[wiki="File:Distorted_Display.jpg"]200px|thumb|right|A Commodore PET display suffering from severe distortion.[/wiki]
There are many forms of distortion on a [wiki]CRT[/wiki] display and just as many causes. Distortions can range from a subtle quivering to a completely illegible image.
Loss of Sweep

Under some conditions the beam will not scan across the display, either horizontally or vertically. On a color monitor this is a dangerous condition! DO NOT operate a CRT without sweep, it will damage a monochrome CRT and can become a severe safety hazard with a color CRT resulting in burn-through of the CRT face by the electron beam. Most CRTs won't do this in brief operation, but some will.
If you wish to repair such a monitor, disconnect the CRT until the sweep circuits are repaired.
Vertical distortion, horizontal or both?

Distortion may be limited to one axis of the display. As each axis is controlled by separate circuitry, distortion in a single axis can help diagnose the origin of the problem. If distortion is present in both axes, it is probable that a faulty power supply is the cause.
Distortion in one axis is often caused by failures in signal amplifier components for that axis, For example, a weak transistor in the vertical amplifier may result in an image which will not extend the full height of the display area, regardless of adjustments. Failure of the vertical amplifier may result in the image being only one scanline high in the center of the display. This can cause damage to the display.
A weak transistor or failing capacitor in the sweep circuit of a raster display can result in an image that is compressed horizontally, or which does not trace the full width of the display.
Power Supply

Arcing or poor regulation in the power supply will cause erratic distortion of the display. Arcing is often accompanied by a snapping sound, though if the arcs are following carbon paths established by earlier arcs they may not be easily audible.
Black or Bright Display

A black or intermittently blanking display may be caused by a failed G1 output on the power supply. Normal output on a typical color CRT monitor will range from 0 to -90 volts. If the output stays at -90 volts it will block the electron beam, causing the display to go dark.
If it fails at a voltage near 0 volts, it will fail to blank the display during video retrace, resulting in both horizontal and vertical retrace lines being seen on the CRT.

The electron beam is normally focused for a fixed distance from the electron gun. To allow CRT screen to be flatter, a focus output adjusts its voltage as the beam sweeps across the screen to allow the beam to stay focused as the distance from gun to phosphor varies. If the display can be focused only across part of the display, like the center but not the edges, or the edges but not the center, then the focus circuit is the likely culprit.
Focus outputs (400 to 1800V) are not as high a voltage as the anode voltage (usually 14,000 to 25,000VDC), but are high enough to be dangerous. The type of focus circuit used, and how flat the screen is (how much it varies from a sphere) will determine the focus voltage and its range of change.
During proper operation, the focus voltage will start at one extreme when the beam is at the left edge of the display. It will swing to its other extreme at the center of the display, then back again as it reaches the right edge.