Here's an update.

In tracking down and replacing numerous burnt lamps, The back half -
the side closest to the far end of the string - suddenly lit up.

What is strange is that I was working on the end closest to the plug
at the time.

Also, I have replaced all burnt bulbs with new ones that were tested
prior to installation. Now I have half a string that does not light
for no obvious reason.

Keep the thoughts coming . . . I'll keep playing with this as long as
my sanity holds out . . . ..

Thanks.


Put one probe in the neutral slot of the outlet, then pull the bulbs out one
at a time and check for mains voltage at one of the contacts. If you start
in the center you can figure out which direction the open is, and quickly
narrow it down. Keep in mind you might have multiple faults.