansion connector.
   • For some model 1s, the BIOS will be on a socketed EPROM. If your system has such a BIOS chip, you can ignore the rest of this procedure and use a chip puller to remove the BIOS. Alternatively you can use a flathead screwdriver to pry the chip out, being very careful as you do so. After the chip is removed, proceed to the BIOS Programming and Installation procedure below.
5. Flip over the motherboard and apply fresh solder and/or flux to the 40 solder joints of the BIOS chip. Desolder the points using a desoldering gun, being careful to avoid heating one area for too long. Each pin should be able to move freely when all the solder has been removed. If the pin still isn't moving, resolder it, apply more flux and desolder it again until it is free.
6. When the BIOS chip is desoldered, it should simply fall out of the motherboard when flipped over. Inspect your work so that no pads or traces have been damaged, and thoroughly clean the area with swabs and isopropyl alcohol or flux remover.
7. Take a 40-pin DIP socket and place it where the BIOS chip originally was, making sure that the orientation matches the silkscreen on the board. While holding it onto the board, solder two opposite corners of the socket in place so it can be anchored to the board. Ensure that the socket sits flush with the board.
8. Finish soldering the socket to the board, making sure not to apply too little or too much solder to each joint.
9. When the socket has been installed, take the original BIOS chip and place it into the socket. The chip must be fully inserted but make sure not to bend any pins in the process.
10. Reassemble the Sega CD to the point where a Genesis can be attached to it for testing, and set it up as normal. If the Sega CD continues to function normally, you may proceed with programming and installing the region-free BIOS.

BIOS Programming and Installation

to be continued