SNES problems

I didn't really know what forum to post this in but I recently got a SNES from a flea market and when I turned it on it would show the intro for the game then flip back to the blue screen showing no signal. I tried this with two games and it does the same thing. I know it's not the cables itself because I used two different ones and they work fine on my N64. I cleaned out both the games and the cartridge slot with q-tips and rubbing alcohol. It is an american first model SNES and it seemed to be working perfectly fine at the store. What should I do?

@Leon618:

Sounds like you got screwed. And why would you use alcohol? Alcohol is NEVER the answer lol. Back then blowing on the cartridges was all you needed. I suggest you take it back to the store and say this doesn't work, try hooking it up to your tv and if he refuses well you know he screwed you.