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Old 15th Mar 2011, 2:43 pm   #1
Andy Doz
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Default To balun or not to balun, that is the question.

Having got my greasy, overkeen paws on a shiny new Aurora this morning, I'm in rather a hurry to connect it to the Ferguson 992T to see if I can get a picture out of it.

Do I need a Balun to feed the telly? or shall I just "wing it" for now?
Also is it OK to "Megger" the aerial socket to check it's integrity isolation-wise?
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Old 15th Mar 2011, 2:50 pm   #2
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Default Re: To balun or not to balun, that is the question.

First I've heard of that one... most if not all of us use a standard 9v 250mA power supply which will be fine. Just change the iso caps on the aerial input on your telly, if your worried.


See here for further discussions.

https://www.vintage-radio.net/forum/...ad.php?t=64760

My Aurora goes into a amp distribution for 1 to many outputs, this is mains powered and earthed so it provides protection for the aurora.

Edit:
Just googled balun, interesting I was not aware of that device.

P.S
you cannot beat the feeling when you first see Testcard C on your old 405 , enjoy

P.P.S
next thing to do is diasble the test tone then feed in some testcard music for the ultimate step back into the past( you can buy cd's full of them) i'm getting the tweed jacket with leather elbows next

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Old 15th Mar 2011, 2:52 pm   #3
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Default Re: To balun or not to balun, that is the question.

Make sure the TV chassis is connected to mains neutral and connect up the Aurora. Don't worry about any possible mismatch, it won't matter.
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Old 15th Mar 2011, 3:17 pm   #4
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Default Re: To balun or not to balun, that is the question.

Quote:
Originally Posted by ppppenguin View Post
Make sure the TV chassis is connected to mains neutral and connect up the Aurora. Don't worry about any possible mismatch, it won't matter.
Thanks. TV Chassis is definitely neutral. I'll wing it then
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Old 15th Mar 2011, 3:18 pm   #5
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Default Re: To balun or not to balun, that is the question.

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First I've heard of that one... most if not all of us use a standard 9v 250mA power supply which will be fine. Just change the iso caps on the aerial input on your telly, if your worried.
Not quite what I meant ! A Balun on the antenna side .. to match the 75 ohms to the 300 of the tuner.
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Old 15th Mar 2011, 3:51 pm   #6
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Default Re: To balun or not to balun, that is the question.

90% of 405 line tv's will be 75 ohms as they are mostly an unbalanced input, the Aurora is also 75 ohms, so what do you need to wing?
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Old 15th Mar 2011, 3:52 pm   #7
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Default Re: To balun or not to balun, that is the question.

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90% of 405 line tv's will be 75 ohms and the Aurora is also 75 ohms, so what do you need to wing?
because 100% of the one I own isn't
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Old 15th Mar 2011, 4:04 pm   #8
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Default Re: To balun or not to balun, that is the question.

What is it you have?
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Old 15th Mar 2011, 4:21 pm   #9
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Default Re: To balun or not to balun, that is the question.

Ferguson 992T
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Old 15th Mar 2011, 4:28 pm   #10
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Default Re: To balun or not to balun, that is the question.

As I said before, don't worry about a balun. You're not short of signal and the reflections on a couple of yards of cable will be of no significance.
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Old 15th Mar 2011, 4:43 pm   #11
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Default Re: To balun or not to balun, that is the question.

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As I said before, don't worry about a balun. You're not short of signal and the reflections on a couple of yards of cable will be of no significance.
I intend to , I was just referring to Murphy's post ..
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Old 15th Mar 2011, 6:00 pm   #12
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Default Re: To balun or not to balun, that is the question.

You don't really need a balun, as others have said, but you can probably convert your balanced input to unbalanced very easily. If the balanced input is 300Ω it would be a good idea anyway, then you won't ever be bothered by reflections.

Assuming that there is an input cable[1] between the input and any other electronics (other than isolation components), at the other end you should find something like one of the top figures in the diagram below - either a transformer with two windings or an autotransformer with just one. Either way, there should be a centre tap on the input. Although the turns ratio is 2:1, the impedance ratio is 2² - which is 4:1, so each half is 300Ω/4 = 75Ω, which is what you want!

In the lower figures, I've shown a coaxial socket connected to form a 75Ω connection but, if you want to keep the existing socket arrangement, it won't do any harm but, either way, you should, ideally, replace the 300Ω feeder with a length of 75Ω coax.

[1] I had a Pye export set once that had the balun mounted directly onto the input socket with 75Ω coax feeding the tuner!
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Old 15th Mar 2011, 6:49 pm   #13
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Default Re: To balun or not to balun, that is the question.

The 992T is not a balanced input. 75-80 ohm co-ax. A lot of makers refused to use the Co-ax plug and socket due to cost. J.
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Old 15th Mar 2011, 7:29 pm   #14
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Default Re: To balun or not to balun, that is the question.

Yes indeed John is correct, the service sheet states quite clearly the set has a 75 ohm unbalanced input.
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Old 15th Mar 2011, 8:38 pm   #15
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Default Re: To balun or not to balun, that is the question.

Indeed you are right ... I am 100 % wrong and 75 ohms

but... and here's the but ... right at the moment I have THE biggest grin on my face .....

Only one thing ... my focus knob turns but doesn't move the magnet ... I'm not sure what it's supposed to do ...I presume the magnet is held againsst the knob by the spring ... don't fancy forcing it though ... any tips?
Focus is actually quite a bit better than shown in the picture.
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Old 15th Mar 2011, 8:47 pm   #16
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Default Re: To balun or not to balun, that is the question.

ah ... a tiny squib of WD40 sorted it
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Old 15th Mar 2011, 9:12 pm   #17
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Default Re: To balun or not to balun, that is the question.

Great result Andy. Now the bug has bitten what's next?
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Old 15th Mar 2011, 10:03 pm   #18
Andy Doz
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Default Re: To balun or not to balun, that is the question.

That's the question ... a friend has a TV22 at his mums ....

The Ferguson is great ... slight motorboating effecting the frame when the volume is low ...
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