Miracles do happen from time to time....
I put it all together again, and now it's working perfectly. Didn't change anything....
It must have been something like you said, maybe a short against the metal case.
Just made sure this time that the connectors to the pins on the PCB were bent slightly more away from the
bottom of the case (although I used heat shrink, so no visible exposed connectors).
Possibly, this was the cause of the trouble.
Anyway, U3S is working fine now, and I have my first long distance report (to ZS1LS, almost 10,000 km), using only 200 mW in
a small dipole antenna. Not the best of circumstances here, but a nice result.
Thank you, and everyone else, for the support.
On to my next project!
Regards and 73,