From: "Dave Dunfield" <dave04a at dunfield.com>

If the above gets you nowhere, then it's just a matter of sitting down with the schematic and a scope and following the signal from that switch all the way through to the bus at some point you should see that the zero from the switch is not making it through a component or connector. I don't recall the details of the IMSAI panel, but you might have to trigger on the read strobe to catch the signal being clocked through the panel.