1, the phenomenon of failure: power on, the machine was "three no" state, but the power indicator light on the panel is normal
Analysis and maintenance: The power indicator light is normal, indicating that the switching power supply is working normally. Detection of switching power supply +B voltage is 129V normal, indicating that the switching power supply has been operating in a wide power pulse oscillation state, it is estimated that the fault is mostly caused by abnormal scanning circuit. The main reasons that can cause the line scan circuit to work abnormally are: line oscillation circuit operating power supply voltage is too low or no voltage: line excitation circuit does not have normal operating conditions or line excitation amplifier tube damage; line output circuit DC supply line open circuit.
Therefore, to repair such faults, the output voltage of the output transistor V402 should be measured first. The actual measurement of the voltage is 130V. Next, the 36-pin power supply input terminal N201 (TDA8362) of the oscillating circuit is tested to jump between 2.1V and 4.2V, which is obviously far from the normal value of 8.2V, indicating that the line oscillation circuit is not working. This voltage is the output of the power supply +24V true current through the resistor R845 step down, 8.2V regulator VD805 regulator, capacitor C480 filter formed, and by the standby control circuit V861, V862 as a state control, the relevant circuit As shown.
This situation is mostly caused by problems with the 8.2V power supply. The V862 collector is welded open and the standby circuit is controlled by the 8.2V power supply circuit. The voltage across the C480 is still fluctuating between 2.1V and 4.2V. This eliminates the possibility of a failure of the standby control circuit component. Should focus on inspection of 8.2V power supply circuit components, the test found that the filter capacitor C480 serious leakage, replace the C480, restore the welding open V862 collector power test machine, the whole work to restore normal.
2, the phenomenon of failure: cable TV test machine, found that most stations have obvious snow point interference
Troubleshooting: First detect the I2C bus circuit and enter the I2C bus to view the relevant data, and there is no exception. Therefore, the RF AGC voltages of the high-frequency head U1011 pin and N201 (TMPA8829) pin 43 were respectively measured, and the measured voltage values â€‹â€‹were all fluctuating around 0.8V to 1.1V (normally, the voltage should be about 2.7V to 2.9V. Fluctuation) From the analysis of the dynamic voltage fluctuations when there is a signal, it shows that the RF AGC control circuit is effective, but the control ability is reduced, and the possibility of analyzing the N201 anomaly is small. Therefore, we mainly check the height of N201 (TMPA8829) to U101 (TECC7949VG35E). The circuit element between the first leg and the second leg.
During the inspection, it was found that the solder joint of R123 was not smooth enough, and the test machine had no effect after it was soldered again. It was suspected that there were serious leakage currents in the two capacitors C122 and C121. Remove the C122 test, the capacitor has a leakage resistance value of about a few k, due to C122 serious leakage, while pulling down the R121, R123 voltage point voltage, while the RF AGC control voltage is mostly shorted to the ground, so AGC control ability is reduced So that the gain decreases. Replace the same specifications of genuine capacitor after the test machine, troubleshooting.
3, the phenomenon of failure: light does not turn on Troubleshooting: In the boot, the indicator light. After the second power on, a few buzzing sounds. During the inspection, it was found that the line circuit works abnormally. +B is 132V. The current is not large. The sound is from the high voltage package. Acceleration voltage is 90V. The other winding voltages also decrease in phase. When the VD908 diode was tested, this set of voltage was found to be 0V. Power down, test and found that the diode is short circuited. After switching on normal. The machine is damaged by high packages, or there is a problem with the secondary rectification circuit. There will be no power-on, and the high-voltage package will emit hum. +B normal phenomenon.
4, the phenomenon of failure: the upper black screen, the lower image shaking Troubleshooting: First check the power supply output voltage in each group is normal. At this point the field supply voltage is normal, test 8823 44 foot voltage is 3 ~ 6V change, not normal. Normal should be stable 5V. Check the 5V-2 voltage regulator circuit V905, VD921, R960, test V905 C, B, E voltage is 8.9, 4~7, 3~6, â€‹â€‹the base should be stable 5.6V. The inspection found that the VD921 performance was poor. After switching on normal. Therefore, voltage supply 8823,44 feet. Internal and field circuit related. Causes field oscillations, sawtooth waves and other abnormalities.
5, failure phenomenon: black screen, no sound
Troubleshooting: Power-on check, switching power supply output voltage is normal. The CRT filament is lit. Test three gun cathode voltage is 184V cut-off state. After the accelerating pole voltage is turned up, the whiteboard raster with reduced line width appears without character display. Both remote control and keying are malfunctioning. It is suspected that the CPU (N204 8823) is not working. Test N204,9,45 Pin 5V is normal, 5 pin reset voltage is 0.6V anomaly, normal should be 5V high level. Check the components of the reset circuit r946 1.5k, r947 8.2k, vd902 3.6V regulator, r945 4.7k, r941 680 ohms, C903 0.01UF, and other components, found C903 capacitance leakage. After the change, the boot sound is normal.
6, failure phenomenon: light does not boot
Troubleshooting: Check found that the machine indicator light, can not be the second boot. Check the boot, found that the line damping diode, field block TDA8350Q damage, change the normal boot.
7, failure phenomenon: no character maintenance
Troubleshooting: Observe that the image display is normal, analyze the line retrace pulses to reach the 28,818 feet. May be a retrace pulse -- pulse inversion amplification -- CPU. Or the field retrace pulse loop check is that the resistance of the resistor R822 is infinite, and the power is normal after the change.
8, failure phenomenon: TV no image, there are snowflakes
Troubleshooting: Input AV signal sound image is normal. Description The fault may be at high, intermediate frequency circuits. First, the voltage of each pin of the high-frequency head A101 is normal. The swap data memory failure remains. After changing the A101 (5V frequency synthesizer tuner), start the automatic selection. The image sounds are back to normal and repaired.
9, the phenomenon of failure: the power indicator light does not boot
Troubleshooting: Test output voltage of each group of switching power supply is normal. Test N204 (8823) 17 feet to start the power supply is 0.7V. This voltage is through the CPU (64) pin, control V570 B764, N902 LA7809 supply 17 feet 9V line oscillation power supply. By controlling the working state of the V570, a segment of 9V is cut to achieve the standby purpose. Test CPU64 pin is 3.5V high level. V571 C945 base is 0.71V, V570 emitter voltage is 12.7V, the base is 12.1V, the collector is 0.7V is not normal, try to change V570 after the boot sound map is normal, troubleshooting.
10, the phenomenon of failure: irregular sounds sand noise sounds
Troubleshooting: First test N701 cd2611, the sound power amplifier block 1 foot voltage is 19.8v., At the same time the sound returned to normal! Visually check that the N701 pin is not connected. The silent transistor V701 c945 is normal. Suspected N701 internal abnormality, after a new one, a long test machine, the failure completely ruled out.
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