Focus changes with white screen
 

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

Focus changes with white screen
 

Emilio wrote:
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.

Focus changes with white screen
 

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


"Emilio" wrote in message
...
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