TV - bad reception on one set with one socket

[TD]

Hi all,

I hope this is vaguely on-topic. This is a really weird one, hoping somebody can help. We've moved into a new flat, which has been wired up with a Loftbox amp to distribute within the property. We're only using the terrestrial signal. We bought an LG M2794DP TV[1] for use in the master bedroom, and it's getting poor reception from the aerial socket in there. The analogue looks like it has inteference, and the freeview is non-existent.

We had the installers in and they were flummoxed. I saw with my own eyes, when they plugged their tester into the bedroom socket, the picture was great, with a signal level between 60-70, and he claimed everything looked normal. Also, we plugged another TV into that socket and it was fine. We also plugged the new LG TV into the master socket in the lounge and it was fine. The Loftbox has been changed, no difference.

The installer phoned a friend who suggested that the TV may be picking up interference from something else. In this bedroom, our mobile phones don't work well, and nor does the DAB radio, but I'm not sure if that is a red herring. We always assumed that the bad mobile phone reception in that room was due to the building structure, but maybe we were wrong. And besides, can interference affect a 3G signal? This is a pure guess, but even the "professionals" are running out of ideas.

This is the second one of these TVs that we have had, having sent the first one back assuming it was faulty. It doesn't make sense - to recap:

This TV has a bad signal in a certain socket, but appears to work in the main socket. Another TV and the installer's test box "thing" work fine in said socket, and show good signal strength. The "dodgy" TV works fine when connected to the flat's incoming feed i.e. upstream of the loftbox.

I've tried turning off all mains circuits apart from the one powering the TV - nothing. I'm at the end of my tether.

How can we get to the bottom of this? To paraphrase Spinal Tap, you can't dust for interference. We're absolutely desperate, so any advice is much appreciated.

[1]

[TD]

Oops, bad form, but I forgot some additional information. Forgot to say that the installer has fitted an attenuator between the Loftbox incoming feed and the Loftbox itself, as the signal was a bit strong. I watched him turn the attenuator down and up and at no stage did the signal to this TV become anywhere near clear.

[The Natural Philosopher]

Ok...

this may make some sense.

IF that particular TV has a totally crap input stage and is entirely mismatched to the 75ohm load that it should be AND that particular feed has a bad connection somewhere then you may get resonances in the cable.

First thing to do is to plug that TV directly into the distribution amp on the socket that it would be on in the bedroom. I.e. to take that piece of cable out of the equation.

If that works, you have a dodgy cable. If it doesn't, its a dodgy amp.

Typically bad connections in belling-lee connectors are the norm rather than the exception.

I doubt its interference.

Its far more likelyy a resonance issue causing low signal.

[TD]

I tried this. It was no better, so the cable must be fine. As for the Loftbox (amp) being dodgy, I'm not convinced - as I said in the OP, it has been replaced, and other TVs work fine connected to the Loftbox outputs. I just tried this TV on a Loftbox output in another room (second bedroom) and it was equally crap. Yet I normally have a 37" LG LCD and MythTV box running from that socket with no problems.

What con I try next?

Is it? I'm still stumped.

[marpate1]

Hmm, does the _faulty_ TV have an option to enable dc output on the aerial (to power an amp ... on my LG, I think that this is selected when trying to tune the digital TV? Try disabling this. It may be that the coax in the bedroom is fine and the dc gets to the loftbox and does something untoward whereas the assumed good cable in the lounge is actually dodgy (or capacitively coupled somewhere) and does not maintain dc connectivity to the loftbox.

I guess to save hunting through the menu's, perhaps just try the centre pin of the antenna connection into the TV in the bedroom (ie, no shield) and see if it makes any difference.

If it helps, I'm stumped too :)

Cheers, Mark

[TD]

Thanks for the input (drumroll). My results are: No DC output option anywhere that I can see. No difference with a cable with a cut shield.

BUT!

The aerial socket on the bedroom wall is actually a TV/Radio dual socket. Just for a laugh, I plugged it into the Radio one, and what do you know (yes you guessed it)... The analogue picture is passable, and the Freeview is working!

Obviously I'm happy that it's "working" but I'm a bit nervous that all is not as it should be - I would after all expect it to work with the TV socket, not the Radio socket.

I don't know anything about these "diplexer" sockets (as I believe they're called from googling) - with this new information, can anyone elaborate on what might be happening, and offer a "proper" fix?

NB It's probably like one of these:

the description of the item, it boasts "Non Isolated (passes DC voltage through TV connector) - fully compatible with the tvLINK Magic Eye and Loftbox systems". I assume that my wallplate is similarly non-isolated, and I wonder if whatever DC voltage it's passing through, is causing the problem with my TV - maybe I need something to "isolate" the connection? Not sure if this explains why it works with the Radio socket though.

[marpate1]

Ah, nice one! Presumably this is a PCB held in with a couple of screws and when installing they removed the PCB to more easily attach the coax and reinstalled wrong way around?

Either way I would not be too concerned. I assume that the unit had simple capacitor/inductor filters to create wide bandpass filters for each output from the single coax. The fact that it works with the radio hole shows that the wrong filter is being used.

Cheers, Mark

[marpate1]

OK .. I jumped in too early here... didn't wonder why other TV's work and not this one .... strange.

[Al]

First thought: Try uk.tech.digital-tv :)

Second thought: I went through a lot of agro with my parents bizarre mix of aerials, splitters, VCRs, TVs. I found that a cheap'n'cheerful 7 quid aerial amp between dodgy feed and dodgy TV cured a host of problems by putting some isolation between the dodgy TV and the rest of the aerial wiring.

Al.

[The Natural Philosopher]

So the TV doesn't work on ANY of the box outputs. I thought you said it did..

try removing the attenuator from in front of the loftbox.

Its not normal to back down an aerial for a distribution amp. They have very little gain. It might be of course that you are overloading that TV..in which case put and attenuator in its feed.

[The Natural Philosopher]

Ok its being overloaded in all probability.

Fit a 10db attenuator and try again.

[The Natural Philosopher]

these:

>> Reading the description of the item, it boasts "Non Isolated (passes DC

This ones got a poor front end that cant cope with 3v signals.

[Dave Plowman (News)]

Is the screen of the 'strange' outlet connected to mains ground?

[Dave Plowman (News)]

Many installers fit a DA where one isn't needed - passive splitting will work ok.

[Alan]

In message , TD wrote

On my (triplex) faceplate I found I had to terminate the unused outputs with 75 Ohm to get a sensible TV signal levels across all 6 Freeview MUXs. My Freeview box can show signal strengths and quality, and with just a TV connected to the faceplate and the unused outputs left open the signals levels were?all over the place? indicating a transmission line impedance mismatch.

numbers AP01796 AP01797 AP01799

Or plug the TV into the correct socket and connect a radio tuner to the other output (or as a test, a second TV to the other socket to provide a 75 Ohm load)

[Dave]

test

url: