My N64 powers on but no A/V

In general not having an A/V signal is not a very big problem (if you know how to do soldering), In general the problem comes from the chip "RCP" it manages the audio and the image of the console and is not far from cartridge slot so it is very simple that dirt or corrosion settles on the pins of it. Is it often enough to properly resolder the pins to the motherboard. if the problem is not fix it can come from other chips so clean (and resolder if necessary) all the chips should fix the problem. check the solder points on the A/V port to see if the problem is not there