[chimera]

That was a cool effect. I learned later that it was due to the sweep circuits for horizontal and vertical deflection turning off more quickly than the electron gun in the picture tube. The sawtooth wave was generated by vacuum tubes and as soon as the “grid” voltage dropped below a critical level the tubes would shut off. The lack of sweep would cause a brief centering of the beam, and that coupled with the persistence of the phosphors would result in the slowly-fading central “dot”.

[weegee]

I’m glad I never had to explain to my parents what I was observing. I’m just glad that I got to extend my bedtime by like 5-10 seconds waiting for it to disappear.

[macamadamia]

I learned later that it was due to the sweep circuits for horizontal and vertical deflection turning off more quickly than the electron gun in the picture tube. The sawtooth wave was generated by vacuum tubes and as soon as the “grid” voltage dropped below a critical level the tubes would shut off. The lack of sweep would cause a brief centering of the beam, and that coupled with the persistence of the phosphors would result in the slowly-fading central “dot”.


That's the most abstract and unusual description of political failure I've ever read.............oh you're talking about old TV sets.......nevermind....

[Erasmus]

The lack of sweep would cause a brief centering of the beam, and that coupled with the persistence of the phosphors would result in the slowly-fading central “dot”.

The persistance of the phosphors was below 1/30 of a second. The persistence of the "dot" was due to the slow cooling of the electron gun filament and the comparably slow bleedoff of the high voltage ("ultor anode") supply.