picture question
 

A couple of weeks ago I finished working on my 1950 Crosley tv. I noticed after it is turned off that the picture shrinks down to a little dot in the center of the screen for a few seconds and then goes dark. Is this normal?

Yes. Alot of old tv's do that. Don't remember what causes it, but, nothing to worry about.
Bill Cahill

Thats one of those things that makes a tv "CLASSIC"

When i restored the tv in my avatar, that was one of the first questions my older brother asked me..

"Does it have the shut-off-dot?" hehe :D

Steve

Anyone here a fan of Bob & Ray? They once did an hysterical bit on how the dot gets there.

veg

On a related note, I just finished recapping a GE set with a U4 chassis (21" B&W, 1960). It worked OK before, but I replaced the electrolytics in an attempt to bring back full vertical deflection. That didn't do it (it ended up being a drifted resistor), but I did test after each can replacement. After I replaced the main three section filter, the turn-off spot became much longer lived and brighter. So much so that I'm worried about damaging the picture tube. Before, the picture would shrink into a dot on turnoff, and the dot quickly faded. Now, it shrinks into a dot and stays there - very bright. Even turning down the contrast all the way to produce a dark screen before killing the power won't prevent it (although it's less bright). The replacement caps are the same values (within tolerance) of the originals. What happened? What can I do to minimize this dot? The turn-off dot is a nice classic thing, but it shouldn't last this long, or be this bright.

-Ian

Haven't heard that one...everything they did was funny!

That brings back some memories....

In extreme cases a persistent electron beam "spot" after shutdown has left a phosphor burn mark on the screen, but in most cases the duration is not sufficient to cause harm.

Some later model TVs had a "spot killer" circuit added. Perhaps a relay in the focus circuit, or cathode circuit of the CRT could provide such feature in an older set... :scratch2:

I had a zenith that had an honest to god HOLE in the center where the turnoff dot is.

I've read Asian TV sets have beam killers because the Japanese are really weird about TV set operation.

Some color sets would have tricolor dots/blobs on the screen after turn off...

Well, a TV makes a picture by sending a focused electron beam (a dot) flying across the screen.

When you turn it off and lose both vertical and horizontal deflection, the undeflected dot rests at the center for a moment -- I guess until the CRT heater cools off and the CRT stops emitting completely.

Phil Nelson

My two Electrohome built TVs (1956 Viking and 1960 Electrohome) have a
section on the power switch that will apply a strong positive bias to the
CRT cathode (from the B+ bus) when in the OFF position. This may have
failed at some point in the past as the Electrohome (which still has its
factory CRT) has a small burn spot in the center of the screen.

That seems like overkill to have to hit the cathode with B+. Why not just open the cathode circuit with the separate switch secton--or use a relay?

Hello Old1625,

To answer your last question, I guess the following:

1 - Opening the cathode with the switch would mean routing the video
signal through long wires going to the power switch, not a good way
to deal with a wideband signal like this.

2 - A relay is costly.

Actually, I have reviewed my schematic. The cathode bias is applied via
a 220Kohm resistor connected to the wiper of the brightness control.
The positive supply is also connected to the wiper through a 3.3Mohm
resistor. One end of the control pot is grounded, the other one is
open. As I understand it, maximum brightness is achieved with the wiper
near the ground, as this makes the cathode less positive (the normal
cathode D.C. voltage is given as 78 volts). As for the spot killing function,
the free end of the brightness control gets connected to +135 volts
when you turn the TV off, thus briefly raising the cathode voltage as
I told you earlier.

I have scanned the relevant section of the schematic (.gif file is about
18 kbytes). If someone could explain to me how I can post this (I am
still new to this forum), it would surely help this discussion. Regards.

Here is that schematic
 

Sorry I hadn't read the FAQ beforehand. Hope everyone can see...

That Bob and Ray bit was the greatest. I still dont quit looking at my old tv's till the dot dissapears.
dan

I never saw a t.v. that didn't have the dot until i was a teenager. I was raised by my grandparents and in our house the sets dated from the 50's to early 60's. We had a CTC-7 in the living room, a 12" Admiral in my grandma's sewing room, and my brother had a CTC-5 in his room that he bought at a thrift store for the whopping price of $5! I had a G.E, combo from 1952 in my bedroom, and my brother (who started me in this hobby over 30 yrs. ago) had
a Capehart and two HUGE Curtes Mathes combos in the garage. I was about 14 or so when I saw my first Sony Trinitron and noticed that the dot was not there.


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