Not sure if this may be related to the problems we’re seeing with this board, but I was looking at my CompuPro backplane and I noticed that pins 20 and 70 are grounded per the IEEE696 specification, while on the IMSAI, only pin 70 is grounded. Pin 20, I believe, is the UNPROT signal.
Maybe it’s not the problem, but I think everyone should try isolating pin 20 from the bus by leaving both J3 and J4 open or otherwise confirming that pin 20 is not connected to ground at the bus. The only issue with this is if you use an early memory card that requires a high on pin 20 to enable writing it won’t work. I remember having to isolate 20/70 on certain CompuPro cards in order to get them to work with the IMSAI.
Just a thought…