Repair of digital television

The park of analog TVs is rather reluctant to give way to digital equipment, gradually occupying "second" places - in the kitchen, in offices, garage workshops, etc. At the same time, DVB-T2 set-top boxes are also transferred. We have already managed to evaluate the advantages of the latter, while some owners also appreciated the shortcomings - the rather low reliability of these devices. As a rule, one of the weaknesses of this kind of equipment is a pulsed power supply unit - most of the failure cases are connected precisely with a malfunction of the power supply, and a PSU malfunction can lead to such serious consequences that it will be impossible to repair the device. Nevertheless, on the example of two set-top boxes for receiving digital TV, the possibility of their independent repair will be considered here. The first device is TVK 3101. When turned on, the image had strong distortions and periodically completely disappeared. A few days later, the prefix began to turn off a few seconds after the manufacturer's logo appeared on the screen.
The second device is the Oriel 740 prefix. This device did not respond to commands from the remote control, the indicator glowed slightly red.
After opening the cases of the prefixes, it turned out that in both cases the electrolytic capacitors of the secondary power filters were swollen.
It should be borne in mind that when working with the prefix, special care should be taken - the primary power supply rectifier converts the 220-volt AC voltage to a constant voltage of the order of 300 volts, and this potential remains at the terminals of the high-voltage electrolytic capacitors for some time after the power is removed - up to several tens of seconds . In the pictures they are located between the pulse transformers and the plugs of the power cords. Before working with the device board, these capacitors must be short-circuited through a resistor with a resistance of 51-62 kOhm.
Both faulty capacitors turned out to be almost the same - 1000 μF, 10 V. The picture shows one of them. The fact that its lid seems to be slightly deformed should not give you the slightest confidence that the part is in good working order - even if some part of the tank is preserved, such a part will have an increased leakage current, which is unacceptable. When replacing, you should select capacitors with the same operating voltage or slightly larger than in the picture - instead of the 10-volt part, 16-volt is shown, with the same dimensions. Of course, the defective parts should be replaced with new ones, and not used ones - otherwise the repair will soon have to be repeated.
After the replacement, we turn on the prefix - the indicator lights up brightly, the device responds to the remote control commands, the image is stable. But the repair is not over yet ...
There were traces of flux on the board - solder paste, rosin ... On this coating, high-frequency currents can quite easily pass wherever they want. As a result, over time we can get an unstable image, noise, etc. troubles. Therefore, I thoroughly wash the board with a cotton swab moistened with alcohol or acetone. After such cleaning, wipe the board with a dry cotton swab
We install the board in place, we check - it works.
Now we collect the prefix fully and once again we check the operability.
We check the second device in the same way - the device works fine, the repair is completed.
In conclusion, I add that the quality of the power supply of the antenna amplifier also greatly affects the operation of the console. So, with insufficient capacity of the filtering capacitor, signal loss may occur - there have been cases of complete disappearance of the channels of the second multiplex. To detect a malfunction of the antenna power supply unit, it is enough to replace it with a DC source with a voltage of 9-12 volts (for example, a Krone battery or a computer battery). If the reception quality improves, then replace the antenna power supply with a known-good one.