If the brightness is set to full and it's still too dark, then you either have a fault, the tube is low in emission, or you need to check and see if there is a sub-brightness preset on the main board. [Some sets have them, not sure if this model does as it's not familiar to me]
I'm guessing there is a fault in the luminance section or your G2 voltage isn't correct.
Don't play around with the R-G-B cutoffs or the R-B drives - you will end up with a strange coloured picture and a tube that does not track properly.
Bookmarks