Why do old tube TV’s power off into a bright dot

Question

Hey Reddit, help me figure out why old tube TV’s when they shut of have a shrinking dot in the centre that slowly fades out.

Thanks

In: Technology

Answer 1

A CRT TV uses an electron beam to excite phosphor coating on the screen to glow. While the TV is turned on, there is circuitry which causes the electron beam to rapidly move back and forth and up and down across the screen, drawing an image. When you turned off the TV, the circuitry moving the electron beam would lose power before the electron beam itself. So the electron beam would stop moving, and just shoot at the center of the screen, creating a bright dot that was the last thing to fade away.

As the circuitry to move the electron beam lost power, it would be able to move the beam less and less, so the image would kind of shrink down towards the center of the screen.