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Nintendo Famicon + Composite Mod

So, I impulse-purchased this original Famicom from a Recycle Shop next to Higashi-Yodogawa Station in the Shin-Osaka area. I wasn't in the market for a Famicom originally, until I found out that the original A-Train was released for it (as well as the MSX.)


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The box didn't have an AC adapter. After a little googling, I purchased one from the stalls under the train station in Akihabara. BE VERY CAREFUL if you're buying an AC adapter for a Famicom, they're non-standard. See further below for potential issues.

Of course, Japan was NTSC back in the day and so this unit outputs an NTSC signal over the RF output. This is no good for TVs in Australia as none of them are capable of decoding an NTSC RF signal. Fortunately, they can decode an NSTC Composite signal, so we'll need to modify this unit to bypass the RF modulator and provide the signal we require.

Outputting a composite signal

I initially thought I'd use this simple modification and bought the required components. At home I realised that I actually had the 1989 version Famicom with Motherboard revision HVC-CPU-GPM-02. Searching further, I found this modification which was stated to work with my revision of the motherboard.

I purchased the required components:

  • 2x 1uf Capacitors (I used Monolythic. The Author says Ceramic, but the picture he shows uses Monolythic caps.)
  • 33uf Electrolytic Capacitor
  • 220uf Electrolytic Capacitor
  • 100ohm Resistor
  • 150ohm Resistor
  • Two RCA jacks for mono audio and video

There is also a transistor involved. This component is already on the motherboard and does not need to be purchased.

Applying the modification

First, de-solder the transistor.

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The next step is to de-solder pin 21 of the PPU and then bend the pin out of its hole. This is to prevent signal noise which shows up as vertical bars in the signal (aka jailbars.) I had issues with the desoldering and so cut the leg at the base with the scissors on my swiss-army-knife. I don't recommend this process as you scratch the crap out of the pcb. Fortunately, there are no tracks underneath, so I fluked it.


Now, flip the board over and scratch the track that 21 was attached to as below. Although you've already removed the signal running on it, all articles recommend that you also scratch the track. Supposedly the thin track next to it absorbs noise from its neighbour. I'm not so sure that the track needs to be isolated, as we've already lifted the pin and therefore there's no circuit... but I did it anyway.

Next, we need to work with the transistor that was removed. Fold one leg horizontal and then solder it to the locations as shown below.


Finally, add the 33uf Cap and resistors. The negative leg of the capacitor is your video output, so wire this (with sheilded wire if possible!) to the yellow RCA jack.

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Add the two 1uf capacitors to the following locations to further suppress noise.


The second-last step is to attach the 220uf capacitor for sound output. The sound output comes from pin 46 of the cartridge port, so connect the capacitor there and the other end to the black/white RCA socket. Of course, both sockets also need to be grounded! I initially thought that I could just lie the capacitor on the board and connect the leg to pin 46... turns out that the case presses against this capacitor when closed, so I had to move it and wedge it in the gap to the left.

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Now the final step is simply to mount the RCA sockets on the side of the unit. I chose the front right as there is heaps of room behind.

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Now... to test...

Incorrect AC Adapter

Nothing... no power... dead... I hadn't actually ever gotten a signal out of this unit; so could it be completed dead? A quick investigation with my multimeter showed that the voltage was backwards on the 7805! Oh shit. For anyone playing at home, the link here gives you all the information you need on the power supply, especially pointing out that the adapter needs to provide negative on the center pin. Of course, I had bought a power supply with positive in the center and had tried to tune the RF for a few minutes. This completely cooked the 'slow-blow' fuse in my Famicom!

See here for more detail on fuses and replacing them. I was unable to find an exact replacement at my local electronics store, so I just shorted it to start with... but there was no way I was going to leave it in this configuration. If you want an exact (or best match) replacement, then search eBay for 'SNES Fuse' and you'll get quite a few results.

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Update: The fuse came and was installed with ease. It's actually from a Super Famicom, so looks different, but does exactly the same job.

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After this, I reversed the polarity on the AC Adapter.


And then....?


Winner! Nearly...


I tried everything to get the picture correct. There's a trimpot, but this didn't help. I also grabbed some tinfoil to shield my video cable to the port, but that didn't help either. After a little googling, I realised that the NTSC timings of the Famicon are slightly out-of-spec and that my TV would never actually decode the signal. I know it can decode a proper colour NTSC signal, so I was a little concerned.

As a final test, I plugged the machine into the big TV.


Muhahahahahaha... and then for the final icing on the cake... (Thankfully I was wearing pants...)

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Now to play the game!

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