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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 1st Oct 2007, 9:56 pm   #1
johnlees
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Smile Aurora RF isolation

My Aurora is on its way but I have just spotted this comment from Darryl in an old thread of October 2006 :-

"As for protection [for the Aurora] , I'll let others provide feedback, but the biggest thing would be to not connect the converter to a hot chassis television without an isolation transformer in the RF output. The unit is not inherently referenced to earth ground unless the mains adapter your using grounds the negative power lead, but the video source you connect to may be. It's always best to isolate "hot" equipment from the rest."



Can someone explain, please, whether there is a problem here or can I go ahead and connect my Aurora directly to the 405 television aerial input (via the F-to-coaxial lead which I gather I shall need). (Exact model of Defiant 405 set unknown until I pick it up tomorrow.)
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Last edited by johnlees; 1st Oct 2007 at 9:58 pm. Reason: Addition of "Defiant".
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Old 1st Oct 2007, 10:05 pm   #2
paulsherwin
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Default Re: Aurora RF isolation

The aerial socket on the TV should be isolated from the chassis as otherwise the TV aerial would be live. Of course, the isolation components may be faulty if the set is unrestored.

Paul
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Old 1st Oct 2007, 10:22 pm   #3
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Default Re: Aurora RF isolation

Thanks, Paul. Now I understand. The set is fully restored by a member of this forum and is at present working with an Aurora, so everything should be OK.
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Old 1st Oct 2007, 10:29 pm   #4
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Default Re: Aurora RF isolation

An isolator from a scrap set (tested with a Megger to prove it's OK) is a good idea. Or a couple of high voltage ceramic caps, one each in series with the centre and braid of the co-ax. These could be mounted in a small plastic box with a co-ax socket on one end and an F lead on the other to fit the Aurora.
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Old 1st Oct 2007, 10:45 pm   #5
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Default Re: Aurora RF isolation

Thanks, PPPPenguin. What value should these caps be, please?
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Old 1st Oct 2007, 10:59 pm   #6
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Default Re: Aurora RF isolation

Not critical. 1000p to 10000p is fine. Voltage rating should be 1000V or greater. You'll find this sort of ceramic cap in some line output stages.

Ideally you should use class Y capacitors but I suspect the self inductance may be a problem. Since this isolator is purely a backstop and not likely to have mains voltage across it continuously I would be happy enough with 1000V ceramics.
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Old 1st Oct 2007, 11:21 pm   #7
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Default Re: Aurora RF isolation

Quote:
Originally Posted by ppppenguin View Post
Ideally you should use class Y capacitors but I suspect the self inductance may be a problem. Since this isolator is purely a backstop and not likely to have mains voltage across it continuously I would be happy enough with 1000V ceramics.
X2 caps from a scrap PSU should be OK too.

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Old 2nd Oct 2007, 12:22 am   #8
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Default Re: Aurora RF isolation

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Originally Posted by paulsherwin View Post
X2 caps from a scrap PSU should be OK too.
In practice they will be fine except for their self inductance which will be a problem at RF. Strictly speaking no, they should not be used, as they are effectively connected between live and earth so failure could expose a user to hazardous voltages.

This raises an interesting question of what to use to replace a failed capacitor in an aerial isolator. Unlike the usage with Aurora, where it's purely a backstop against failure of an isolator, in a TV it may have to withstand mains indefinitely and not cause a hazard if it fails. This means a class Y part is mandatory yet the self inductance of film caps means it may not be very good for passing RF. You really want a ceramic capacitor but are any of these properly class Y rated?
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Old 2nd Oct 2007, 9:09 pm   #9
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Default Re: Aurora RF isolation

Thanks for the advice, both of you. I am now in possession of this Defiant set (exact model unknown and can't find it anywhere but will post a picture in the relevant section soon to ask for help identifying it) and I have checked that the chassis is connected to mains neutral. Also (but only with X100 on my Avo Multiminor) there is no connection between the outer or inner on the aerial socket with the chassis.
As the seller had it working for ages with an Aurora (and I saw it working at his house today) I think I can safely connect mine up to it when it arrives.
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Old 2nd Oct 2007, 10:10 pm   #10
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Default Re: Aurora RF isolation

Worth checking that no amateur has removed the insulating washers /paxolin disc often used between a live chassis and the mounting plate the aerial socket is mounted on!
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Old 2nd Oct 2007, 10:17 pm   #11
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Default Re: Aurora RF isolation

Thanks, Plumbweiss, will check that, though my recollection from earlier this evening is that the aerial socket is mounted on a paxolin board.
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Old 7th Oct 2007, 6:49 pm   #12
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Default Re: Aurora RF isolation

Never use class x or y caps for isolation purposes,they are not safe enough,if they go leaky across the mains,thats ok,because they are supposed to be self healing,but I have seen many explode.
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Old 7th Oct 2007, 9:03 pm   #13
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Default Re: Aurora RF isolation

Ok, the original question has been answered, and only relates to the protection of the Aurora.

Lets not have the great X, Y, Z debate eh?

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Old 8th Oct 2007, 2:58 pm   #14
David_Robinson
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Default Re: Aurora RF isolation

You could always just buy an isolated socket, example below.

http://cpc.farnell.com/jsp/search/pr...sp?SKU=AP01093
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Old 16th May 2008, 11:52 pm   #15
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Default Re: Aurora RF isolation

Not wishing to start a new thread as the title is relevant, what would people suggest for protection against surges when feeding the output of an Aurora into a domestic CATV distribution system? I know of several instances where equipment has been damaged during storms, in one case the surge definitely came up the signal leads (blew my cousin's modem). I wouldn't want to take unnecessary risks so RF isolation is a must for a permanent installation



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Old 18th May 2008, 12:17 pm   #16
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Default Re: Aurora RF isolation

Hi.
Surely the easiest way is a saimple 1 to 1 RF transformer.
It would be very easy to make and of course they are available from the likes of Farnell etc, and so much safer than a cap.
Trevor
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Old 31st May 2008, 2:50 pm   #17
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Default Re: Aurora RF isolation

Hi, looking for a mains isolation transformer on a auction site that shouldnt be mentioned ( I think.) by mistake left off the 'mains' bit and came across the following.
North Hills signal processing, wideband isolation transformer.
I thought this may be of some interest.
Cheers.
Geoff.
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