[Ron Ramirez]

If the foil is indeed all gone, you will want to wrap that piece of cardboard in aluminum foil, and put it back in. The foil was there to help totally shield the inside of the chassis, since this is a regenerative set.

Edit: It is hard to tell for certain, but it appears that the original foil may still be present on your cardboard piece - just very dirty. Gently wipe a small part off with a damp, soft rag. If you then see a metallic surface...gently wipe down the rest and you're good to go. Otherwise...new foil is indicated for the reason described above.