Troubleshooting CRT Displays

Troubleshooting techniques for displays vary greatly from display to display. It is imperative that you refer to the service manual for any given display to ensure the most accurate troubleshooting information.

If adjusting the user controls discussed earlier in this lesson does not fix any perceived problems with a CRT display, try following the instructions on color calibration and geometry adjustments. If the CRT is still not functioning satisfactorily, the problem is likely to be the result of internal hardware failure. In that case, you must give serious thought to whether repair is an economic alternative to replacement.

There are dangerously high voltages inside a CRT, and very few serviceable parts or adjustments. Therefore, it is not recommended that you attempt to repair a CRT yourself. At current prices, it probably costs more to diagnose a defective CRT than to replace it outright.

Color Calibration

Customers may complain that their display's colors are “off”—not realistic, not what they expected, not matching output, and so on. The colors you per ceive on a display are affected by many factors, including ambient light, wall colors, the position and angle of the display, and the age of the display.

Mac OS X relies on the Apple ColorSync color management system to produce accurate, predictable color from a wide variety of displays, scanners, printers, proofing systems, and presses, all of which represent color differently.

From a technical standpoint, how ColorSync works is fairly complex, but from a user's standpoint, it's pretty simple. You need only select color-calibrated profiles for each device and let ColorSync handle the tricky part. If you work with web or print publications, you are strongly urged to review the Help material on ColorSync.

To make color adjustments:

1.	Open the Displays pane in System Preferences and click the Color tab. Take note of the currently selected display profile.
2.	Click Calibrate to launch the Display Calibrator Assistant application, which takes you through the necessary steps for creating a ColorSync profile for the display. [View full size image]
3.	Follow the directions in Display Calibrator Assistant to create a new ColorSync profile.
4.	If you are satisfied with the new profile, continue to use it. If not, you can return the preference to its original setting by selecting the computer's original display profile on the Color tab of the Displays pane.

Geometry Adjustments

Geometry is a general term describing a CRT's ability to reproduce shapes and images accurately without distortion. You can adjust the shape and position of the display's image by making geometry adjustments. The Displays pane in System Preferences displays a Geometry tab for CRTs only, not LCDs.

Geometry adjustments apply only to CRTs because magnetic fields in the environment can distort the electron beam used in a CRT and cause, for example, images to appear misaligned. In contrast, the pixels in an LCD have fixed positions and don't suffer from geometric distortion.

You can use the radio buttons on the Geometry tab to adjust various CRT attributes. Keep in mind that image geometry is somewhat subjective, so always adjust these settings to the customer's satisfaction.

Position

When the position is properly adjusted, the image is centered on the screen with black borders, all of the same width, around the image.

Height/Width

Because of the normal curvature of the CRT face, distortion is most prominent at the edges. Therefore, a properly adjusted CRT should display the entire image with black borders all around. Attempting to make the image as large as possible to fill all the visible area of the display usually results in noticeable distortion.

Pincushion

Borders that look bowed or curved indicate a pincushion error.

Rotate

If an image appears tilted clockwise or counterclockwise, rotate it until the image is squarely on the display.

Keystone

When the keystone is adjusted properly, the top and bottom edges of the image are the same length.

Parallelogram

When the parallelogram is adjusted properly, the sides of the image are perpendicular to the top and bottom of the image.

Factory Defaults

Click the Factory Defaults button to restore the original Apple settings for the display.

First Checklist: Common Problems

As you're gathering information to diagnose a faulty display, and before you ever return a module for repair, be sure to run through this checklist of front-line quick fixes. This helps to prevent needless module replacement and unnecessary time delays.

• Verify that the display is compatible with the attached computer.

  For best display performance, operate the display in one of the factory-preset screen resolutions.

• Be aware that the CRT raster (the area of the CRT where the image is produced) does not always resemble a perfect rectangle.

  CRT tolerances allow for some distortion. Additional distortion can be caused by nearby magnetized metal objects, such as desks and file cabinets. Rotate the display or move it to a different location if you notice raster bowing or bent raster edges. For newer displays, you can also use the Displays pane in System Preferences to make geometry adjustments to optimize the screen display.

• Check “the jitters.”

  Jitter, faint lines, or screen movement can be caused by the computer or external interference, such as electronic devices and fluorescent lights. Fluorescent lights, computers, other displays, or electronic appliances, such as coffee makers and copy machines, can cause raster distortion. Move the unit to another room or building to determine if external interference is the source of the problem.

  Note that if the raster has shifted up, down, right, or left only, you can adjust it using the geometry controls in the Displays pane in System Pref erences. Keep in mind that if you then move the display to another location, you may need to readjust the controls. If the display changes (for better or worse) when you move it to another location, the environment is the source of the problem. Move the display or the distortion-causing object.

• Determine the cause of variances in screen color and brightness.

  These variances are usually caused by the setup controls or the environment. Color purity over the entire screen may not always be perfect. CRT tolerances allow for some distortion. Try adjusting the rotation, brightness, or contrast to reduce or eliminate the symptom.

  Color imperfections can be caused by nearby magnetized metal objects, such as desks and file cabinets. If you notice color blotches or a change in brightness on an area of the screen, move the unit to a different location. If the display changes, for better or worse, when you move it to another location, the environment is the source of the problem. Relocate the display or move the distortion-causing object.

• Be aware that a misadjusted screen can mimic the symptoms of main deflection board or CRT failures.

  By performing the adjustment procedures, you may be able to determine if one or more of the adjustments is the cause of the problem.

Symptom Charts

When you come to the point of using quick fixes in the troubleshooting process, you will find that the general troubleshooting symptom charts help you diagnose general symptoms related to displays. Every service manual for a display unit contains these symptom charts. Because cures are listed on the charts in the order of most likely solution to least likely, try the cures in the order presented. Verify whether or not the product continues to exhibit the symptom. If the symptom persists, try the next cure.

For procedures that require the display to be turned on, allow the display to warm up for 10 minutes. When troubleshooting a front-of-screen symptom, allow a 30-minute warm-up, unless instructed otherwise.

Check the symptom charts periodically for updates and new symptoms and cures.