[Winky Dink]

Thanks, guys.
When I first removed the connection panel, after tagging all the wires, I noticed that there was nothing soldered to the two ground lugs on the metal dividers.  I could have unknowingly broken the wire then, but I don't recall seeing any evidence of a broken wire--and the lugs still look like they've never had anything soldered to them.

So I'll start peeling layers off the transformer and hope I can see the end of a broken copper wire or foil tab.  Resistance across the the winding is 314 ohms, so I'll be looking for ~150 ohms to either side of the plate connections.
If you hear a "Eureka!" emanating from southwest Idaho, you'll know that I found something.