This could be an AGC problem if the picture appears overloaded. However, if
the picture is normal except unstable, the sync separate is the place to
|NotTaR of Television Sets : Jumping picture on white scenes
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| Interference when using V.. >>
(From: Jack Schidt (email@example.com).)
White screens are a worst case video pattern for sync separators, and will
cause an erratic shift in the vertical multivibrator trigger level unless
the horizontal and video information is filtered out [integrated] prior to
driving the vertical sync input of the processor IC.
This will show up with a scope as high frequency noise going into the
vertical sync input.
Look for a small electrolytic [in fact, all of them], around 1-10 uF or so
near the deflection/sync processor IC. Often simply increasing the value of
this cap will help.