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Front Projector Focus

Focusing a CRT is a daunting task because it involves two projection systems which operate in series - optical and beam focus.

Problems with one type of focus make it difficult to see problems in the other. As a result, people are sometimes at a loss as to which system is the problem. 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.

A good pair of binoculars which can focus at SHORT distances is very helpful if not essential. Do yourself a favor and obtain a pair. Also pick up a roll of 3M blue easy release masking tape.

Mechanical Gun Aim

If you have a projector using lens tilt rings, set them to factory spec for your projection distance before doing any aiming. You must first mechanically aim the guns properly. One method for doing this is to display a white field pattern and shift the pattern edges equally spaced relative to the phosphor edges. Look into the lenses while doing this, not at the screen. You will need to do this for both red and blue guns separately. The pattern will be widest at the CRT bottom (assuming ceiling mount) and that is where it should be balanced left/right to assure the raster is well clear of the phosphor edges at its closest points. Then go to the top of the projection screen and examine the left/right relationship of the pattern's edges and the screen edges. Swing the gun to balance the edge relationships left right. Actually, the projected edge farthest from the gun can be set slightly wide of the screen edge than the nearest edge, but the difference is minimal and it's okay to make them equal. Lock the CRT's into position.

Next pay attention to how the white field pattern is positioned vertically in the green phosphor. Shift it to make the top and bottom distance from phosphor edges equal. Adjust projector up/down tilt to make the projected green white field edges equally balanced relative to top and bottom screen edges.

New owners confuse the two, but mechanical aim is not the same as lens flapping (aka Scheimpflug). Mechanical aim of the CRT/lens assembly is the same as taking a telescope and physically pointing it at something. Lens flapping is adjusting the mounting angle of the end lens without changing where the telescope is pointed.

The red and blue guns of a CRT projector have mounting screws which when loosened allow the two outer guns to swing left/right. Sometimes the screws can go into different holes for different convergence angles or there is a slot for the screw which allows free motion. Your installation manual should cover which screws are involved. The central green gun usually isn't adjustable left/right. This is one reason to be very accurate in getting the projector centered and square to the screen when mounting. Up/down mechanical aim is accomplished by altering projector tilt.

Lens flapping doesn't change the direction of aim (well just a little), but compensates for the planes of the screen and phosphor being non-parallel. If the lens is kept parallel to the phosphor surface, it would be impossible to focus throughout the screen at the same time. The flapping places the lens into a plane which is intermediate between those of the screen and phosphor and that makes global focus possible.

Do a rough beam and optical focus. Mark the exact screen center with a lightly applied triangle of 3M's blue easy release masking tape. *** do NOT substitute another brand tape here **** Also mark the center of the top, bottom and sides of the screen frame to make geometry easier to set later. Bring up a center cross pattern. Neutralize each gun's position shift controls and then use the projector's raster centering controls or magnets (just behind the yoke if present) to align the center cross of each gun on screen center. Remember, you have already mechanically aimed the lens and gun assemblies so you centering the projected center crosses on the marked screen center automatically centers things on the phosphor.

Notice that I've had you use a white field instead of a center cross to do the mechanical tube aim. This technique gets the lenses and CRT's well aimed even if the raster hasn't been accurately centered on the CRT yet. Now if you happen to know your center cross pattern is already precisely centered you can just use the center cross to aim the tubes. The best way I know of doing that accurately is to pull the lenses off, center the cross on the phosphor while measuring with a ruler, then remounting the lenses. The white field pattern edge comparison method allows easy, accurate physical aim without pulling the lenses and gives a subtle plus for the red and blue guns as I?ll explain later.

I know this method seems backwards, but balancing the edges of the centered white field pattern against the edges of the phosphor and then the projected edges relative to the screen edges achieves precisely mechanical aim in an easy manner. The advantage to this method is basically the difference between having someone mark the middle of a piece of paper without aid of a ruler vs aligning a slightly smaller piece of paper so it is uniformly spaced inside the larger piece of paper. The latter is easier to do accurately.

Consider now the off center red and blue guns. If you aim the actual center of the phosphor of those tubes to project at the center of the screen, you'll note that the phosphor usage distribution is unequal left/right due to the throw angle. Graph it out and you will see that the farther half of the screen gets illuminated with a smaller area of phosphor. Ever notice how the side of the screen opposite the side of the gun is less well focused? This is part of the reason.

Centering the raster edges relative to the phosphor and then using those lit up edges to guide lens aim will actually place the red and blue guns so they are mechanically slightly off true center. The left lens ends up pointed slightly left of center and the right lens ends up slightly right of center. At first blush, this seems wrong, but this can actually be advantageous because it makes the raster usage, resolution, and illumination more uniform across the screen. Less horizontal linearity compensation and lens flapping are needed.

Now if you are a traditionalist and want the center of the phosphor actually aimed at the center of the screen, you can pull the lenses off, set the center cross with great precision and then use the projected center cross position to guide mechanical aim. This is the usual way things are done, but I present an alternative approach with some advantages.

Only after the guns are physically aimed and the raster centered, should one begin final optical and beam focus adjustments.

Rough Optical Focus

Display a crosshatch. Adjust optical focus using the two knobs. The front most knob controls edge optical focus. The rear most knob adjusts center optical focus. Dial in center focus first. Then adjust the outer (front knob) focus to while watching the corner lines of the crosshatch flare inward and outward. Try to minimize flaring using the outer focus knob. Go back and forth between the two optical focus knobs to get both center in focus and outer edge minimally flared. Use of binoculars aid this process tremendously.

Rough Scheimpflug (Lens Flapping)

This is for projectors like Sony's which have continuously variable lens flapping. For other types of projectors simply place the lens rings at the spec setting for distance and skip forward.

Display a focus pattern. Look ONLY at the center of the top edge. Adjust center optical focus to make that edge sharpest. Note the position of the focus knob. Pay attention to the center of the bottom edge. Adjust lens center focus to make the bottom edge sharpest. Is the lens center focus control in the same position as when the top was sharpest? If so, vertical flapping is correct. If not, adjust vertical flapping using a wrench. Note the wrench position when the bottom edge is focused vs top edge. Set the vertical flapping to put the wrench in exactly halfway between the two positions. Horizontal flapping is performed in an analogous fashion but paying attention to only the left and right edges.

Now go back and redo center and edge lens focus. Once that is done you should have the lenses in fairly good optical focus and can begin work on astigmation without the special lens cover mentioned later.

Electron Beam Astigmation

Warning!!!!! If you are not a technician skip the following electron beam astigmation step. Adjustment of CPC magnets should ONLY done by an advanced setup technician. Improper technique will render focus impossible. High voltages and tube neck fragility are also significant hazards during CPC magnet adjustment!!!!

Beam astigmation fine tunes the electron beam lens to create a uniform electron spot with with minimal flaring. Poor astigmation can make electron beam focus impossible.

Electron beam astigmation is carried out using CPC (called Color Purity Control for historical reasons having little to do with CRT projection) magnets on the tube neck and/or electronic astigmation controls. If both CPC magnets and electronic astigmation controls are present, it is best to let the CPC magnets do most of the work and fine tune with the electronics. That means neutralizing the electronic astigmation controls and then adjusting the CPC magnets.

CPC magnets are arranged as pairs of rings about the end of tube neck near the socketed drive board. Projectors don't always have the full set of 2, 4, and 6 pole CPC magnets. Sometimes the ring pairs have a small knob allowing one to adjust the angle between the two rings of a pair. More often one merely sees tabs with which to manipulate the rings. The CPC have 2, 4, or 6 magnetic poles, but don't confuse that with the number tabs on the rings. One cannot actually see the poles. By varying the angle between the two rings of a pair (moving tabs in opposite directions) one varies the intensity of the effect. Rotating a pair about the axis of the tube neck (moving tabs in same direction) changes the directionality of the effect.

If all three sets of CPC magnets are present, the rearmost is the 2 pole (centering). The middle is the 4 pole (ovalness). And the most anterior (if present) is either non-functional or a 6 pole correction (triangularity).

Be sure to neutralize all electronic astigmation controls prior to working with CPC magnets. Also, on projectors which lack separate electronic astigmation controls, do CPC and electronic astigmation while the highest scan frequency to be used is displayed.

I assume you know how to change the electron beam focus and don't get that confused with optical focusing.

The 4 Pole alters the ovalness of the electron beam lens. Adjust this while displaying a dot pattern while contrast is moderately high. Intentionally UNDER focus the electron beam making the dots into uniform blobs. Adjust the 4 pole magnets to make the blobs as perfectly circular at screen center as possible. Attach a perfectly round piece of 3M blue tape on the screen to aid in judging shape. Turning the small knob or moving adjustment tabs in opposite directions alters the amount of ovalness. Spinning the 4 pole rings around the axis of the tube neck changes the direction of the ovalness axis.

The 2 Pole centers the electron beam in the electron beam lens. Adjust this while displaying a dot or crosshatch pattern while contrast is moderately high. Intentionally OVER focus the electron beam making the dots into a flare with a bright central core. Turning the small knob or moving adjustment tabs in opposite directions alters the amount of deflection. Spinning the 2 pole rings around the axis of the tube neck changes the direction of deflection. Make the bright core centered in the flare.

The 6 Pole adjustment is also done with the dots pattern in under focus. If the 6 pole works (it may not do anything) it creates a triangular astigmation change. Use it to correct any residual triangularity which you could not correct using the 4 pole.

Go back and forth between the 2 and 4 pole adjustments to get things right. As a final check, carefully watch the dots as you go from under to over focused. The dots should stay almost motionless as you vary the beam focus.

Once CPC magnets are set, fine tune using electronic astigmation.

You will have to redo raster centering after adjusting the CPC magnets. If this is the blue gun, you'll probably want to leave the electron gun underfocused enough to make its light output measure about 20% higher than its fully focused state to improve grayscale tracking at higher light output.

Phosphor Grain Optical Focus Technique

At this point most people have difficulty deciding whether optical, beam or both kinds of focus problems are present. Here is a method for setting excellent optical focus without being confused by beam focus. You must have a good pair of binoculars which focus at a short distance to use this technique.

The phosphor surface of a CRT has an inherent grain pattern. Because this grain is visible and is always exactly at the plane of light generation, one can use the grain to set optical focus independent of beam focus. A small piece of 3M easy release blue masking tape aids in keeping your eyes correctly focused on screen. Display a bright window pattern and intentionally defocus the electron beam to make the scan lines disappear. Adjust center optical focus while viewing the screen through binoculars. When optical focus is correct, the inherent grain pattern of the phosphor surface suddenly snaps into view. This is nearly impossible to see with just your eyes, but binoculars make it readily evident.

Once optical focus is sharp enough to see phosphor grain refocus the electron beam finely.

Focusing Lens Cap

The phosphor grain optical focusing technique largely eliminates the need for this method, but I mention this for completeness. Some projectors come with a special lens cap having a central hole approx 1 inch in diameter. This is intended to reduce the aperture of the optical lens and allow examination of beam focus even when optical focus is not quite correct. Because the phosphor technique achieves good optical focus independent of beam focus, I recommend setting the optical focus using the phosphor grain technique first. This allows greater light availability while setting beam focus than reducing the lens aperture.

Final Beam and Optical Focus (3 x 5 Card Technique)

At this point in the process, both beam and optical focus should be excellent, but further refinement is sometimes possible.

Use a plain white 3 x 5 card for finding the exact focal distance of the projector. Do this by moving the card fore and aft in front of your screen to see at where a fine focus pattern is best in focus. If it is already exactly at the screen surface plane throughout the screen, then you are done. If it is more than 1 cm in front of or behind the screen do the following. And yes, this will temporarily undo the hard work done getting the phoshor grain sharp.

Display a fine detail focus pattern and intentionally overfocus the optics (rear lens control) so that the center is best optically focused about 2 cm short of the screen. This is in the direction that extends the lens barrel forward. Bringing the focal plane slightly short of the screen lets you more easily examine the focal distances throughout the screen. Check the focal distance for each screen edge by moving the 3 x 5 card back and forth in front of the screen. You'll be able to see very accurately the distance at which things are best focused on the 3 x 5 card. Note particularly if the distances are uneven between left/right (indicating a horizontal lens flapping error) and top/bottom (indicating a vertical lens flapping problem). Fine tune lens flapping to make the focal distances for top = bottom, and left = right. This is the time to fine tune lens flapping rings. The 3 x 5 card check is so precise that you'll notice the flapping changes caused by uneven tension on the lens mounting screws.

Next, pay attention to how the left and right edge distances compare to the distance at screen center. They should be about equal between center and edges. If not, slightly adjust the inner and outer optical focus to bring both the edges and center to focus about 2 cm in front of the screen. Notice that I don't have you check the screen corners. That's because there will almost always be a difference in the extreme corners and center and using the left/right edges gives a good compromise which preserves the central focus where the video image is going to be sharpest portion of a movie frame anyway.

At this point the optics are perfectly balanced in terms of Scheimpflug and inner vs outer lens focus. The only thing left is to shift the entire optical plane to the screen surface. Make tiny movements of the rear lens focus knob while repeatedly checking the optical focus position with the 3 x 5 card. One thing to consider is that you probably did all this with the projector's lens hood off and the lenses are probably a little cooler than normal. As the lenses warm up, their index of refraction decreases and the focal plane moves slightly outward. You may need to leave the focal plane a cm or so short of the screen surface so it hits exactly on when the optics are at normal operating temperature an hour or so after the lens hood and hushbox are closed.

If you use the 3 x 5 card for final focus, you'll note that it is so sensitive an indicator that even the slight shift of the lens while tightening the focus knobs will be detectable. Expect to have a great deal of exercise moving between screen and projector using this technique.

With optical focus perfected, set final red and green beam focus, and then gray scale/blue gun defocus. Recheck raster centering especially if the projector uses electromagnetic focus. Now you are ready to begin the "little" job of geometry and convergence for the projector.