Why fuse and switch in the neutral rather than the live?

[Refugee]

Back in the 1970s I visited a house where the TV had a hole about half an inch across in the middle of the screen.
It sure was a problem. Perhaps the set concerned had an open circuit pull up resistor.

[pwdrive]

If it was an old TV it might have been an ion burn.

Lawrence.

[Refugee]

It was a 1960s set in the mid 1970s.
It had blasted the phosphor off completely to produce a clear hole about a third of an inch across.

[pwdrive]

Normally spot burn was usually quite confined, it could be that the aluminized layer in that area was destroyed.

Customers often left the sets on for the sound when a set had frame collapse, that often ended up with a horizontal burn line.

Another possibility is a scan coil plug/skt coming loose, the set also being left on for the sound.

Lawrence.

[Refugee]

The diameter was a little large for a focused spot but perhaps with a loose scan coil the spot would be far enough out of focus to get the effect on the screen.

[pwdrive]

Yes that's what I was referring to in the latter part of my last post.

At switch off with no spot killer it's mainly down to the cathode temperature as to how long the spot lasts.

Lawrence.

[Kalee20]

I remember 1960s TVs, mainly Ferguson, that had a high value resistor wired to the mains neutral side of the double pole mains switch. It fed, I think the CRT cathode cct. Its purpose being to blank the CRT at switch off to prevent the spot burning the phosphor.

Not quite - the purpose of the resistor was to drive the grid positive (or put AC on it so that it was positive at least some of the time). Then the CRT would be turned hard on.

The idea was to encourage lots of beam current on switch-off and discharge the EHT capacitance as fast as possible, while the scanning circuits were still doing something. Then, there would be no concentrated tiny bright spot in the middle of the screen and no screen damage.

It's a very inelegant way of doing it, crossing the on-off switch with a resistor to the middle of the circuitry, but Mullard did suggest it!

[pwdrive]

Mullard developed a VDR to give the same effect.

Lawrence.