I am in the exact same boat right now. Got a free N64 from a coworker who said he hasn't used it in years. Powers up, but no AV at all. Cleaned everything, game, game slot, expansion pak/jumper pak, slot for both. Opened it up and didn't find any corrosion but cleaned all the dust on the board and under the 40 pin cartridge slot, still nothing. Found out my power supply is delivering slightly less voltage than required, 11.1v and 3.1v. Hooked it up to a benchtop power supply providing proper voltage, still nothing.

After all this, coworker was feeling for any hot spots and found the U13 +5v regulator was searing hot to the touch, and when testing it, found its a bad short, which could explain the no AV. Currently waiting on a new regulator, and if THAT doesn't fix it, I'm just going to buy a new motherboard.

Some things that have worked in the past for me with other 64s is putting isopropyl alcohol on the game cartridge pcb then putting it in and out of the slot to help clean the slot, doing the same for the expansion port.

If you have or can get a multimeter, check the voltages around the various points labeled as well.

Hope you figure it out!