The switch matrix only shows electrical connections, it in no way represents the wiring sequence for the switches.

You wouldn't happen to have a logic probe, or better yet, an oscilloscope would you?

WOW. terryb This is the first time I looked at your guide to logic probes and what a great guide it is; I'm a couple years late, but thanks for taking the time to creating this guide for folks to use.

I suspect you have a poor connection, most likely at J208/209-7 as this appears to be effecting all of column 6.

Your 10 opto board only controls optos in columns 7 and 8. Realistically though, you can have a bad connection on Row 6 (White-Blue) at other points back from the CPU that would effect all switches in the row prior to that point. I'm going to rule out the 10 opto board completely as a power issue would effect all rows and a row 6 issues would not prevent any un-associated switches from being driven low.