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Joined: Aug 2005
It's possible there's a logic fault, but I think it's fairly unlikely. I still suspect the keyboard itself.
As I mentioned, the keyboard is a 4*14 matrix. The 4 columns go to the ACT chip via the first 4 pins on the 18 pin connector on the keyboard PCB (the pins are numbered from the end furthest from the 9 pin connector, that is to say they are numbered from the printer side of the machine).
The 14 rows are scanned by the display cathode driver chip -- the 20 pin chip on the keyboard PCB, possibly marked 1820-1629.
Remove the keyboard from the machine (or at least remove the printer, logic PCB, and support plate). Now connect a continuity tester between pin 1 on the 18 pin connector and pin 4 of the cathode driver chip. You should get continuity when the 'f' key is pressed.
Then try connecting the tester between pin 4 of the 18 pin connector and pin 12 of the cathode driver chip. This time you should get continuity when x^2 is pressed.
If the keyswitches are OK, then it would appear to be a ROM problem on the logic board. As you have a parts unit (I think), you might try swapping out the 18 pin chip (1818-0225) ROM0 on the logic board, and then the 5 little 8 pin chips on the same board with the ones in your other machine. Those are the ROMs.