Putting this here as a placeholder - I release I’ve swapped a screen from a different machine to create the basis for a working machine, and none of that is documented here yet.
With the keyboard constantly displaying random characters as the next step, I thought I’d pop up my fault finding update. I’ve reached out to the TRS-80 crowd to get suggestions, and the likely candidates are the keyboard buffer chips. I’ve ordered some spares now (for both board variations so some will be… spare spares). Looking forward to trying to swap them out and see if that works!
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Some notes so far - the cables for powering the screen are loose - so the screen can appear to not be working even though it works well. Those are the orange and black wires on the nearest side of this socket, and are in a separate plug.
Current state: When powered on (and break held down to not boot from floppy), random characters appear either at the cursor or sometimes all over the screen. Then the computer locks up.
Next steps: try disconnecting the disk interface card and the modem card, and if that doesn’t improve behaviour, chip test the RAM, then ROM, then CPU.
Hot tip on the power wires - turns out there was a manufacturing issue with the wave solder machine, and the power pins will be dry jointed by now. Needs a solid re-solder on that socket. Check the video https://www.youtube.com/watch?v=76bVeQP8m3Y - this was covered by Mav 11 years ago, around the 10:57 mark in the video.