The new audio board and isolation transformers (shown above) have been installed and working exactly as I'd intended; to quote Hannibal from the A-Team: I love it when a plan comes together!
Operationally, there are two input channels for transmit audio: The microphone and CW sidetone on one, and the soundcard interface on the other. This separates the audio paths, allowing the level of each to be optimized without affecting the other. Also, as I mentioned in my last post, it mutes the microphone audio during digital transmissions, preventing any unintended voice transmissions while operating the digital modes.
I have had intermittent problems with hum on receive and/or transmit due to ground loops. Mounting a bunch of modules to a chunk of wood is nice for experimenting, but it's not necessarily the ideal platform when it comes to ensuring no differences in ground potential exist between the circuits. I'd tried a number of "easy fixes", none of which ended up being particularly easy or fixing the problem, so I finally said "screw it", pulled everything off the board, laid down a sheet of copper, and remounted everything. This seems to have all but eliminated the problem, and has the added benefit of being shiny - at least for now.
So, for all intents, I'm calling this one "done". Over the past few weeks, I've made dozens of QSOs on CW, SSB and JT65/JT9, where, just a few minutes ago I was able to "cross the pond" for the first time, working PB8DX in The Netherlands. Even after all these years, it still amazes me to think that a bunch of parts on a piece of wood, connected to some wire hanging from a tree branch, can be used to talk to someone 4000 miles away. Wild!
73! Steve N8NM