I bought an Apple IIe in a second hand shop. Looked ok, complete with dual diskdrive and monitor (no cables, though). When I switch on the computer it beeps after some time, the power light goes on, but I get no picture on the screen. I tried another monitor, but I get nothing at all. On the main board, a white socket is empty. Should there be some chip? What type? What else could be wrong? HELP!!!