White Spot On TV Screen On Turn Off & What Make

[FireballX]

I remember staring into the screen as a child in the early 60s and seeing how long I could still see the spot after turning the set off before it (the spot, not the set!) vanished. It will have been a rental set (Rediffusion?) but no idea what make or model.

[Hartley118]

I recall that our family Pye FV1 (1951) was great at the brilliant switch-off spot. And it seemed to avoid screen burn despite the fact that switch-off happened twice per day then - one after the 5pm children’s programmes and again after the 8pm grown-ups’ programmes.

Interesting that, after years of nothingness after switch-off, our current Samsung QLED shows a passable imitation of frame collapse - enough to worry any CRT TV service technician.

Martin

[raditechman]

The comment from FireballX (post 21) remined me that I am sure as a child I would get a strong magnet and have fun putting the magnet next to the screen and moving the switch of spot about while it faded away.

John

[Sideband]

Originally Posted by raditechman

The comment from FireballX (post 21) remined me that I am sure as a child I would get a strong magnet and have fun putting the magnet next to the screen and moving the switch of spot about while it faded away.

John

Oh yes I remember doing that as well!

[Sundog-]

As a field engineer you came across some odd situations. One call said "Spot on centre of screen".

I was surprised to see the inside of the house was a film set. A large man dressed in outrageous colours (for the time), came over and said "I'm the director, it's about the spot on that TV". The TV was on and I saw no spot. I switched it off and there was no spot, so I asked "What spot?" "EXACTY" he boomed - "there's no spot as it turns off."

When I told him it wasn't feasible to make the set do this, he just brushed me away muttering he'd have to fix it in post-production.

[peter_scott]

The switch-off spot was not a problem with the pre-war EMI sets. They did instruct you to turn the brightness control down before switching on but surprisingly they made no suggestion that you should turn it down before switching off!

Peter

[thermionic]

There was, at least one set, where the on/off switch was ganged with the brightness control, so eliminating this problem.

Before I go trawling through all my sets, can anyone suggest which models incorporated this?

Cheers.

SimonT.

[kalee20]

So, when you switched off, was that at maximum brightness (so that the high EHT current discharged the tube capacitance quickly, before the scanning circuit died), or was it minimum (so that the tube was cut off, and it contrived to stay cut off till the cathode fell below emitting temperature)?

Either scenario is possible...

[thermionic]

Originally Posted by kalee20

So, when you switched off, was that at maximum brightness (so that the high EHT current discharged the tube capacitance quickly, before the scanning circuit died), or was it minimum (so that the tube was cut off, and it contrived to stay cut off till the cathode fell below emitting temperature)?

Either scenario is possible...

It was at minimum brightness. The CRT grid was brought to near chassis potential.


SimonT.

[Graham G3ZVT]

Originally Posted by Sundog-

As a field engineer you came across some odd situations. One call said "Spot on centre of screen".

I was surprised to see the inside of the house was a film set. A large man dressed in outrageous colours (for the time), came over and said "I'm the director, it's about the spot on that TV". The TV was on and I saw no spot. I switched it off and there was no spot, so I asked "What spot?" "EXACTY" he boomed - "there's no spot as it turns off."

When I told him it wasn't feasible to make the set do this, he just brushed me away muttering he'd have to fix it in post-production.

See my "Bulman" anecdote at the end of this post:
https://www.vintage-radio.net/forum/...62&postcount=9

I did subsequently find the episode on Youtube, but it later got removed for copyright reasons.

[thermionic]

Originally Posted by thermionic

There was, at least one set, where the on/off switch was ganged with the brightness control, so eliminating this problem.

Before I go trawling through all my sets, can anyone suggest which models incorporated this?

Cheers.

SimonT.

The Ambassador TV7, is one such set! Does anyone know of other manufacturers who incorporated the brilliance control with the on / off switch to limit the white spot problem?

Cheers.

SimonT.

[Graham G3ZVT]

You could argue that ganging the volume control with the on/off switch is the worst possible choice!

The popular Bush TR82 radio has the on/off ganged with the tone pot, so the volume is the same as the last time you used the set.

With a TV on/off ganged with the brightness you may still have to readjust, but there again with mean-level AGC, AC coupling etc conspiring together with picture content, there is no "ideal" brightness setting on a typical vintage TV anyway.

[Focus Diode]

I've noticed the Plessey MK1 chassis warming up to display a spot when the HT fuse fails!

[Sideband]

My Ferguson '45' (1958) portable has a switch-off spot but it fades fairly quickly.

[AdrianH]

And a switch off spot on my Ferguson 991T 1954.

Adrian