One can do so by centering the electronic controls prior to centering the raster with the centering magnets. It is best to use minimal electronic correction to reduce strain on the convergence circuitry. Most projectors also have electronic static position controls for fine adjustment of the raster centering. By rotating the rings relative to each other and also around the neck of the tube one can shift the raster about the phosphor surface. The centering magnets are a pair of rings with small tab handles. Some installers will intentionally displace the raster slightly leftward so the active image is displayed on the later, more stable portion of the raster.Ĭentering the raster is performed by use of centering magnets on the CRT necks just behind the deflection yokes. During the first portion of the horizontal movement the beam has not settled completely and one may see some waviness in the image if the extreme left edge of the raster is used. Looking into the projection lens the orientation is backwards. When viewed on screen, the leftmost portion of the raster is drawn first after the electron beam completes horizontal retrace. Since the raster is usually larger than the actual image area and it is actually the energy of the image area that can cause damage, some installers will allow the raster to extend beyond the edges while still maintaining image area within the safe portion of the phosphor. Keeping at least 7 to 10 mm of unused phosphor on all edges helps prevent the catastrophic tube failure that will occur if active image is projected beyond or too close to the phosphor edges. The rasters are best kept small enough to ensure active video image is never extended beyond or near the phosphor edges. Size of the raster is adjusted using vertical and horizontal size (aka amplitude) controls. It may be necessary to open the left, right, top, and bottom blanking controls to allow visualization of the entire raster. This makes the entire raster light up dimly. The raster can be seen by peering through the lens after turning down contrast nearly all the way and then raising brightness to make the normally black raster light up. Normally, only part of the raster is actually used to produce the image. Raster – The area of the tube phosphor that is painted by the electron beam. You should begin by hooking up and displaying a video source. Raster, Image Area and Internal Test Pattern Centering on Tube FaceĬentering of the image on the tube phosphor surface is done by centering the raster on tube phosphor and then centering of the actual image within the raster. Laser Pointer or CLEAN straight edge for finding screen center.Tape measure to find center of each screen edge.One roll of 3M brand blue easy release masking tape for marking the center of screen surface and edges.A good pair of binoculars which can focus at SHORT distances is very helpful for critically observing the effect of adjustments.An external test pattern DVD such as AVIA or S&V HTT or a test signal generator provide the signals needed for alignment.One revisits earlier steps because later steps make earlier ones easier to do more accurately. I have attempted to present this in sequence but one must perform tasks iteratively to achieve the final result. At first, it will seem as though some steps are discussed in jumbled order. Read through this document in its entirety before proceeding. Add to that the need to astigmate the electron beam and adjust lens flapping (Scheimpflug) and the novice CRT setup can fall far short of the projector’s optimum. As a result, first time owners are sometimes at a loss as to which system is the problem. Problems in one system make it difficult to see problems in the other. It involves two projection systems which operate in series – optical and beam focus. This guide was written by Guy Kuo of Ovation Software, Inc.įocusing and aiming a CRT projector is a daunting task.
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CRT Projector Focus & Mechanical Aim Basics