Question about White dot
 

Hi guys, I recently got my Admiral 27" Black and White about 75% operational. But I have a question when I turn the Tv off there is a white dot in the center of the screen that sits there for between 1 and 2 minutes. I remember as a kid the dot on the tv disappeared quickly. Is this a problem with the set? Will that dot burn the prosper? I've been turning the brightness and contrast all the way down before turning the TV off so it won't make the Dot.

This happens when the voltages on the picture tube remains after sweep drive stop. Many early sets did nothing about it or happened to have circuits arranged in a way it didn't happen. Later sets included a circuit just to prevent this happening.
It kind of depends on when the TV was made.
Also it could be that capacitors or resistors have changed value in a way that allows this to happen.
You don't say what brand or what year this TV was made.
If the spot is not extremely bright it will not cause any harm.

You may wear the controls out before the picture tube.

Its my 1962 Admiral model L2711 with the 20L8M chassis. My thinking was is collapsed vertical burns a line into the phospher quickly maybe the dot would too?

This could be a tricky question. Turning the brightess and contrast down may be making the dot last longer, but with reduced intensity. If so, it's like braking your car gradually from high speed - raises the brake temperature much less than trying to stop quickly from high speed.

Did you try it without reducing the brightness and contrast at first and then decide to do that? If so, how quickly did the spot go away with the brightness and contrast at normal settings?

I re-read your post, and now I see that there's no dot when you turn the contrast and brightness down, and the long-time dot is at normal settings (correct?).

One or two minutes does seem like a long time. Don't know what to say other than double check circuits near the CRT; but if the controls operate normally, it's probably OK.

One thing you might be able to do to kill it quickly is if you have a damaged HV meter that no longer works, you could use the biggest resistor from it as an HV bleeder. There were electronics mags back then that had spot suppressor articles with gun bias spot suppressor circuits.

I have a 1960 Zenith where the spot burned it's self into the phosphor, and I ended up swapping the CRT.

That's exactly what I'm worried about. I don't have any of that what should I be looking for on the schematic to figure out if it has a bleed circuit?

I enjoyed seeing the dot. I would switch off the room light to see it longer.

I am surprised it was reported to have caused a burn. With the power off, the CRT cathode is cooling and the capacitor storing the acelerating voltage is generally the CRT itself so there should be minimal energy to cause any lasting damage.

I wouldn't worry about it.

I've seen a burned spot in quite a few sets from the late 50s/early 60s. That's when the switch to 110 degrees and high anode voltage happened. The images became brighter but spot burn seemed to become a more serious issue. Especially on 21" and larger screens. I'm thinking because they had more surface area for the aquadag forming a HV filter cap.

Can I build a bleeder circuit?

A design that allowed spot burn should have been corrected quickly, but sometimes a problem escapes engineering until the last minute, or happens only in marginal cases where all the component variances are in the wrong direction.

Motorola's early small portable solid state B&W sets had a problem that only appeared if you pulled the plug with the power switch on. The CRT bias went to max off and the video output stage had a slow bleed. The high voltage itself had no way to bleed at all until the video amp supply bled down, so the spot wasn't there for a few seconds, but then the gun turned on again while the cathode was still hot and burned a hole in the phosphor. The circuit was such that the problem didn't occur if the set was turned off with the power switch. At the end of the assembly line, the sets were being quickly checked for operation and then put into the box after pulling the plug. The problem was not discovered until samples were opened (some after shipping) and the CRT was burned. As you can imagine, there was a crash problem-solving effort to add a spot killing circuit.

The point is that it is possible in unusual circumstances to have enough energy stored in the dag to make a burned spot. So, I would never say never.

If you want to get specific about your TV you need to post a schematic that shows at least the circuit around the CRT base.
I have seen designs that use a dual contacts power switch where the extra contacts connects to a circuit that changes the CRT bias to turn off the gun as soon as the power switch is turned off.
Assuming your set does not have a circuit that turns the gun off, one could be added.
It could be that your set does have a circuit to do this but a capacitor or resistor has changed value.

I'm not sure why I can't get the PDF to upload onto the site and when I turn the PDF into a series of images they come out blurry. So here is a link to my Google Drive with the Sam's on it. Thank you guys again for the help.

https://drive.google.com/file/d/1D56...w?usp=drivesdk

Would capacitor 36 and Resistor 44 be my bleeder circuit?

No those are in the focus network.
The grid and cathode voltages determine whether the CRT gun is emitting the beam. To cut off the beam the grid would have to be more negative than the cathode.
You didn't say that you have finished re-capping the set. I know opinions differ but you cannot expect a set to be working as it should if it still contains some old caps.
Possible paper caps in the grid and cathode circuit would be C32, C33 and C34.
The grid is indirectly biased from the 245V supply, which also supplies the sweep.
The cathode is indirectly biased from the 135V supply which comes from the audio output cathode, that is a common "split supply" circuit.
The values of C2 and C3 could be intended to hold the cathode more positive while the grid supply decays. Those caps would be suspects.
Of course a resistor might also cause the problem.

I think it's more an issue of the image collapsing to a dot 100s of times and the damage slowly accumulating.

I've mostly seen it in 21" sets from the late 1950s. CRTs like the 21CEP4, 21ESP4, 21FDP4, etc.

If by "bleeder" you mean High Voltage bleeder, it would mean adding it between the CRT HV anode and ground. It would be a very high value resistor (tens of megohms) such as used in a HV probe (as Electronic_M mentioned in post# 6).

A 'spot killer' circuit would work by making the CRT control grid (pin 2) more negative relative to the cathode (pin 11), thus turning off emission. Same result by making the cathode more positive than the grid (as Notimetolooz mentioned in a previous post).

Thank you I will check them. To be honest I just haven't recapped the set yet because I'm so excited to actually have something that works. I've watched it for about 8 hours so far in the last 2 days. If you don't mind explaining how can you identify these circuits I'm really trying to learn. It's pretty cool that you were able to pick them off quickly

Well first of all it isn't a sure thing that I got it right, its a idea.
You have to understand how vacuum tubes work.
The rest is reading schematics and knowing how resistors and capacitors work in circuits. Recognizing sections of circuits I've seen before, etc.
There are some good vintage books available for free download on the web.
RCA tube handbooks have a section explaining a bit about how tubes work and for a more detailed explanation is in books like "Radiotron Designer's Handbook" also available.
Actually it seems to me that the way that C2 is connected it may make the problem worse. It may have been connected the way it is so as to reduce C2's needed voltage rating.

Since this particular design injects video via the CRT's cathode, I wouid be disinclined to mess with the cathode circuitry at all. But rather concentrate on how to make the grid go negative at switch-off. A dedicated negative supply of maybe 16V or so, charging an electrolytic, would kick the grid negative thru a diode reversed-biased. Or sumpthin' similar or other, just musing.

Caution! A spot killer that works by biasing OFF the CRT has to hold the negative bias until the cathode cools or the high voltage decays, whichever comes first. Read my post about the burn problem on Motorola's first small transistor sets.

Many color sets killed the spot by turning the beam ON fully while the raster had not yet collapsed completely. You would get a short bright flash of a small raster, discharging the high voltage through the CRT before the raster had shrunk to a dot.

Would that short bright flash put excessive wear on the crt?

Interesting discussion. Back in the day, the bright dot was never known to be a problem, service wise (at least in our experience). But musing on fixes nowadays is good mental exercise anyway.

No, it's equivalent to having a bright image for a short period. So for the cathode, it's no worse than having a bright scene for a fraction of a second. For the phosphors, it's not enough to produce visible darkening even after many cycles. The only time visible darkening happens is in studio monitors that run 24/7 with a test pattern on, or a message display that has text 24/7, or a video game in constant used with permanently stationary image elements / menus, etc.

With relays being inexpensive these days I'd be tempted to power a 6VAC relay coil off the tube heater winding and use it to gate a voltage to one of the CRT when the set goes off....A 4 pole relay could probably switch a lytic to charge when the set is on, and to bias the tube off when the set is off.

In the mid-late 90's Funai built a great number of TV/VCR combos for themselves and many brands including Magnavox. I remember a Magnavox service bulletin regarding "the spot" in some models and a fix for it because it was so bright and long lasting that it had the potential to burn the CRT.

I finally tracked down where I had seen a "spot killer" using a dual section on/off switch. A 19" RCA from 1969. It may be there for customer satisfaction rather than protect the CRT.
Basically the cathode bias is removed and goes to near ground. I suppose if you have a set where it bothers you you could add a relay powered by the filament supply (as mentioned) or the switched power to the set.

I wouldn't get overly concerned with this issue. In a properly operating set it seldom is a problem. There would be a lot of CRTs around with a small area damaged in the center of the screen if it was really a problem and there isn't.

You're right, it's probably NOT a real problem and I didn't mean to make it out like it was.