Northstar Advantage 8/16 problems

Hello all, I have a Northstar Advantage 8/16 that I am having some problems with. It is the 2-floppy version, and it does the same thing whether or not the ADV-16 CPU card is installed.... It does not recognize the keyboard. The system turns on, beeps, and displays "Load System" (all normal, according to the docs). However, when I insert a disk, and press enter, no attempt is made to access the disk. Also the "Control-C" combination does not drop you into the monitor, as the docs say it should. One quick diagnostic test said to press the "All CAPS" key repeatedly. The light on the key should go off, then on, then off, etc. It does NOT. It stays lit no matter how many times I press the key. Same for all other lit keys ("Crsr Lock", etc.). I took the case apart, and reseated the cables, made sure all the chips were correctly socketed, looked for burns, hot spots, etc. All seemed fine... Unfortunately, most of the paths in the tech manual end in "replace the main board". I cannot for obvious reasons do this, so my question: Has anyone had an Advantage 8/16 with a seemingly unresponsive keyboard, and how did you fix it? Thanks! Rich B.