Every console, even the Atari 2600 has an audio and video "output" and it can be wired to a portable tv's av input. In the Atari's case, the audio and video points are in the middle of the board somewhere, depending on the board revisions, and I'm wondering where these points are on the famicom are. Then, maybe I could wire it to my portable dvd player and make a portable famicom!
The composite video output is found on PPU pin 21 (https://wiki.nesdev.com/w/index.php/PPU_pin_out_and_signal_description). You need to build an amplifier for this since the Famicom only uses an RF modulator. This is the same as when making an AV mod.
On the RGB PPUs the RGB video comes from pin 14, 15 and 16 for red, green and blue respectively, while pin 21 is composite sync as can be seen in the above link.
APU audio comes from pin 1 (Pulse channel 1 and 2) and pin 2 (Triangle, Noise and DPCM channels) on the CPU chip (https://wiki.nesdev.com/w/index.php/CPU_pin_out_and_signal_description), but this is not amplified nor is the expansion audio included. Final audio output can be tapped from cartridge pin 46 (https://wiki.nesdev.com/w/index.php/Cartridge_connector) on Famicoms (but not on NES). This is amplified and have expansion audio from cartridge mixed in.
Audio output available in the Famicom's 15-pin expansion port, differs between the red & white Famicom and AV Famicom. On red & white Famicom, the expansion audio isn't mixed in and I think it isn't amplified either. I don't know about Twin Famicoms. Cartridge pin 46 is the way to go.
THX a lot! That should help me quite a bit!