The most popular Qingdao brand 47cd840qd color TV

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Examples of soft fault maintenance for Qingdao 47cd840qd color TV

1. When watching a Qingdao 47cd840qd color TV, sometimes the brightness suddenly darkens, only the dark color image outline appears, and in serious cases, a negative image appears. The frequency of failure varies from good to bad. Check that C301, r301, c320, r320 and L301 in the brightness processing circuit are also normal. Finally, it is suspected that the S301 maintenance switch has poor contact. Connect pin 1 of ic301 and C35 with wires Permanent deformation: after removing the load, the negative pole of 01 capacitor is directly connected without maintenance switch. After a long time of watching, everything is normal. It is proved that the fault is caused by poor contact of maintenance switch S301. Most maintenance switches have poor contact due to oxidation of internal contact reed. Use a medical syringe to inject alcohol into the maintenance switch, move the maintenance switch handle back and forth from left to right for several times, and then dry it with an electric hair dryer. When the machine is turned on for a long time, just put the spring into the test panel and watch it normally

2. Everything was normal at the beginning of startup, but after a few minutes, the image began to jump up and down irregularly (the jump range was large and small), but the accompanying sound was always normal

adjust the field synchronous potentiometer r407, and the field runout can be stable for a short time. It is confirmed that the field oscillation frequency can reach the normal value, but the stability is poor. After careful observation and analysis, it is considered that the field amplitude image does not move rapidly by establishing a macro-control and market conditioning mechanism, but by jumping up and down sharply, which indicates that the field out of step is not the loss of field synchronization signal. It is estimated that the field synchronization separation and integration circuit are not a problem, and the fault is likely to be caused by the poor thermal stability of a component in the field oscillation circuit. Measure the voltage of each relevant pin of ic501 (bl5435), and it is found that the voltage of {13} pin fluctuates between 5 ~ 7V, so it is suspected that the field sawtooth wave forms a capacitor c404 (3.3 μ F/16v) performance becomes worse, replace it and remove the fault

3. Everything was normal at the beginning of startup, but the contrast gradually faded after watching. The contrast potentiometer changed but was not obvious

the voltage of each pin of ic301 (bl5612) is normal. No abnormality is found by repeatedly checking the peripheral circuit of ic301. Replacing the contrast potentiometer R310 (10K Ω) is invalid, and the failure of ic301 is still the same. During the maintenance, the maintenance switch S301 was inadvertently moved and then moved back to its original position. The fault suddenly disappeared, so it was suspected that the maintenance switch was in poor contact. After repeated cleaning with anhydrous alcohol, observe for a long time and troubleshoot

4. The black-and-white image is normal, but there is no color or accompanying sound

from the fault phenomenon, it seems that the accompanying sound and color circuits have faults at the same time. Measure that the voltage of each pin of the sound integrated circuit ic201 (bl5250) is normal. Inject the human body induction signal from its {14} and {15} pins, and the speaker has a sense sound, indicating that the sound amplifier and power amplifier circuits are basically normal. Measure the voltage of each pin of ic301 (bl5612) and ic601 (bl5622) related to color, and check that the peripheral related circuit components are also normal, so the maintenance is in trouble. According to the electrical schematic diagram, the simultaneous occurrence of these two faults is related to the synchronous separation stage. After ic501 (bl5435) is replaced as a scientific and fair production and inspection basis, the two faults are eliminated at the same time

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