Can Anyone Remember The White Spot on the TV Screen when Turned Off & What Make

[bobhowe]

As per title i can remember my Grand parents TV used to have a white Spot in the centre of the screen when they turned the TV off after watching the wrestling on world of sport with Dickie Davis kind regards Bob

[Mike Watterson]

Scanning drive would fail at once. The cathode and insulated heater would take a little while to cool. The CRT, except on oldest sets, has internal metallisation and outer conductive graphite to make a glass EHT capacitor. Later models (later 1950s or early 1960s?)had a spot suppression circuit, but I don't know how it worked. A few colour sets would briefly show three coloured circles.
The spot quickly defocused and faded.
Wireless World January 1960 has "Aspects of Design" Prevention of switch off spot.

Projection TVs had special protection circuits due to the high beam current.

[BusyBee]

I do remember, many years agowhen I was a child seeing sets which had a fading spot. Some could be seen for quite a while (seconds) before they finally disappeared but I suspect it was probably only in the first second or two when the spot was capable of causing problems. I also remember instantaneous frame collapseon turn off with the spot going a fraction of a second later so that was probably early spot suppression. One of the few faults that can cause a spot would be simaltaneous frame and linescan failure which is obviously what is happening on power off. Normally the linescan provides EHT so it would not be a problem. OC scan coils would cause the same but maybe only likely if an engineer forgets to re-connect. I have seen an ion burn in the centre of a CRT tube though so it did happen in some way sometimes.

Tracy

[Hartley118]

I well recall that our original Pye FV1, installed in 1951 once the Holme Moss transmitter opened, produced a very intense spot when switched off, but I don’t recall any screen burn occurring as a result. It was just an educational reminder of the way that the picture was produced by line and frame scanning. And of course, back in those early days, TV sets would have probably been switched on and off more frequently than today, with a big gap in programme output between children’s programmes around 5 pm and ‘grown up’ programmes not starting until 8 pm.

It may be that as viewers we rather liked the white spot as a response from the set acknowledging that it had been switched off. I find it interesting that our current Samsung QLED simulates frame collapse on switch-off as a similar acknowledgment. It’s enough to cause a sharp intake of breath from any CRT TV technician!

Martin

[Doodlebug]

I used to watch the fading white spot for some years on my old Viewmaster TV.
Sylvia Peters disappearing in a puff of white smoke. :-)

[peter scott]

Am I right in thinking that on some sets you got a view of the emissive surface of the cathode?

Peter

[Kalee20]

I have read that, by appropriate adjustment of grid, first and final anode (EHT) voltages.

[Hartley118]

Am I right in thinking that on some sets you got a view of the emissive surface of the cathode?

Peter

I recall as a teenager experimenting with an MW6-2 projection CRT and its associated lethal power supply. This in a defocused condition produced a screen image not unlike the surface of the moon.  I assume that I was seeing irregularities in the cathode emissive surface.

Martin