Master System:Mark III SMSFM Installation

Although the Sega Mark III had an official FM add-on (the FM Sound Unit), it is fairly difficult to find and also does not pass through the Mark III's RGB output. The SMSFM board by Tim Worthington is compatible with the Mark III, though it requires a fair degree of manual wiring since it was largely intended for install in a model 1 Master System.

An alternative FM mod also exists specifically for the Mark III, and does not require a large number of wires to be soldered. However, it is not usually available in assembled form and sourcing certain parts like the YM2413 may be difficult.

Materials and Tools

Materials

• SMSFM board
• Leaded solder
• Flux (not required but recommended)
• Hookup wire

Tools

• JIS/Phillips head screwdriver
• Soldering iron
• Desoldering braid or desoldering gun
• Flush cutters
• Wire stripper
• Drill with step bits (optional)

Procedure

1. Flip over the Mark III and remove the six JIS screws. Do not remove the top shell yet as the card slot is screwed into it.
2. Carefully maneuver the ribbon cable connecting the card slot PCB to the motherboard and remove the two screws securing the card slot PCB.
3. Remove the two screws on the cartridge slot (one on each side) and use a flathead screwdriver or spudger to undo the two bottom catches securing the motherboard to the bottom case. These will require some effort so proceed carefully.
   • The cartridge slot will also have a black paper cover placed around it, which hides the internals when the cartridge slot flap is open. Make sure to keep this in a safe place during disassembly.
4. The SMSFM will need to be wired manually to the Mark III's motherboard, consisting of about 20 wires. Follow the SMSFM wiring diagram and the Mark III schematic to determine how to wire up the SMSFM. Many of the signals are part of the main system bus and can be found in multiple places, though the front expansion port is most convenient.
   • Note that if you decide to wire the SMSFM to the expansion port pins, this will prevent you from using the SK-1100 keyboard or an original FM Sound Unit. If you wish to retain this function, wire the SMSFM to the appropriate pins of the Z80 CPU or the cartridge port instead.
   • Also note that you do not need to connect the KBSEL signal, as the need for this was eliminated in more recent versions of the SMSFM.
5. For audio, locate resistor R12, which is located directly below the power jack. Flip over the board and cut the audio trace after it combines with R12 and the emitter for Q13. Solder the white wire of the SMSFM to the rightmost lead of R12 and the red wire into the via labeled "AUDIO" near the RF modulator. Ground can be wired to any ground source, such as the RF modulator shielding.
6. If desired, the three-way switch included with the SMSFM kit can be used to turn the FM audio on or off. Use a step bit to drill a small hole on the rear of the bottom shell and install the switch so that it does not interfere with the internals or screw bosses from the top shell.
   • Note that the region switching feature of the SMSFM does not work on the Mark III, it will always remain in Japanese mode regardless of switch position.
7. Place the motherboard back into the bottom shell and test with a compatible game. If both PSG and FM audio function as expected, reassemble the rest of the console.