While adjusting my older RPTV, I noticed that the picture blurried when a WHITE (like snow) scene appeared on the screen. The picture would sharpen on a dark screen; bright color scenes OK. I monitored the B+ on the power supply and did not see any changes. Is this normal for older CRT? Is there somethig else I should monitor? (Set is a 1988 Magnavox PTV325 chassis).
Thanks for any insight.
Emilio
Yes, that is a normal sign of an old worn out picture tube. I would also be suprised if there is not coolant contamination in the picture tubes as well which would further degrade the image quality.
This happens due to focusing voltage drop on CRTs anodes. With bright scenes beam current is at maximum and if there is an increased impedance somewhere in anode circuit it will create voltage drop sufficient to screw focusing.
There could be multiple reasons for increased impedance -- CRT deterioration, flyback transformer, voltage multiplier or low voltage flyback control and switching circuitry.
-- Anatoly
Have something to add? Share your thoughts — no account required.
Ask the community — no account required