TV / Cable fault

[Paul205]

Dear All

I have a Sony TV which is around 10 years old and in the last 2 weeks has started to display a fault message which is intermittent.

The TV is fed from an Antiference amplifier in the loft and the fault message is in place when the amplifier is in or out of circuit ( I have bi-passed the amplifier by connecting the TV directly to the roof aerial). When I move the cable from the amplifier to the TV to another 'output' on the amplifier the fault is still in place.

The TV sometimes has a message stating 'no signal'. I haven't remembered to ascertain whether this message is on some / all channels but it seems intermittent and just before seeing this message for the first time there was heavy pixilation in the picture and then the message appeared. No birds were sitting on the roof aerial. When the fault is in place it doesnt appear on other TVs in the house (which are all fed from the same amplifier). When the fault is in place the DC voltage on the aerial lead is 8.98, AC Voltage is zero and centre core to outer core is open circuit, this is identical to readings from other TVs where the fault isn't in place.

Just priot to writing this email I have retuned the TV 'automatically' (I cannot remember if the fault was in place at the time of the retuning) which identified lots of channels when undertaking a 'digital' search. It did not identify any channels when undertaking an 'analogue' search and asked for an aerial to be connected.

I would appreciate your thoughts. I have fitted a new aerial plug at the TV (which seems to work as this is what I used to take the above readings). I have also closed up the female socket in the TV where it accepts the male plug pin. The TV is swivelled daily on a '2 joint' bracket at the wall but this doesn't seem to create or remove the fault. The aerial lead does seem to have 'snagged' in the loft as I cannot pull more cable through. The cable is genuine Webro CT100. I would appreciate any thoughts on this please.

I would rather not remove the TV from the wall at this stage to move it to another fully functioning aerial lead without gathering thoughts here first

 

[Lucid]

Hello Paul, and welcome to the Audio Visual section of DIYnot forums.

You've done some useful troubleshooting already, so thanks for that info; it helps.


I'm going to recap on the main points:

* Roof aerial

* Intermittent issue

* It's on the Sony TV from any output on the Antiference power splitter. (Question: does it appear on any other TVs?)

* No Signal for some channels only (on the Sony TV)

* No Signal message with the splitter in place or bypassed

* Also seeing heavy break-up / pixelation of signal intermittently

* Auto tuning finds lots of digital stations - No analogue stations

* New aerial plug fitted at Sony TV end

* No effect on the issue from the articulation of the TV bracket for the Sony


Some additional questions from me:

How long has your current roof aerial been installed?

Do you know which transmitter your aerial is pointing at?




Initial thoughts:

First off, I'd be very surprised if the Sony TV is at fault based on what you've described so far.

Not getting any analogue stations picked up is entirely as expected. We haven't had analogue TV broadcasts now for a few years, so the only sources of analogue RF signals would be things such as a VCR or older Sky box RF out, or an analogue RF modulator from a video sender. You haven't mentioned those, so my guess is that the Sony TV just gets a signal from the amplified splitter rather than picking up analogue RF along the way from some intermediate box. Incidentally, many TVs have the option to search for Digital Only, so if you do auto retune then you can cut down the waiting time by selecting that option.


Bypassing the amplified signal splitter was a smart move on your part. It suggests to me that the issue is something to do with the aerial or cabling down to the amp point.


Narrowing things down:

If I was on-site diagnosing, my first actions would be to measure the signal level and quality for the various muxes from your local transmitter. If you're not familiar with the term 'mux' it's part of the way digital TV works. It stands for multiplex. This is where one transmission frequency carries a group of channels. Another mux frequency carries another group of channel, and so on.

Where you live will determine whether you get a Freeview Lite service from a minimum of three muxes or the full service Freeview with as many as 8 muxes.

Measuring the signal would show me whether there's enough coming from the aerial to drive that Sony TV properly. I suspect that there isn't. Here's where things move from being 'black-and-white' / 'yes-or-no' to shades of grey. I'll explain...

When we had analogue, losing some signal strength would make the picture go grainy. The bigger the loss the more grainy it got until eventually it was just snow. Digital doesn't work like that. With digital, losing signal quality and strength won't make much of a difference to the picture until it reaches a critical tipping point. We call this The Digital Cliff. When you get the 'no signal' message displayed on the TV, the signal has already fallen off the cliff. When you're seeing pixelation, the signal is teetering on the edge.

Everything in the digital aerial system plays a part in affection signal strength and quality: The type of aerial and whether it's still a good match for your local transmitter frequencies; the alignment of the aerial; the age and condition of it; the condition of the cable; how much the amplification/splitting is affecting the signal; the signal loses due to the lengths of cable; the quality of any joints/plug-end-fittings and of course, the sensitivity of the TV's tuner. On this last point, they're not all the same. It's entirely possible to swap a TV and find you get more or fewer channels depending on the tuner's sensitivity.


Pulling all of this together:

The signal path down to the Sony TV could have been just scraping by for years, but something changed recently such as a Freeview retune or the aerial moving a little more which has pushed certain muxes right to the edge of the digital cliff. Throw in the effect of atmospheric conditions on signal levels which, by their very nature, vary from day to night and with the weather and there you have a recipe for an intermittent problem.


Solutions:

Where there's not quite enough signal coming off the aerial, then adding extra powered amps won't fix that. They'll make a bad signal 'louder', but it'll still be a bad signal, so although the strength will go up, the quality - which is the essential part - will go down, and that will make the TV tuner's job even harder.

The answer here is to get the aerial sorted. Even if you're current aerial isn't that old, the UK has gone through a lot of channel changes which means that for a lot of us our relatively recent aerials are no longer a good match for the local transmitter channel spread.

Contact a local aerial installer. Make sure it is someone who can do a metered (measured) installation. You're looking for a signal level of around 55dBuV at the TV's aerial socket, but more importantly, the signal quality as reported by your TV should be a minimum of 80% across all the different muxes.

 

[Paul205]

Thanks for the comprehensive effort put into such a detailed response.

The TV antenna on the roof is 15 years old and was fitted by a reputable local contractor.

The issue does not occur on any other TV in the house (they are all fed by the amplifier)

The TV is pulled out from the wall on the stand each evening so maybe this is putting mechanical stress on the centre core of the cable in some way