Apple IMAC Service Source - Page 165

When displaying a single color over the screen area, the LCD panel shows one or more pixels that are not properly lit Active-matrix LCD technology uses rows and columns of addressable locations (pixels) that render text and images on screen. Each pixel location has three separate subpixels (red, green, and blue) that allow the image to be rendered in full color. Each subpixel has a corresponding transistor responsible for turning the subpixel on or off. There are typically millions of these subpixels on an LCD display. For example, the LCD panel used in the Apple Cinema HD display is made up of 2.3 million pixels and 6.9 million red, green, and blue subpixels. Occasionally, a transistor does not work perfectly, which may result in the affected subpixel being turned on (bright) or turned off (dark). With the millions of subpixels on a display, it is quite possible to have a low number of faulty transistors on an LCD. Therefore, a certain number of subpixel anomalies is considered acceptable. Rejecting all but perfect LCD panels would significantly increase the retail price for products using LCD displays. These factors apply to all manufacturers using LCD technology-not just Apple products. To determine whether or not the display has an acceptable number of pixel anomalies, follow the steps below: 1. Set the display image to one of the following colors: all-white display, all-red display, all-green display, or all-blue display. 2. Using a jeweler's loupe, pocket microscope, or other magnifying device, identify and count each subpixel anomaly: • Bright subpixel anomaly = subpixel that is always on • Dark subpixel anomaly = subpixel that is always off 3. Important: Check the number of subpixel anomalies with the following chart: LCD Size (inches) 17 to 20 Acceptable Number of Subpixel Anomalies Bright Dark Combination up to 4 up to 6 up to 8 Replace the Display Bright Dark 5 or more 7 or more Combination 9 or more 4. If the number of subpixel anomalies exceeds the acceptable number listed in the chart, replace the LCD panel. 5. If the number of subpixel anomalies is acceptable, explain to the customer that the pixel anomalies are within specifications, and no repair is necessary. Important: Do not release the specifications to customers. Instead, inform them that a certain number of subpixel anomalies is considered acceptable, and these factors apply to all manufacturers using LCD technology-not just Apple products. 20 - iMac (USB 2.0) Troubleshooting Display Tilt Symptom Charts