Well I was able to get things fixed. As best as I can tell it was a cold solder joint somewhere - it didn’t seem to be in the cable though (I found quite a few bad solder joints for the cable connections, but this wasn’t one of them). Nonetheless, I reflowed some solder and now it seems to be in good shape!
You are a saint, Wilba!
Aha! Let this be a word of warning to others! Turns out, the method I was using to test the connectivity of the cables didn’t account for the orientation of the control surface. When I was testing it, the boards were folded, more or less, together. But when I actually supply power to the board, I stand the control surface up so it doesn’t short out anything.
In that orientation, there were shorts both with the 8th column and a few of the encoders (which explains why I was having issues in another post). Trouble is, I’m not sure how best to test things easily when in this mode but at least I know what the problem is.
So, for anyone else building an MB-6582, be sure to take into account board positions when checking for continuity!
Good to hear you got things working!