In Switch test mode (and these errors are occuring during play as well)... ONLY WHEN THE DROP TARGET AND RIGHT TROUGH SWITCHES ARE CLOSED... any remaining Column 1 switch closed manually also closes the Column 7 switch within the same row. This is true for every switch down the column.
ie. closing the outhole switch (18) shows both the outhole (18) and shooter switches (78) closed
closing the center trough switch (16) shows both the center trough and the ball popper switch (76) closed
This is true on up the column, while both the right trough and drop target switches are closed.
Diode? Transister? This one is weird and apparently causing a lot of problems. I'm not super smart on circuit boards etc, but if you have ideas of where to start, please let me know. I have looked the diodes over, they are installed correctly.