Broken Genesis?

Hi all,

a friend of mine gave me a Genesis (actually, it is a Megadrive, PAL) which seems to have some problems:

it switches on, but all you can see onscreen (even if I use the RF instead of the AV out) are vertical bars. The game runs in background because I can hear the music, choose options (I hear the sounds), and sometimes between the vertical bars you can see something of the game running (messed graphics, of course). I tried several carts, so I'm sure it's not a cart issue.

anyone who ever fixed something like this?

Is there any way you can test the RGB output? This would allow a pretty solid determination as to whether it's a problem with the VDP/VRAM or a failed encoder.
Check the RGB output? I would, but I never did, so the only way is some1 to tell me how it is to be done, so that I'll do as soon as possible and I'll post the results here.

So guys, that poor MD is still there, waiting for someone who point me to the right direction so that I can fix it!

I hate having consoles broken, and I hope some1 will have some useful (every help is useful of course, but... well you know what I mean) tip.