4000 LX (250-5100) Math Co-processor Faxback Doc. # 1507 Contact the nearest Radio Shack Store for the most recent math co-processor for this computer. There is a socket on the main logic board at location U15 for the math co-processor. Install a jumper on E3-E4 to initialize the math co-processor option. *You will need to use a 20mHz Intel 80387 Math Co-processor. *Installation is recommended, damage to chip possible if improperly installed.