[old_tv_nut]

Only the DC convergence of a CRT needs to be plus/minus. Because the radius of curvature of the screen is/was always larger than the yoke to screen distance, there is always a positive polarity dynamic waveform required. In other words, the screen edges are always farther from the yoke deflection point than the screen center is. Much later designs that simplified the convergence were based on deflection yokes with a center of deflection that varied with the direction of the beam.