Color display power supply characteristics and maintenance - Power Circuit - Circuit Diagram

2) The filament voltage of a color TV is typically supplied by the line output transformer (AC), whereas the filament voltage of a color monitor is directly provided by the switching power supply (DC at 6.3V). The adoption of this power supply method in color monitors primarily serves energy-saving purposes. When a computer remains unattended for a certain period, the monitor enters an energy-saving mode. During this mode, the field scanning ceases, drastically reducing overall power consumption. However, the filament continues to heat up. Once someone interacts with the computer, the monitor immediately resumes its normal operating state. 3) The output level power supply of a color TV is generally around 110V and fixed, while the output voltage of the color TV's line output stage power supply typically ranges between 45V and 135V and is not usually fixed. This difference arises because the scanning frequency of a color TV is fixed, whereas modern SVGA color monitors have multiple scanning modes, with the scanning frequencies of these modes being closely correlated. For instance, the line frequency range for monitors with screens smaller than 15 inches is 30~54KHz, while for larger screens, the range is broader. To maintain consistent line scan amplitude and reverse pulse amplitude (appropriate high voltage) across different scanning modes, the line power voltage in color monitors adjusts according to the line scan frequency. Higher line frequencies result in higher voltages. Most common switching power supplies provide a fixed output, which is then regulated by a secondary switch to achieve varying outputs. A typical fixed output might be 45V and 150V, where 45V is further regulated to produce 45V`~135V for the raw pressure type; 150V is regulated to produce 45V-135V for the step-down type. Due to the power supply circuit, the main fault symptoms involve either no raster or raster abnormalities. 1) The first step in troubleshooting is observing the power indicator light to determine if the picture tube filament is functioning correctly. If it’s not functioning properly, the issue could be with the switching power supply itself or a short circuit in the load. If the light is normal, you need to proceed to measure the output voltage of each group of the switching power supply. Whether the switching power supply is functioning correctly is primarily determined by the accuracy of the 12V output. If 12V is correct, the switching power supply is generally fine. 2) Assuming the switching power supply is functioning normally, possible causes for no raster include: the first possibility being a fault in the line scan circuit or the picture tube circuit, similar to color TVs; the second possibility being an abnormality in the secondary switching power supply, which can be checked by measuring the line output transformer's power supply voltage; the third possibility is a fault in the energy-saving circuit or the pattern recognition circuit, which might prevent the secondary switching power supply from working. It should be noted that some color monitors require being connected to the host to start up. In the offline state, the energy-saving circuit automatically shuts down the line scan due to the absence of a synchronization signal. 3) Raster anomalies often occur when the width is either too wide or too narrow, and the width adjustment knob on the control panel cannot adjust it to the normal amplitude. This type of fault is mostly caused by an abnormal line voltage, a fault in the line pulse amplitude sampling circuit, or an issue in the line power secondary buck circuit. Additionally, line amplitude abnormalities can occur in the circuit involving the reverse circuit capacitor and the S-circuit positive capacitance automatic adjustment circuit, and the maintenance methods for field amplitude abnormalities are the same as those for color TVs.

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