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Step-by-step guides to understanding digital TV

ABC guide to...

Fault finding and diagnosis

summer 2010

Freesat Freeview euro tv skyThe best kit, The best programmes

2 What Satellite & Digital TV summer 2010

When your kit goes wrong, a few simple rules can help you identify the cause of a problem. Even if you can’t fix it, it will help the people who can

unfortunately, like other things, TV systems can go wrong. A satellite or DTT system that is

working perfectly one day may no longer do so the next. many problems that develop are quite easy to fix – just so long as you know what’s causing the trouble. That’s the hard bit, because there are many stages in the signal’s path from the airwaves to your TV screen where the problem can hit.

The most common faults are no signal at all, missing channels, or a lousy picture, and most of us will experience one or the other at some time.

so when the screen next goes blank (and it isn’t because the satellite has stopped transmitting – yes, even that can happen), don’t panic, and don’t immediately phone for an engineer. Just take a little time to see if you can’t diagnose the problem yourself.

What to do with no picture/soundWhen there’s no picture or sound

from the TV (and the mains is on, and the TV works when checked with another signal source such as DVD), check your receiver. Are there indicator lights or a front panel display working? If not, it’s possible that the receiver has failed – most likely its power supply – but turn it off at the mains (and check the mains plug’s fuse) and then on again; it may simply have crashed.

Next, examine the cables to the TV, checking they all make a firm connection. Try swapping from, say, an HDmI connection to scart to see if a picture materialises that way.

If you can get the receiver’s menus up on the TV screen (or even a message saying ‘no signal’) but no broadcasts displayed then the receiver itself is probably OK. Now’s the time to see if the antenna cable has been broken, cut or disconnected, or if the dish or aerial has been knocked off alignment, fallen off the wall, or even been stolen!

It’s also possible that the fault is still with the receiver, or (for a satellite system) that the LNB has failed overnight

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(unlikely). This can be established by trying your receiver connected to another working dish and another receiver with your antenna.

If everything works on its own, but not all together, then double-check that antenna cable. It may have corroded away invisibly inside.

What to do when some channels go missing

more usual than a complete lack of channels is the mysterious departure of just some channels.Continued reception of other channels would suggest that the receiver is fine, but do check that your viewing card (if required) is present and paid up, and it’s not been inserted the wrong way up. If you can easily substitute a fully working receiver, that can rule out a diagnosis of a receiver fault. For DTT systems, are you sure that

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the transmission frequencies have not been changed?

missing satellite channels are not usually caused by a misaligned dish – although if you find that you have lost, say, all Astra 2D channels but still have the others from 28.2˚e, it possible that your dish has shifted just enough to stop reception from the much weaker 2D satellite only.

so the real task is to see what the channels that you are missing have in common. Write a list of the ones missing along with their transponder parameters, particularly frequency and polarisation.

Missing satellite bands or polarisations

If your list reveals that satellite channels lost are all of one polarisation (horizontal or vertical), or one frequency band (Low Band in the range 10.70-11.70GHz,

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summer 2010 What Satellite & Digital TV 3

ABC guide to...

Many outside sources of interference can ‘attack’ your signal at different stages in its journey, and with different effects

or High Band from 11.70-12.75GHz) then the problem is likely to be with the LNB, as that is where the reception of each polarisation and each frequency band is determined.

Water in the LNB, or simply old age, can cause one polarisation of one band to fail, and replacement of the LNB is usually the only solution (a simple job if you can access the dish safely). Occasionally, water getting into the LNB cable can have the same effect, but you can check that the cable is at fault by running a new length of cable (temporarily, to the nearest window) from the dish to the receiver.

A missing transponder/multiplexIf you find that all the channels

missing are on the same transponder or multiplex, or perhaps two adjacent in frequency, but other satellite signals of the same polarisation and the same frequency band are received loud and clear, then the problem is either the receiver (mis-programmed or faulty – again, check with a working substitute) or with local interference swamping reception at that frequency.

The problem doesn’t have to be at the transmission frequency; it could be at the LNB IF frequency (for satellite), another intermediate frequency internal to the receiver, or even a harmonic of one or other of these. This makes it hard to predict what may be causing the interference, but typical suspects include local point-to-point microwave land links and mobile phone masts.

The only solution is to try better-shielded cable and/or move the antenna so it is shielded from the interference (but can still ‘see’ the satellite or transmitter). This usually involves a lot of trial and error, and may require professional help.

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What to do about on-screen faultsYou may receive all the channels

you expect but suffer a sudden drop in quality. Problems due to a weak signal may have similar causes to missing channels and the same diagnosis process should be followed. signal weakness usually shows itself as pixilation or ‘blocking’ of the picture and occasional dropouts of sound.

Don’t forget to check for new growth of trees or even new buildings erected between your antenna and the satellite or transmitter. It’s even possible to have enjoyed DTT reception previously via a reflection from a building – which considerably worsens when the building is demolished!

Picture degradation in the shape of increased graininess, poor colours, or lines or bands of snow across the screen, is less likely to be a signal problem but instead to be a fault with the TV or the connections to it.

Check the connecting cables are plugged in correctly and that in the spaghetti at the back of the set, you keep apart signal cables and mains cables as much as possible. use better connecting cables if necessary.

Faults that come and goNot all signal problems manifest

themselves continuously. some picture problems appear just briefly. some occur only very infrequently and can be ignored (they usually can’t be fixed, anyway) but some occur more often although only lasting a short time, so are more annoying.

Brief picture break-up, flashes on the screen or bursts of ‘static’ in the audio can come from many interference sources, including a mobile or DeCT phone,

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glossaryInterferenceAny additional unwanted signal that interacts with a desired signal to degrade its proper reception. Interference can be caused by desirable transmissions close in frequency or geographically, reflections of the same signal, or by spurious radiation and wideband noise from other devices.

PolarisationOrientation of the oscillating electromagnetic waves of a broadcast signal. Satellite and terrestrial TV signals are usually polarised vertically or horizontally

wireless home network, microwave oven, taxi radio, radar detector, power tool, thermostat in a heating system, fridge or freezer, and even a street light or fluorescent light.

You can even get interference from other equipment under the TV – a DVD player or satellite or DTT receiver can produce radio signals that interfere with the TV itself – so try moving the boxes from under the TV and away from one another.

using better-shielded cables (behind the TV and to the dish and aerial) can sometimes solve the problem, as can moving your equipment away from the source of the interference. Otherwise, you should track down the source and, if possible, simply switch it off.

And don’t forget to check your satellite dish for intermittent problems. many apparently random picture fluctuations have been traced to an itinerant pigeon perched on the LNB n Geoff Bains

Continuous interference from microwave links, mobile masts, etc

Intermittent interference from DECT phones, taxi radio, thermostats,

etc

Continuous interference

from other AV and computer

equipment