Collapsing picture into a bright spot on power off.

[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.