The Japanese Sega Master System included a Yamaha YM2413 FM synthesis chip along with the standard PSG sound. Many games, some of which were never even released in Japan, contain extra FM sound an music. Installing this board will add FM sound capability to any Master System, Mark III, Game Gear*, or Megadrive* game console. The first post in this thread contains a list of games with FM sound. A toggle switch is provided with the board to disable the FM chip and revert to PSG sound whenever desired.
*With SMS cartridge adapter.
Version 1.0
Circuit Diagram
There was a small problem with this board which made it incompatible with a couple of
games. Read this discussion
on the SMS Power forums for more information and a fix. Many boards also contained a
defective YM2413 sound chip, read more about it here.
Version 1.1
Circuit Diagram
Installation requires some soldering for audio mixing. A capacitor in the audio circuit
is removed and a two core shielded cable soldered into its place. The standard (SN76489)
audio signal is sent into the expansion board via the white wire, is mixed with the FM
sound signal, and sent back through the red wire. The cable shield should be connected to
ground to reduce noise.
PAL SMS (original 1986)
Capacitor C48 (10µ aluminium electrolytic type) is removed, the white wire is soldered to
the +ve hole and the red wire to the -ve hole. There's a large ground plane
nearby to attach the shield. Access to this cap first requires the removal of the voltage
regulator heatsink. This is done by first removing the screw on the 7805 voltage regulator
with a long shaft screwdriver, then unscrewing the two screws holding the heatsink to the
PCB. Don't forget to reattach the heatsink before you finish!
PAL SMS 2
Capacitor C31, located beneath the video encoder IC, is removed. White wire to -ve,
red wire to +ve, and shield to nearby ground point at the -ve leg of C44.
NTSC SMS
Remove C37, white wire to -ve, red wire to +ve. There's a ground plane around the
base of the capacitor so just scrape off a bit of the green stuff and solder the shield to
that. The heatsink on the voltage regulator many need to be removed for access (remember
to put it back!).
Late model PAL SMS (VA3)
Remove C62, white wire to -ve, red wire to +ve. The shield is connected to a
nearby ground plane - the leg of C59 looks like a good choice.
Brazilian Tectoy SMS (original version)
Locate CI-5, the 315-5124 VDP. Find the trace running from pin 10 and cut it
close to the IC. Solder the white wire to pin 10 and the red wire to the positive leg of
C45 (that's where the trace goes). I don't have any photos for this one but I do have a circuit diagram for reference.
Generic SMS (those not listed above)
Trace the audio signal from both the RF modulator and A/V port (if there is one) until
they meet at a common point. Trace the audio signal back further (it may go through an
opamp or audio buffer inside the video encoder IC) until you find an electrolytic
capacitor in the signal path. White wire = input to expansion board, red wire = output.
Find a ground connection close by and attach the shield. Keep in mind that the input and
output are electrically the same as those on a stereo system. There's no reason why
standard phono sockets can't be attached and the mixing done with external leads.
Solder Version
This is simply a board without a 50 pin edge connector fitted. In order to install the
board into a SMS2, Megadrive, or Mark III you will need to do a bit of soldering.
In addition to the above, the solder in version requires 18 wires to be connected from the
expansion board to the SMS board. Thin solid core wire (like wire wrap wire) is
recommended for all connections with the exception of +5v and Ground, which should be
thicker. View from the bottom of the board.
Solder diagram for PAL SMS 2 (1MB)
Info on connecting
the board to a Sega Megadrive
Plug In Version
Punch out the piece of plastic over the expansion slot and plug it in. Do not plug it in
up side down!
Enable Switch
Drill a 6mm hole and mount the enable switch in a convenient place. If you don't want a
switch then place a jumper over the switch pins.
Testing & Adjustment
Insert a game cartridge which is known to support FM sound, turn on, and listen. If you
like you can adjust the volume of the FM sound to match the standard sound by twiddling
the trimpot (RV1).
Changelog
29/8/09 - Updated with details on the new v1.1 board.
10/1/08 - Page created
