Where did you get this Famicom? Unless there was some official derivative I don't know about, this sounds like a botched mod job. Open it up and see if there's any sloppy soldering or something like that.

I did an AV mod for an Atari VCS and the picture was too dark despite using the same components listed in the circuit diagram. I wound up replacing the resistors with potentiometers that I could tweak until I got a good picture. When I tried that same console on another TV, the picture was bad again. (But still fine on the former TV)