Diagram of the power supply for a digital set-top box for a TV. Reasons for poor reception or lack of signal reception on DVB-T2 set-top boxes
In the article, the author shares his experience of repairing a tuner for receiving terrestrial digital television programs. The troubleshooting technique he describes is applicable to other electronic products, where one or more functional units are powered by a voltage stabilizer with unknown output parameters.
The "Globo GL50" tuner is designed for receiving DVB-T/T2 terrestrial digital television programs, as well as for playing multimedia files from external media connected to the USB port of this set-top box. After working for about a year, this tuner broke. The malfunction looked like a complete inoperability of the device, while the LED on the front panel glowed yellow. Repairing such devices is usually impractical, but since modern digital devices have few interesting parts for further use, it was decided not to disassemble the receiver into parts, but to try to repair it.
The troubleshooting began by checking the functionality of an external power supply with a stabilized output voltage of about 5.2 V at a load current of up to 1.5 A, which turned out to be serviceable. Next, the integrated voltage stabilizers were tested on the block board (its marking is M3103-0C). At the output of one of them (KV3VC - according to the U4 microcircuit circuit) there was a voltage of 0.537 V, while the microcircuit body was heated to a temperature above 100 o C in five minutes. This stabilizer directly powers the tuner's central processor, which remains cold. The initial version that one of the ceramic blocking capacitors C23, C24, C35 was broken was not confirmed.
Disabling the stabilizer load did not change the situation: the chip body also became very hot. Since no useful information could be found either about the failed microcircuit or about the board itself, it was necessary to find out what voltage was required for the operation of the central processor experimentally. To do this, the U4 chip was soldered out of the board, and the output of a powerful laboratory regulated power supply was connected to the printed conductor intended for soldering its output pin (the closest contact next to the inscription “U4”). At a voltage of 1.1 V or less, the tuner processor worked with errors or froze. At a voltage of 1.2 V, the processor worked without errors in all operating modes of the set-top box, consuming a current of about 0.6 A.
Because even if it were possible to find and purchase the same microcircuit to replace the failed one, its installation, in the author’s opinion, is devoid of practical meaning (what burned out once will burn out a second and a third time); It is not enough to simply eliminate the malfunction; you need to eliminate the causes of its occurrence. Probably, the KV3VC integrated stabilizer burned out from overheating - a typical malfunction of modern digital devices, it ranks first in the number of failures of devices for household and industrial use (in second place is printed wiring, and together they “take away” more than 90% of all malfunctions that did not arise due to improper use).
It was decided to replace the faulty switching stabilizer chip with a linear stabilizer with an output voltage of 1.3 V on the KR142EN12A chip, the circuit of which is shown in Fig. 1 (positional designations of its parts begin with the prefix 1). The output voltage is set by resistors 1R1, 1R2. The lower the resistance of the first of them (with the second remaining unchanged), the lower the output voltage. Capacitors 1C1, 1C2 are blocking. Diodes 1VD1 - 1VD3 protect the load from damage if 1DA1 malfunctions. Taking into account the voltage drop on the connecting wire, the output voltage of the stabilizer was approximately 1.28 V.
Rice. 1. Stabilizer circuit
The parts of the new voltage stabilizer are placed on a circuit board measuring 22x22 mm. The KR142EN12A microcircuit is installed on a ribbed duralumin heat sink, which is tightly pressed to the bottom and side walls of the metal case of the set-top box. Since the heat sink flange of the microcircuit is electrically connected to pin 2, it is fixed to the heat sink through an insulating gasket; a PVC tube and a getina washer are put on the mounting screw for insulation. All mating planes are lubricated with heat-conducting paste. Instead of KR142EN12A, you can install KR142EN12B or one of the imported ***317 series in the TO-220 housing (for example, LM317, KA317). The purpose of the pins of all these microcircuits is the same. A view of the installation of the stabilizer and the “stuffing” of the attachment is shown in Fig. 2.
Rice. 2. View of the installation of the stabilizer and the “stuffing” of the console
The stabilizer input is connected to fuse link F1 (Fig. 3, red wire), the output is connected to inductor L4 (Fig. 4, green wire), and the common wire is to the negative plate of capacitor C35 (Fig. 4). Instead of 1N4001 diodes, you can use any of KD208, KD243, KD247, 1 N4002-1 N4007.
Rice. 3. Circuit board
Rice. 4. Circuit board
The M3103-0C circuit board can be used in other models of DVB-T2 receivers. Since the author has never come across “cold” DVB-T2 set-top boxes (they all get very hot during operation), after the end of the warranty period, it is advisable to measure and record the input and output voltage values of the stabilizers installed on the board, and also photograph the circuit board on both sides like this so that all the inscriptions are visible, this can be useful when repairing a failed device. Keep in mind that several operating voltages can be generated simultaneously at the output of a pulsed integrated stabilizer. To improve cooling, you can install a small “laptop” fan inside the console.
After the repair, the current consumed by the tuner from the power supply was about 0.75 A, which means that the external complete switching power supply with the output parameters indicated at the beginning of the article is capable of providing power not only to the DVB-T2 set-top box, but also to the external one connected to it. hard drive form factor 2.5 inches. If the set-top box is equipped with a power supply built into its case, it is advisable to power 2.5" hard drives from an external unit to reduce the load on it and, accordingly, to reduce the temperature inside the case.
Publication date: 20.01.2016
Readers' opinions
- Genn / 11/12/2017 - 07:30
Well told - thank you - admin / 10/31/2017 - 10:17
Unfortunately, there is no diagram! - ALEXANDER / 10.30.2017 - 19:00
Globo GL50 diagram where to find - Victor / 03/19/2017 - 11:47
Thank you! Interesting. Educational. - Vladimir Vasilievich / 29.12.2016 - 13:59
I’m 69. And I’m still interested. I liked the site and the sections. Since I’m still doing repair work. since 1967.Thank you 73. - Guest / 02/07/2016 - 20:23
The article will be useful to a certain circle of people, I liked it.
Since the late 90s of the last century, audio-video technology has evolved greatly. From VCRs and DVD players, CD and MP3 recorders, to omnivorous media players that allowed you to read media files from a USB flash drive. Such devices at one time cost 3-4 thousand.
Now every DVB-T2 receiver can do this. The receivers are quite cheap - from 900 rubles, and in addition to reading media files from a flash drive, they allow you to watch television for free in digital quality, even if only 20 channels. And everything would be fine if the Chinese, in pursuit of cheap devices, did not put low-quality parts there. I have had cases where, in a receiver with a built-in power supply, after 2 years of operation, a small electrolytic capacitor had .
Small electrolytic capacitor
And accordingly, the receiver did not turn on, after measuring the equivalent series resistance with an ESR meter, and replacing the three-ruble capacitor, everything returned to normal and the receiver turned on. But this, as they say, was just luck. Much more often, DC-DC converters burn out in receivers. Sometimes, fortunately for the user who decides to independently repair the set-top box, stabilizers with 3 legs are installed instead; replacement is not difficult, but sometimes the boards have unreliable five-legged converters, and we will analyze this case. There are 3 of them there - small microcircuits in the SOT-23-5 package.
Chip Converter - drawing
They produce, respectively, 3.3 volts, necessary to power the RAM chip, 1.8 volts and 1.2 volts, necessary to power the processor.
Converter chip dimensions
It is easy to determine where the output of the microcircuit is, even without having a datasheet for this microcircuit; the output of the converter can be connected by a track to the inductor necessary for the operation of the converter. You can familiarize yourself with one of the typical converter circuits by looking at the figure below:
What if your set-top box refused to turn on, you opened it and, after ringing, found two or more terminals in a short circuit or low resistance? Such converters, due to the fact that their wiring is sometimes individual and incompatible with other types of converters, it is necessary to replace them strictly with exactly the same ones, or, in extreme cases, with complete analogues taken from the datasheets.
Converter chip pinout
The connection diagram, component ratings, output current, and, of course, the output voltage must completely match. I received one of these set-top boxes for repairs with the 3.3 volt converter power input punched to ground. A quick search in radio stores in our city showed that we have neither such a microcircuit nor complete analogues anywhere.
The fact is that for design on Arduino and microcontrollers, special small-sized boards of converters and stabilizers are produced in China, immediately with the necessary kit soldered on the board for their operation. These are AMS1117 stabilizer chips familiar to many electronics engineers.
AMS1117 stabilizer chips
These microcircuits are produced both adjustable, which in this case is unnecessary for us, and with a fixed output voltage, but we are interested in voltages of 1.2, 1.8, 3.3 volts. For all these voltages there are ready-made converter boards based on these stabilizers. How can you distinguish converter boards if, for example, you bought them earlier and forgot what voltage they were for?
On the housing of the microcircuits, in addition to the model name, for stabilizers for a fixed voltage, the voltage that will be at the output of the converter is written, that is, the same 1.2, 1.8, 3.3 V we need. How to place these converters in the receiver housing? They won’t take up much space, I wouldn’t think twice about it, solder the MGTF to three of the contacts on the converter board, there are 4 in total: input plus power, output plus power, and two contacts, a common ground for input and output.
I think it’s clear why we use three out of four contacts. How can we check ourselves whether we have found the correct pinout of the microcircuit, if the Chinese datasheet found, for example, raises doubts? Call the output indicated by the datasheet Vin, often if the set-top box comes with an external power supply, it is directly connected to the power socket. Also, between the ground and the power input, an electrolytic capacitor is often installed on the board, 220 uF x10 or 16 volts.
Capacitor 220 x 25 volts
The plus of the capacitor is connected to the power input of the converter microcircuit. What if you don’t know what output voltage this converter was for, that is, what voltage do you need to buy a converter for? You can try, after dismantling the burnt microcircuit and cleaning the contacts on the board from solder, apply power to the receiver and measure the supply voltage on the two remaining converters. And determine the voltage at the output of the remaining microcircuit by elimination. Solder this burnt-out converter using a soldering gun, or by applying a drop of solder, Rose or Wood alloy to all contacts, and warming them up quickly alternately with a 25-watt soldering iron.
If you are sure that the converter has burned out and are not afraid of burning it with the high temperature of the tip, during dismantling, you can apply a little ordinary POS-61 solder to all contacts and alternately heat the 40-watt terminals with a soldering iron, trying to move the microcircuit. If, after desoldering, it turns out that the short circuit was “underfoot” on the board, and not in the microcircuit, to finally make sure of this, you need to clean the contacts of the old soldered converter from solder using a dismantling braid, applying alcohol-rosin flux to the contacts with a brush (SKF).
Alcohol rosin flux SKF
Then we simply place the braid on top of the contacts and heat the leads on top of the braid with a soldering iron. Our solder will transfer to the clean braid. For better absorption, the tip of the braid can even be dipped in alcohol-rosin flux. As the solder is absorbed, the tip of the braid should be cut off and the procedure repeated from the beginning. The same should be done with the contacts on the board remaining after the soldered converter.
Dismantling braid
There, as usual, we will be left with “snot” from the solder applied during dismantling - they must be removed. Then you can solder the MGTF wire connected to the contacts of the converter, finding from the datasheet for this microcircuit where we have the power input, where the output is, and where the ground is. It will be possible to check, as I wrote above, the contact connected to the ground by the power supply minus, we can call by touching either the polygon on the board, or if you are a beginner and are not sure of the correctness of the connection - the metal case of the USB connector.
Ethyl alcohol photo
After everything has been soldered, do not rush to connect the set-top box to the network, wash off traces of flux with alcohol, especially if you never know, you used a low-active flux, which in this case is a prerequisite for long-term operation of the device. Then look under a strong magnifying glass or take a photo with a phone with a good camera and make sure that you haven’t put “sniff” on the adjacent contacts, because they are located quite close to each other.
Multimeter in audio mode
To be completely sure that there is no short circuit, or if you couldn’t find a strong magnifying glass, test all adjacent contacts relative to each other for short circuits with a multimeter in audio testing mode. All these procedures with replacing the converter make sense only in one case - if, after checking the datasheet, you did not find a short circuit of the power input pins to the power output, since in this case your processor or RAM chip has already burned out due to the supply of an excessive supply voltage.
Which, of course, is sad, since then it will be impossible to repair it either at home, or even in a good workshop, due to the laboriousness of the repair and its high cost - higher than the cost of a new set-top box, it will not make sense.
Conclusion about the work done
Any more or less trained radio amateur can easily cope with this repair, and due to the low cost of the board for replacing the converter, it can be recommended as a means of albeit a “collective farm”, but very budget solution, in the event that a thrifty radio amateur does not have extra funds for buying a new console. Or you simply have a desire to prove to yourself that sometimes it is quite possible to carry out repairs of complex digital equipment on your own. Happy repairs everyone! AKV.
Everyone who lives in the age of high technology is sure that these technologies are designed to make life easier. This is true, although there are situations that can contribute to nervous tension rather than relaxation. For example, when some of your favorite TV channels disappear. Naturally, this is far from the best experience, especially when you want to watch TV, but it throws up similar, not very pleasant surprises.
If you also encounter a similar problem, then there is no need to rush to a television repairman, especially since this may take some time. If you wish, you can try to restore digital channels on the TV yourself. Of course, a 100% return of their broadcast cannot be guaranteed, but, as a rule, independent manipulations help.
Reasons for missing channels
As a rule, the TV shows two dozen standard digital channels. But sometimes even they disappear, if not completely, then partially. Considering that they are free, problems with broadcasting can be associated with many reasons, but the main ones can still be identified:
Problems with the program;
Problems with the signal receiving equipment.
Both the first and second situations can be solved, but they require completely different manipulations. If the problem is with the equipment, then the attempts of an ordinary user do not always help, and can even do harm. If there is a problem with the TV channel itself, then this happens with one, maximum, two programs, but not with half of the channels broadcast at once.
Let's figure out what the number represents before doing any manipulations related to recovery processes. So, the advantages of digital television channels are improved signal reception, cooler sound, picture and other characteristics, compared to other free systems. But only 20 channels are free, which are periodically switched off and changed, so in a situation where some TV channels have disappeared, there is nothing surprising.
A paid channel symbol appeared on the screen. This is a sign that the subscription to services is ending. If you didn’t pay for your digital subscription on time, the answer to the question of why the channels on your digital set-top box disappeared is obvious. You need to top up your account and then they will start working again, of course, if this message does not pop up on. In this case, it is better to contact your provider for clarification;
There may also be difficulties with the software, or more precisely, its compatibility. If this is a GS-HD receiver, then you need to go to the menu and press the “Restore channel list” button. If the new software is not suitable, then on the same receiver you need to: find the TV/RADIO and CHANNEL buttons on the remote control, and also press the same buttons on the receiver panel in parallel. But it is very important to do this at the same time. Then press the STANDBY button on the remote control;
The inscription “No signal”. This is already a standard sign of problems with the antenna. If the antenna does not work as it should, it is not surprising that dozens of channels began to disappear. If the device is new, then it may not have configured programs, so this fact should be clarified. But if the programs used to work, but disappeared in an instant, you should still go outside and see if the antenna has moved due to strong wind or something like that;
Sometimes the connection between the antenna and the cable, or the cable and the set-top box, is disrupted. If such suspicions arise, you need to make sure that the cable is connected to the LNB IN socket.
When none of the above list helps, there is only one way out - call for help. The master will definitely figure out what’s wrong and return your favorite programs.
Why are only 10 out of 20 digital channels shown?
Over the past two or three years, digital television users have often complained that after updating channels, half of the programs that were shown before disappear. Since then, people have a logical question: why did 10 channels disappear if they used to work and were free?
They are trying to find out the answer to this question on many forums, which is why different versions are put forward. For some, the channels of the first multiplex do not work, for others, the second. This suggests that the reasons for these troubles lie in various difficulties in signal reception associated with technical work or the peculiarities of the functioning of digital television.
Perhaps after some time the channels will restore their broadcasting, so you shouldn’t worry in advance about where ten popular channels have disappeared at once. RTRS clarifies that for now the work is being carried out in test mode, so for some regions such difficulties will become the norm. Sometimes searching for channels again or reflashing the set-top box helps, but if this is not your case, you can use analog or satellite television, where the signal works without such significant interruptions.
The fleet of analogue TVs is rather reluctant to give way to digital equipment, gradually taking “second” places - in the kitchen, in offices, garage workshops, etc. At the same time, DVB-T2 set-top boxes are also carried. We have already managed to evaluate the advantages of the latter; some owners also appreciated the disadvantages - the rather low reliability of these devices. As a rule, one of the weakest points of this kind of equipment is the switching power supply - most cases of failure are associated with a malfunction of the power supply, and a malfunction of the power supply can entail such serious consequences that repairing the device will be impossible. And yet, using the example of two digital TV set-top boxes, the possibility of repairing them independently will be considered here. The first device was TVK 3101. When turned on, the image had severe distortions and periodically disappeared completely. A few days later, the set-top box began to turn off a few seconds after the manufacturer's logo appeared on the screen.
The second device is the Oriel 740 set-top box. This device did not respond to commands from the remote control, the indicator barely glowed red.
After opening the console cases, 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 set-top box, special care should be taken - the primary power supply rectifier converts an alternating voltage of 220 volts into a constant voltage of about 300 volts, and this potential remains at the terminals of 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 looks barely deformed should not give you the slightest confidence in the serviceability of the part - even if some part of the container has been 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 with a slightly higher one, as in the picture - instead of a 10-volt part, a 16-volt part is shown, and with the same dimensions. Of course, faulty parts should be replaced with new ones, not used ones - otherwise the repair will soon have to be repeated.
After replacement, we turn on the set-top box - the indicator lights up brightly, the device responds to remote control commands, the image is stable. But the renovation is not finished yet...
There are traces of flux on the board - solder paste, rosin... Through such a coating, high-frequency currents can quite easily pass wherever they want. As a result, over time we may get an unstable image, noise, etc. troubles. Therefore, carefully 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, check - it works.
Now we assemble the console completely and check its functionality again.
We check the second device in the same way - the device is working normally, the repair is completed.
In conclusion, I will add that the quality of the antenna amplifier power supply also greatly affects the operation of the set-top box. Thus, if the capacity of the filter capacitor is insufficient, signal loss is possible - there have been cases of complete loss of channels of the second multiplex. To recognize a malfunction of the antenna power supply, it is enough to replace it with a DC source of 9-12 volts (for example, a Krona battery or a battery from a computer uninterruptible power supply). If the reception quality improves, you should replace the antenna power supply with a known good one.
06.01.2017
Denis Bolshekov
If you are wondering: Why does the digital set-top box not display or display poorly? You are at the right place.
Let's understand this situation.
Digital DVB-T2 television does not show and what to do about it?
First, you need to make sure that you are in an area with reliable DTV reception. If the set-top box used to show and then suddenly stopped, a possible reason is that the antenna equipment was connected incorrectly. Or the DVB-T2 receiver is out of date, this can be solved by downloading new firmware from the manufacturer’s website.
We will assume that the console is in full working order. In this article we will configure only the set-top box itself and the antenna.
Problem: not all channels were scanned and caught
You did an auto search and your set-top box did not find all the channels - you need to conduct a manual channel search. Most likely, the set-top box did not receive the signal from the second multiplex, and you will need the frequencies of the nearest transmitting tower.
If the TV shows nothing at all
You selected the wrong video input or did not switch the TV to AV mode or HDMI. Or they simply didn’t turn on the set-top box; it should have an indicator that lights up green when turned on. The set-top box is turned on either with a button on it or with the remote control.
The set-top box is turned on, but the TV says "No signal"
If this happens, most likely the antenna is turned off, or its amplifier is not turned on. Check if the antenna is connected, maybe it has come loose, pull it out and put it back in. Repeat auto search.
The set-top box says "No services" auto search found nothing
If the channels stop showing and the set-top box says “No services,” then the problem is a bad signal. You need to repeat the search manually. The fact is that with a weak signal, auto search can skip channels. On the contrary, a manual search by frequency will show the strength of the signal, even a very weak one.
Picture with friezes squares
We need to look at the signal level. Press INFO on the remote. The same behavior of the digital set-top box is observed if you are far from the tower, or if there is a large building that interferes with signal reception.
The solution here is that it is necessary to amplify the signal using an active antenna, or put the antenna outside, raise it higher, and the like.
If the signal and picture quality is poor, we adjust the antenna
If the signal intensity is high but the picture quality is poor, then you need to either replace or try to adjust the antenna. Literally turn the antenna in different directions. The mustache is responsible for the meter range, and the ring for the decimeter range. Here we turn the ring in different directions. It is advisable to move around the room or place an antenna on the windowsill.
Digital television also works great with a reflected signal. But it is very sensitive to obstacles from reinforced concrete walls and plastic window frames.
As a result we get
If you need to understand why digital TV doesn't work? If the signal is weak, then it must be amplified using an amplifier or an active antenna. Find the optimal location of the antenna in the room and “rotate” it.