If you are seeing "no signal" try removing and re-seating your jump pak or expansion pak.
If you are seeing color bars your Super 64 most likely is fine.
We understand this sounds odd, but what we would recommend is that you re-clean your game using 91+% Isopropyl alcohol and a q-tip or 1Up Card.
If this doesn't work you can use an eraser to clean your cartridges contacts.
DO NOT USE BRASSO.