Protecting the Aurora RF output

[high_vacuum_house]

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

[dazzlevision]

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

[Rubberfingers]

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.

[kan_turk]

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

[Duke_Nukem]

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

[kan_turk]

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

[kalee20]

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.

[G6Tanuki]

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.

[Synchrodyne]

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,

[winston_1]

Originally Posted by kan_turk

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.

That sounds very short sighted if it is true. Surely any isolator sold should be OK for band 2 at least.

[kan_turk]

You'd think so but I'm afraid this is not the case -

[Restoration73]

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

[KeithsTV]

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

[jonathanbaldry]

I have overcome this worry by running my 405 line Emerson on a mains isolation transformer.

Jonathan.

[Heatercathodeshort]

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.

[Radio Wrangler]

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

[Mark James]

Would this do the job http://www.ebay.co.uk/itm/231800074179 - if so how would one wire it?

Mark

[Gridiron]

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.

[Mark James]

Thanks for the advice.

Mark