Setup Adjustments For CRT Colour Monitors

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Generic instructions for adjusting color purity, convergence and greyscale.

Original Author: Ray Carlsen CET (CARLSEN ELECTRONICS... a leader in trailing-edge technology.)

Latest updates and corrections: 7-7-05

Introduction

If you've never adjusted a monitor before, keep in mind there are dangerous voltages inside. The most obvious is the high voltage on the picture tube, but you're not likely to lift the cap on the CRT during your setup adjustments, so don't worry about that. The not-so-obvious danger is that monitors and TV sets are "line operated" devices. That means that the chassis is "hot" to ground at all times and poses a lethal potential between chassis parts and other grounded objects around it. Technicians use a 1:1 isolation transformer to reduce the risk of shock and damage to equipment. You can work in reasonable safety if you keep other grounded objects (computer, drives, etc.) away from your work area. Touch only with one hand (put the other hand in your pocket, a habit I got into long ago) to reduce the risk of electrical shock. Taking a hit from arm to arm across the chest can stop your heart. Now, down to business...

Advances in technologies along with cost-cutting measures by all electronic equipment manufacturers have produced a display tube that needs less support hardware and fewer setup adjustments than earlier dot-matrix types. The most common Cathode-Ray Tube (CRT) now has three inline (side by side) electron "guns" rather than the triad arrangement of the earlier types. The newer tubes are sometimes referred to as "slot-mask" or just "inline" CRTs. With inline tubes, the deflection yoke is designed to match the tube so only simple corrections are needed to produce an acceptable picture. Setup adjustments are done by moving several sets of rings on the neck of the tube. These rings are made of magnetized material to direct the CRT electron beams to the desired locations on the tube face. The two major alignments are screen "purity" and "static beam convergence". The term "static" simply means beam correction with magnets rather than with electronic circuitry. The latter is usually referred to as "dynamic" convergence and is only used in high-end monitors and large screen TV sets.

There are three pairs of rings, each designed for a specific adjustment. The ones closest to the yoke (the large deflection coil assembly, mounted near the bell of the tube) are the two pole purity rings. Purity adjustments are necessary only if a blank white screen shows "blotches" of color. The next pair of rings are a four pole type that controls the RED and BLUE static convergence (horizontal and vertical lines in the center of the screen). The last pair of rings are 6 pole to control the static alignment of MAGENTA (RED + BLUE) with GREEN. Behind that pair of rings there is often a locking device. Note: not all makes and model use a locking ring.

Manufacturers usually put a line of glue across the assembly, and/or there is an ink line drawn across all the rings to indicate their position after factory alignment. Mark your own line if there is none. That's useful in case you get "lost" when attempting realignment so you can put them all back in order again. Each ring has two tabs that stick out to allow for adjustment with your fingers, a rounded end and a square end. The rounded ends are usually pretty close together in normal use and are generally the ones accessible with your fingers.

A picture tube is said to be in convergence when all three beams (primary colors of RED, GREEN and BLUE) overlap in all places on the screen. Misconvergence shows up as color "fringing" around the edges of objects anywhere on the screen. The convergence adjustments are normally "roughed in" before purity is adjusted because they interact with each other somewhat. Unless someone has been "diddling" and has it completely out of alignment, coarse settings will not change and you can just "tweek" the convergence to optimise it.

All ring pairs share one thing in common: when the rounded adjustment tabs are set together (aligned directly over each other), the magnetic fields of the two rings cancel each other out. If they are then rotated together, nothing happens. If a picture tube and yoke were perfect from the factory, they would need no correction, but that never happens. When pairs of rings are offset with respect to each other, magnetic fields are generated in specific directions relative to the tabs. One convergence adjustment is done by splitting or separating the tabs of a pair of rings in the necessary direction. That moves the beams in a horizontal direction and so affects vertical lines on the screen. The other adjustment is done by rotating both rings together, and that moves the beams in a vertical direction, affecting horizontal lines. Of course these adjustments interact with each other, so you must go back and forth between the two settings for optimal convergence. It takes very little movement of the rings to affect the image you see on the screen. All this may seem confusing at first, but it becomes clear when you see for yourself how movement of the rings changes the screen.

To be able to see the effect adjustments are having on the tube face, test patterns are recommended, the most common being a "crosshatch" pattern of a dozen or so horizontal and vertical white lines on a black background. Some techs are more comfortable with a white dot pattern, but it doesn't really matter. A screenful of zeros or + signs on the monitor will work as well. The front panel color level control must be turned down so there is no residual color on the screen to confuse the readings.

Important First Steps

If a lock is used on the convergence ring assembly, it needs to be unscrewed (rotated counter-clockwise, looking at it from the rear of the tube) to unlock it. Otherwise you will break the rings attempting to move them. A locking ring will be similar in appearance to the adjustment rings and will have tabs for "adjustment", but it will be thicker than the others and will be the last one on the stack towards the rear of the tube. Back off the lock ring about one-half a turn. If it is left too loose, the adjustment rings will slip out of adjustment easily while you are working... too tight and they might break. If there are any wires around the assembly, they must be moved out of the way so they don't snag on any of the ring tabs underneath.

The glue (if it was used) on the rings holds them even if the lock is released, so you need to break the glue bonds between rings to be able to make adjustments. Use a sharp knife or flat blade screwdriver to gently pry between rings to free them from the assembly and from each other. Don't scrape the glue off. It's useful as a marker to see where the adjustments were... just in case. If there is no glue or marks of any kind, draw one with a magic marker or felt tip pen across the entire assembly before you begin. Glue or not, it's a good idea to make your own mark anyway. That way, if you get totally confused and mess up the adjustments, you can always put it back the way it was if you line up all the rings on your mark.

Since you will be working on a live chassis, it is assumed you already know your way around inside electronic equipment and will observe the proper safeguards.

Colour Purity Adjustments

Screen color purity is normally the first CRT setup adjustment to be made, but it doesn't normally drift very much over time and therefore doesn't require routine adjustment. Purity is said to be good if, when each color is turned on and the other two turned off, the entire screen is all one color. Bad purity will show up as wrong colors on what should be a pure color field, or color blotches on what should be a white screen (all three guns turned on). If you can already get a good white screen, don't fool with the purity adjustments. But, if you must...

There are internal controls for each CRT "gun" to set the brightness level for that color. To adjust green screen purity (the center gun and the most common one to view during purity setup adjustments), turn down the red and blue guns with their respective controls and/or turn up the green. If you can, somehow mark the position of the controls beforehand so you can put them back when you're finished. Each TV or monitor has it's own terminology for these controls. Some call them "low light", others "screen" or "G2" or "cut off", and still others "bias". These controls need to be reset to their proper points when purity adjustments are completed or the resulting offset "white balance" will show as "tinting" of one color over the entire screen when the brightness is changed.

An alternate way to do the setup without having to adjust any CRT level controls is with a generator or via a program in the computer feeding the monitor. For example, a simple BASIC program can be written to set forground and background color to GREEN only. Patterns of lines or dots can be likewise generated in a program.

To adjust purity, set up a green screen as indicated above. Loosen the yoke clamp and pull the yoke back towards the convergence assembly as far as it will go. If there is any glue or tape holding it, it may be necessary to gently twist the yoke to break it loose from the tube. A wide vertical bar of solid green should appear in the center of the screen. If it's off center, the purity rings should be rotated and/or separated to center the green bar. Then slide the yoke forward just enough to get an overall green screen without contamination by red or blue. Use a bar or crosshatch pattern to make sure the yoke is straight (rotational misalignment will cause the picture to be tilted), and then gently tighten the yoke clamp. It's a good idea to check the red screen purity, then blue. Slight adjustments of the rings or yoke position may be required to optimize the purity of each of the three color fields. Some compromise may be necessary, but it's usually not important to get it perfect. It has to be pretty far off to show up on a white screen.

If you turned any of the level controls to do the purity adjustments, reset them to their original spots. The screen should be a neutral grey. To check for proper "grey scale" or "CRT tracking", turn down the brightness control and see if the white screen goes to any color as it darkens. If it does, adjust the level controls a bit to get a neutral grey dark screen.

Convergence

For convergence adjustments, you need to use something to generate a pattern of horizontal and vertical lines on the screen called a "crosshatch". Some techs prefer a pattern of dots. If your source video is from a computer, a screenfull of zeros or + signs will work OK. As stated before, the purity rings are the first set on the convergence stack behind the yoke. Don't adjust those unless you have to. The second pair of rings is used to converge the RED and BLUE lines at the center of the screen. Separating the rings will move the beams horizontally (side to side), so for that adjustment, you must look at the vertical lines of the crosshatch to see the effect. Holding and moving both rings together moves the beams vertically. To see that effect, you need to watch the horizontal lines at the center of the screen.

The third set of rings set the convergence of MAGENTA (RED+BLUE) AND GREEN. AS before, Looking at the center of the screen, separating the rings allows for horizontal beam movement (observe vertical lines) and rotating both rings together moves the beams vertically (observe horizontal lines). All slot-mask picture tubes use a variation of this setup procedure.

If the center of the screen is properly converged but there is still misconvergence at the screen edges, you must tilt the -front- of the yoke up or down (without loosening the mounting clamp), or side to side for compensation while observing a crosshatch pattern on the screen. Note that up/down movement of the yoke will affect convergence at the screen left and right sides, and side to side movement of the yoke will affect screen edge convergence at the top and bottom of the screen. Smaller tubes don't normally require adjustment, but larger screen sizes can show quite a bit of misconvergence at the edges. When adjustment is optimised, small rubber or plastic wedges are used to hold the yoke in place. Sometimes these blocks fall out. If edge convergence is bad and center convergence is OK, look for loose or missing blocks. They are fitted with double-sided tape or glue to keep them in position between the yoke and the bell of the tube so the yoke can't move. Silicon rubber sealer works well as glue.

Greyscale

The setup procedure can be called black and white tracking, color balance, white balance, CRT tracking, or just greyscale adjustment. CRT monitors are similar enough that this generic information can be used with most of them. If you somehow mark the controls before you make any adjustments, you can return them to the starting point just in case you get "lost". A dab of paint or ink from a marker pen works well, but whatever you use, don't get any inside the control itself.

There are two sets of controls (usually three per set, but some monitors have only two "drive" controls) to set the picture highlights and lowlights separately. These controls set the voltage levels that appear on each of the three color "guns" of the picture tube or CRT. The highlights or bright areas of the image are set with the "background" or "drive" controls, and the lowlights are set with the "screen", "cutoff" or "G2" controls. Each manufacturer has their own names for these controls, and of course each model has the controls in different places physically. Some will place all controls on the CRT neck board (the board that plugs directly onto the picture tube), and some will have some or all on the main chassis, usually along the rear panel for easy access. They are normally adjusted by use of a small, flat blade screwdriver with an insulated (plastic) handle.

To prepare the monitor for adjustments, turn the color level (intensity) control all the way down. You don't want any color from the video to offset the adjustments you are trying to make. All other controls are set at normal viewing levels. Feed a video signal from a video game or computer into the monitor so you have a fixed stable image with an average of highlights and lowlights. A moving image from a TV station is not a good choice because the forground/background levels keep changing. As an alternative, don't connect a video signal, but just use the brightness control on the front panel to run the "raster" (blank screen) brightness up and down and make the adjustments for low and high levels that way. It's not as "obvious", but it works.

For only a minor correction of CRT greyscale, inspect the image to see which areas of the picture are not neutral grey or white. If it's the dark areas (low brightness off-color), use the "cutoff" control for the predominant color (green in your case) and back that control down until the screen looks grey. Adjustment of the other two cutoff controls may be necessary to get it looking just right, but don't adjust any control more than a few degrees. The adjustments are rather coarse and a small control movement is all that's normally necessary. Colour balance offset in the dark areas will show up more than a similar offset in the highlights, and some "drift" is normal as the monitor ages.

If the white areas of the image are affected, back down the drive control of the predominant colour or raise the others to match so you have a neutral white in the highlights. You may have to make minor adjustments to both sets of controls so all levels of brightness are neutral grey when you're done. Run the front panel user brightness up and down to see if the raster "tracks" properly (doesn't change colour off neutral grey).

Some monitors have an internally adjustable brightness control that sets a normal range for the front panel control and/or limits the maximum brightness you can get without overloading the HV circuits. It's normally located on the main board and may be called "sub-bright" or "bright limit". It is normally set to produce a good picture with the front panel brightness control set in the middle ("detent" position) of it's range.

One last setting you might need to make at some point is the master screen or G2 adjustment. It's a -very- coarse adjustment that sets the overall level of brightness. As a tube ages, the G2 may need to be set higher to compensate for a low-brightness problem. If this control is set too high, the image may be too bright and/or take on a milky appearance with faint diagonal "retrace" lines in the picture. If set too low, you will not have enough brightness even with the front panel brightness control at maximum. This master screen is on the flyback (line output) transformer, the large black plastic block usually located at the right rear of the main chassis. For identification purposes, note the thick (usually red in colour) wire that runs from the flyback to the top of the picture tube. The two controls on the flyback are the focus (top control) and master screen or G2 (bottom control).