Looking for a little help from folks who are really good with circuits...
I've had a TRS-80 Model III for a few decades. A bunch of keys on the keyboard stopped working, so I assumed they were dirty and went through the process of desoldering/cleaning/resoldering all of them, only to discover that the same keys still don't work. I then found the Model III service manual online, which has a diagram of the keyboard interface on the CPU board. See the attached PDF for the diagram.
From the diagram, I discovered that lines 9, 10, and 15 account for all of the keys which do not work. So the problem is most likely on the CPU board and not the keyboard itself. The problem is, I'm no electrical engineer and I might be a bit out of my depth here. Looking at the diagram, it looks like each of those lines is connected to a resistor (RP5) and maybe a diode (D0A, D1A, etc.) but I don't know if I'm even reading that correctly. I can't really tell if those are all discrete components or if some of them are the same component shared by multiple lines, though. So I'm wondering which parts are likely the culprit here, and how I might locate them if I pull the CPU board from the computer.
I tested the 20-pin ribbon cable end-to-end and it seems fine, so the issue isn't in the keyboard or it's connection. It definitely seems to be something on the CPU board based on this diagram.