Genesis:CD BIOS Mod

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Warning 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/Multi-Mega, both Wondermega models and the X'Eye. The latter few will require a different installation as the BIOS chips on these consoles are SMD SOP-40 package chips which do not have modern equivalents. A BIOS modification can also be performed on the Aiwa Mega CD, which uses the exact same through-hole BIOS chip as the Mega CD 2.

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)
    • MultiBIOS replacement PCB from Will's Console Modifications (for combination 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)
    • Soldering iron (preferably temperature-controlled)
    • Desoldering gun (not required but highly recommended)
    • Hot air rework station (for systems with an SMD BIOS)
    • Multimeter
    • EPROM programmer
    • Hex editor software or other program which can perform byteswapping

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. For the Aiwa Mega CD, flip over the "Game Unit" section and remove the ten screws on the bottom.
  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. On the Aiwa Mega CD, it is a 40-pin DIP chip located on the right-hand edge of the motherboard, right next to the main 68000 CPU.
    • 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.
    • For the model 2, the BIOS chip is located directly underneath the expansion connector, giving you far less room to work with. Be very mindful of this as you proceed.
  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, proceed with programming and installing the region-free BIOS.

BIOS Programming and Installation

  1. Download the region-free BIOS that matches your specific Sega CD model. Any region BIOS can be chosen, but the model must match (i.e. you cannot use a model 2 BIOS on a model 1).
  2. Before the BIOS can be programmed, it must be byteswapped if it hasn't been already. Some EPROM programming software has the ability to byteswap files, but if not, you will need to use another program such as Hex Workshop or AFS ROM Suite MD which can perform this function.
  3. Take a 27C1024 EPROM and insert it into the EPROM programmer. Set up the programmer's software so that it is configured for your particular EPROM.
    • If you are unsure about the various voltage settings for your EPROM, look up the exact part number and see if there is a datasheet available. The datasheet should provide full specifications of your particular EPROM including the proper voltages for reading and programming the chip.
  4. Use the blank check function of your programmer's software to determine if the EPROM is blank or not. The EPROM must be completely blank or else it cannot be written to.
    • If your EPROM is not blank, it will need to be erased by exposing it to strong UV light. Inexpensive EPROM erasers are available through various stores including Amazon.
  5. Load the BIOS file into your programmer software. No other changes need to be made before programming the chip aside from making sure the BIOS file is byteswapped.
  6. Program the EPROM. If the programming and verification were successful, proceed with installing it into the system.
    • If the programming failed, double check that the programmer is properly configured for your EPROM and that no other settings were changed. Erase the EPROM before making another attempt.
  7. Take the programmed EPROM and insert it into the BIOS socket on the Sega CD. Make sure all the pins are lined up and carefully, but firmly, insert the chip all the way into the socket.
  8. Reassemble the Sega CD again to the point where it can be tested, it does not need to be fully reassembled at this point. If the system boots as normal, proceed with testing by using a game which is of a different region than your system (it does not matter if it is an original pressed disc or burned copy). The other region game should now play, and you can fully reassemble your system when testing is complete.

Installation (for JVC X'Eye)

  1. Flip over the console and remove the six screws on the bottom. Remove the top case and set it aside.
  2. Unscrew the five screws securing the motherboard to the bottom case, then carefully disconnect the three connectors going between the motherboard and CD processing board. Lift the motherboard out and set the rest of the console aside.
  3. Locate the BIOS chip, which is labeled "IC104" and is directly above the large 315-5632 ASIC. Apply a generous amount of flux to all the chip's pins.
  4. Place Kapton tape or tin foil around the components in the vicinity of the BIOS chip so that they will not be damaged or accidentally removed during the removal process.
  5. Set your hot air station for low to medium airflow and set the temperature for around 335 degrees Celsius. Hold the nozzle within an inch of the chip and swirl it around so that the heat is applied evenly. After about 10-15 seconds, the chip should become loose. Carefully remove it from the board and set it aside. Do not force it or make any fast movements as you could damage or lift pads.
    • Alternatively, Chip Quik or similar solder which melts at a very low temperature can also be used to remove the BIOS chip. The pads will need to be thoroughly cleaned with desoldering braid afterwards as the low melt solder can contaminate your solder joints when the new BIOS is installed.
    • If the original BIOS will not be salvaged, cutting the pins of the original BIOS with a pair of flush cutters is also possible, but great care must be taken as the force of the cutters may cause damage to the pads.
  6. Clean the pads by applying flux and then carefully and lightly dragging desoldering braid over them. Once the old solder has been removed, clean off the area with IPA or flux remover.
  7. Take the MultiBIOS board and align it so that the jumpers are on the same side as the notch in the silkscreen. Place a piece of Kapton tape over the board to help keep it aligned to the pads. Solder opposite corners to anchor the board. Keep in mind that the solder pads for the BIOS chip are fairly short so make sure there is enough room to solder the board down.
    • It is also highly recommended to place a piece of Kapton tape over the pins of the ASIC directly across from the BIOS, as a stray solder ball can easily lodge itself into the chip's fine pitch pins and cause problems!
  8. If the alignment is good, apply a generous amount of flux to the board and then put a small amount of solder on the tip of your iron. Drag the tip across the pads so that the solder flows evenly; the flux will do most of the work for you. If you wind up with a solder bridge, apply more flux and "sweep" the excess solder away onto your iron tip.
  9. Once soldered down, inspect your work and use a multimeter to verify that all connections have been made. Consult the service manual if necessary.
  10. Reassemble the X'Eye to the point where it can be tested, it does not need to be fully reassembled at this point. If the system boots as normal, proceed with testing by using a Japanese or PAL game (it does not matter if it is an original pressed disc or burned copy). The other region game should now play, and you can fully reassemble your system when testing is complete.