Nintendo64 turning on but no picture!!???

Hi guys, I recently bought an N64 second hand from a retro games store, and they tested it in the store, it was working fine, until me and my friends brought it home... We plugged it in to a 40" Samsung HDTV, and it turned on, but there was no picture and sound... We had tried with a RGB to scart adapter, and the RGB wires directly in to the TV. We also tried it on a 30", and a 19" desktop monitor. It still didn't work. We then took it back to the store, and the guy tried it on an old 15" TV,(the ones with the VCR integrated in the TV), and it worked fine!!??? The only game we tried was DooM 64, and the guy in the store used the exact same one. So we took it home, thinking it was fixed, and it still won't work.. We tried it on about 4 different old TV's, and nothing.

We aren't using an expansion pak, just the standard jumper pak. What can we do!? Thanks :D