Aurora RF isolation - UK Vintage Radio Repair and Restoration Discussion Forum

johnlees · 1st Oct 2007, 9:56 pm

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.)

paulsherwin · 1st Oct 2007, 10:05 pm

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

johnlees · 1st Oct 2007, 10:22 pm

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.

ppppenguin · 1st Oct 2007, 10:29 pm

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.

johnlees · 1st Oct 2007, 10:45 pm

Thanks, PPPPenguin. What value should these caps be, please?

ppppenguin · 1st Oct 2007, 10:59 pm

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.

paulsherwin · 1st Oct 2007, 11:21 pm

Quote:

Originally Posted by ppppenguin

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.

Paul

ppppenguin · 2nd Oct 2007, 12:22 am

Quote:

Originally Posted by paulsherwin

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?

johnlees · 2nd Oct 2007, 9:09 pm

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.

plumbweiss · 2nd Oct 2007, 10:10 pm

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!

johnlees · 2nd Oct 2007, 10:17 pm

Thanks, Plumbweiss, will check that, though my recollection from earlier this evening is that the aerial socket is mounted on a paxolin board.

G8DLG · 7th Oct 2007, 6:49 pm

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.

Sean Williams · 7th Oct 2007, 9:03 pm

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?

Sean

David_Robinson · 8th Oct 2007, 2:58 pm

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

http://cpc.farnell.com/jsp/search/pr...sp?SKU=AP01093

BGmidsUK · 16th May 2008, 11:52 pm

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

BG

murphyv310 · 18th May 2008, 12:17 pm

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

Geoff 555 · 31st May 2008, 2:50 pm

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.

1st Oct 2007, 9:56 pm	#1
johnlees Hexode Join Date: Oct 2004 Location: Cheltenham, Gloucestershire, UK. Posts: 283	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.) __________________ Now that's the kind of Long Wave I like ! (Severn Bore near Gloucester) Last edited by johnlees; 1st Oct 2007 at 9:58 pm. Reason: Addition of "Defiant".

1st Oct 2007, 10:05 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 26,762	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

1st Oct 2007, 10:22 pm	#3
johnlees Hexode Join Date: Oct 2004 Location: Cheltenham, Gloucestershire, UK. Posts: 283	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. __________________ Now that's the kind of Long Wave I like ! (Severn Bore near Gloucester)

1st Oct 2007, 10:29 pm	#4
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	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.

1st Oct 2007, 10:45 pm	#5
johnlees Hexode Join Date: Oct 2004 Location: Cheltenham, Gloucestershire, UK. Posts: 283	Re: Aurora RF isolation Thanks, PPPPenguin. What value should these caps be, please? __________________ Now that's the kind of Long Wave I like ! (Severn Bore near Gloucester)

1st Oct 2007, 10:59 pm	#6
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	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.

2nd Oct 2007, 9:09 pm	#9
johnlees Hexode Join Date: Oct 2004 Location: Cheltenham, Gloucestershire, UK. Posts: 283	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. __________________ Now that's the kind of Long Wave I like ! (Severn Bore near Gloucester)

2nd Oct 2007, 10:10 pm	#10
plumbweiss Retired Dormant Member Join Date: Nov 2006 Location: Manchester, UK. Posts: 157	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!

2nd Oct 2007, 10:17 pm	#11
johnlees Hexode Join Date: Oct 2004 Location: Cheltenham, Gloucestershire, UK. Posts: 283	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. __________________ Now that's the kind of Long Wave I like ! (Severn Bore near Gloucester)

7th Oct 2007, 6:49 pm	#12
G8DLG Retired Dormant Member Join Date: Aug 2006 Location: London, UK. Posts: 48	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.

7th Oct 2007, 9:03 pm	#13
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,511	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? Sean __________________ Engineers make things work and have spare bits when finished

8th Oct 2007, 2:58 pm	#14
David_Robinson Retired Dormant Member Join Date: Feb 2004 Location: Nottingham, UK Posts: 94	Re: Aurora RF isolation You could always just buy an isolated socket, example below. http://cpc.farnell.com/jsp/search/pr...sp?SKU=AP01093

16th May 2008, 11:52 pm	#15
BGmidsUK Retired Dormant Member Join Date: Oct 2007 Location: Leicester, UK. Posts: 809	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 BG

18th May 2008, 12:17 pm	#16
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,376	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 __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

31st May 2008, 2:50 pm	#17
Geoff 555 Heptode Join Date: Jan 2008 Location: Bristol, UK. Posts: 981	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.