Cable TV networks for the smallest. Part 10: Troubleshooting KTV Networks
The final, most boring reference article. It probably makes no sense to read it for general development, but when it happens, it will help you a lot.
Content of a series of articles
- Part 1: General architecture of the KTV network
- Часть 2: Состав и форма сигнала
- Часть 3: Аналоговая составляющая сигнала
- Часть 4: Цифровая составляющая сигнала
- Часть 5: Коаксиальная распределительная сеть
- Часть 6: Усилители RF-сигнала
- Часть 7: Оптические приёмники
- Часть 8: Оптическая магистральная сеть
- Часть 9: Головная станция
- Часть 10: Поиск и устранение неисправностей на сети КТВ
So, your grandmother stopped showing TV. You bought her a new one, but it turned out that it wasn’t the receiver — it’s worth a closer look at the cable. Firstly, often wrap connectors that do not require crimping miraculously twist themselves from the cable, which leads to loss of contact with the braid or even the central core. Even if the connector has just been re-crimped, then you should make sure that at the same time not one of the braiding hairs is connected to the central conductor. By the way, the diameter of the central core is usually noticeably thicker than the hole in the receiver socket - this is necessary for good contact due to expandable petals in the connector. However, if you suddenly replaced the connector with one in which the central core does not come out “as is”, but passes into the needle (as in the parts shown by me in 5connectors for RG-11), or changed part of the cable and the new core was thinner, you may encounter the fact that tired petals in the socket will not provide good contact with the central core.
When measuring with a device, all this can be easily seen by the shape of the slope of the signal spectrum, which I wrote about in 2 parts . So we can immediately monitor the signal level (it should be reminded that according to GOST it should be at least 50dBμV for a digital signal and 60 for an analog one) and evaluate the attenuation in the low and high frequencies, which will give us hints for further searches for the problem.
Let me remind you: the attenuation of the lower frequencies is usually associated with problems in the central core, and the strong degradation of the upper ones indicates poor contact with the braid, and this is usually associated with crimping (well, or the general poor condition of the cable, including excessive length).
Having examined the cable with the TV connector, you should track it throughout the apartment: since the coaxial cable is not just an electrical conductor, but a waveguide, not only breaks and other mechanical damages, but also bends and bends are significant for it. It is also worthwhile to find all the signal dividers and calculate their total attenuation: it may turn out that before that everything worked to the limit and slight cable degradation led to complete inoperability. In this case, in order not to re-lay the cable hidden behind the finish, you can more correctly select the values of the dividers or put a small amplifier at the entrance to the apartment.
If none of this is observed and with the cable everything is in order to the very low-current shield on the stairs, then it is necessary to measure the signal level that goes into the apartment. If the level and shape of the signal at the tap of the subscriber divider is normal, then you should evaluate the difference between the values on the TV and in the shield and think about where and what we missed. If we see that the attenuation to the TV was some reasonable amount, but at the same time we see problems with the signal at the tap, then it’s worth moving on.
Having seen a problem on a subscriber tap, you should make sure that it is not the divider itself that is to blame. It happens that one of the taps immediately or gradually worsens the signal parameters, especially in dividers for a large number of subscribers (more than 4). To do this, you need to measure the signal level on another branch (preferably as far as possible from the problem), as well as on the incoming trunk cable. Here again, an understanding of what shape and level the signal should be is useful. The attenuation value indicated on the divider in the marking on the subscriber tap (for example 412 - 4 tap -12dB taps) should be subtracted from what was measured on the trunk. Ideally, we should get the figure that was removed from the subscription. If it differs by more than a couple of dB, then such a divider is better to replace.
If we see that the signal is already coming along the highway with a strong slope or a low level, then we will have to either familiarize ourselves with the riser project, or, by turning on the logic, estimate two things: the riser is built from above or below and how far from the nearest branch we are. The first one can be understood from where the cable comes from that is connected to the input of the divider and where the one from the output goes. It is usually not difficult to directly trace the trunk cables directly in the shield, if they are not visible, then you can go to the floor above (or below) and see what rating the divider is there. From the fifth partyou probably remember that with a distance from the beginning, the denomination should decrease. I wrote there about dividing the riser into parts (we usually call them “pilasters”, I’m not sure that this is generally accepted). Usually one pilaster spreads over 5-6 floors and at its beginning there are dividers with nominal values of 20-24 dB, and at the end - 8-10. When there is confidence that the problem is outside the floor, you should find the beginning of the pilasters and make a measurement from the main divider from which it starts. Here the problems are still the same: both the divider itself and the damaged cable and poor-quality crimping can have an effect. It even happens that after moving the connectors, the signal is restored (but more often disappears completely). In this case, you have to re-compress everything, and it will be just wonderful if the installers, having foreseen this, left a supply of cable. After all, when re-crimping it has to be shortened. The wrong crimping problem is very common on the RG-11 cable: this is either non-compliance with the stripping standard, which leaves the central core too long (as a result, the connector is loose and the cable may pop out), or the same, but because of too much section A (see the figure below).
Separately, it is worth saying that even the correct stripping will not protect against error if the crimp does not completely upset the connector and the central core does not enter the “needle” of the connector. In this case, the needle has mobility, if you shake it with your finger. When the vein entered well, it is impossible to move it. This must be checked for each untwisted connector.
With the divisors themselves in houses that are more than 10 years old, what can be known among collectors of large-scale models as the “zinc plague” can happen.
Photo from a-time.ru
Cases of dividers made of obscure alloys and located in poor climatic conditions can literally crumble in your hands when you try to unscrew the connector, or even just when moving the cables in the shield. And usually this happens when installers are working in the dashboard, handing someone the Internet, or some other intercom.
If the divider from which the pilaster starts does not fall in half, and the signal level on it is as bad as in the apartment, then you should find the divider on which the very first branching occurs and measure the signal that comes to us from the active equipment from the basement (or attic - as built). Having walked the riser in this way and not having solved the problem, you have to go look for active equipment and take measurements already on it.
First of all, it is worth paying attention that between the optical receivers and amplifiers there is also a distribution network built on the same principles as the risers, and therefore having the same kind of problems. Therefore, everything that is written above must be checked here, and only then sin on the good condition of the iron.
So, we are in the basement (attic, main switchboard), in front of a box with amplifiers.
And it happens ...
If there is no signal at all in the riser and there is a suspicion that the amplifier is dead, then it is easiest to determine which one is most sensitive to its temperature by touch. Even in severe frosts in unheated rooms, the working amplifier will be warmer than the environment, and from the burned it will blow cold. If the temperature difference is not noticeable enough, then an autopsy will definitely show that the power indicator does not light up inside the amplifier. Such an amplifier is replaced with a known working one, and subsequently it is repaired using a conventional soldering station, because almost all the failures are connected with banal swollen capacitors. When replacing amplifiers with remote power supply, it is necessary to disconnect the entire network to avoid a short circuit. Although the voltage there is not very large (60 V), but the current is the power supply that I showed you in the sixthcan give a considerable one: when you touch the central residential building on the building, a strong firework is provided. And if such amplifiers do not always successfully survive power outages at home, then with these special effects there is a non-zero probability to disable several more devices, which you will then have to look for throughout the house.
But it also happens that the amplifier is alive, but at the same time it gives a lot of noise to the network, or simply does not swing to the signal level set by the project (usually 110dBμV). Here it’s worthwhile at the beginning to make sure that the signal is not already corrupted by measuring the incoming signal. Of the characteristic incurable problems of amplifiers, the following can be distinguished:
- Gain reduction. Due to the degradation of part or all of the amplifier stage at the output, we have the same signal level as at the input (or more, but not enough for normal operation).
- Signal Noise. The operation of the amplifier distorts the signal so that the Carrier / Noise ( C / N ) parameter removed at the output goes beyond the norm and interferes with signal recognition by receivers.
- Scattering of the digital component of the signal. It happens that the amplifier satisfactorily passes the analog signal, but at the same time can not cope with the "digit". Most often, the MER and BER parameters described in 4 parts are outside the acceptable range and the constellation turns into a chaotic mess, but it’s also funny when, for example, the amplifier forgets about one of the modulation parameters and draws a ring or circle on the device’s screen.
When these malfunctions occur, the amplifier must be replaced, but there are troubles that can be eliminated by adjustments. Usually the signal at the output of the amplifier floats away and it is enough to reduce the value of the input attenuator. And sometimes vice versa, the amplifier starts to make noise due to the increased level at the input, then we press it with an attenuator. All adjustments should be made on one problematic amplifier, because if, for example, we lower the signal coming out of the optical receiver, then this will affect other, working, amplifiers and all of them will have to be manually reconfigured for the changed parameters. Also, due to overpowering, the figure may also crumble (with a slight noise on the analogue). Amp settings I described in detail in the sixth part .
You can try to correct the settings and tilt. Often, during commissioning of a newly built network, a large initial slope is not required to ensure good parameters at the ends of the trunk. But over time, due to degradation of the cables, it may be necessary to increase the slope, which, as we recall, increases due to the lower level of the lower frequencies, which will need to be compensated by the attenuator.
Optical receivers most often die just for food. If at the input it has a sufficient signal level (which - I wrote in part 7), then with the release of problems usually does not arise. Occasionally the same thing happens - increased noise and insufficient output level, but due to the stinginess of the settings, this is usually not amenable to treatment. The diagnosis is the same - we check whether it is warm or not, after which we measure the signal from the output.
Separately, I will say about the test connectors: it is not always worth trusting them. The fact is that even if everything is working properly, then a signal underestimated by 20-30 dB may not have the problems that the “real” output has. But it often happens that problems in the tract occur after the test tap, and then everything seems to be fine - but in fact it’s horrible. Therefore, for complete certainty, it is always worth checking the exact exit that we are looking at the highway.
You can talk a lot about problems and their search on optics, and it is very cool that they already did this before me: Welding of optical fibers. Part 4: measurements on optics, taking and analysis of reflectograms . I’ll just briefly say that if we see a signal drawdown on the optical receiver and it’s not connected with something like this:
We have cormorants in St. Petersburg - you yourself know. And they’ll get optics underground.
cleaning or replacing the final patch cord can help. Sometimes it happens that the photodetector or optical amplifier degrades, here medicine is of course powerless. But in general, without detrimental external influences, the optics are extremely reliable and problems with it, as a rule, come down to a tractor grazing on a lawn nearby.
In addition to the obvious problems with power supply and connectivity with sources over IP networks, one of the main factors in the operability of the headend is weather. Strong winds can easily tear off or deploy antennas, and wet snow, clinging to a satellite dish at times worsens the reception quality. It is difficult to deal with this, because the antennas are located as high as possible, where the weather is raging and even the anti-icy heating of plates does not always save, so sometimes you even have to clean them manually.
PS That concludes my brief excursion into the world of cable television. I hope these articles helped broaden my horizons and discover something new in the familiar. For those who have to work with this, I recommend to deepen the book "Cable Television Networks" by SV Volkov, ISBN 5-93517-190-2. It describes everything that is needed in a completely accessible language.