History of the Spot-Killer circuit

[zenith2134]

I was curious: Which sets were the first with a spot killer? I bet pro video monitors had the circuitry before most consumer sets since their CRTs were higher-resolution and were therefore more expensive to replace if the phosphor burnt.

Always liked how a really old color set (or a later, cheap model b&w) will give that BRIGHT spot upon shutdown. I'm sure you guys know what I mean...

How about O-scopes and vector monitors? With no input signal, the phosphors are very susceptible to burning.

[old_tv_nut]

I suspect this will be hard to identify, since some designs naturally discharge the high voltage when the set is turned off and do not need a specific spotkiller.

Regarding oscilloscope burns, these were very common in the old scopes like the Tek 535. Later, thin-film phosphors that were much more burn resistant were developed, although they may not have been suitable for all applications.
I encountered these when working on the "EVR" flying-spot video player. The old type of phosphor would be burned badly if the vertical sweep failed, but the thin-film (which was proposed to us by Tektronix, but too late to get into production before the project died) could sit with the vertical collapsed for minutes at a time with almost no burning.
The flying spot raster was double-height during normal motion play, in order to follow the continuous film motion. For still images, the raster collapsed to normal 3x4 ratio, but this meant the center half of the motion-play raster would get burned in and then show as a dark horizontal band in the center of the screen during normal play. The player included a random centering shift when going to still mode to blur the burn band.

I believe some scopes may have had anti-burn circuits of some kind - can someone verify?

[wa2ise]

I think all you really needed to do to build a spot killer was to bypass to B+ the brightness line feeding the CRT grid 1. This in sets that fed the video into the CRT cathode. SO when the set was turned off, the B+ would collapse, and the bypassed grid would be pushed very negative, cutting off the cathode. and the cathode cools off by the time the time constant of the cap and resistances around it bleeds off charge.

[ChrisW6ATV]

I remember the spot on our old B&W TV when I was a kid... "The man in the TV with a flashlight".

[ha1156w]

Of all my sets, I think my GE bakelite "Locomotive" set is the only one that does the power-off spot. And it does it for quite a while, enough to make me nervous. Is this normal behavior for this set? I'd be totally aghast if I had to replace the currently-bright 10BP4. Is there an issue with Ion Burn due to the spot?

Now my 1970 Zenith hybrid color SC400 set, it makes that really cool tube-color-set swirl effect, but at a very low brightness. My nephew b.1989 was completely entranced the first time he saw it!

Thanks!

[zenith2134]

My coolest 'turn-off-phenomena' in a set is probably my 1978 Magnavox 12" b&w. THE brightest spot I've ever seen, and the raster collapses in a slow kinda way when it's shut-down...like a slow-motion collapse. Probably due to the horizontal drive draining away last.

[Tony V]

I had a Zenith roundie once that had the tri-color dot when turned off. It looked just like the tri-color knob and emblem on the set. If i had to have turn off spots...that was the one to do it. I liked it actually and didnt seem to effect the crt any. The Zenith roundie i have now doesnt do this.
-Tony

[zenith2134]

Used to have a Sony KV-1712 from 1977 that had the R G B spheres upon shutdown. Funny, the CRT was very weak (instant-on) but the spheres were pretty vivid nonetheless.