Intro: NES Stereo Sound Mod.
drill and bits
(2x) uf 1 50 volt capacitors
(1) 47k ohm resistor
rca audio jacks (red/white)
around 2 feet of wire (I used speaker wire)
Step 1: Confirm You Have Proper Components & Disassemble the NES.
Gather up what you need & confirm that every component you have is what's specified to do this project. Remove the 6 screws from the bottom of the NES using philips screwdriver. Flip it back over and remove the top setting it to the side out of the way. Now, using the philips screwdriver remove all the screws holding the metal shielding on & set it to the side. Remove the remaining screws holding the motherboard down as well as the cartridge tray. Raise up the motherboard & slide the tray off the 72 pin connector.
Step 2: Find Soldering Pins for 47k Resistor.
Once you get the cartridge tray off the motherboard find the pins that are above the expansion slot that sticks down to the bottom of the NES. There are 48 pins & it's easy to find because there's a black tab sticking through the motherboard from the expansion slot on each end of it's pins. We are interested in pin 3 from left and pin 9 from left. Refer to the pics I've posted to see which row of pins from expansion port I'm talking about & solder one leg of 47k resistor to the 3rd pin from left & the other to the 9th pin from the left.
Step 3: Solder Wires to Resistors Below CPU.
Now you are going to need to solder some shielded wire (I used speaker wire) to either the 1st 2 pins on lower left side of CPU or the better option in my opinion is to solder them to the 2 resistors (R3 & R4) below those CPU pins. Reason being is you don't have to worry about damaging the CPU & there's already a nice solder joint there for the resistors. This is where the stereo sound is coming directly out of the CPU.
Step 4: Solder in Capacitors, Rca Jacks, Ground Wire, Then Drill Holes for Jacks.
Now that you have your wires soldered to the CPU put the NES motherboard back in place. Route your wire up around the back corner of the NES lower housing & solder each wire from CPU to the negative side of your 1 uf 50 volt capacitors. Then cut 2 pieces of wire to run from the positive side of each resistor to the positive post on your RCA jacks which center hole is positive. Solder it in place then cut 2 more pieces of wire to run from the ground post of the RCA jacks which stem off the out metal ring of the jack. Run these ground wires to the metal box that houses the RF modulator components that sits at the back corner of the NES. Clean off a spot on the metal & solder your ground wire directly to the metal. Tug on the wire to make sure you have a good solder joint for your ground. Be sure to use shielded wire for all this so as there's not any shorts or interference. Figure out where you are going to put your RCA jacks & drill out small hole first then use bigger bits to work your way up to the bit that's the size of the jacks. I chose to place mine at the back corner of the NES opposite of the RF modulator since the corner has plenty of space & would provide more strength. Make sure you place them low enough so as the upper console housing will fit back into place. Use a generous amount of hot glue around them as you hold them in place giving the hot glue time to dry before letting go.
Step 5: Test Before Reassembling.
Give everything a good check making sure your capacitors are isolated along with all your soldering joints. Hook up your console to test it before reinstalling the screws. I'm using powered pc speakers for my audio & I'm quite happy with the results so far. I've read various articles across the web on different methods to do this & this was the simplest way & gets you the true audio signals directly from the CPU without mixing in mono. Next I'll probably try to pick up a rca to headphone adapter to see how headphones sound now that the audio is separated. Enjoy & game on!