I'm Getting a bunch of ?????? on my Defender setup menu screen

If you have a bad 5101 CMOS chip (located at "1E" on the CPU board) you may have several different symptoms.  Like a bunch of random ??????? in the setup screen or high score table.  Also, if you can't get past the set up screen you may have a bad 5101 chip or associated circuitry to the 5101. These chips are static sensitive and should only be replaced with a 22pin socket.

1) The most common problem is a bad 5101.

2) The next most common problems is the socket used to replace the 5101 or broken traces
under the 5101. You may want to ohm out the pins to the 5101 and make sure that they are actually connected.

3) Next in line are the signals going to the 5101 are bad or missing. Pins 19 and 20 of the 5101 are the control signals
to the 5101. If one or both of these are bad you will also get the ?????? error.