B&W TV White Spot On The Screen When Turning Of TV & What Make
Hi can anyone remember the White Spot on the screen when turning of TV and what make ? kind regards Bob
We had a Bush, I think the model was M69, but it was the same design even if I have the model number wrong, 21 inch console, bought around 1958.
That had the white spot on switch off, bottom end of the brightness control direct to chassis.
Looking through the 1960/61 Radio and Television servicing book we find TV manufactures employing the simple brightness control to unswitched mains neutral spot suppression circuit were Pye and associated brands Invicta and Pam. Others were the Decca DM35, Kolster-Brandes, Sobell and McMichael.
A much better solution to eliminate the white spot is the insertion of a VDR in the earthy end of the brightness control.
An alternative is a third set of contacts on the on-off switch.
Hi Frank, a third set of switch contracts would have been a much more satisfactory method of implementing the spot suppression circuit. Much more desirable in the sixties when many TV sets were connected to the mains with reversible mains plugs.
In Australia, and probably elsewhere, Philips and others used the VDR-in-the-bottom-of-the-brightness-pot technique. But it requires a small capacitor connected to a +ve going line pulse of 100 to 200 volts connected to the junction. The pulse keeps the VDR at a low resistance in normal operation.
Philips engineers seemed to be fond of VDRs, in the same way that Bosch engineers like diodes.
The other popular technique is a large capacitor at the top of the brightness pot. This would hold up the brightness voltage longer, as the CRT cathode voltage fell rapidly.
The whole idea is to implement what is called a "spot swallower", turning the CRT hard on and thus discharging the EHT rapidly.
Philips turned this into an art form with the 9000, 11000 and later B&W chassis. The time constants were so arranged that, when the set was switched off, the screen would go black for about 50mS, then a white block about half the screen size would flash up, expanding and fading as the EHT was discharged. The effect was quite pleasing to watch! So much better than the bright line flash that was more commonly seen.
The Pye T26 solid state B&W did it another way. Switching off the set would hold the CRT instantly in cutoff for about 20 or 30 seconds, long enough for the selenium rectifier to discharge the EHT.