27-10-2014, 12:53 PM
(23-10-2014, 11:08 PM)Eddystonepete Wrote: 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!
