Toshiba vertical problem

[David Wolff]

Repaired a Toshiba t.v. with a vertical problem caused by bad
capacitors. After fix, vertical is short on top and bottom equally.
Attributing this to set needing adjustment upon new components. Size
can be properly adjusted with vert height adjust. Problem now is that
vertical size flucuates a bit depending upon brightness of scene being
displayed. It isn't very noticeable, but I am wondering if it is
normal operation or if there is some other problem. Maybe it's just
the way it's designed because I see no regulation of the vert out
supply. Seems like if there is a problem it would be with voltage
regulation, but vert out supplies seem reasonably stable (a bit of
variation, maybe .2 V). It's not vert supply filter capacitors,
ripple remains constant. I don't have a schematic, but the supply to
vert out comes from flyback so if vert supply is varying, then B+ must
be varying too much? Customer saw the set and didn't notice the
problem, am I just being too critical?

 

[Sofie]

David Wolff:
With a variac do a basic main power supply B+ regulation check to verify
that it holds regulation when the AC Line input is varied from about 105 to
120 V. You also may want to check and/or replace the filtering
electrolytics in the main B+ supply...... and also check the flyback derived
B+ electrolytics. Also you could be seeing the result of screen supply
regulation problems, particularly if the picture size changes when the
average brightness level changes..... around the flyback look for low uf /
high voltage electrolytics...... i.e. 10uf @250 v .... or similar.