Common maintenance methods of power board

(1) Off board maintenance method

In order to ensure the safety of the power board and load circuit, it is recommended to use the method of off board maintenance, remove the power board from the TV and repair the power board separately.

At present, most of the maintenance is on-site maintenance. All the maintenance operations are completed at the customer’s home. Due to the restrictions of conditions, it is often necessary to remove the power board and take it back to the maintenance department for off board maintenance. The normal operation of the power board is often controlled by the switch of the main board control system. It is often unable to start and enter the working state after leaving the host. It is necessary to simulate the start-up control voltage. Since the start-up state of the switch control voltage of most power boards is high level, a 1 ~ 3.3k Ω resistor can be used to bridge between + 5V or + 3.3V between the switch control terminal and the auxiliary power output, Input the analog on-off control voltage for the power board to force the power board to start working.

In addition, when the switching power supply circuit is off board for maintenance, due to the no-load circuit, its output voltage is often different under no-load and load conditions. Some power boards can not be started because the no-load circuit will enter the protection state, which is easy to cause misjudgment for maintenance. It is necessary to connect a false load at the output end of the switching power supply to simulate the power consumption of the load circuit.

Generally, 12V or 24V motorcycle bulb is selected as the dummy load for the switching power supply, or 120 ~ 330 Ω high-power resistor can be selected as the dummy load, which is connected between the 12V or 24V output end and the cold ground end. The connection position between the dummy load and the starting voltage is at both ends of the power board output connector or the power board secondary output voltage filter capacitor. First, select the connection point through the pin function and voltage marking of the circuit diagram output connector; Second, the pins near the output connectors of most power boards directly mark the function and output voltage.

The specific operation is shown in figure 1-47. The power board is Konka KPS + l707c2-02 power board. According to the circuit diagram of the machine and the marks on the power board, the power board has two output connectors. The pin functions of xs951 are: pin 1 and pin 2 are 12V voltage output terminals, pin 3 is on / off lighting control terminal, pin 4 is dim brightness control terminal, pins 5 and 6 are grounding terminals, and pin 7 is PS / on switching control terminal; The pin functions of xs956 are: 1 ~ 5 pins are 24V output terminals, 6 ~ 10 pins are grounding terminals, 11 pins are empty pins, 12 pins are lighting control terminals, and 13 and 14 pins are brightness control terminals.

Common maintenance methods of LCD color TV power supply board

According to the above functions, the 24V bulb is connected between pins 1 ~ 5 and 6 ~ 10 of xs956, and the 12V bulb is connected between pins 1, 2, 5 and 6 of xs9 51 as the dummy load of the power board. Due to the dense pin functions of the connector, the bulb lead is directly welded to the appropriate solder joint of the corresponding pin copper foil routing under the power board. Since the output voltage of the power board is 12V and 24V, and the start-up control voltage is 3 ~ 5V, 2K Ω and 1K Ω resistors are used in series to divide the voltage, which is connected between the 12V output of pins 1 and 2 and the grounding terminals of pins 5 and 6 of xs951, and about 4V is divided to provide the start-up voltage for pin 7 of xs951. After the above dummy load is connected with the start-up voltage dividing resistance, connect the voltmeter in parallel at the 24V output or 12V output end to power on the power board for test run. In order to prevent failure of the power board and damage to other components of the power board caused by long-time power on, it is recommended to use a socket or plug-in row with key switch when power on. When power on, half press the switch of the socket or plug-in row by hand (do not press it to the end to prevent the switch from locking), power on the power board, observe the brightness of the bulb and the voltage of the voltmeter. If the voltage is too high or smoke and scorch occur, Press the power switch and release the power switch immediately.

The components on the power board are mostly special components, and the original accessories are generally required. During emergency repair, in addition to the fact that the electrical performance parameters of the replaced components must be consistent with or higher than the prototype number, the volume and appearance of some components need to be the same as the prototype number, otherwise the whole machine will be poorly assembled or the components will not fit in, and there may be a short circuit with other components. In addition, due to the small space in the screen and high working temperature, the replaced components have certain requirements for temperature, such as capacitance. It is best to choose 105 ℃ capacitance. Otherwise, the power supply is prone to thermal stability problems or reliability problems.

(2) External voltage method

External voltage method is to connect the voltage or signal suitable for the demand outside or inside the machine to the power supply voltage. Two voltages are generally provided.

First, through the VCC voltage for the drive control circuit, use a DC power supply with an output of 12 ~ 20V to connect to the VCC power supply input of the drive control circuit, and then power on the test machine to measure whether the circuit starts to work. If it starts to work, it is the VCC control circuit fault, otherwise it is the drive control circuit fault. Since the drive control circuit is located at the hot low end, it is prone to electric shock and damage to the power board or alternative power supply. It is recommended to use isolation transformer, pay attention to safety, and distinguish between the hot low end and the cold low end.

Second, provide startup voltage for the on-off control circuit. Generally, operate on the output connector. First find the on-off control pin and 5V power output pin. Since the on-off control voltage is divided into high level and low level, the on-off control pin is directly connected with standby + 5V for high-level startup; For low-level startup, directly connect the switch pin with the cold low end. After connection, it can be powered on to measure the output voltage of the power board.

(3) Short circuit method

Short circuit method is to short circuit the control voltage or protection trigger voltage, and then detect the voltage change after short circuit to judge the fault range. There are two main short-circuit methods.

One is the optocoupler of short-circuit voltage stabilizing circuit or protection circuit. The switching power supply of LCD color TV mostly adopts the direct sampling voltage stabilizing control circuit with optical coupler. When the output voltage is high, the short-circuit method can be used to determine the fault range. The application steps of short-circuit maintenance method are: first short circuit the two pins of the photosensitive receiving tube of the optocoupler, or short circuit with tens of ohms resistance, which is equivalent to reducing the internal resistance of the photosensitive receiving tube. If the measured main voltage remains unchanged, it indicates that the fault is on the primary circuit side of the switching transformer Conversely, the fault is in the circuit before the optocoupler.

The second is the trigger voltage of short-circuit protection. When the protection detection circuit detects a fault, it often sends a high-level trigger voltage to the protection execution circuit to cause automatic shutdown fault. During maintenance, the key points of the trigger voltage can be found, such as the B pole of analog thyristor and the positive pole of isolation diode at the output end of the protection circuit. Short circuit it to the ground to remove the protection and then repair the switching power supply.

It should be noted that the short-circuit method should be adopted on the basis of being familiar with the circuit, and can not be short circuited blindly to avoid expanding the fault. In addition, from the perspective of maintenance safety, the load circuit shall be disconnected before short circuit.

(4) Open circuit method

It is to remove the key points or components, remove the impact of the circuit on the switching power supply, and then start the machine to judge the fault range. If the fault is eliminated, the fault is in the removed part. There are two open circuit methods.

One is the trigger voltage of open circuit protection. In case of protection fault in the power supply, the connection between the protection detection circuit and the protection execution circuit can be disconnected for fault judgment; If the power on is no longer protected after disconnecting the protection detection circuit, it is the protection caused by the detection circuit.

The second is the unit circuit with open circuit fault. When some circuits are damaged and suffer from no accessories, the circuit can be cut off, and then a normal message can be simulated for the control circuit. For example, when some external control elements of PFC are damaged, the external control elements can be removed and the control information can be directly transmitted to the PFC circuit, so that the PFC can be powered and work normally. Once you buy accessories, try to restore the original appearance of the circuit.

It should be noted that when repairing the power board, it is necessary to disconnect part of the load or circuit, but the voltage stabilizing circuit and peak absorption circuit must not be disconnected, otherwise the components may be damaged.

(5) Substitution method

When the power circuit board cannot be repaired for some reason, the whole board replacement method can also be used for maintenance. When replacing, select the power board with the same output voltage, output current and power equal to or greater than the replaced power board, and pay attention to the matching of the control circuit of the switch circuit with the new power board.

When the selected power board lacks a group of voltage output, if the missing group of voltage is low, a higher group of output voltage can be used and replaced with a three terminal voltage regulator. For example, the selected power board outputs three groups of + 24V, + 12V and + 5V, while the replaced power board needs four groups of + 24V, + 12V, + 9V and + 5V, The + 12V output terminal can be externally connected with 7809 three terminal voltage regulator to generate + 9V after voltage stabilization, so as to meet the replacement demand.

Common maintenance methods of LCD color TV power supply board

Troubleshooting of LCD TV power supply board

Two methods are usually used: static test method and dynamic test method.

1、 Static test method

The static test method refers to using the resistance gear or diode gear of the multimeter to test the components in the board to find out the fault point when there is no power supply. This method is mainly used to judge whether the power module is faulty when the fuse is blown out and the device is obviously burnt out.

2、 Dynamic test method

Dynamic test method refers to the method of testing the voltage of key points of the power board through the voltage range of the multimeter, and then judging the faulty components according to the voltage. This method is mainly used when there is no obvious device burnout, the fuse is intact, the relay has “ticking” sound, some voltage output, and the fault cannot be detected by “static” method. The specific methods are as follows:

(1) Disconnect the load, that is, unplug the connector connected to the power board and forcibly ground the power control pin STB (to connect the relay).

(2) Turn on the power supply, turn on the relay, measure the output voltage of each group, find out the abnormal voltage group, then measure the voltage of each pin of the corresponding key chip, and then find out the bad devices according to the abnormal points.

(3) If the STB pin is forcibly grounded and the relay is closed and then disconnected with a “tick”, it is necessary to first determine whether there is a short circuit to ground at the voltage output end or the protection circuit is started due to overvoltage. In case of short circuit, the fault point can be found by testing the resistance to ground; If the above phenomena are not found, disconnect the protection circuit to find the fault, and pay attention to the action of the relay.

The failure rate of LCD TV power board on the whole machine is quite high. It is also one of the key and difficult points for us to repair LCD TV, which is easy to confuse people. A considerable part of his energy is supplied to the lamp board driving circuit (divided into high-voltage board and LED lamp board according to different light sources) and the main board. Once the TV does not turn on, black screen, ripple interference, irregular shutdown and other phenomena, we often do not know whether the fault is on the power board, main board, lamp tube (strip) or lamp driving board, causing many detours for maintenance.

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