NotTaR of Television Sets : Keystone shaped picture                 
 Copyright © 1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying. I may be contacted via the Sci.Electronics.Repair FAQ (www.repairfaq.org) Email Links Page.

  << Single Horizontal Line |  Index  | Loss of Horizontal Sync (.. >>

Keystone shaped picture

This means that the size of the picture is not constant from top to bottom (width changes) or left to right (height changes). Note that some slight amount of keystoning is probably just within the manufacturing tolerance of the deflection yoke and factory setup (geometry magnet placement, if any). On a TV, this is only noticeable with scenes having straight edges (e.g., video games) in relationship to the CRT bezel.

However, a sudden increase (and it will usually be rather substantial in a TV) may indicate a problem with the deflection yoke.

An open or short in a winding (or any associated components mounted on the yoke assembly) will result in the beam being deflected less strongly on the side where that winding is located. Typical TV yokes have fewer individual windings in parallel than high scan rate monitors so the effects of one such fault are more dramatic. See the section: Deflection yoke testing.

If the set has been dropped off a 20 story building, the yoke may have shifted its position on the neck, of the CRT resulting in all sorts of geometry and convergence problems (at the very least).