The test screen shows the stuck bits as BYTE 0/ BIT 0 and BYTE 0/ BIT1.
I spent a good deal of time connecting our digital scope to each data line (0-7) and running the stuck bit test. Found that data lines 0,1 and 4 had much more activity (like 27 downward spikes) than the other data lines on the bad board (is it re-trying??). On the good board there were only a couple of downward spikes for each data line when the stuck bit test was run.
When zoomed in on the spikes, each spike is actually 2 sets of 4 spikes. Not sure what that's all about.
The only single IC that's connected to data lines 0,1 and 4 is the "inverter hex bus driver OC TTL". I replaced that IC but the issue didn't change. The IC was harvested from a scrap board, so it serviceability is unknown.
In the attached pics I have one scope channel connected to data line 0 on the good board and the other scope channel is connected to data line 0 on the bad board. I set the trigger for falling slope and the threshold abt 0.5V lower than "high' and started the stuck bit test; first on the good board (#1), then on the bad one (#2). I then repeated the tests for each of the 8 data lines.
We don't have a logic tester available at this time to use. No thermal camera either. I did try touching each suspect IC but couldn't find one that stuck out thermally.
We have datasheets for the IC's, but like I said I'm still a bit green with digital circuits.
What else can I try??
Just saw armadillo's reply. That sounds good and it's an easy thing to check.
Still would like to understand what the scope is trying to tell me.