Re: " What does a single vertical line in the middle of a display indicate?" This is a symptom that is normally never seen. It would, in theory, indicate loss of horizontal deflection. The problem is that loss of horizontal deflection in virtually all monitors also causes loss of high voltage, which keeps you from seeing ANYTHING. [The high voltage is obtained from the horizontal output transformer ("flyback" transformer) in virtually every TV set and monitor made.) One possibility: If the horizontal deflection yoke was open, there could be a loss of horizontal deflection while you still had output from the horizontal output transformer. Usually, an open yoke would shut down the horizontal output and high voltage, but it's the only way I can think of that you might get loss of horizontal deflection and still have high voltage.