Com 226 comp trouble shooting II theory book
Troubleshooting a Color Monitor
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| Troubleshooting a Color Monitor Any chapter about monitor troubleshooting must start with a reminder of the dangers involved. Computer monitors use very high voltages for proper operation. Potentially lethal shock hazards exist within the monitor assembly—both from ordinary ac line voltage, as well as from the CRT anode voltage developed by the flyback transformer. You must use extreme caution whenever a monitor’s outer housings are removed. If you are uncomfortable with the idea of working around high voltages, defer your troubleshooting to an experienced technician. Symptom 27-35. The image appears to flip or scroll vertically A synchronization problem is in your vertical raster circuit. Begin by checking the video cable to be sure that it is installed and connected securely. Cables that behave intermittently (or that appear frayed or nicked) should be replaced. If the cable is intact, suspect that the problem is in your vertical circuit. If a vertical-sync (or vertical hold) adjustment is on the raster board, adjust it in small increments until the image snaps back into sync. If no such adjustment is on your particular monitor, try resoldering all of the connections in the vertical-processing circuit. If the problem persists, replace the vertical oscillator IC or replace the entire raster board. Symptom 27-36. The image appears to shake or oscillate in size This might occur in bursts, but it typically occurs constantly. Inmost cases, this is caused by a fault in the power supply—usually the VB) output. Try measuring your power-supply outputs with an oscilloscope and see if an output is varying along with the screen-size changes. If you locate such an output, the filtering portion of that output might be malfunctioning. Track the output back into the supply and replace any defective components. If you are unable to isolate a faulty component, replace the power supply. If the power supply is integrated onto the raster board, you might have to replace the raster board entirely. If the outputs from your power supply appear to be stable, you should suspect that a weak capacitor is in your horizontal circuit. Try resoldering the FBT, HOT, and other horizontal circuit components to eliminate the possibility of a soldering problem. If the problem remains, you will have to systematically replace the capacitors in the horizontal circuit. If you do not have the time or inclination to replace individual components, replace the raster board outright. Here’s an unusual problem. The shaking you see might be related to a problem in the degaussing coil located around the CRT funnel. Ordinarily, the degaussing coil should unleash the most of its energy in the initial moments after monitor power is turned on. Thermistors (or posistors) in the power supply quickly diminish coil voltage—effectively cutting off the degaussing coil’s operation. A fault in the degaussing-coil circuit (in the power supply) might continue to allow enough power to the coil to affect the image’s stability. Try disconnecting the degaussing coil. If the problem remains, the degaussing coil is probably operating properly. If the problem disappears, a fault is in the degaussing-coil circuit. Symptom 27-27. The fine detail of high-resolution graphic images appears a bit fuzzy At best, this kind of symptom might not appear noticeable without careful inspection, but it might signal a serious problem in the video amplifier circuit. High resolutions demand high bandwidth—a video amplifier must respond quickly to the rapid variations between pixels. If a weakness in the video amplifiers) occurs, it can limit bandwidth and degrade video performance at high resolutions. The problem will likely disappear at lower resolutions. The particular problem with this symptom is that it is almost impossible to isolate a defective component—the video amplifier board is working. As a result, your best course of action is to first check all connectors for secure installation. Nicked or frayed video cables can also contribute to the problem. If the problem remains, replace the video amplifier board. Symptom 27-28. The display changes color, flickers, or cuts out when the video cable is moved Check the video cable’s connection to the video adapter at the PC—a loose connection will almost certainly result in such intermittent problems. If the connection is secure, an intermittent connection is in the video cable. Before replacing the cable, check its connections within the monitor itself. If connections are intact, replace the intermittent video cable outright. Do not bother cutting or splicing the cable—any breaks in the signal shielding will cause crosstalk, which will result in color bleeding. Symptom 27-29. The image expands in the horizontal direction when the monitor gets warm One or more components in the horizontal retrace circuit are weak—and changing value a bit once the monitor gets warm. Turnoff and unplug the monitor. You should inspect any capacitors located around the Horizontal Output Transistor (HOT). The problem is that thermal problems, such as this, can be extremely difficult to isolate because you can’t measure capacitor values while the monitor is running after the monitor is turned off, the parts will cool too quickly to catch a thermal problem. It is often most effective to simply replace several of the key capacitors around the HOT outright. If you don’t want to bother with individual components, replace the raster board. Symptom 27-30. The image shrinks in the horizontal direction when the monitor gets warm This is another thermal-related problem that indicates either a weakness in one or more components or a mild soldering-related problem. Turnoff and unplug the monitor. Start by checking fora poor solder connection—especially around the horizontal deflection yoke wiring, the Horizontal Output Transistor (HOT), and the flyback transformer. If nothing appears obvious, consider resoldering all of the components in the HOT area of the raster board. If problems continue, suspect that a failure is in the HOT itself. Semiconductors rarely become marginal—they either work or they don’t. Still, semiconductor junctions can become unstable when temperatures change, and result in circuit characteristic changes. You could also try replacing the HOT outright. It is also possible that one or more mid- range power-supply outputs (i.e., 12 or 20 V) are sagging when the monitor warms up. Use a voltmeter and measure the outputs from your power supply. If the 12- or V outputs appear to drop a little once the monitor has been running fora bit, you should troubleshoot the power supply. Download 1.89 Mb. Share with your friends:
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