Distribution of Television signals - UK Vintage Radio Repair and Restoration Discussion Forum

Sean Williams · 18th Oct 2006, 11:58 pm

Ok, un-officially split from the Aurora thread......

Doctor Who wrote I have a cabled system throughout the house, there are approximately 24 sets connected throughout the house via distribution amps and co-ax outlets. The UHF and VHF signals are combined by a distribution unit and then sent all around the house. Even thought this is purely a cabled system 405 can be picked up indoors on a set with a set top aerial! Amusingly the 7 modern colour sets also connected up are also being fed with a VHF signal as well as UHF, sadly though even those with VHF tuners do not tuen down low enough to pick up even the audio.

The one and only problem with the whole system is that, when 405 is switched on, it causes interference on UHF channels, any ideas how this can be overcome as, ideally, I would like to have 405 on all of the time?

Ok, lets have a go at this.

Firstly what sort of signal levels are we talking about here - if you have loads of gain in the amplifiers you may well be overloading the front ends of the UHF sets - this will then make them more likley to take offence at the VHF signal's harmonic content (there is a significant amount in nearly all modulators currently available).

It may be worth experimenting with attenuators - just to see what level of signal is enough - this might cure the problem. Otherwise you will need to see if a high pass filter fitted in the coax feed to the affected set cures the problem - you may even need both....

Turning to the distribution amplifier - Does it have a variable gain control? If you are able to watch a picture on a set top antenna from the 405 line stuff then you either have very poor screening on the coax, or far too much signal!

Try turning the gain down.

If you have any splitters on the coax feeders check the soldered joints (worth checking the plugs too) a poor conection here can lead to rectification of RF and hey presto a makeshift diode mixer - the result of this would be to create all sorts of mixing products that may interfere with all sorts of things!

If you have a professional antenna installation company nearby they should have test equipment to measure signal levels at the tv set - they might be able to advise if the levels are too high.....

Cheers
Sean

paulsherwin · 19th Oct 2006, 12:48 am

This is what I wrote in the other thread before reading this:

I'm certainly no expert in this area, but I would think that either the VHF modulator is producing a dirty harmonic-laden signal, or the signal strength of the VHF modulator is overloading the UHF tuners.

In either case, reducing the modulator output with an attenuator may improve matters.

Paul

Dave Moll · 19th Oct 2006, 12:04 pm

From the other thread:

Quote:

Originally Posted by DoctorWho

Amusingly the 7 modern colour sets also connected up are also being fed with a VHF signal as well as UHF, sadly though even those with VHF tuners do not tuen down low enough to pick up even the audio.

Presumably, even they could, they would be expecting FM sound rather than the AM of the System A standard.

The only point in my aerial network that I have UHF and VHF signals diplexed together is the (rather long) downlead from my aerials (on the end of my barn) into the start of my network, where the UHF and Band II signals go into a splitter. I then have separate networks for UHF and Band I (from my Domino) with co-ax cables all over the place. Now I'm no longer allowed to lay mains cables, I avoid withdrawal symptoms by laying co-ax and telephone cables.

DoctorWho · 19th Oct 2006, 3:35 pm

Thanks for the inout here. I have tried attenuation but this cuts off the 405 signal before removing interference from UHF, so that doesn't seem to be an option here. There's no variable gain on the distribution amp, but it is a good quality unit. I don't have any splitters in use, just another amp in the loft to send VHF and UHF on to the rest of the house and extension, but the problem is before this point and is only present when 405 is switched on. You can use either 405 or 625 individually with perfect results, the problem only arises if I am watching 405 when someone else in the house wants to watch 625.

It seems to be suggested that a filter would cure this, any ideas on where I can get one which will work on Channel 1?

Thanks for the help.

Peter.

Sean Williams · 19th Oct 2006, 3:46 pm

Hi Peter,

Erm, you are a bit limited for filter outlets these days - you need a High pass filter for each of the UHF sets - Try one, and if it works then all is good!

I do have one here that I built to attenuate 50 Mhz interference - I could measure it and see what it can do ....

If it is any good you are welcome to try it (I can always give you the build info.....)

Cheers
Sean

Paul Stenning · 19th Oct 2006, 6:17 pm

This is covering the same subject as another thread started today, so I'm closing this one.

Please continue here https://www.vintage-radio.net/forum/...ad.php?t=11914

18th Oct 2006, 11:58 pm	#1
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,518	Distribution of Television signals Ok, un-officially split from the Aurora thread...... Doctor Who wrote I have a cabled system throughout the house, there are approximately 24 sets connected throughout the house via distribution amps and co-ax outlets. The UHF and VHF signals are combined by a distribution unit and then sent all around the house. Even thought this is purely a cabled system 405 can be picked up indoors on a set with a set top aerial! Amusingly the 7 modern colour sets also connected up are also being fed with a VHF signal as well as UHF, sadly though even those with VHF tuners do not tuen down low enough to pick up even the audio. The one and only problem with the whole system is that, when 405 is switched on, it causes interference on UHF channels, any ideas how this can be overcome as, ideally, I would like to have 405 on all of the time? Ok, lets have a go at this. Firstly what sort of signal levels are we talking about here - if you have loads of gain in the amplifiers you may well be overloading the front ends of the UHF sets - this will then make them more likley to take offence at the VHF signal's harmonic content (there is a significant amount in nearly all modulators currently available). It may be worth experimenting with attenuators - just to see what level of signal is enough - this might cure the problem. Otherwise you will need to see if a high pass filter fitted in the coax feed to the affected set cures the problem - you may even need both.... Turning to the distribution amplifier - Does it have a variable gain control? If you are able to watch a picture on a set top antenna from the 405 line stuff then you either have very poor screening on the coax, or far too much signal! Try turning the gain down. If you have any splitters on the coax feeders check the soldered joints (worth checking the plugs too) a poor conection here can lead to rectification of RF and hey presto a makeshift diode mixer - the result of this would be to create all sorts of mixing products that may interfere with all sorts of things! If you have a professional antenna installation company nearby they should have test equipment to measure signal levels at the tv set - they might be able to advise if the levels are too high..... Cheers Sean __________________ Engineers make things work and have spare bits when finished

19th Oct 2006, 12:48 am	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Distribution of Television signals This is what I wrote in the other thread before reading this: I'm certainly no expert in this area, but I would think that either the VHF modulator is producing a dirty harmonic-laden signal, or the signal strength of the VHF modulator is overloading the UHF tuners. In either case, reducing the modulator output with an attenuator may improve matters. Paul

19th Oct 2006, 3:35 pm	#4
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Distribution of Television signals Thanks for the inout here. I have tried attenuation but this cuts off the 405 signal before removing interference from UHF, so that doesn't seem to be an option here. There's no variable gain on the distribution amp, but it is a good quality unit. I don't have any splitters in use, just another amp in the loft to send VHF and UHF on to the rest of the house and extension, but the problem is before this point and is only present when 405 is switched on. You can use either 405 or 625 individually with perfect results, the problem only arises if I am watching 405 when someone else in the house wants to watch 625. It seems to be suggested that a filter would cure this, any ideas on where I can get one which will work on Channel 1? Thanks for the help. Peter.

19th Oct 2006, 3:46 pm	#5
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,518	Re: Distribution of Television signals Hi Peter, Erm, you are a bit limited for filter outlets these days - you need a High pass filter for each of the UHF sets - Try one, and if it works then all is good! I do have one here that I built to attenuate 50 Mhz interference - I could measure it and see what it can do .... If it is any good you are welcome to try it (I can always give you the build info.....) Cheers Sean __________________ Engineers make things work and have spare bits when finished

19th Oct 2006, 6:17 pm	#6
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,073	Re: Distribution of Television signals This is covering the same subject as another thread started today, so I'm closing this one. Please continue here https://www.vintage-radio.net/forum/...ad.php?t=11914 __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster Last edited by Paul Stenning; 19th Oct 2006 at 6:49 pm.