About 60 Sega Master System games include music data for Frequency Modulation (FM) synthesis. Yamaha’s YM2413 (or OPLL) sound chip, employing only 2 operators, was a cheap alternative to the FM synth chips found in its flagship DX7 keyboards and frequently employed in advanced arcade cabinets. Sega’s Mark III console, released only in Japan, could play the FM data when an external sound unit accessory was connected, and, later, the Japanese version of the Master System included the YM2413 chip in the internal circuitry. Unfortunately, the North American and European releases of the Sega Master System were not built to support the expanded audio, even though most game cartridges retained the FM sound data.
I recently modded two North American Sega Master System consoles to read and perform the FM sound data, using a PCB with the YM2413 chip that was built and designed by Tim Worthington. These mod and kits are explained on this website. Although diagrams and schematics are posted online, I have not found a detailed, step-by-step guide; therefore, I am blogging my experience with installing two kits for those, like me, who may need more guidance. I am not an electrical engineer by any means, so please comment below if you have any suggestions for improvement. My approach worked well for my two consoles, but please be aware that differences exist in the variant releases of the Sega Master System.
EDT’s video proved an invaluable resource to my approach.
-Tim Worthington’s FM Sound Kit (available for purchase here).
-Phillips head screwdriver
-Solder and flux
-Drill with 5/16 bit
-Small gauge wire cutters
-Desoldering pump or wick
-Jeweler’s flathead screwdriver
-91% Isopropyl Alcohol
PART 1: Preparing the PCB
-Slide the pin connector through the holes in the PCB and solder each pin on the rear side. To ensure perpendicular alignment, use something to prop up the board so the two rows of pins are even. Careful not to bridge adjacent pins (see picture); use a desoldering pump or wick to remove any excess solder.
-Slide in the toggle pin connector so that the long end runs parallel to the front of the PCB. Then solder the 3 short end pins on the rear of the board (where the text FM and Japan FM is printed.
-You’ll see that the plug on the provided wire for the toggle switch will slide right onto the 3 pins that are running parallel to the front of the PCB.
Part 2: Preparing the Toggle Switch
-While the plugs on the wire provided with the kit fit snuggly on the pins for the PCB, I found that they were too small for the terminals on the toggle switch.
-Therefore, I cut off the plug on one end, stripped the 3 wires, and then threaded through and soldered to the 3 terminals, being careful not to bridge any of the three wires with solder on the back of the toggle switch.
-Note: Keep the plug on the other side of the wire, as it will slide easily onto the PCB pins.
Part 3: Preparing the SMS Console
Disassembly. This video may be helpful.
-Remove the six screws on the rear of the console case with the Phillips screwdriver and pull apart. Put screws and console top aside.
-Then remove the 5 screws around the base of the RF shield as well as the screw on top of the RF shield. NOTE: the screw on top of the RF shield is different and includes a washer.
-Finally, remove 7 additional screws that hold the motherboard in place. See the diagram pictured below (one of the two screws on the bottom left is slightly cut out of the photo).
-You can now remove the motherboard.
Wiring SMS motherboard
-Remove the electrolytic capacitor at C37 by locating the traces on the rear, warming the existing solder with you iron, and pulling the capacitor away from the other side when the solder is molten.
-Locate ground plane to the left of the same capacitor field and scratch away the green coating with a jeweler’s flathead screwdriver.
-Prepare the wire provided by stripping the sleeves to reach the target locations on the motherboard and then tin the ends.
-Thread the red and white wires though the positive (+) and negative (-) polarities at C37, and solder the traces on the rear of the motherboard. NOTE: The image provided on the website above has the red and white wire locations reversed. The white wire should be on the right (+ve), and the red wire on the left (-ve).
-Apply flux to the ground plane you just scraped on the component side and solder the threaded copper wire to it. Once completed, wrap electrical tape around exposed wire threads to keep them from reacting to nearby components.
Part 4. Final assembly.
-Clean the pins on the expansion port with some alcohol and cotton swabs.
-Slide the pin connector on the PCB onto the expansion port pins. It should slide all the way in tightly.
-Attach the motherboard to the bottom casing with the 7 screws left to the side.
-Use shears or tin snips to cut the RF shield and roll the fingers up so the FM sound board will clear without making contact with the shield.
-Attach the plug on the toggle switch wire to the PCB (if you haven’t already) and put the RF shield back into place with the 5 screws left aside, as well as the smaller one for the top of the shield that has a washer.
-Use a 5/16 bit and drill a small hole through the lid of the console in the desired location. I used the rear left side, so it would have the same clearance as the power and AV cables wherever I set up my SMS.
-Make sure the hole is just wide enough to slide the threaded part of the toggle switch through and fasten with the included washer and nut. Electrical tape can be used to hold the wire in place, so it will not block access to the cartridge and card slots.
-Carefully place the console lid into position, making sure that the toggle switch wire doesn’t get pinched. (Note: I took photos during both console mods, so pay no attention that the toggle switch wire colors are different at times).
-Put the 6 outer casing screws back in. Done!
Optional. Label the toggle switch. The direction will depend on how the wires were connected to the pins with the plug on the PCB.
Once assembled, you should be able to boot the FM sound data from the Sega Master System cartridges that have it (they are listed at the website above). Some games will require you to use the Japan FM setting on the toggle switch. Other games will require a patch or ROM from a flash cart like the Master Everdrive. You cannot toggle during gameplay, as the FM data is only loaded at boot up.
Many thanks to Tim Worthington for an outstanding mod and kit!!!
Please leave comments below if you have any questions or any suggestions for improvement. I hope this step-by-step guide will be helpful! =)
Like many others, I modified my original NES with a 47k resistor to support expanded audio playback. While connecting expansion pins 3 and 9 does access the additional channels, the balance is very inconsistent. This is due to variability in the chips added to Famicom cartridges and to the Famicom Disk System and how they were mixed with the primary sound created in the console. Each expansion included both the additional audio data and the instructions for how it should be mixed with a Famicom. A fixed resistor therefore results in a fixed mix with the console.
A potentiometer, used to vary the ratio of voltage divided between two circuits, can be wired as a variable resistor (i.e. rheostat) to provide dynamic control of the extra channels. Instead of connecting all three pins, you just wire one of the outer terminal pins and the middle pin (wiper). I found this image helpful in demonstrating the difference. You could choose to bridge the extra terminal pin to complete the circuit in case the wiper should fail over time.
Since there were no visual guides specifically for a 100k pot mod to the NES, I decided to post my experience. I’m no expert, so I’m grateful for the post by Wesley Almond that guided me along the way. Also, thanks to Brad Smith (a.k.a. rainwarrior) for the 100k pot mod idea.
A)-regular philips head screwdriver
B)-jeweler’s flathead screwdriver
D)-wire stripping pliers (for small gauge)
E)-soldering iron and solder
F)-flux: to prevent oxidization at soldering joints
G)-drill with 3/8 bit
H)-100k Potentiometer. TIP: Some come with “off switch” that is engaged with a click when turned all the way, counter-clockwise.
1-Remove the 6 outer casing screws with the philips head screwdriver.
(NOTE: All of the screws removed in this mod are the same, except for 2 slightly longer ones later in the process. I still recommend keeping them grouped and on a surface where they won’t roll away!)
2. Remove the bottom casing to reveal the heat shield.
4. Lift the shield slowly to navigate around any obstacles
5. Remove the 6 screws from the cartridge tray. Note that the inner screws of the bottom pairs (blue) are slightly longer than the other screws.
6. Pull the cartridge tray out and then lift up.
8. Locate the Nintendo lockout chip and pry off the 4th pin from the left—I used a jeweler’s flathead screwdriver to pop off and bend back.
9. Slowly flip the board back over and set back into the console casing.
10. Prepare Pot: At this time, warm up and tin your soldering iron. Cut a pair of 5-6 inch wires and strip the coating back on both ends. Warm wires with soldering iron, dip in flux, then warm solder with iron and connect one wire to the middle terminal on the pot, which controls the wiper, and the other to the right terminal on the pot. I shaped my wire like a hook and looped in the pot rings before applying solder.
11. Once the pot has cooled, set it outside the console between the reset button and the controller jacks. Mark your drilling location on the plastic casing where the pot knob will go, leaving enough clearance above the pot to reassemble the console.
12. Drill a small pilot hole and then use a 3/8 drill bit to create an opening for the threaded pot knob. Careful not to drill into the wiring, which can be pulled back slightly and even held out of the way with electrical tape if needed.
13. Apply some side pressure (or use a dremel) to widen the opening as needed until the threaded knob pulls through snug, and then fasten with the nut.
14. Locate pins 3 and 9 on the board, used to channel expanded audio.
15. Solder the wire attached to pot’s middle (wiper) terminal to pin 3 and the wire attached to the pot’s right terminal to pin 9. TIP: Make sure there is enough slack for the wire to run over the board and loop along the bottom of the casing before routing back up to the pot.
15. Use side of drill bit (or dremel if you have one) to carve out two small openings along the edge of the cartridge tray where the wire will not be pinched when pushing tray down during reassembly. Alternatively, you could use longer wire and run completely around the tray and approach pins from the side.
16. When reassembling the cartridge tray, make sure to slide the wires into the openings before pushing tray down. Also, remember to use the two longer screws in the correct locations.
17. Re-attach the heat shield and the bottom casing, and flip over. DONE!
18. Test your modded NES! NOTE: This mod will not work with VRC7 (Lagrange Point) unless you are using an actual Lagrange Point cartridge, with the 60-to-72 pin adaptor wired to route the cartridge’s audio. Most other expanded audio will work via Powerpak flash cart. Use pot knob to mix expanded audio with console audio to your liking! Turn the pot knob off when not in use.
Hope this helps newbies like myself! Please comment with any questions or feedback; if there’s a better way to do this, let me know and I’ll update!