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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions. |
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3rd May 2015, 11:44 am | #1 |
Octode
Join Date: May 2003
Location: Belper Derbyshire
Posts: 1,936
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Protecting the Aurora RF output
Good morning,
I have just purchased an aurora from Crowthorn tubes. I was thinking if there is a suitable method to protect the RF output should the aerial isolator caps or the insulation around the aerial socket fail shoving some nasty things back into the RF input. I was thinking something like a small box with a UHF plug and socket with 2 low pf capacitors wired between them effectively ensuring that no DC or 50hz signals could get back to the RF output. Would this work or would something else be needed. Many thanks, Christopher Capener
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Interests in the collection and restoration of Tefifon players and 405 line television |
3rd May 2015, 12:41 pm | #2 |
Dekatron
Join Date: Sep 2003
Location: Near Swindon, North Wiltshire, UK.
Posts: 3,621
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Re: Protecting the Aurora RF output
Hello,
Many UK made colour TVs used an isolated aerial input socket, combined with a short coax lead terminated with a coax plug, to the UHF tuner RF input. These would be ideal. An example would be those used in the Thorn/Ferguson TX9 chassis. Regards, Dazzlevision |
3rd May 2015, 3:56 pm | #3 |
Pentode
Join Date: Oct 2012
Location: Dorset, UK.
Posts: 240
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Re: Protecting the Aurora RF output
You can buy isolated TV aerial sockets, intended for wall mounting in buildings, you could use one of these http://www.speedyelectrical.co.uk/1....FSQewwodMLkA2w (no connection with supplier, etc, just first Google hit looking for an example) Make sure you get an isolated one though, as there are non isolated ones available.
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3rd May 2015, 7:55 pm | #4 |
Hexode
Join Date: Mar 2014
Location: Dublin, Ireland
Posts: 398
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Re: Protecting the Aurora RF output
Just a word of caution - output of aurora is VHF (at least 405 line version) and many (but not all) isolators are designed for UHF and may introduce significant loss at VHF. While this not be a problem as output level of aurora is very high it should be borne in mind. Better to get an isolator intended for the wider European market which will be intrinsically VHF/UHF if you can.
Just a few words of advice based on my own experience. Best regards John |
3rd May 2015, 11:16 pm | #5 |
Octode
Join Date: Jan 2003
Location: Birmingham, West Midlands, UK.
Posts: 1,268
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Re: Protecting the Aurora RF output
I have an aurora and connect the RF via a couple of caps - 10nF 600V ceramic if memory serves but chosen simply because they were to hand, much smaller cap values will work just as well. I usually add an additional 18db attenuator for most sets simply because there is just so much signal.
TTFN, Jon |
5th May 2015, 3:20 pm | #6 |
Hexode
Join Date: Mar 2014
Location: Dublin, Ireland
Posts: 398
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Re: Protecting the Aurora RF output
Just to be very clear - as far as I am concerned safety of humans comes first so ground side of Aurora output connector must have a continuous dc path to antenna isolating connector where the isolation from potentially live chassis takes place (including through any splitters, amplifiers or attenuators etc in the path) - isolating capacitors in the RF live (centre) conductor of Aurora are advantagous but I would advise higher voltage rating (c 1.5Kv).
The problem with this sort of issue is that many schemes will work ie equipment will function but it will not be safe. Apologies for labouring the point but it needs to be made and is pften not adequately understood. Regards John |
5th May 2015, 3:52 pm | #7 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,088
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Re: Protecting the Aurora RF output
Can you not make a little RF transformer with a small ferrite toroid and some insulated wire? With plenty of RF available, matching and losses should not be an issue, and you'll have full galvanic isolation.
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5th May 2015, 6:35 pm | #8 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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Re: Protecting the Aurora RF output
Right behind the RJ45 port of any 'Gigabit Ethernet' equipped PC or laptop motherboard is a quad 1:1 ferrite isolating transformer with usually at least a 1Kv isnulation rating.
Not sure if these will work up to UHF but I've successfully used them in the past as couplers for a 28-to-50MHz double-balanced modulator [diode ring] in an amateur-band transverter without excess loss. |
5th May 2015, 9:26 pm | #9 |
Nonode
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,944
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Re: Protecting the Aurora RF output
Would a commerical "braid breaker" type unit work in this case? These are usually 1:1 RF isolating transformers for 75-ohm lines packaged with Belling Lee or F connectors. I am not sure who makes them these days. I have some from AKD, but these would be from around 25 years ago.
As they are designed primarily to block interference, I guess it would be necessary to check their AC voltage blocking capability. Cheers, |
10th May 2015, 3:07 pm | #10 |
Heptode
Join Date: Oct 2012
Location: London, UK.
Posts: 500
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Re: Protecting the Aurora RF output
That sounds very short sighted if it is true. Surely any isolator sold should be OK for band 2 at least.
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10th May 2015, 6:05 pm | #11 |
Hexode
Join Date: Mar 2014
Location: Dublin, Ireland
Posts: 398
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Re: Protecting the Aurora RF output
You'd think so but I'm afraid this is not the case -
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29th Jun 2015, 7:23 pm | #12 |
Nonode
Join Date: Jun 2013
Location: Surbiton, SW London, UK.
Posts: 2,801
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Re: Protecting the Aurora RF output
I believe the Aurora does produce harmonics, and a transformer will have a frequency
cutoff that might be useful - not expensive http://uk.farnell.com/coilcraft/pwb1...mhz/dp/2458064 |
29th Jun 2015, 7:39 pm | #13 |
Nonode
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,573
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Re: Protecting the Aurora RF output
The Aurora certainly creates a lot of harmonics well into the UHF TV spectrum.
In the 2013 Golborne talk on Alignment I showed the Aurora spectrum which is in the notes for the talk. https://www.vintage-radio.net/forum/...=100384&page=2 There's also a table showing what combinations of band 1 & 3 channels should be avoided in a 2 channel system due to the harmonics of the band 1 signal falling in band 3. Keith |
3rd Dec 2015, 7:27 pm | #14 |
Diode
Join Date: Oct 2012
Location: Boston, Lincolnshire, UK.
Posts: 8
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Re: Protecting the Aurora RF output
I have overcome this worry by running my 405 line Emerson on a mains isolation transformer.
Jonathan. |
6th Dec 2015, 5:18 pm | #15 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,147
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Re: Protecting the Aurora RF output
If you are really concerned about this I would suggest something along the lines of this birds nest that I knocked up a few moments ago. The input and output coils are four turns each wound on a ferrite ring. They need to be 'in phase' easily checked by swapping over one pair of leads of either the primary or secondary coils.[but not both!]
There is a slight attenuation of the signal but the AURORA has more than enough output to cope with the minor reduction in output. It could be fitted in a small die cast box for neatness. Some experimentation may be needed depending on the core material as there are many variations in 'Ferroxcube'. The picture was taken on a 1950 Murphy V180. This chassis does not have vision AGC so a good comparison can be made. Just an idea, John. |
6th Dec 2015, 5:49 pm | #16 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,901
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Re: Protecting the Aurora RF output
You can buy isolating transformers designed to pass TV signals and to break both the inner and braid paths, already housed and fitted with Belling-Lee connectors. Available from amateur radio suppliers under the name "Braid breakers"
David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
13th Jan 2016, 10:19 am | #17 |
Hexode
Join Date: Apr 2012
Location: Chorley, Lancashire, UK.
Posts: 260
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Re: Protecting the Aurora RF output
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13th Jan 2016, 10:33 pm | #18 |
Heptode
Join Date: Apr 2010
Location: West Midlands, UK.
Posts: 663
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Re: Protecting the Aurora RF output
Looking at the specifications, the response of these transformers starts to roll off above 3.5 MHz so probably not much good at TV RF frequencies (41 MHz and above ), loads of signal attenuation.
Mike. |
14th Jan 2016, 10:38 am | #19 |
Hexode
Join Date: Apr 2012
Location: Chorley, Lancashire, UK.
Posts: 260
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Re: Protecting the Aurora RF output
Thanks for the advice.
Mark |