[Color TV Repair] What should I do if the color TV is faulty?
(1)Color TV repair: Current measurement method
The “current method” is mainly used to judge whether there is an overcurrent fault in the line output stage and whether the line output transformer is partially short-circuited. During maintenance, it is usually to disconnect a certain point in the working voltage supply circuit of the row output stage, and then connect an ammeter in series at the disconnection point according to the current flow. Figure 2 is a diagram of the amperometric measurement of the operating current of the row output stage. The range of the multimeter is DC 2.5 A. The measurement results and the problems explained are as follows:
1. If the current is greater than 50 mA during measurement, it means that the line output stage has serious breakdown or short-circuit fault. It can be roughly judged that it is caused by the partial short circuit of the line output transformer, the breakdown of the line output tube, and the breakdown of the reverse capacitor.
2. No current, that is, the needle does not move. This means that the measured line output stage circuit is not working or the line output tube is open.
3. The working current of the normal line output stage is generally 350 to 400 mA, indicating that the line output stage and the line oscillation stage are normal.
(2)Color TV repair: Direct observation method
Color TV “intuitive method” repair can be divided into There are two types of faults: “Phenomena Intuitive Method” and “Component Intuitive Method”.
1. “Intuitive method” to detect the fault phenomenon Use the “intuitive method” to repair the fault phenomenon, first of all to classify the fault phenomenon, here we summarize the various fault phenomena of color TV into the following several category.
(1) “Nothing at all” for a color TV is any manifestation of the TV working without electricity. The fault may be short circuit or overcurrent of switching power supply, line scan, field scan, audio channel, etc. For remote control color TV, the remote control system should also be considered.
(2) “Three Nos” for color TV
Namely: no grating (no characters should be added to the remote control color TV), no picture, no accompanying sound, but with ” “Squeak” sound or other response to power-on work, the fault should be in the switching power supply or line scan circuit.
(3) There are no faults in the image and sound. Generally, it is in the public channel and the adjustment unit. For the remote control color TV, the remote control system should also be considered.
(4) Image failures for color TV maintenance include: no image, poor image effect, image color cast, video channel and adjustment unit between image and sound separation. For remote control color TVs can also happen in remote control systems.
(5) The sound includes no sound, small sound, poor sound effect, and the sound channel after the image sound is separated, and the remote control system should also be considered for the remote control of the color TV.
(6) The grating includes no grating, the grating amplitude is small, there is a black background of characters, the brightness is dark, the brightness is too bright, a bright line, etc. The fault is generally in the scan channel.
Through the “intuitive method” to narrow down the scope of fault repair by the small differences of similar fault phenomena. The fault phenomena of color TVs vary widely, and there are thousands of fault phenomena. The above are only classified into six categories, and the judgment of the fault location is roughly in that location. If we subdivide each type of fault phenomenon, and use the fine distinction between different phenomena in the same type of fault, we can further reduce the scope of fault maintenance, and some can even narrow the fault to a specific component.
In color TVs, the same component may be damaged in different ways or to different degrees, which may cause different malfunctions. If the sampling amplifier tube in the switching power supply is open, it will cause the output voltage of the switching power supply to be high; if it breaks down, it will cause the output voltage of the switching power supply to be low or no voltage output.
In contrast, failure of different components in different unit circuits can sometimes cause the same failure phenomenon. Such as “nothing” fault, it may be caused by a problem with the switching power supply, or it may be caused by a problem with the line scan circuit.
Therefore, when repairing color TVs, it is necessary to correctly classify the fault phenomena to narrow the fault to one or several unit circuits; at the same time, it is necessary to find out the characteristics of different fault phenomena on this basis. , so as to further reduce the fault to a unit circuit or a specific component.
The most common failure symptoms of color TV repairs
(1)color TV picture tube Magnetic
The grating and the image have colored spots or rainbow bands locally. This kind of fault is called the magnetization of the picture tube. The cause of the failure is the color TV degaussing circuit. The solution is to directly replace the degaussing resistor or degaussing coil.
(2)Color TVThere are 1~10 white bright lines (retrace lines) on the upper part of the grating
The cause of the failure Pump power circuit with OTL field output stage. The solution is: replaceable field output stage 4.7μF, 10μF, 100μF, 220μF boost capacitors, blocking diodes or field output thick film blocks.
(3)Color TVTreadmill
When the program key is pressed lightly, such as black bars on the grating , directly replace the program key.
(4)Color TVPoor picture in non-preset state
Close the door on the TV panel , you can turn on AFT (ordinary color TV) or preset switch (remote control color TV), the image effect becomes worse, directly adjust or replace the AFT mid-cycle (phase-shifting network) and 38.0 MHz mid-cycle in the common channel.
(5)Color TVTreadmill, unlocked
It’s easy to change programs on a normal color TV, It is difficult to find a good image in the search station, and the remote control color TV cannot lock the program, so the AFT mid-week and 38.0 MHz mid-week in the public channel should be replaced first.
(6)Color TVRaster and image color cast
First distinguish the cause of the failure: receive a standard color signal, put the The color saturation is turned off, and the screen has vertical bars that change from white to black. If the low-brightness vertical bar has a certain color, it means that the dark balance is poor, and you can adjust the three equidistant dark balance adjustment potentiometers on the video amplifier board; if the brighter vertical bar has a certain color, it means that the bright balance is poor, The two adjacent bright balance adjustment potentiometers that are far away from each other on the video amplifier can be adjusted.
Brightness balance adjustments are collectively referred to as white balance adjustments.
There is a little green or red noise in the black and white noise of the raster, and the color of the image is wrong, you should adjust the two adjacent potentiometers that are far apart on the video board.
For strong green gratings, strong red gratings, strong blue gratings and white retrace lines in the gratings, you should first check the discharge gap or triode between the corresponding cathodes of the tube base of the picture tube.
(7)Color TVOnly slanted black and white bars on the screen
This failure is called line frequency misalignment , the line frequency potentiometer or 500 kHz crystal in the line scan circuit should be replaced.
(8)Color TVBlur
The black and white noise on the raster is very large or blurred, after 30 minutes of power on Whether it becomes normal automatically, if it can automatically return to normal, replace the tube base of the picture tube; if there is no change, it is called defocusing, and adjust the focus potentiometer on the line output transformer.
Maintenance of internal component failures of color TVs
“Intuitive method” to detect unit circuits and components There are as few as hundreds of components inside a color TV. There are more than 2,000, and it is impossible to observe one by one.
When repairing, it is necessary to focus on observing certain components in a unit circuit in combination with the failure phenomenon.
“None” type fault unit circuit The main components of the unit circuit to detect “none” type faults The unit circuit that should be observed is the switching power supply and the line scan circuit. Among them, the main observation in the switching power supply is the fuse tube, the large filter capacitor, and the power supply thick film circuit. The line scan circuit mainly observes the line output transformer, and also observes the remote control transformer for the remote control color TV. The main objects of observation of the above components and the problems explained are as follows:
(1) Whether the inner wall of the TV fuse is seriously blackened or whether the internal fuse is broken
a. If there is one situation, it means that there is a short circuit fault in the switching power supply, mainly: switching tube, power supply thick film circuit, large filter capacitor of 100μF/400 V or above, breakdown of bridge rectifier diode, internal breakage of degaussing resistor crack. These components are the most vulnerable components in the switching power supply except the switching tube. It is easy to observe that other components in the switching power supply and components in the load will not directly cause the fuse to fuse. The above may cause failure. The structure and real object of the components in the schematic diagram are shown in Figure 3.
b. If the fuse tube does not turn black and the internal fuse is normal, it means that the switching power supply has no breakdown or short-circuit fault.
(2) Whether the top of the large filter capacitor is burst, the top is bulging, or the pins are corroded
This indicates that the large filter capacitor is faulty and should be replaced .
(3) Whether the appearance of the remote control transformer is burnt.
If yes, it means the remote control transformer is broken and should be replaced. Observe the remote control transformer only when servicing a color TV.
(4) “Three no”, with “squeak”, sound.
The main components of the unit circuit are mainly observed in the filtering large electrolytic capacitor in the switching power supply and the line output transformer in the line scan output stage. For the 4.7-100μF electrolytic capacitors in the switching power supply, the main observation targets are whether the top is burst, and whether the pins (removed) are rusted. The observations and descriptions of the line output transformers are the same as in “Nothing” above.
Image failure of color TV repair – detection of main components of unit circuit
(1)color TV poor image effect Unit circuits and components that should be observed if the program cannot be locked with the search station
Mainly observe the two adjacent mid-cycles in the common channel in the shield, such as the 38. O MHz image detection mid-cycle If there is no capacitance in the middle of the AFT, if there is no capacitance in the dotted box, the observation is over. If there is a capacitor in the dotted frame, remove the two middle circles and observe whether the internal capacitors are partially or completely blackened or darkened, as shown in Figure 6. If it is black or dark, it means that there is a problem with the mid-week and should be replaced; if it is clean as new, it means that the mid-week is normal.
(2)Color TVThe unit circuit and main components should be observed for blurred image
The blurred image should be observed on the video board Check whether the focusing pole of the tube seat of the picture tube is corroded and polluted, and whether there is patina powder, as shown in Figure 7. If there is, it means the socket is leaking and should be replaced.
Note: To observe the focusing electrode, open or remove the small cover at the end of the tube base by pressing the pin in the small cover inward and disassemble it at the same time.
(3)Color TVThere are horizontal bars or “S” distortion on the picture
The unit circuit and main The component is a large filter capacitor of 100μF/400 V or above in the switching power supply. The observation target and result are the same as the above “nothing” introduction.
(4) Trouble with color TV sound
The unit circuit that should be observed when detecting sound-like faults of the main components of the unit circuit is the sound channel
(1) Observe whether there are cracks, bursts, etc. on the appearance of the integrated circuit of the audio amplifier. If so, judge that the integrated circuit of the audio amplifier is broken down and should be replaced.
(2) Observe whether there is any capacitance in the single sound detection mid-cycle in the shielding cover and whether the capacitance is black and deteriorated.
(5) There is a sparking sound or smoke in the TV, and the main components of the unit circuit are detected. (1) The unit circuit and main components should be observed if there is a sparking sound.
If there is a very loud “beep” sound, the main observation is whether the line output transformer in the line scan unit circuit is ignited. If so, it means that the line output transformer is broken down or the insulation performance is degraded , should be replaced.
If there is a “silk” sound of fire, the main observation is whether there is purple light at the high pressure cap of the picture tube. If there is, it means that the insulation performance of the high-voltage cap of the output transformer has deteriorated. Use cotton balls to clean the high-voltage cap of the picture tube and replace the high-voltage cap. When replacing the high-voltage cap, firstly, tin the welding point of the high-voltage line; secondly, apply a little silica gel (orphan killer) 2-3 cm below the welding point, so that the high-voltage line can be smoothly penetrated into the high-voltage cap. In addition, silicone is also coated near the high pressure nozzle of the picture tube, as shown in Figure 8. Prevents re-occurrence of spark failures and moisture protection.
Maintenance of color TV Smoke failure observation method
You should try to observe the smoke in the shortest time after starting up components, and shut down quickly. If the smoking component is a capacitor or a line output transformer, you can directly replace the smoking component; if it is a resistor, it means that the current flowing through the resistor is too large, which means that the circuit behind the resistor has a short circuit or overcurrent fault. The smoking resistor is sometimes blown out or its resistance value becomes larger; sometimes it just looks scorched, but its resistance value is still normal, and it can continue to be used after other faults are eliminated.
(3), maintenance “substitution method”
The “empirical method” and “intuitive method” mentioned above are used to repair some characteristic faults When the phenomenon occurs, the method of introducing the first replacement is actually the “substitution method”.
“Replacement method” is to use other methods to preliminarily determine the fault after certain components, and replace the suspected components with new components. If the fault is eliminated, it means that the replaced component has a problem : If there is no change in the fault, it means that the replaced component is normal, and it should be replaced with the original TV.
(4), maintenance “touch method”
“touch method” is to shut down after 5 minutes or more disconnect the power plug), and then purposefully touch some components to see if the temperature is too high.
The main components checked by “touch method” are: degaussing resistance, switching tube, thick film block, electrolytic capacitor in switching power supply; line output transformer and line output tube in line scanning circuit; field scanning circuit The field output block in (or output (a) structural diagram output tube); the audio power amplifier block in the audio circuit. Except for the electrolytic capacitors, if the temperature of the above components is too high (except the degaussing resistance), it means that the heating element may have an overcurrent fault. Because the temperature of each component is different under normal conditions, even for the same component, for example, the temperature of the line output transformer in different machines is also different. As a maintenance person, pay attention to accumulating experience in daily repairs.
A special reminder is required here: when performing touch maintenance on a component in a unit circuit, it must be based on the “voltage method”, “current method”, and “empirical method” mentioned above. In order to preliminarily determine which component of which unit the fault is located, the “touch method” can be used to check this component. Do not touch the components of the switching power supply without purpose, so sometimes due to the different temperatures of different components or different movements, you will misjudge the fault location and lead to a detour in maintenance. Be careful not to touch it with electricity.
Application of “touch method” in color TV repair circuit
1. “Touch method” to detect degaussing resistance Under normal circumstances, the temperature of the degaussing resistance is higher than high, it should feel hot to the touch. If the touch result is hot, it means that the degaussing circuit in the TV is working normally; if there is no temperature, it means that the degaussing resistor is not working and should be replaced.
2. “Touch mode method” to detect components in switching power supply
(1) Application of “touch method” in switch tube Touch the temperature of the switch tube to judge the work of the switch tube Whether the current is too large. Under normal circumstances, the switch tube has a certain temperature. If the touch temperature is the same as the normal temperature or lower than the normal temperature, it can be judged that the working current of the switch tube is normal or less than the normal value; if the temperature is much higher than the normal value, it means that the switch tube is working. The current is large, that is, the load of the switching power supply (circuits such as line scanning) has an overcurrent fault, and the reason for frequent damage to the switching tube is often the overcurrent thermal breakdown.
Note: If the maintenance failure is repeated burning of the switch tube, you must find out the cause, completely remove the faulty components, and then replace the new switch tube.
(2) Application of “touch method” in electrolytic capacitors Touch the temperature of electrolytic capacitors to determine whether it leaks or fails. Theoretically speaking, electrolytic capacitors should have no temperature, but in fact they all have lower temperatures. If the temperature of the capacitor is high, it means that the capacitor is leaking and should be replaced; if there is no temperature, the capacitor may fail and should be removed for inspection.
3. The “touch method” detects the components in the line scan circuit are the same as the other circuit components mentioned above. The line output tube and line output transformer in the line scan circuit have a certain temperature under normal circumstances. If there is no temperature by hand, it means that the line output stage is not working, but it will not cause the output voltage of the switching power supply to be low (it is an excitable switching power supply, such as the Panasonic M11 switching power supply); The leakage of the components makes the output stage work a large current, which is the reason for the low output voltage of the switching power supply or the small amplitude of the grating.
4. The “touch method” detection integrated circuit “touch method” application of integrated circuits is different from the “touch method” of other components mentioned above. After the integrated circuit breaks down, turn on the power for a while after replacing the new components, and then touch the temperature after turning off the power. If the temperature is normal, it means that the working current of the field output block or the sound block is normal, and other reasons should be checked; if the temperature is high, it means that there is an overcurrent fault, and its output stage circuit should be checked.
5. The “touch method” detects other components, such as video, CPU and other large-scale integrated circuits, which have lower temperatures when they are working normally. If there is no temperature, it is likely that the integrated circuit is not working.
For electrolytic capacitors with larger capacity (above 100μF) in other parts, the lower the temperature, the better; if the temperature is higher, you need to check whether the capacitor is leaking.
There are many methods for repairing color TV sets, including: voltage method (also known as key point test method), current method, resistance method, intuitive method, substitution method, disconnection method, short-circuit method, Connect the dummy load method, the empirical method, the step-down method, the induction method, the waveform method, the vibration method (knock method), etc. What I have introduced earlier are the most common color TV repair methods in different maintenance situations of color TVs. I hope it can help everyone.
