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 has an electron beam that is scanned across the screen in a raster pattern left-to-right, then down a row, left-to-right, etc. It covers the whole screen 60 times per second. The scanning is controlled by magnetic fields.

When you turn off the TV, the magnetic fields go away slightly before the electron beam. So, for a fraction of a second the electron beam is concentrated in one place (or a line) leading to a bright point or line.

Here’s a demonstration of how magnetic fields effect a crt picture http://i.imgur.com/Nz2HqmX.gif