The contestant teaches you how to repair the switch power supply of the monitor

Nowadays, most of the power supplies in monitors use switching type voltage regulators (switching power supplies for short). The so-called switching power supply refers to the adjustment tube in the switching power supply working in the cut-off area and the saturation area. When the adjustment tube is cut off, it is equivalent to the disconnection of the mechanical switch, and when the adjustment tube is saturated, it is equivalent to the closing of the mechanical switch. This triode that acts as a switch is called a switching tube, and the power supply that uses the switching tube to stabilize the voltage is called a switching type voltage regulator. Most common faults in monitors are switching power supply faults.

According to the connection method of the switching power supply and the load, the switching power supply can be divided into two types: series type and parallel type. The output end of the series switching power supply is directly connected to the power grid through the switching adjustment tube and the rectifier diode, so its bottom plate is charged, commonly known as the "hot bottom plate", which brings great safety problems to maintenance. Everyone should pay attention when disassembling the machine; while the parallel switching power supply, its output end and the AC 220V power grid are isolated by the primary and secondary sides of the switching transformer. Therefore, except for the part connected to the primary side of the switching transformer, the rest of the circuit board of the whole machine is not charged, so it is called the "cold bottom plate". The parallel switching power supply has good safety, but its circuit is relatively complex and has high requirements for the switching tube. If the working state of its protection circuit is unstable, the resulting fault will be more serious.

Repairing the switching power supply is the key point and difficulty in monitor repair (accounting for about 70% of the monitor repair in daily maintenance). When repairing the switching power supply, it is best to add an isolation transformer, which can avoid accidents of electric shock caused by the grounding terminal being charged. For 14-inch to 17-inch monitors, a 70W to 100W isolation transformer is sufficient.

When repairing the switching power supply, you can use the voltage reduction method. The method is: connect the power plug of the monitor to an AC voltage regulator, then adjust the output voltage of the voltage regulator to about 100V, then power on for maintenance, and gradually increase the power supply voltage for maintenance.

Fault example 1: The fuse burns out when the machine is turned on, and the output voltage is zero. This situation is generally caused by the breakdown of the switch tube and the short circuit of the emitter and collector. At this time, you can first remove the switch tube and measure the resistance of its emitter and collector to ground. If it is zero or very small, you can replace it. But you must also check whether there are any problems with other components before turning on the machine.

Fault example 2: The grating is distorted in an "S" shape. This problem should focus on checking the filter circuit and voltage stabilization circuit. Generally, it is because a diode is broken, and the full-wave rectification becomes half-wave rectification. This may also be caused by the reduction of the filter capacitor capacity.

Fault example 3: AC 220V rectifier filter circuit has short circuit fault, and the fuse burns out when the machine is turned on. First check whether the rectifier diode is short-circuited and whether the filter capacitor has serious leakage. You can also unplug the degaussing coil plug and check whether the degaussing thermistor has short circuit fault. If so, replace it with a new one.

Fault example 4: No grating, no display, power indicator light off when the machine is turned on, but the fuse is not burned. At this time, check whether the AC mutual inductance transformer is open-circuited, whether the current limiting resistor of the rectifier circuit is open-circuited (burned) or failed, or whether the rectifier diode is open-circuited.

Fault example 5: No grating, no display, and abnormal sound from the machine. If a "squeaking" sound is heard, it means the oscillation frequency is low, and the oscillation-related components should be checked. If a "clicking" sound is heard, it means the power supply is over-current protected, and the over-current protection circuit should be checked.

Fault example 6: Output voltage is higher or lower than normal. The output voltage is higher or lower than the normal value by more than 10 volts to several dozen volts, but it is not zero, and the protection circuit does not work. The fault phenomenon at this time will vary with the voltage change. You can adjust the voltage regulator potentiometer. If the output remains unchanged or changes very little, it means that the sampling differential amplifier circuit is faulty, and the voltage regulator diode providing the reference is likely to be broken down or short-circuited.

2. Repair of high voltage ignition failure of picture tube

CRT high voltage ignition failure is also one of the most common problems in monitor repair.

The main manifestation is that many irregular bright spots appear on the screen grating, and in severe cases, they become dot-shaped lines. Sometimes you can hear a "squeaking" sound inside the machine. If the spark is serious, the image will be blurred, and the grating may disappear after a few seconds or minutes. There are many reasons and locations for the high-voltage spark of the display tube, mainly: high-voltage spark inside the tube socket; spark of the high-voltage nozzle or high-voltage cap of the display tube; spark of the high-voltage lead end of the high-voltage package; spark of poor contact inside the focus potentiometer, etc. Let's talk about the maintenance methods separately below.

1. Repair of sparking of picture tube socket

The spark of the CRT socket is usually at the focusing electrode. The main reason for the spark inside the CRT socket is that the environment in which the monitor is used is too humid or it is not used for a long time. Since the voltage of the focusing electrode is very high, between 4KV and 9KV, the repair method is to turn off the power, remove the tube socket from the tail plate, scrape off the rust on the tube pin with a knife or fine sandpaper, clean it with pure alcohol, and clean the rust on the plastic inner wall of the tube socket with alcohol, then blow it dry with a blower, and reinstall it.

2. Sparks around the high-voltage nozzle or high-voltage cap of the picture tube

Sparks around the high-voltage nozzle of the cathode ray tube are caused not only by the humid environment, but also by the loose contact between the high-voltage nozzle and the cone glass of the cathode ray tube or the presence of impurities. To repair the sparks of the high-voltage nozzle, you must first discharge it. The method is to insert a large wooden handle flat-blade knife into the high-voltage nozzle and touch the bottom plate several times. Sometimes you can hear the "pop" discharge sound. Then remove the high-voltage cap, check whether the high-voltage nozzle and high-voltage retaining spring are rusty, and whether there are signs of sparks and dust accumulation around them. If there are, you can clean them according to the previous method, and then blow them dry with a hair dryer. Pay attention to the temperature and time to prevent the local overheating of the cathode ray tube from bursting. Then evenly apply some butter around the high-voltage nozzle, and then buckle the high-voltage cap.

3. The high voltage package and the focus potentiometer ignite

Sparking of the high-voltage package can cause grating blur or no grating. If the high-voltage package itself sparks, you have to replace it with a new one. If the lead end of the high-voltage package sparks, you can wipe it clean with pure alcohol and drop a little insulating varnish. If the focus potentiometer on the high-voltage package sparks, you can turn off the power and adjust the focus potentiometer several times to see if the internal contact can be improved. If not, it is best to replace it.

3. How to degauss the monitor

The display screen is magnetized and has color spots, which is also a common problem. The main reasons are: the display is magnetized when it is close to magnetic objects; after the display is moved, the deflection coil inside the machine is shifted, resulting in poor color purity; the demagnetization circuit is damaged. Although some displays have a certain demagnetization function, they are somewhat powerless for more serious magnetization.

For poor color purity caused by interference from an external magnetic field, an external demagnetizer can be used for demagnetization. Demagnetizers can be purchased or made by yourself. In an emergency, you can find a used electric meter (1A to 5A), remove the voltage coil, remove its "gate"-shaped wire core, leaving only the T-shaped iron core and coil that penetrate the coil, and then connect the power cord and switch plug to the two terminals of the coil, and then wrap it tightly with plastic cloth. But no matter which demagnetization method is used, you must pay attention to safety. After power is turned on, hold the demagnetizer and shake it continuously, gradually approaching the fluorescent screen, and repeat it for the magnetic parts, then shake the demagnetizer while retreating to about 2 meters away from the fluorescent screen and turn off the power. The power-on time should not be too long each time. If the demagnetization effect is not good once, it can be repeated several times.

If the color purity is poor due to moving the display, you can open the back cover of the display to restore the deflection coil to its original position and tighten the deflection coil screws. For poor color purity caused by damage to the degaussing circuit inside the machine, you can first check whether the thermistor is damaged, remove it, and shake it by hand. If it makes a "swooshing" sound, the thermistor is broken. Use a multimeter to check the resistance value of its pins. If the resistance value is less than 8 ohms or greater than 50 ohms, it means that the RTC element in the degaussing resistor is broken and should be replaced. If the resistance value of the degaussing resistor is normal, you should focus on checking whether the lead, plug, and socket of the degaussing coil are loose or in poor contact.

In addition, if the degaussing resistor is short-circuited or leaks electricity, it will cause the fuse to burn out when the machine is turned on, so everyone should pay attention.

4. Repair of screen display failure

After a monitor has been used for a long time, the screen display area may become larger or smaller. At this time, adjust the horizontal and vertical width potentiometers. If there is no effect after adjusting them to the maximum, you need to open the back cover to adjust them. For example, a 14-inch monitor has the above fault after three years of use. In the 640×480 display mode, the screen is basically normal and adjustable, but in the 800×600 display mode, the screen is seriously small, with black borders of about two centimeters on both sides of the screen, and the same is true when the external knob is adjusted to the maximum; what is even more annoying is that in the 640×400 DOS mode, the screen is seriously large, so that only 2/3 of the startup screen can be seen during startup, and adjusting the external knob also has no effect. Remove the back cover of the monitor, and you will see that there are four sets of fine-tuning potentiometer knobs on the main circuit board, namely H.WIDTH, PIN, V.HEIGHT, and H.PHASE. In addition, there are three synchronous potentiometer knobs of 31KHz, 35KHz, and 38KHz, which are responsible for the three display modes of 640×400/640×480/800×600 respectively. Connect the monitor, turn on the power, and use a small screwdriver to adjust the potentiometer knobs in the three display modes of Windows and DOS (640×400) until you are satisfied with the three resolutions. Because these knobs of each monitor are similar, you can try them repeatedly several times. After the adjustment is completed, enter the corresponding display mode, and adjust the focus and brightness as needed until you are satisfied. These two knobs are generally on the high-voltage package. Just pay attention to safety when adjusting them, because the voltage on the high-voltage package is very high. The key to such adjustments is to be bold and careful.