Examples of simple electronics repair

At the time of Russia’s transition to digital broadcasting, the population still had many TV sets in use without support for the DVB-T2 digital standard. And although there is cable TV, the Internet, but the population is more accustomed to watching the usual broadcast.

Manufacturers did not hesitate and a lot of TV set-top boxes immediately appeared on sale. Moreover, the most popular were simple inexpensive consoles.
The price for them now starts from 1000 rubles, and it’s really inexpensive. But, as they say, expensive, but nice, cheap, but rotten.
Set-top boxes began to fail en masse. In this article, I will try to tell you what breakdowns can be fixed at home without special skills, education, tools, and I will share my own repair experience.

About power supplies in set-top boxes
Very often in set-top boxes, the power supply fails. In this context, they can be divided into two categories: with a built-in PSU and with an external one.

Here with the outside, for example:They are inexpensive and repairing them is not economically feasible. Well, only if you have absolutely nothing to do and the necessary parts to replace the faulty ones are available.

If the PSU is built-in, then it is quite easy to repair it.
Set-top boxes consume very little (about 10 W), so flyback converter circuitry is most often used. Now there are a lot of specialized microcircuits, for example, the TNY174-TNY180 series of chips. The wiring diagram is very simple.

 

Most often, the built-in transistor burns out (in this case, these are contacts D (Drain or Drain) and S (Source or Source)) . You can check it with the usual continuity, usually when the transistor fails between D and S, there is a short circuit. In this case, the diode bridge may burn out (in the picture it is from individual 1N4007 diodes, but there are also such things as:

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and fuse. All these parts are inexpensive, so the repair is simple.

By the way, instead of a fuse in such inexpensive devices, there is often a special breaking resistor. It performs two functions: the fuse itself, and also limits the charging current of capacitors.

 

Such resistors are made using a special technology, and the Chinese often replace them with ordinary ones with the same resistance, which is unacceptable. An ordinary resistor can ignite when the power is exceeded, and such a special non-combustible coating on top. What, in fact, is the whole “feature” of the manufacture of such resistors. In general, it is more correct, of course, to put a fuse and a thermistor to limit the current. But such a scheme will cost 2.5 kopecks more.

As I said above, in such consoles they save on everything, and electrolytic capacitors also fall into this savings. You won’t see any Hitachi or Rubycon here. Most often there are rootless noname capacitors.

Naturally, in the absence of active cooling, even with a very small consumption, they do not last long.

First HD 1702M refurbishment series

Here is an example from my practice. Prefix DEXP HD 1702M.

 

BP built-in. In use for a little over a year.
The nature of the breakdown: when plugged into the network, the indicator LED does not light up, the prefix does not respond to any manipulations.
We open the case and immediately see the reason.

 

The power supply filter capacitor marked LQ (it is not clear, the manufacturer’s name, or series), designed for voltages up to 16 V and with a capacity of 470 microfarads, is swollen. At the same time, I also changed the high-voltage capacitors, they were in order, but from the same manufacturer. After that, the prefix began to turn on and work normally.

By the way, even in such a cheap product soldering is excellent.

 

If normal parts were soldered …

The second series of repair DEXP HD 1702M, the final (hopefully)
A couple of months passed and I again complained about the work of the console. The nature of the malfunction: the image “freezes” at times, sometimes for a long time and only a reboot saves. Sometimes the problem occurs for half a minute or a minute and then everything works fine again.

The user suspected the antenna, but I disagreed. Location a couple of kilometers from the TV tower, in the zone of reliable reception, allows you to catch a signal even on a piece of wire inserted into the connector.

The prefix was disassembled and all the capacitors were soldered. As a result, one was found that outwardly did not cause any suspicion, but according to measurements, its capacitance instead of 100 microfarads was slightly less than 10 microfarads.
Yes, bad capacitors don’t always look like this:

 

After replacement, the device works great.

 

Just in case, I changed all electrolytic capacitors in general, which was necessary immediately, during the first repair, since we have very reliable Jamicons for sale in our city.

 

Well, what to do, the author is a fool and comes to him slowly.

It would seem that everything is very simple, but it is by replacing the electrolytic capacitors in such devices that most problems can be cured. Fortunately, they are inexpensive. Buying new capacitors cost me about 150 rubles.
In case of problems with the microprocessor, memory, the repair of such set-top boxes is impractical, as it requires a fairly high qualification and the availability of special equipment.

I forgot to say that climbing into such devices that are directly connected to a 230 V network is dangerous to health.