Tarmo Tammaru wrote:

Dave,

Thanks for the info. In my case, the source seems to be a bank of 4
transformers on a pole about 300 feet away. Weirdest thing I had was S9
white noise from about 40 to 55 MHz with about 50% amplitude modulation at
around 50 KHz. Turned out to be a 60 amp cartridge fuse in my fuse box about
to go bad. I think the 50 KHz was a mechanical resonance in the fuse.

...

There are lots of weird noise sources. As a kid in the 40s, I was part
of a family visit to an old farm west of Bethlehem Pa., in what was
definitely "fringe area" TV reception. The man of the house was telling
my father about the strong zig-zag pattern that sometimes showed up on
the screen, blanking out the video and often lousing up vertical synch.
He wanted to know if the FCC would track down the interference; he
thought it was a ham about half a mile away. Just then, the interference
started, and right after, his son came downstairs to join us. A little
while later, his daughter went upstairs, and the interference cleared
when she got to the top. Without saying anything, I went over to the
stairway switch and turned it on and off a few times. "On" caused the
interference. When we swapped the bulb with one in a floor lamp, the
floor lamp caused the same interference. It has been one of my enduring
regrets that I didn't ask to keep the bulb. It was probably thrown out.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯