AGC adjustment typically has a low signal (blank raster/snowy pic) range, a mid range with a sweet spot, and an overload range where sync is lost and or grossly excessive contrast/negative pic occur.
Typically you turn AGC up till synch is lost and or pic contrast becomes overloaded, then back it down until sync stabilizes and contrast is in a reasonable range for it's control....It may be worth tweaking the AGC again to see if it is having an effect.
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