Collapsing picture into a bright spot on power off.

[DeLorean00]

I am working on my 1950 Mercury TV with 16KP4 picture tube. When I turn the TV off the picture collapses into a bright spot. I know other vintage TVs do this and have been told before it is the sign a worn picture tube. My concern is burning a dot into the phosphor. Is there any way to prevent this? Something I should check?

Thanks in advance,
Chris.

[old_tv_nut]

It is not necessarily a problem. It mainly depends on the high voltage and focus voltage decaying more slowly than the video and sweep power supplies. I don't know how to tell you how to distinguish if your case is unusually long and bright, but if it hasn't burned a spot in the phosphor yet, it probably never will.

If you could post a video of it, someone might be able to take a guess as to whether it really is trouble or not.

[MadMan]

How long does it persist?

[old_tv_nut]

Is there a schematic somewhere we could look at?

[DeLorean00]

Originally Posted by old_tv_nut

It is not necessarily a problem. It mainly depends on the high voltage and focus voltage decaying more slowly than the video and sweep power supplies. I don't know how to tell you how to distinguish if your case is unusually long and bright, but if it hasn't burned a spot in the phosphor yet, it probably never will.

If you could post a video of it, someone might be able to take a guess as to whether it really is trouble or not.

Thank you! Here is a video.
https://youtu.be/py9yY6at_GM

Originally Posted by MadMan

How long does it persist?

About 7-10 seconds. I posted video in the comment above this.

Originally Posted by old_tv_nut

Is there a schematic somewhere we could look at?

I uploaded the PDF for the schematic as an extension on this post. Let me know if there is a better way to post it.

[MadMan]

I feel like it persists long enough to do damage, especially because it's so focused. But I'm new to all this, so I really don't know.

[decojoe67]

I always thought that the dot in the middle of the screen at turn-off was normal for vintage tube TV's, but actually it's not. I forgot what the reason is for it, as I'm not a repairman, but it's really not what you want to see.

[Chip Chester]

It's not TV, it's HBO. Re: their iconic power on/power off bumpers.

I've got flat panel TVs that take longer to boot up than instant-on CRTs of years past.

[old_tv_nut]

It looks to me like it's not really tightly focussed. To do damage, it has to really burn the phosphor, and it's not doing that.

Looking at the schematic:
1) I haven't spotted where point A 9 the B+ supply goes to, but it must be connected through some point to eventually feed C1.

2) The video signal is AC coupled to the CRT grid through C17; the grid DC is set by the cathode voltage of V13. I think this sets sync tips at ground (or close to that when the left half of V13 conducts.

3) The CRT cathode is connected to a variable DC by the brightness control.

When you turn the set off, I think the grid drops to zero (no more video), and the cathode voltage decays as the voltage on C1 decays. So eventually the CRT draws more current, but it's not enough to discharge the HV capacitor before the raster collapses. It seems to me it's designed that way, and that is that.

The focus voltage will also be collapsing, and my guess is it has gone to zero by the time the spot forms, so the spot is larger than during normal operation. I played your video at 1/4 speed to have a better look, and it seems the spot is large and not changing in size. Try that and see what you think.

Some sets were designed in such a way that the CRT bias voltage collapsed very rapidly, so the HV capacitance was discharged before the raster completely collapsed to a spot, but this apparently isn't one.

[old_tv_nut]

Is the 16KP4 the original CRT type for this chassis?

[DeLorean00]

Originally Posted by old_tv_nut

Is the 16KP4 the original CRT type for this chassis?

The tube chart says 16RP4.

[DeLorean00]

Originally Posted by old_tv_nut

It looks to me like it's not really tightly focussed. To do damage, it has to really burn the phosphor, and it's not doing that.

Looking at the schematic:
1) I haven't spotted where point A 9 the B+ supply goes to, but it must be connected through some point to eventually feed C1.

2) The video signal is AC coupled to the CRT grid through C17; the grid DC is set by the cathode voltage of V13. I think this sets sync tips at ground (or close to that when the left half of V13 conducts.

3) The CRT cathode is connected to a variable DC by the brightness control.

When you turn the set off, I think the grid drops to zero (no more video), and the cathode voltage decays as the voltage on C1 decays. So eventually the CRT draws more current, but it's not enough to discharge the HV capacitor before the raster collapses. It seems to me it's designed that way, and that is that.

The focus voltage will also be collapsing, and my guess is it has gone to zero by the time the spot forms, so the spot is larger than during normal operation. I played your video at 1/4 speed to have a better look, and it seems the spot is large and not changing in size. Try that and see what you think.

Some sets were designed in such a way that the CRT bias voltage collapsed very rapidly, so the HV capacitance was discharged before the raster completely collapsed to a spot, but this apparently isn't one.

Thank you for the detailed post and the time you spent looking into this. I will hook up my meter and watch some of the voltages as it powers down. I am going through things I am having a hard time seeing anything so far that is out of whack. However this TV has had a lot of done to it as it appears the picture tube was changed form a 16RP4 to a 16KP4A, the flyback is a replacement, and it even appears the yoke might have been changed.

I have a couple of questions if anyone has time,

On the schematic, you can see R31 a big 15W 2500ohm power resistor and it is connected to the picture control on the front of the set. I have not changed that yet as the part hasn't come in, I haven't pulled it out to test it so I don't know if it is good or bad. Is it possible that could have any effect? I should have the resistor tomorrow and I can change that.

What is the difference between the 16RP4 and the 16KP4? I pulled the data sheets and a quick view I can only see a difference in horizontal deflection angle of 1 degree.

Also, I remember old TVs doing this all the time when I was a kid. I am just a little paranoid and there might not be anything wrong.

[DeLorean00]

Originally Posted by MadMan

I feel like it persists long enough to do damage, especially because it's so focused. But I'm new to all this, so I really don't know.

I am new to this too. That's why I am asking so many questions. I have restored a number of TVs before I joined but I made a lot of guesses and I googled a lot.

Originally Posted by decojoe67

I always thought that the dot in the middle of the screen at turn-off was normal for vintage tube TV's, but actually it's not. I forgot what the reason is for it, as I'm not a repairman, but it's really not what you want to see.

Yeah, I remember as a kid back in the 1980's seeing TVs do this all the time. But I seem to remember my parent's TV collapsed into a line that got smaller.

Originally Posted by Chip Chester

It's not TV, it's HBO. Re: their iconic power on/power off bumpers.

I've got flat panel TVs that take longer to boot up than instant-on CRTs of years past.

I know those HBO bumpers. Yeah, I have some slow flat-panel TVs, I have often wondered if they had broken when waiting for them to boot.

[old_tv_nut]

Originally Posted by DeLorean00

The tube chart says 16RP4.

Looks like they have identical specs. I was wondering if there could be a difference in the HV capacitance, but there is not. So, unless you find a wrong component value somewhere, I think the set was expected to work as you see it.

[DeLorean00]

Originally Posted by old_tv_nut

Looks like they have identical specs. I was wondering if there could be a difference in the HV capacitance, but there is not. So, unless you find a wrong component value somewhere, I think the set was expected to work as you see it.

I really appreciate all your help.