tl;dr: use RF adapter from SEGA or Nintendo, set Famicom switch to CH1 and then tune in your TV to channel 95.
Last year I bought an original unmodified Famicom from a local seller. It is a bit dirty, white plastic has some yellowing, but otherwise it’s in great cosmetic condition. It also came with japanese Final Fantasy III cartridge.
Today I decided to test it. Original Famicom only outputs RF. I didn’t have a Nintendo RF adapter handy, but I got one from SEGA Master System. I tested it with a SEGA Master System first. It worked great, image was almost as clear as from composite video.
I did a quick online research about Famicom power supply needs. I learned it’s the same as for Super Famicom: 10v, center pole negative. SEGA Genesis model 1 power supply is ideal.
Next I hooked up everything, set Famicom RF output channel to CH2 and tuned my TV to channel 2. This didn’t work. After some more online searching, it turned out japanese channel 1 is roughly equivalent to american channel 95 and japanese channel 2 is american channel 96. Both produce poor quality output, but CH1 / channel 95 works better.