[electroking]

Hello Old1625,

To answer your last question, I guess the following:

1 - Opening the cathode with the switch would mean routing the video
signal through long wires going to the power switch, not a good way
to deal with a wideband signal like this.

2 - A relay is costly.

Actually, I have reviewed my schematic. The cathode bias is applied via
a 220Kohm resistor connected to the wiper of the brightness control.
The positive supply is also connected to the wiper through a 3.3Mohm
resistor. One end of the control pot is grounded, the other one is
open. As I understand it, maximum brightness is achieved with the wiper
near the ground, as this makes the cathode less positive (the normal
cathode D.C. voltage is given as 78 volts). As for the spot killing function,
the free end of the brightness control gets connected to +135 volts
when you turn the TV off, thus briefly raising the cathode voltage as
I told you earlier.

I have scanned the relevant section of the schematic (.gif file is about
18 kbytes). If someone could explain to me how I can post this (I am
still new to this forum), it would surely help this discussion. Regards.