Yes, the idea of bringing 50Hz 240V right up to the signal is just plain daft! Irrespective of the extra wear on the potentiometer track.
I tend to use a switch on the power point to switch on and off any equipment with a combined switch and potentiometer. It avoids the second problem although there is still the first.
Quote:
Originally Posted by Argus25
So if there was ever a case where a rotary control should be connected with an on-off power switch, it would be to put the power switch on the CRT's brightness control, in a TV, so it was zero when the set was switched off to suppress the white spot.
Of course designers wised up to this, and in better designs had "spot killers" that take the CRT's grid very negative at turn off, to prevent the white spot and phosphor burn, regardless of the angular position of the brightness control.
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Though, some TV's did exactly the opposite - take the CRT grid to zero bias, or even to a positive potential, at switch-off. I recall a Mullard article where the potential divider for the CRT grid, instead of going to circuit 0V, was taken to the supply-side of the two-pole mains switch in the neutral line. When the switch was turned off, the CRT grid would be fed with raw mains, albeit with a lot of circuit impedance in series.
The idea was, to get a really high beam current, maximum brightness, while there was still enough voltage left in the reservoir and smoothing capacitors to run the line and frame scan. So the tube's EHT capacitance would discharge rapidly due to the high beam current, and by the time the scans collapsed there would be no EHT left to produce a spot.