where did the shut off dot go?

[radio nut]

To help the performance of my Muntz tv I added a 3 amp power cord interference filter and then mounted a voltage dropping resistor so line voltage should be about 110 instead of 125 from the wall. I did notice my B+ was high before but I waited til now to work all the bugs out and now the set has a cleaner picture, and better sound.
Curious though......When I turn the set off I no longer have the shut off dot that this set did have.....Where did it go?

[earlyfilm]

Originally Posted by radio nut

......When I turn the set off I no longer have the shut off dot that this set did have.....Where did it go?

Did you accidentally bump or bend the contact for the CRT external coating?

If a metal CRT, did the (probably .005 Mfd) HV capacitor fail or become disconnected?

All of the pins of the CRT socket were not making contact when you first worked on the set. I've had a grid to fail to make contact on a 16 inch CRT and the only visible symptoms were a much longer spot and vertical retrace lines on the screen! These went away when the CRT socket was cleaned.

Some sets intentionally used a bleed resistor on one of the CRT grids to automatically bias the CRT to cut-off, immediately on shutoff, to prevent the spot showing, and the lower operating voltages may have been just enough to allow this feature to work again as it did when the set was new.

James

[Jeffhs]

That the dot doesn't appear anymore shouldn't be a problem. In fact, it is probably just as well that you don't see that dot, as it could burn a small spot on the CRT phosphors. One way to eliminate the dot is to increase the brightness just a bit before turning the set off. This dot appeared on older TVs (it doesn't on later b&w sets and certainly not on flat screens, as the latter do not use CRTs) because the electron gun inside the CRT stayed hot enough for several minutes to create an electron beam after the set was shut off. As was mentioned, some TVs had a bleeder resistor that forced the CRT into cutoff just before the set was switched off, and some sets had an actual "spot killer" circuit using a relay that performed the same function.

I have never seen the dot on a color TV; don't know why. I would imagine, however, that this would appear as three separate dots on the CRT screen, since a delta-gun color tube has three guns, one for each primary color. Therefore, if the "dot" were to appear on a color set, you would see not one but three separate dots arranged in a triad in the center of the screen. I would also guess that most color sets had a spot killer system to eliminate these dots and, therefore, protect the CRT phosphors from burns.

[crtfool]

Originally Posted by Jeffhs

I have never seen the dot on a color TV; don't know why. I would imagine, however, that this would appear as three separate dots on the CRT screen, since a delta-gun color tube has three guns, one for each primary color. Therefore, if the "dot" were to appear on a color set, you would see not one but three separate dots arranged in a triad in the center of the screen. I would also guess that most color sets had a spot killer system to eliminate these dots and, therefore, protect the CRT phosphors from burns.

My Mom had a 19" Zenith Color set from 1993 that did this when it got older - I was never able to figure out why or even fix it. The dots were not concentrated at the center of the screen - they were not very bright or focused, so they did not cause any damage. She had an identical set that was bought at the same time, and had just as much use, but it never exhibited this failure. I thought that it was kinda cool!

On a side note for the original topic - my Grandparents had an old B&W console set when I was a little kid in the 60's. It had a hole burned in the phosphors at the center of the screen - that TV scared the crap out of me everytime I went near it!!

[radio nut]

Originally Posted by earlyfilm

Did you accidentally bump or bend the contact for the CRT external coating?

If a metal CRT, did the (probably .005 Mfd) HV capacitor fail or become disconnected?

All of the pins of the CRT socket were not making contact when you first worked on the set. I've had a grid to fail to make contact on a 16 inch CRT and the only visible symptoms were a much longer spot and vertical retrace lines on the screen! These went away when the CRT socket was cleaned.

Some sets intentionally used a bleed resistor on one of the CRT grids to automatically bias the CRT to cut-off, immediately on shutoff, to prevent the spot showing, and the lower operating voltages may have been just enough to allow this feature to work again as it did when the set was new.

James

The coating on the Glass crt is still grounded to the chassis, and the high voltage cap is new to the set....so I will clean the crt connector.
The picture always shrunk down to a line first then the dot, now just the line and darkness..
just thinking that I did fiddle around with the crt socket while I was adding on the parts mentioned earlier......

[radio nut]

this is a black and white set

[Ed in Tx]

Originally Posted by crtfool

On a side note for the original topic - my Grandparents had an old B&W console set when I was a little kid in the 60's. It had a hole burned in the phosphors at the center of the screen - that TV scared the crap out of me everytime I went near it!!

Maybe you saw "The Twonky" movie when you were a kid, too? The old Zenith we had like in my avatar had a dot that would fade away after turn-off. Spooked the heck out of me after seeing "The Twonky" on the Dialing for Dollars movie one morning.

[Sandy G]

We had a big ol' Zenith that had the dot when I was a tadpole, I'd run up to the screen & plaster my eye to it, hoping to see what was STILL goin' on in the program that was just turned off..

[decojoe67]

As a kid we had a '60 Admiral briefcase set that displayed a lingering bright dot. When I started collecting vintage TV's one thing I was actually hoping to see is that dot again. Of about 30 vintage b/w (1946-65) TV's that passed through my hands I never had one that had that same center dot that stayed on the screen when the set was turned off. On some there is a dot that completely fades in seconds. It must be due to the newer components installed.

[radio nut]

I did have a CBS-Columbia set with a 21yp4 crt that when shut off would go dark instantly......but if you waited for about 10-15 seconds would have what seemed to be lightning on various spots of the crt on and off for another 20 seconds.
It was kinda like watching a thunderstorm!
I sold the set to a CBS-Columbia collector who had all their sets but the one I owned.
I still kick myself for not getting video of it!

[old_tv_nut]

In later color sets with a focus bleeder hanging from the HV, that would discharge the HV and prevent a spot. Sets without a fixed load on the HV were in a race between the sweep collapsing and the video output (CRT cathode) voltage collapsing. Motorola's first solid state B&W chassis had a way of killing the spot when the power was switched off, but they didn't think about the case of someone pulling the plug while the set was on. In that case, the video output voltage had a very slow decay because the output transistor bias voltage died immediately and that transistor was the only load on an independent video b+ supply. About 15 or 20 seconds later, (as the cathode voltage dropped) the beam would come on full blast (because the HV had not been discharged and the cathode was still hot enough) and burn a big black spot into the CRT. Unfortunately (or fortunately for discovering the problem), pulling the plug was the last step in production before dropping the set into a carton.This was only discovered when the first ones were unpacked at the distributor to check a few. There was a rush effort to add a spot killer that did not depend on the power switch.

[peter scott]

Seems to be on folks minds at present...
http://www.forum.radios-tv.co.uk/vie...php?f=5&t=8647

Peter

[compucat]

Originally Posted by radio nut

I did have a CBS-Columbia set with a 21yp4 crt that when shut off would go dark instantly......but if you waited for about 10-15 seconds would have what seemed to be lightning on various spots of the crt on and off for another 20 seconds.
It was kinda like watching a thunderstorm!
I sold the set to a CBS-Columbia collector who had all their sets but the one I owned.
I still kick myself for not getting video of it!

My Motorola 9VT1 does this. After I switch it off, if the room is completely dark, occasional flashes can be seen on the screen. This goes on for several minutes while the voltages discharge. Interestingly enough, my little 2" Broksonic TV/radio combo has a dot that appears on the screen for a fraction of a second at switch off. It is very unusual to see a solid state battery portable do this.

[wa2ise]

To kill the spot in an old Admiral bakelite, I connected a 100uF cap between the brightness line and B+. Idea being that the charge in the cap would push the brightness way negative when B+ collapses at turnoff, cutting off the CRT.

[walterbeers]

Originally Posted by Ed in Tx

Maybe you saw "The Twonky" movie when you were a kid, too? The old Zenith we had like in my avatar had a dot that would fade away after turn-off. Spooked the heck out of me after seeing "The Twonky" on the Dialing for Dollars movie one morning.

Got me interested, I love old sci-fi, horror shows. Found that it is free on You Tube. Now I've got to make some time to watch it.