[Jeffhs]

If the raster collapses to a dot (or a horizontal line, for that matter), don't leave the set on for long; the CRT phosphors will be burned, resulting in a permanent brown line across the center of the screen -- ruining the tube, of course. A horizontal line across the width of the CRT indicates the vertical sweep has failed, possibly due to a defective vertical oscillator and/or output tube (some sets use one tube for both functions), an open vertical output transformer, an open vertical sweep winding on the deflection yoke, or even an open vertical hold control.

In any TV with a flyback HV system (all sets made since the early 1950s until the solid-state scan-derived systems of the '80s-'90s), it is impossible for the raster to collapse to a dot, as this would mean both sweep systems have failed. The horizontal sweep must be operating in these sets in order to produce high voltage for the CRT's second anode; no horizontal sweep always means no HV. Very early TVs from the late 1940s, however, used an RF high voltage system; this system could generate the necessary CRT second-anode voltage regardless whether or not the horizontal sweep was operating. This type of sweep system could cause either a dot (if both sweeps were inoperative) or a vertical or horizontal line on the screen, the latter depending on which sweep system had quit. If the vertical osc/output circuit was dead, no vertical sweep (horizontal line across the width of the screen); if the horizontal sweep circuit was inoperable, no horizontal sweep, resulting in a vertical line extending from the top of the screen to the bottom. In either case, shut off the set immediately; do not use it again until the problem has been corrected.