What is the reason why the monitor has no image?

Posted in Feb 19, 2020 in Tutorials, no comment

Everything is fine when the monitor is turned on, but after a while there is no raster, no image, and a slight squeak. From the point of view of the fault phenomenon, it is the unstable fault of the display. Using the cooling method and heating method to check the main circuits of the display, no problems were found, so the following checks were carried out.

monitor has no image

1. Check whether the output voltage of the power supply (90V) is normal, and the measured result is OV, which indicates that there may be a fault in the power supply or a short circuit in the main load. The existence of either of the two problems will cause the output voltage of the switching power supply to be OV, that is, the power supply has no output voltage. However, because there is a squeak, indicating that the power supply has oscillated but the frequency is abnormal, it can be considered that the power supply is basically normal, and the load circuit should be checked emphatically.

2. Cut off the connection between the line output transformer and the power supply voltage, and the measured power supply output voltage is 90V normal, indicating that the fault is really in the load circuit, and the line output circuit is the main load, so the output circuit is likely to be short-circuited.

3. Check the line output tube, line output transformer and other components, all in good condition, no breakdown short circuit phenomenon, why as long as connected to the uplink load fault occurs, while the inspection line load circuit did not find the broken components. It is estimated that there are two reasons: one is that under the condition of high voltage pulse, a certain insulation material of the line output tube and line transformer is broken down to form a short circuit, and the other is that the insulation resistance of the 90V power supply to a certain part of the line output circuit decreases, resulting in a similar short circuit phenomenon. the above two points cannot be detected with a multimeter.

4. By using the method of replacement, the mica sheet between the collector of the row output tube and the heat sink is replaced with a new one. Boot test, troubleshooting.

 

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