The rear-projection circuit has many similarities to our current multiplier CRT TV, the only difference being the addition of a convergence circuit. The idea of ​​overhauling rear projections can be used to learn from the experience of repairing color TVs, such as power supply, audio amplification, video amplifier circuits, and scanning circuits. Of course, there are still some repairs to the rear-projection that are different from the maintenance of ordinary color TVs. When you open the rear-projection color TV back cover, you will feel that the rear-projection movement core board is too large, and there are many high-level components. Relatively speaking, the space inside the machine is very Small, it is a daunting feeling for those who are in contact with the rear-end color TV repair.
The projection tube of the rear projection TV also needs high voltage. Because the distance between the high pressure nozzle and the deflection coil is short, the distance between the projection tube and the movement plate is also short, which brings the problem of increasing the chance of sparking discharge, if it is usually Failure to pay attention to the insulation work of the high-voltage part during maintenance may cause blackout, confusion, or damage to components due to discharge and ignition. What's more worth mentioning is that the rear projection mostly uses a 7-inch projection tube. The anode high voltage required for such a small-sized projection tube CRT and our usual large-screen CRT anode high voltage are also not much lower, so rear-projection maintenance High-voltage ignition failures are commonplace. Problems with the projection tube itself can also cause internal ignition, causing failure to start, boot protection, poor convergence, and even burning tube failure. From the current situation of rear-projection maintenance, the poor contact of the high-voltage partial pressure component box and the improper convergence caused by improper handling of the high-pressure cap account for a large proportion of bad faults in the rear-projection color TV convergence. This point must be paid attention to by the maintenance personnel.
The design idea of ​​the rear projection is to minimize the interference caused by the ignition opportunity and the distributed parameter circuit, so the movement of the movement must be short, so the current rear-end movements sold on the market are not exhausted. It is impossible to flip the movement board with a wire plug; when repairing a normal TV set, the movement can be easily turned over by pulling a few plugs, and the working voltage and waveform are checked for maintenance.
The purpose of pulling the wire to flip the core plate is to measure the voltage, the on-resistance and replace the components. It is really difficult to test the voltage on the back of the movement when the machine is powered, unless there is a special extension cord or rear projection tooling. These external conditions are generally not available to non-factory maintenance personnel. Is there no special extension cord or rear-projection tool to repair the rear-projection color TV? The answer is no. In actual maintenance, even in the case of special extension cords or rear-projection tools, it is common to measure the voltage at the critical test points on the front of the road and the front of the movement board for fault determination.
If the convergence circuit is the most prone to problems, because the DC voltage of the signal output from the convergence component is almost zero, the change of the measurement voltage is not significant, but the resistance to the ground is not the same, if the convergence of the red gun is not adjusted. Or, when the adjustment is small, the method of measuring the resistance to the ground is much easier.
The convergence circuit is actually a high-power amplifier circuit, which is somewhat similar to the common CPT color TV audio OCL power amplifier. The TV sound amplification channel has two to three channels, and the convergence amplifier circuit has more than three channels. There are six independent amplification channels (the three primary color projection tubes each use two convergence amplification channels: the line H convergence signal and the field V convergence signal amplification). ). The six-channel channel does not affect each other. The six-channel power amplifier is integrated into a thick film block with three groups as one device. This is the Japanese STK-392 series convergence amplifier. Two identical convergence power amplifiers take up the power amplification of the convergence signal, and the periphery of the integrated block consists of six sets of identical circuits and components. Since the circuit structure is the same, then the peripheral resistance values ​​are measured and compared one by one, and the fault point is found. The red gun is not well adjusted, indicating that the magnifying channel of the red gun may not be the same as the green and blue two-channel resistance (must be a problem with the converged power amplifier circuit), and various problems caused by the convergence of the power amplifier can be quickly eliminated by testing. Of course, the problem of convergence circuits is far more than that. Convergence components, converged power supply systems, and convergence coils can cause problems. In the rear-projection color TV, the convergence problem caused by the power system also accounts for a large proportion.
The units of the rear projection TV have a lot of processing circuits, and different power supplies are supplied to the respective power units. The movements of the rear-projection color TV are different, and the power supply mode is also very different. At present, the convergence circuit of most rear projection TVs is directly powered by the switching power supply. The convergence and line scan power supply of the Changhong PDT-6 movement and the DP series is supplied by a single switching transformer of the power board, but the convergence power of the PDT-3 movement (HP, KPJP series) is provided by the auxiliary power supply on the power supply board. Instead of using the same switching transformer, the voltage is supplied by another switching transformer.
The power circuit of the rear-projection color TV is working in a state of high current, so the probability of failure is large. The power supply part of Changhong rear-projection adopts the high-power thick-film block power supply of Japan Sanken Company. The circuit principle is almost the same, but some powers and auxiliary circuits are different. As long as the specific division of labor of these auxiliary circuits is clarified, Then the maintenance idea can be very well determined. Repairing the rear-end power supply only needs to sum up a kind of power supply--the experience of the STR66×× series maintenance example, it can repair the fault caused by the power supply problem of Changhong rear projection.
The difficulty of repairing the abnormality of the rear-projection color TV convergence is actually the determination of the fault range. When there is a fault in the convergence of the rear-projection color TV, do not rush to adjust the convergence. First, observe the convergence distortion phenomenon in detail, and distinguish whether it is a hardware problem of the circuit or a debugging problem. It is important to note that some convergence distortions are caused by faults in the geometric distortion correction circuit. When the geometric distortion circuit fails to cause convergence distortion, the image has a large image and the left and right sides are substantially symmetrical. Of course, if the AFC circuit fails, the image center is severely offset, which also causes convergence distortion. Generally speaking, when the fault is detected and the convergence is not caused by the AFC circuit and the geometric distortion correction circuit, the convergence is adjusted and the convergence is not detected. malfunction.
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