After getting some really good advice from the Heathkit Radios group, I did some further troubleshooting. I re-soldered much of the RF driver board, having found a spot that would change the symptoms when it was hit with freeze spray or tapped with a wooden probe.
At one point, I just got curious as to what the SSB performance was like compared to CW. Surprise! In SSB mode, I could key the mic and get pretty good output. That told me the problem was limited to CW and tune mode, which pointed only one place… the mode switch! I had cleaned that switch at least once before, but I did it again. A squirt of the nearly depleted can of Radio Shack tuner cleaner, a dozen or more rotations in each direction, and … yes, now it’s working.
After completing the transmitter alignment procedure, I put the whole thing back together (mistake, as it turned out) and took it all upstairs where I have my other rigs. I tuned the TS-850S to WWV and zero-beat the 100 kHz calibrator as closely as I could. Then I figured out that i really needed to move the dial pointer on the LMO shaft. Remember how I said I’d put everything together? It’s a real bear trying to get at the set screw to make that adjustment when it’s in the cabinet. I managed, but I may have used a word that I would not repeat on the air.
With all that done, I resorted to a sked and had a nice chat on 20 meters with Dan, W7RF. I was tuned up for close to max output, and got a good report for audio and signal strength given my little 70′ wire in the trees in the back yard. The rig is now officially “in use”!