Question about White dot

[old_tv_nut]

Originally Posted by Penthode

...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.

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.

[Notimetolooz]

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.

[bandersen]

Originally Posted by old_tv_nut

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....

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.