Atari 5200 power but black screen

[musicman130]

I recently got a hold of an atari 5200 from a friend of mine from HS. When it was put away, it was in working condition. It has been sitting in his parents basement for years and I got it and tried to power it up. The only game he found right now is frogger, so I have a game to try and 2 controllers. The console powers up alright but all I get is a black screen without a cartridge in it. I put frogger in and got a green screen, turned it off and then back on and just a black screen. Any idea what I can do to fix this problem?

Forgot to mention that it is a 2 port system, so separate power and RF. I plugged another RF cord from my jr atari into it to see if it would work and same thing, so I know it's not the cord.