I'm having difficulty troubleshooting an older H8-4 Serial Board (#010781 or #010881). I need the correct schematic and board X-ray view if possible. The documentation I downloaded from SEBHC website is for the later version (#050978).
I currently have channels 2 and 3 working while unable to get anything going with channels 0 and 1. I thought I found the problem when I discovered an open (ground) via located near channel 1 but still having difficulty tracing signals due to the differences in the boards. IC's 102, 112, 122, 132, 147 and 148 are wired differently than shown in the later version schematic / board X-ray view.
I've swapped IC's between the good and bad channels, the problem remains with the bad channels, no issues with the swapped IC's.
I remember when Heathkit updated the 8250 serial I/O circuit on the H89A CPU board due to some issue(s) and I wonder if they applied the same change to the H8-4? (Not that that is going to fix my problem but just throwing it out there as a possible reason why this board is different.)