Are you using an isolation transformer to power the scope? It occurred to me that the ground is not isolated on most, just the line. That's why I suggested the adapter as a ground lift.
I had used a power adapter without a ground for the scope, which caused the noise. I just tried an isolation transformer (see attached), and the noise is gone.
I still don't see a significant difference between the good and bad lines, except for a little positive spike at about 450 msec on the bad line. Incidentally I had earlier tried putting a "snubber" (resistor and capacitor in series) on the bad line to suppress inductive spikes, but it didn't help.
My kludge circuit continues to work. If you're curious, I posted the hardware and software design as one of my repositories on Github:
https://github.com/LenShustek/phoneline_fixer. I'm awaiting receipt of the printed circuit board (5 boards in 2 days for $2 from China) to make the final version.
