TV Problem

[PaulT]

I have an Avtex tv connected to a Sky box. For the last few weeks the tv has cut out intermitently. It works fine for a while and then I get interference, then the blue screen with "No signal or retune". After about 10 seconds the picture returns to normal for a while, then the same thing happens. Sometimes it will work fine for an hour or more then the fault re-occurs. Othertimes it happens every minute or so.

My first thought was that the tv was on it's way out so I tried another - same thing. I've tried a different Sky box - same thing. I connected the tv direct to the Sky box (to eliminate the caravan cabling) - same thing. I've tried different leads, operated the tv on mains and 12v - nothing changes.

The only thing I haven't tried is to replace the lead from the dish to the satelite box as this is a bit more involved. However, usually, if this is at fault I'll get the "no satelite signal is being received message" which I'm not receiving.

Can anyone suggest what might be the problem?

 

[ProfJohnL]

Hello Paul,

Based on your description, and a logical assumption, if the fault remains when you change the TV and the sky box, it leaves only the aerial and down lead

Leads can become intermittent, but normally the fault will only clear if the cable is moved, so that leaves the aerial, and I assume as you have a Sky box, its a satellite dish. Again the description of a periodic fault which occurs and then resets, is indicative of some active component often associated with an over temperature conditions.

I don't use a satellite receiver system, but I believe the LNB is usually powered. So it could be power supply for the LNB, or the LNB its self.

 

[philip_stevens]

As the Prof says, it could well be the LNB (Low Noise Block) that is at fault.

There are amplfiers within the LNB that get the (I think) 5 volts from the satellite box via the co-ax cable. It is possible that the amplfiers are intermittently failing. The LNB is not too expensive to replace.

I just googled "LNB supply" and Wikipedia came up with a good answer.

Worth a try.

 

[Woodentop]

As the Prof says, it could well be the LNB (Low Noise Block) that is at fault.

There are amplfiers within the LNB that get the (I think) 5 volts from the satellite box via the co-ax cable. It is possible that the amplfiers are intermittently failing. The LNB is not too expensive to replace.

I just googled "LNB supply" and Wikipedia came up with a good answer.

Worth a try.

Actually slightly more complex than that.

The signal comes in two frequency bands and two polarities. To change from low frequency band to high a 22KHz tone is sent up the cable. To change polarisation the supply on the cable is changed from around 13V to nearer 18V.

The LNB may well have a supply regulator inside it which fixes the voltage at something lower than 13V - maybe 10V but that is a guess. Even though the LNB is in open air and thus cooled, the more voltage drop for a given current the more power that the regulator has to dissipate, but you need typically 2.3V headroom for the regulator to work, so 9V or 10V is probably not far out.

However if there is an issue with the connection that makes it slightly resistive then there could be voltage drop across that connection which could be enough to take the supply out of regulation and stop the LNB working properly but more likely it could reduce the supply enough that the polarity switching is intermittent and that could cause the dropout.

Check ALL connections on the dish cable very carefully to make sure they are good and clean and there are no shards of the braid catching the centre core. After that go for a LNB swap. If it is a Sky dish they you will need one with a Sky dish mount - and there are two types/sizes of those depending on the age of the dish. If it is not a Sky dish then the LNB will be collar mounted - but there are two sizes of those as well so it is important to get the right size. Inverto LNBs are pretty good and can be had on eBay for a little over a fiver with free delivery for a single output unit. (At that price I would carry a spare!)

 

[PaulT]

Thanks for the replies. I will try changing the LNB and the cabling as I have spares. The reason I haven't tried this before is that the error message isn't what I normally get when the LNB and/or cabling is at fault e.g. "No satelite signal is being received". I'm getting "No signal or retune" which normally means that the fault is between the Sky box and the tv. Would a faulty LNB give me this message?

The only other explanation I can think of is that it's some sort of interference. I've turned off all the mains appliances in the 'van but to no avail. The only other piece of equipment which is near the tv is the solar panel and its control box. Would it be possible for these to produce interference if they were faulty?

The really annoying thing is that the fault is intermitent. First thing this morning there was no problem. Just before 11.00 we came in to watch the Rememberence service and the fault ocured every minute or so. As far as I could work out there was no difference in conditions inside the 'van and the weather was the same (cold and rainy).

 

[Dustydog]

Check thecable(s) attached to the LNB. Does yours gave rubber o ring seals?
If not I suggest you remake the cable ends , cutting back to clean copper ends and Earth. Use the o rings too.
Is your Sky Box an Amstrad ? Earlier ones had problems. I suspect it is cabling. Usually an LNB either works or nothing.

 

[PaulT]

It definately is not the LNB or the cabling. I know this because whilst I was changing the LNB and remaking the connections the tv was on. The screen was showing "no satelite signal being received" as would be expected. However, whilst I was making the changes the interference occured and the display changed to "no signal or retune". After a few seconds the interference disappeared and the "no satelite" message came back.

Needless to say changing the LNB etc made no difference.

I've disconnected the solar panel and isolated the caravan completely from the incoming mains. Running the tv on 12 volts makes no difference. I've run an extension lead from the barn and connected the tv to this - no change. I have to assume that the interference is coming from an outside source but how near would a faulty electrical appliance have to be to cause this?

I've got a small media player and if I connect thisto the tv I get no interference whatsoever.

 

[philip_stevens]

When you find out what the problems is/was, let us all know please.

 

[Buckman]

You do not say the age of the Avtex TV or if you are using a HDMI lead or a flylead from the RF out on the Sky box. If from RF out on the Sky digibox, then you may have to adjust the frequency by entering the main menu and then setting it to ch60 instead of ch65.
On some Sky boxes I know there is an issue where the HDMI lead plugs into the box and the fault lies with the socket and not the lead. On a large screen Tv it will appear as if the picture has dropped out for a second or two. On a small TV, the impression is that the TV has switched off momentary and then come back on.
We had a 1 year old Avtex where it appeared to be switching off for about a second or two. We discussed it with Avtex and they got back the TV to check. A fault was found and the TV replaced. Sadly the replacement went dead within 6 months so they replaced it again.

 

[glenno]

Try switching off your mobile phones. I had issues with my mobile causing tv interference.

 

[Buckman]

Technically the signal from a mobile phone should not interfere with the TV signal as the frequencies are so far apart.

 

[PaulT]

Latest update. The Sky Box was in the awning. I've moved it into the caravan but other than that all the wiring remains the same. There is still a small amount of interference but nothing like it was before. My thoughts are that the caravan is acting as a sort of Faraday cage thus screening out interference from an outside source. Is this likely? I've approached my (2) neighbours and neither of them have installed any electrical equipment recently so where the interference is coming from is anyones guess but, for the time being, the problem has gone away.

 

[PaulT]

Problem solved!
It was the mobile phones causing the interference - thanks Glenno for pointing me in the right direction.

 

[Craigyoung]

Don't understand how the mobiles can interfere with a TV ours arn'nt switched off and we don't have any problem with interference, always have a good reception and decent picture ?

 

[HenryB1]

Just back from 10 days away. Good signal on our Avtex but picture breaking at times if my wife's Samsung mini (on Vodaphone) was anywhere near tv. No problem with my phone Motorola G4 on Virgin (EE).

 

[PaulT]

We never had any problems with our old phones (Samsung and Doro) but we've both now got new Huawei's. In the past (with other phones) we've had some interference on the radio but nothing so severe as we experienced with the tv.

 

[Dustydog]

Well done Glenno and Paul
Problem solved.
See this for a more detailed explanation
https://www.gov.uk/government/news/eliminating-interference-with-tv-signals-from-4g-mobile-services

 

[Woodentop]

Quite obvious if it is the phone.

Vodafone by default works in the 900MHz band.
Virgin works on the EE system which by default is in the 1800MHz band.
A signal can overload the tuner input just by its presence - it does not need to be frequency related. As Vodafone is much nearer to the TV band it is more likely to cause interference.

For the record, 4G works in the 800MHz band which the TV band for channels 61-68. Whilst we use the 800MHz band for phones other countries still use 61-68 so as the tuner is a world-wide product it will be open for frequencies up to 863MHz - well into the 4G band.

Satellite receives in the 10.7-12.75GHz band but it is down converted in the LNB to 950-2150MHz band. With a local signal close enough it could conceivably also upset a satellite box.

 

[PaulT]

Reading DustyDogs posting above, it seems as though the UK government was aware of the problem of mobile phone intereference and took steps to do something about it. Here in France that appears not to be the case, the staff in various phone shops that I've been into just look blankly at me when I explain the problem. Does anyone know if the devices to plug in the back of the tv are still available in the UK and, if so, what are they called?

 

[Hutch]

Hi Paul, not sure what devices, are they to boost signal or what. ?

 

[PaulT]

I've got no idea EH, I'm only going from the attachment to Dusty Dogs posting quote "The vast majority of affected households will simply need to fit their TV with a filter that will be supplied by the help scheme. Extra support to fit the filters will be available to the over 75s and people who are registered disabled."

 

[Hutch]

Ok Paul , just to keep the thread going and hopefully others can help, when I was working in Nigeria, in 19filthy lie, we used to have a TV booster from the ariel to the TV that would stop interference but that was Analoge stuff. Lets hope someone comes back with some knoledge.
Hutch.

 

[Woodentop]

What is being talked about is a plug-in filter that will apply quite sharply attenuate the signal above about 790MHz. The effect should be to reduce any signal getting down the aerial and hopefully preventing overload in the early stages of the tuner. They are readily available from Amazon for less than £6 (needs F-type connectors) and Maplins (£12.99). If you are using one in the caravan it better to fit it between the aerial and aerial amp* to stop the amp being overloaded as well. *This only applies to Status aerials with orange writing on them - purple writing indicates the amp is in the aerial not in the box.
A phone causing interference is because the phone transmitter switches rapidly on and off - a technique known as Time Division Multiplex if you want to Google it - and the fast rising edge of the transmitter signal as it switches on can produce wideband interference that gets into audio and video circuits as well as those handling signals and can play havoc.

 

[Dustydog]

Hutch
If you re reread my link you will see HMG have been aware of this issue since before 2011 when they decided to release to the phone companies the licence to use the 800mhz spectrum for 4G. They have made it very clear ALL costs to avoid interference rests with the mobile phone companies. Things have moved on and I don't know where we are exactly today . I wouldn't start spending my own money just yet to find a solution.
A solution is out there and it should be free!
Maybe let the fingers do a bit of walking over a G&T

 

[PaulT]

Update on this - whilst back in the UK at Christmas I bought a filter from BandQ which said it would stop interference from 4G signals. Plugged it in and.......it doesn't work!

If I can explain my set up someone might be able to help. I have an Avtex tv, with a Sky box in the awning. I have a "magic eye" in the system so I can change channels from inside the caravan. I've tried disconnecting the magic eye and the result is the same. I've tried the filter in both the outlets on the sky box.

Something else which might be relevant - the Sky box is connectef to the aerial socket on the TV (i.e not via a Scart lead). The "source" on the tv is shown as analogue. Is this right? It won't work on the digital source although when I'm back in the UK with the 'van I get tv via the status aerial and the source is then digital.

Anyone got any ideas?