There is a small TV in my kitchen; was on.
Went to the basement, turned its light on and the TV lost reception. TV still on. Left the basement, turned its light off and reception returned to normal. It is a weak signal channel but never happened before (Separate breakers/wiring)

 

What type of light? Dodgy switched-mode supply?

 

Are there any LED bulbs in the basement?

 

There is a small TV in my kitchen; was on.
Went to the basement, turned its light on and the TV lost reception. TV still on. Left the basement, turned its light off and reception returned to normal. It is a weak signal channel but never happened before (Separate breakers/wiring)

When something occurs only one time, you cannot rule out coincidence.

 

I often listen to an AM radio station that's on the edge of reception. Any light dimmer in the house makes it completely useless, nothing but 60 cycle hum.

 

Move the RF antennas away from most local EMI/RFI sources and run shielded cables to each TV, radio/AV receiver that needs an antenna.



Make it a good one.

 

There is LED string lamps in the basement, that may be switching mode led drivers.

 

SOME florescent light tubes produce huge amounts of RFI. I have experienced that in person. A different tube in the same fixture produced no audible AM radio interference, while the one tube totally obliterated reception of a local station. All of those elements are history, but the experience was real. So that is one possible explanation.
IF the TV was using a cable TV feed, all sorts of strange stuff can happen. THAT was the case at my dad's house with comcast cable.
It is also possible that an electrical faulty resulted in a voltage drop in the mains power loop that the TV was plugged into. AND, it may have just been a coincidence.

 

Hi. There is zero fluorescent lamps in my house, the misbehavior happened again today. No cable TV provider in my house. The antenna is a magnetic loop unmolested for months 12' above the TV. The circuit breaker box in the basement is ~7' under the TV, if matters

 

OK, so those guesses were not applicable. So it is probably not an interfering signal generated by the wiring. A simple diagnostic test would be to connect a voltmeter in parallel with that small TV set. an open circuit in some common neutral conductor could cause a voltage drop. That is a bit of a stretch, but open neutrals can cause voltage fluctuations in seemingly non-related circuits.

 

There is LED string lamps in the basement, that may be switching mode led drivers.

There's your problem. That and possibly a change in received tv signal strength and/or aging receiver components.