I'm missing tape return channels 3 and 4 of slot 2. I've swapped cables, of course, and also swapped three different AIO-8 cards, all of which pass all eight channels when in other slots) into that backplane slot with the same results. So my conclusion is that the problem is inside the console. First pass was cleaning that slot's backplane connector. No luck there.
With good luck, it'll be a loose or ribbon cable (or one with dirty contacts) between the backplane and the console guts. With bad luck, it could be bad luck.
So, how do you get inside? I have a couple of copies of the service manual, which tells you how to build a console from a stockroom full of parts, but doesn't explain how to disassemble the console for service. Anybody here ever opened one up? Where do you start? And how far is it safe to go? I'm pretty good a working on these things - been doing it for 50 years - but I'm wary of taking out screws without knowing what they hold together.
Does the bottom panel come off? That might be a good place to start. Since this isn't my console (yet) I don't want to dive in without knowing something about how cold the water is.