Tv un-tunes itself

16 Jun 2006
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United Kingdom
An elderly family friend had a 32" Samsung tv that died.

she replaced it with the current model, from memory, that is the UE32T4307.

About 2 weeks after setting it up I had to go back to retune it, she had lost the likes of ITV and BBC1/2.

2 weeks later had a phone call to say that she had lost those channels again..

The day that I committed to turn up, I got phone call to say that the channels were back. Regardless, I told the tv to search for channels

She has a loft aerial which, exclusively feeds the downstairs tv. Noting has changed other than the single TV.

I think it unlikely that that the aerial has changed since the new tv was turned on, but I would appreciate any sensible feedback.
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10 Sep 2013
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United Kingdom
The signals for digital TV reception are subject to something known in the business as 'the digital cliff'. What that means is much like walking blindfolded towards a sheer drop, everything is okay right up to the final step when.... arrggghhhhh....Splat! it's not.

The TV signal being received can be close to the edge of the digital cliff. It's marginal, but still appears to give just as good a picture as if it was strong. However, all it needs is a small variation in reception conditions and suddenly..... arrggghhhhh... Splat!

Things that can change reception conditions include high pressure weather systems - hot weather - where distant signals from other transmitters normally out of range get reflected and appear at the affected aerial. Now the TV is looking at two competing signals. The marginal signal gets weakened until the weather changes again.

[Something to try] Quite a few TVs have the ability to scan for new signals whilst the set is in standby. This Samsung has that too. Look in the menus at Settings > Broadcasting (Auto Tuning Settings) > Standby Auto Tuning. If the option exists to switch this feature off then do so. Changing this alone might well fix the current issue, but it won't solve the underlying reason why the channels were disappearing in the first place. It's a sticking plaster solution.

"Do you just want me to fix it, or would you prefer to know how to fix it?" - Troubleshooting

Signals can also be too strong. It's slightly odd to think of having too much signal as a bad thing, but with digital TV it's definitely a thing. The most common cause is too much amplification. This was something of a problem back from the early days of the introduction of digital when the signal power for it was low compared to analogue. Amps were added. However, when analogue was switched off and the power of digital ramped up then the amps were no longer needed but still in place.

With analogue TV it was harder to over-amplify, but not impossible. With digital though having too much amplification (signal Strength - S) actually reduces the signal Quality measure (Q). Reducing the amplification improves Q. It's Quality that is the most important element in getting a good usable signal for reception, not strength from additional amplification.

In it's simplest form, Quality is the measure of signal versus background noise. It starts with picking an aerial that's a good match for the local transmitter. This relates to where in the frequency band the local transmitter groups its stations (muxes). The frequencies in megahertz can also be expressed as RF channels, and so 474MHz is RF ch21, and 850MHz is RF ch68. (RF ch49 is 698MHz).

An aerial will be a poor match where it is optimised to receive from the higher end of the transmission range when the local signals are grouped in the lower portion. This happens quite frequently now that the Government has sold off the upper 1/3rd of the old transmission range. RF channels 50-68 are now used for mobile phones.

Lots of people bought Hi-Gain Wideband TV aerials without realising that all the "Hi-Gain" was concentrated in the upper 1/3rd of the old ch21-68 range. We're now running on ch21-49 and those aerials don't perform that well in this range. They still work to a degree, but the signal coming off them is much weaker. That means there's less signal compared to any background noise, and so there's a lower tolerance to variation in reception for changes such as high pressure.

Since the reception doesn't stop working completely, then the aerial is thought of as still working fine - after all, it doesn't appear broken, so why suspect the aerial, right? It's obviously the new telly / recorder / 'booster' / coax lead etc [add or delete as appropriate]. The truth is it could be a variety of things, and sometimes several of these things combining.

- The aerial is no longer a good match for the local transmission since the 800MHz and 700MHz clearances
- A shop-bought moulded coax fly lead has poor shielding, and so is allowing in more noise and reducing the system's tolerance to signal variations
- the same moulded coax lead has a crack in the core conductor. It still works because of something called capacitive coupling, but the signal level is much weaker
- there is no moulded fly lead. It's a plug on the end of the coax attached to the aerial, but the plug is loose and swapping over to a new TV has made the issue show up
- the new TV's tuner is less sensitive that the previous one, and again the system is now more sensitive to changes
- The TV has an always-on-by-default auto tuning feature that finds or loses stations. The old TV never had this, so it ignored any minor changes

This isn't an exhaustive list, and for someone with experience and the right gear then they can troubleshoot most of this in a few minutes. I've done the same walking in to jobs. Bad fly leads - especially the moulded ones - is a common issue.

Where the aerial is mounted indoors in a loft then it's already working with a handicap. The tiles/slate and gable ends will restrict the amount of signal it's getting. Add a couple of changes from the TV swap and hey presto, you're right on the edge of the digital cliff.

Start by disabling the "auto update" tuning feature on the TV. If you want to know how to diagnose further then drop a reply here and me or another FM will give you some basic tips. We can also supply hardware such as properly shielded coax fly leads and cheaper than the crappy moulded ones from the DIY sheds.

A word on tuning: If you can, find out the RF channels being used by the local transmitter. There are probably 6 or fewer. Rather than retuning, try doing a manual tuning for the one that's missing.
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