Tap audio from 15-pin front connector on RF Famicom?

Started by bsa, June 25, 2022, 10:29:57 pm

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bsa

Hi all.

Seems that the video output on my junk RF Famicom is broken, but audio is ok. I'm thinking to use it for experimenting with audio.

From the Nintendo schematic for the RF Famicom here (https://console5.com/wiki/File:Famicom-Schematic.png), it seems the only difference between expansion port "SOUND" and cartridge pin "SOUND I" is a capacitor to ground before the cartridge pin. Both pins look to be after mixing the CPU audio channels, and the amp.

I'm a complete novice with electronics, so:

1) If I just add an RCA connector to expansion port P2 pins 2 and 1, would the level be suitable for a sound card line input, without blowing anything up? I understand that I wouldn't get expansion audio added on the cartridge.
2) What is the purpose of the 1 microfarad capacitor to ground (C8) in the internal audio circuit? Should I add one somewhere in my cable, and if so, where?

Thank you!

P

Since AV-mods take the audio output directly from cartridge pin 46 and pin 45 is normally connected directly to pin 46 on most cartridges I think it's safe to assume that pin 45 is line level audio at least.
I don't know what that capacitor C8 is for, but I guess you could add the same capacitor on your expansion port device to ground close to your RCA jack to be safe. It looks like 0.1 μF to me, not 1 μF. The value is also different on different boards, the GPM-02 seems to use a C8 at 0.47 μF, maybe because of different stray capacitance from changes on the board layout or other things.

Note that your schematic is for the original Famicom. On the AV Famicom, the Expansion port pin 2 (SOUND) is supposedly connected to pin 46 instead of pin 45 so you do get expansion audio on AV Famicom, and it has a very different board.

bsa

Thank you! I'll try it out and see how it goes.

Thanks for the schematic links too. My Famicom is one of the old style ones, but good to know that the cartridge audio is mixed in on the newer ones.