Atari 7800 Not working

[cityside75]

I picked this up at a garage sale with a bunch of games for a few bucks. I'm really anxious to try it, but I just can't get the system to work. Here's what I know:

-The power light comes on when it's powered up

-It does occasionally display a very scrambled picture on the screen when turned on, and even made some random sounds on a couple of power-ups

-It usually displays a black screen when turned on, but it's not straight black but rather a weak-looking (if that makes sense), rolling black screen

-It behaves exactly the same way whether a cartridge is in the slot or not, putting in a cartridge does not change anything (I tried both 7800 and 2600 carts)

-It did not appear to have been opened before I opened it up

-I did not see any obviously burned-out or broken pieces while looking at the board

-There was a small piece of broken shiny plastic rolling around inside the unit, but as far as I can tell it did not break off of anything on the board

I want to check the cartridge connector, but I can't seem to remove the plastic shroud that covers it - any ideas on that?

I know everything else involved works OK, I've tested other systems with the RF connection, I've tested the 2600 games on another 2600 and they worked fine. I've tried messing with the channel switch and trying other channels, nothing helps.

I don't usually post my tech problems because I can usually find a repair link with enough effort, but I figure 7800's must not break often as there is very little info available on the net.

Any ideas where to start?