Turn the machine off then on or hold down the "code" key and hit Q "reset" key to get the machine to type the correct keys. This usually happens because something interferes with the free rotation of the printwheel. Check to be sure the ribbon and corredtion tape are installed correctly and check to be sure none of the "fingers" on the printwheel are bent outward. If a finger is bent, bend it back. If everything looks okay, the board or power supply is going bad.