The Serial Box 3 can appear to be broken but it just needs a small adjustment in the Hphase control on the front panel. If you see the picture Luma and Chroma sometimes dip down and come back up totally dependent on picture content, (that is, if it is bright on the left side of the screen then the levels are crushed and if its not bright the levels are not crushed), then the picture content could be shifted into blanking and the SB3 is trying to set its blanking level to the actual picture. Just adjust the Hphase of the box back to the right so that there is no picture content in the blanking area, just black.