The following are some of the experiences that engineers summarized during the maintenance process. They hope to help everyone’s work.
1 No grating, burning fuse
Model: CPMPAQ MODEL N441 15 inch VGA multi-frequency numerical control color display
Fault phenomenon: No grating, burn the fuse.
Failure analysis: No grating, no display, and burning fuses are common failures in displays. This type of failure indicates that there are components in the switching power supply circuit that are broken down or shorted. In general, the switching power supply itself is designed with an over-current protection circuit. Therefore, when the load of the switching power supply fails and a short circuit causes a circuit over-current, the internal protection circuit of the switching power supply operates to stop the oscillation circuit from operating, and thus it will not burn. Components, will not burn the fuse, for the phenomenon of no grating, no display and burning fuse, the fault scope is generally in the power line to the switch transformer primary coil before the circuit and the degaussing coil in these two parts of the circuit, there are damaged components .
Troubleshooting: An effective way to check for such failures is to measure the resistance of the key points after shutdown. Based on the measurement results, determine the display fault. Remove the degaussing coil plug before measuring. The magnetic mode of the 441 monitor is special, and the measurement work can be performed directly.
Begin with the input of both ends of the power cord. As shown in Figure 3-1. Turn on the power switch and measure the resistance at both ends of the power input. If it is normal, the resistance value should be infinite. If the resistance value is very small or zero, R901, R902, R910, R903, C903, C906, and rectifier bridge stacks are generally used. It can be damaged. Upon inspection, the fault in this example is the breakdown of the R901 thermistor, causing a short circuit in the AC circuit and burning the fuse. Replace the R901 and the fuse, and then turn on the fuse after power on.
Continue to check the rectifier bridge reactor and other components on the entire AC circuit, no problems were found. Due to the limitations of this example, it can be inferred that there is no fault in the rest of the machine that causes the fuse to burn.
When the machine is working normally, the AC current is less than 3A, and at the instant of starting, the AC current can reach about 5A. Why is there so much starting current? The experiment and analysis found that the input resistance R901 of the original AC circuit is a thermistor with a positive temperature characteristic. When the AC power is connected, the current through it can reach about 5A. With the gradual increase of the R901 temperature, the resistance value increases. The current is gradually reduced to the rated value. The original 3A fuse has the characteristic of resisting surge current under the condition of 250V AC voltage, and can withstand the impact of large current at the moment of starting. Therefore, the fuse will not be blown at the moment of starting. Later changed 4A fuse replacement, power-on start test, troubleshooting.
Example 2 Characters are fuzzy
Model: COMPAQ MODEL N441 15 inch VGA multi-frequency numerical control color display
Symptom: The character is blurred.
Failure analysis: Characters are vague, indicating that the picture tube is out of focus, and there are four main causes of failure:
1, the focus voltage is wrong;
2, the anode voltage is too high or too low;
3, kinescope circuit is faulty
4, picture tube aging.
Troubleshooting: 1, because the machine is not long, so the general picture tube will not aging.
2. After the monitor is powered on, touch the screen with your hand and there is an electrostatic discharge sound, indicating that the high pressure is quite strong, and it is considered to be normal.
3. The FOCUS potentiometer was adjusted and found to be slightly loosened. After the adjustment, it was sealed with epoxy resin and the above failure did not occur again.
Example 3 Raster red and retrace lines, the raster disappears after 20s
Model: COMPAQ ODEL N441 15 inch VGA multi-frequency digital color display
Failure Analysis: After the monitor is turned on, no display, the screen shows a red grating and a retrace line. About boot 20S or so, I saw a flash of light on the screen, with the grating also disappeared, rest for a while and then boot, the fault occurred again. At first, it was thought that the circuit may have components with poor thermal stability. Therefore, using the “cooling method” to find faulty devices has never been found. Carefully observe the fault phenomenon, analyze the working principle, and finally concluded that: The red grating appears on the display tube, which indicates that the red cathode voltage of the electron gun is too low. Because the cathode potential is low, the current of the electron beam of the picture tube is too large, and the protection circuit operates and shuts down automatically. Based on the above analysis, the fault should be in the kinescope circuit or the video output circuit.
Troubleshooting: 1, first check the picture tube is damaged. If the kinescope filament collides with the cathode of the kinescope, similar faults can occur.
Unplug the tube holder and use a multimeter to electrically block the resistance between the measurement tube filament and the cathode. Normally, it should be infinite. The measured resistance is infinite, indicating that the filament and the cathode do not touch each other.
Unplug the tube holder and turn on the monitor. Measure the red, green, and blue cathode voltages. The red cathode voltage is about 18V. The green and blue cathode voltages are normal. From the measurement results, one is that the cathode of the kinescope filament collides with the cathode; the other is that the red cathode voltage of the kinescope stem is too low. This result indicates that the fault is in the video output circuit or the peripheral circuit of the kinescope.
2, detect the video output circuit. The circuit is shown in Figure 3-2. The red output circuits R734, VT707, R725, R722, VT708, VD709, and VD717 are all detected, and none of them have been listed as abnormal.
3, detection tube circuit. The actual inspection grid and accelerating power supply are normal.
4. After the above tests, no failure point was found and it was difficult to understand. Remove the kinescope socket again, and use a multimeter to measure the resistance of the red cathode to the ground is only about 4.8Ω, or not normal. Carefully check the line, in the case of disconnection resistor R734, the circuit is only connected to SP1, remove it and measure it, its negative value is about 4.81KΩ, the normal value should be infinite, indicating that SP1 is damaged. SP1 is a discharger. Since there is no discharger on hand, there is no further connection to the arrester. After finishing the circuit and powering it on, the monitor will return to normal. If you do not have a discharger, you can temporarily not use it, but it is best to try to buy a change. (for reference only)