Failed model: Jin Great Wall C-I5271 display;
Cause of failure: lightning strike;
Fault phenomenon: No display. The user reported that when the machine was struck by lightning, it made a "bang".
Analysis and maintenance: When I opened the case, I found that the power supply fuse (F901/3.15A) had been blown out and the inside was black, indicating that the power supply had a serious short circuit fault. The power supply adjustment tube drain (D) has a ground resistance of 0Ω, and the gate (G) has a ground resistance of 0Ω, indicating that the power supply adjustment tube has been completely broken down.
Remove the power supply adjustment tube (IRFBC40) from the circuit, use the three-purpose meter (MF-47 type) × 1kΩ gear to measure the resistance between the three electrodes D, S, and G, and found that it has been completely short-circuited. The ground resistance value of the circuit connected to the three electrodes D, S, and G is measured using ×1kΩ and ×100Ω gears, and the ground resistance of the circuit connected to the three electrodes of D, S, and G is shown in Table 1. According to previous maintenance experience, the forward and reverse resistance of the G electrode is obviously abnormal.
Check the circuit connected to the power supply adjustment tube (^50030401e^1), the drain (D) is connected to the power supply pulse transformer, the source (S) passes to ground through a 0.2Ω resistance, and the gate (G) of the adjustment tube is to ground through a diode D910 (BYT42-M). Remove one pin from the circuit and measure the forward and reverse resistance to be 5Ω, indicating that D910 has been broken down.
The power supply of this machine consists of the current pulse width modulation circuit UC3842B and the MOS tube (power supply adjustment tube) IRFBC40. When the power supply adjustment tube breaks down, the UC3842B will also be damaged immediately in normal circumstances. Check the ground resistance of each pin of UC3842B, no obvious abnormalities were found. Check the DC resistance between each pin in the offline state of UC3842B and the check of UC3842B are all in line with the normal values in Table 2 (see Table 2 for normal resistance values, ^50030401b^Table 2, ^50030401c^Table 3 is the typical voltage parameter value of UC3842B). No short circuit was found when checking other parts of the power supply circuit.
Replace the power supply adjustment tube, diode D910, and fuse F901. In order to prevent the load of the power supply (especially the short circuit of the line scanning circuit), disconnect the +80V output terminal of the line scanning circuit, and connect a 40W/220V lamp from the power supply output terminal. Turn on the power supply, and the power indicator light of the display is on, indicating that the power supply short circuit fault has been eliminated. The field scanning circuit power supply + 50V, digital circuit + 15V, control circuit + 25V, video amplifier circuit + 80V are all normal.
Detect the line scanning circuit, the imaging tube circuit, etc. separately. After no obvious open or short circuit fault is found, remove the light bulb, connect the +80V of the line scanning circuit, turn on the power again, and the display power indicator light is on, indicating that the line scanning circuit or line output circuit is normal. The line output tube collector is normal for +80V, and then the line output tube collector is normal for +50V, +15V, +25V, and +80V. Turn on the host, add a signal, and display the display. Change the display refresh rate through the host, change the display mode, and adjust the brightness, contrast and other items of the screen. There are no problems when the display is turned on for two hours continuously, indicating that the fault of the monitor has been eliminated.
Summary: 1. When the monitor is struck by lightning, it will often cause large-scale damage to the circuit, especially power supply circuits, line scanning, field scanning circuits, and interface circuits with the host. When this monitor is struck by lightning, there is a "bang" sound, which roughly indicates that there is a serious short circuit fault in the power supply, and the focus of its maintenance should be on the power supply circuit. Since there is no pattern to follow in the lightning monitor, and there is no idea of some circuits that may occur in the fault, we should establish an idea to correctly analyze and judge the fault range. Since the protection circuits of general monitors are relatively complete, when the power load (scan circuit, etc.) circuit is short-circuited or open, it can be well protected. The power circuit will only stop working, no output voltage, and the power supply fuse of the power supply will not burn. If the power supply fuse is found to be burned, the fault should be in the power supply circuit itself. Starting from this idea, first check the power supply circuit, so that the location where the fault is located can be quickly checked.
2. When lightning enters the unit instantly, its powerful pulse current will generate an instantaneously strong reverse induction pulse voltage on the power supply pulse transformer. Since this pulse voltage is much larger than the reverse breakdown voltage of the power supply adjustment tube, it will cause the reverse breakdown of the power supply adjustment tube and burn the power supply fuse.
3. Since the power circuit of the monitor is a switching power supply, the power load cannot be open. When checking and judging the faults of the power circuit and the scanning circuit, a false load should be connected to the power output end (high voltage output end). Generally, a light bulb is used. The size of the light bulb should be determined based on the output voltage of the monitor's power supply or the size of the power load. A 14-inch to 17-inch display generally uses a 40W to 60W/220V light bulb.
4. Generally speaking, when the user sets the power on the monitor, it is synchronized with the system (Win95/98), that is, when the host power is turned off, the power supply of the monitor will also be turned off, but at this time it is soft shutdown. Although the power is turned off, the power switch is not really disconnected and remains closed, thus providing conditions for the influx of lightning or other strong pulse interference. It is recommended to develop good habits: disconnect the power switches of the host and monitor every time they shut down. If you are used to using soft shutdown, you should turn off the total power supply, which can avoid many unnecessary troubles.
5. Since IRFBC40 is difficult to buy after damage, IRFBC40A and IRFBC80A can be replaced directly, or 6N60 can be replaced directly. For the performance parameters of IRFBC40, please refer to Table 4 of ^50030401d^.
Cause of failure: lightning strike;
Fault phenomenon: No display. The user reported that when the machine was struck by lightning, it made a "bang".
Analysis and maintenance: When I opened the case, I found that the power supply fuse (F901/3.15A) had been blown out and the inside was black, indicating that the power supply had a serious short circuit fault. The power supply adjustment tube drain (D) has a ground resistance of 0Ω, and the gate (G) has a ground resistance of 0Ω, indicating that the power supply adjustment tube has been completely broken down.
Remove the power supply adjustment tube (IRFBC40) from the circuit, use the three-purpose meter (MF-47 type) × 1kΩ gear to measure the resistance between the three electrodes D, S, and G, and found that it has been completely short-circuited. The ground resistance value of the circuit connected to the three electrodes D, S, and G is measured using ×1kΩ and ×100Ω gears, and the ground resistance of the circuit connected to the three electrodes of D, S, and G is shown in Table 1. According to previous maintenance experience, the forward and reverse resistance of the G electrode is obviously abnormal.
Check the circuit connected to the power supply adjustment tube (^50030401e^1), the drain (D) is connected to the power supply pulse transformer, the source (S) passes to ground through a 0.2Ω resistance, and the gate (G) of the adjustment tube is to ground through a diode D910 (BYT42-M). Remove one pin from the circuit and measure the forward and reverse resistance to be 5Ω, indicating that D910 has been broken down.
The power supply of this machine consists of the current pulse width modulation circuit UC3842B and the MOS tube (power supply adjustment tube) IRFBC40. When the power supply adjustment tube breaks down, the UC3842B will also be damaged immediately in normal circumstances. Check the ground resistance of each pin of UC3842B, no obvious abnormalities were found. Check the DC resistance between each pin in the offline state of UC3842B and the check of UC3842B are all in line with the normal values in Table 2 (see Table 2 for normal resistance values, ^50030401b^Table 2, ^50030401c^Table 3 is the typical voltage parameter value of UC3842B). No short circuit was found when checking other parts of the power supply circuit.
Replace the power supply adjustment tube, diode D910, and fuse F901. In order to prevent the load of the power supply (especially the short circuit of the line scanning circuit), disconnect the +80V output terminal of the line scanning circuit, and connect a 40W/220V lamp from the power supply output terminal. Turn on the power supply, and the power indicator light of the display is on, indicating that the power supply short circuit fault has been eliminated. The field scanning circuit power supply + 50V, digital circuit + 15V, control circuit + 25V, video amplifier circuit + 80V are all normal.
Detect the line scanning circuit, the imaging tube circuit, etc. separately. After no obvious open or short circuit fault is found, remove the light bulb, connect the +80V of the line scanning circuit, turn on the power again, and the display power indicator light is on, indicating that the line scanning circuit or line output circuit is normal. The line output tube collector is normal for +80V, and then the line output tube collector is normal for +50V, +15V, +25V, and +80V. Turn on the host, add a signal, and display the display. Change the display refresh rate through the host, change the display mode, and adjust the brightness, contrast and other items of the screen. There are no problems when the display is turned on for two hours continuously, indicating that the fault of the monitor has been eliminated.
Summary: 1. When the monitor is struck by lightning, it will often cause large-scale damage to the circuit, especially power supply circuits, line scanning, field scanning circuits, and interface circuits with the host. When this monitor is struck by lightning, there is a "bang" sound, which roughly indicates that there is a serious short circuit fault in the power supply, and the focus of its maintenance should be on the power supply circuit. Since there is no pattern to follow in the lightning monitor, and there is no idea of some circuits that may occur in the fault, we should establish an idea to correctly analyze and judge the fault range. Since the protection circuits of general monitors are relatively complete, when the power load (scan circuit, etc.) circuit is short-circuited or open, it can be well protected. The power circuit will only stop working, no output voltage, and the power supply fuse of the power supply will not burn. If the power supply fuse is found to be burned, the fault should be in the power supply circuit itself. Starting from this idea, first check the power supply circuit, so that the location where the fault is located can be quickly checked.
2. When lightning enters the unit instantly, its powerful pulse current will generate an instantaneously strong reverse induction pulse voltage on the power supply pulse transformer. Since this pulse voltage is much larger than the reverse breakdown voltage of the power supply adjustment tube, it will cause the reverse breakdown of the power supply adjustment tube and burn the power supply fuse.
3. Since the power circuit of the monitor is a switching power supply, the power load cannot be open. When checking and judging the faults of the power circuit and the scanning circuit, a false load should be connected to the power output end (high voltage output end). Generally, a light bulb is used. The size of the light bulb should be determined based on the output voltage of the monitor's power supply or the size of the power load. A 14-inch to 17-inch display generally uses a 40W to 60W/220V light bulb.
4. Generally speaking, when the user sets the power on the monitor, it is synchronized with the system (Win95/98), that is, when the host power is turned off, the power supply of the monitor will also be turned off, but at this time it is soft shutdown. Although the power is turned off, the power switch is not really disconnected and remains closed, thus providing conditions for the influx of lightning or other strong pulse interference. It is recommended to develop good habits: disconnect the power switches of the host and monitor every time they shut down. If you are used to using soft shutdown, you should turn off the total power supply, which can avoid many unnecessary troubles.
5. Since IRFBC40 is difficult to buy after damage, IRFBC40A and IRFBC80A can be replaced directly, or 6N60 can be replaced directly. For the performance parameters of IRFBC40, please refer to Table 4 of ^50030401d^.