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Old 3rd Feb 2004, 6:17 pm   #1
fableglade
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Default Balance to Imbalance Transformer

I have a couple of 1940's Philips TV sets, both of which were designed to run from a balanced aerial. On the service data it states that, if a coax feeder is already installed then a " Balance to Imbalance Transformer " should be used. What exactly were these transformers and is anything still available that does the same job.

These sets seem prone to hum when in use and I wonder if it is because one of these tyransformers is needed when using them?
 
Old 3rd Feb 2004, 6:34 pm   #2
ppppenguin
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Default Re: Balance to Imbalance Transformer

If you have a good strong signal then the mismatch caused by not using a transformer does not matter. Since you are likely to have a good signal from a modulator then just put low value caps (1000p, not critical) in series with inner and screen to break the earth loop that may be causing the hum.

A balun, as a balanced to unbalanced transformer is often called, may be available or could be made. THey're not anything special at these frequencies. But it does not really seem necessary.
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Old 19th Apr 2004, 3:13 pm   #3
oldeurope
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Default Re: Balance to Imbalance Transformer

http://www.pixum.de/viewalbum/?id=1246463

Darius
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Old 19th Apr 2004, 3:32 pm   #4
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Default Re: Balance to Imbalance Transformer

Hi all

They are available from Maplin Electronics, see link here..
http://www.maplin.co.uk/products/mod...145&Products=1

That and an inline coupler is all you need.

Malcolm
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Old 22nd Apr 2004, 8:37 am   #5
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Default Re: Balance to Imbalance Transformer

The correct modern wire would be that horrible 300 ohm ribbon. Often supplied as an aerial for VHF receivers. For the few inches you will be using it does not matter at all. Bell wire will be fine.

I am guessing that the original 300 ohm feeder was much like modern plastic ribbon stuff but with rubber insulation. The geometry must have been the same, with the conductors about 1cm apart, to give the correct impedance.
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Old 5th Nov 2004, 12:01 pm   #6
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Default Re: Philips TV and balanced aerial feed

Quote:
A dipole aerial is inherently balanced. Many aerials do not have proper baluns. But balanced feeder is more prone to interference than co-ax, more affected by proximity to metalwork, rain etc and even more prone to mechanical damage.

The world settled on 75 ohm unbalanced fairly quickly for TV. There were (still are?) loads of VHF/FM tuners with 300 ohm unbalanced aerial inputs.
Ideally you should attach 75 ohm coax to a dipole type aerial with a balun. Good quality aerials will have a balun built in, but not all do. ISTR you lose about 3db from mismatch losses if a balun isn't used.

300 ohm balanced feeders are still the norm in the US, and possibly in other parts of the world to. In Britain it's actually quite difficult to buy 300 ohm feeder cable.

Best regards, Paul

Last edited by Paul Stenning; 29th Dec 2004 at 1:34 pm.
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Old 5th Nov 2004, 1:19 pm   #7
Ed_Dinning
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Default Re: Philips TV and balanced aerial feed

Hi Guys, a late 50's Radio Rentals (Baird) console set we had was fitted with balanced feeder, the had to be unscrewed from a pax panel to move the set. When it was later convered to ITV it then used a coax type.

Ed
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Old 5th Nov 2004, 5:20 pm   #8
Paul Stenning
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Default Re: Philips TV and balanced aerial feed

I have a Philips radio (Reverbeo) with a 300R balancd aerial input for FM. I have just connected the centre core of a standard 75R aerial feed to one side of the 300R input and the braid to the AM earth socket. It works fine. In fact I need a 6dB attenuator because the output from my FM booster amp is too much for it, so the 3dB mis-match loss is irrelevat.

I think Maplin still sell baluns if you want to do it properly.
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Old 5th Nov 2004, 5:22 pm   #9
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Default Re: Philips TV and balanced aerial feed

Paul's method is the standard trick for connecting co-ax to 300 ohm balanced inputs. It makes the assumption that the balanced input isn't floating. Though if you have plenty of signal then almost anything will work.
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Old 10th Nov 2004, 10:47 am   #10
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Default Re: Philips TV and balanced aerial feed

I have a number of such sets (including the Philips 385U) and I used to get a lot of hum when is use. After advice from the chaps here I bought some baluns from Maplins and also some 300 ohm balanced feeder cable and the sets performed so much better. I've actually found that Philips, Pye and Ekco used this type of aerial in the late 40's era and using the proper balun and cable makes a much greater improvement than you would imagine. The baluns and cable and very cheap from Maplins.
 
Old 22nd Feb 2005, 7:47 pm   #11
Mick Mearman
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Default Re: Philips TV and balanced aerial feed

We had a Philips 385U and from what I can remember (I was only 3 at the time) it wasn't an unscreened feeder, the co-ax had two cores red and green
this was taken to a large "H" aerial there was also a single core cable that came down from the aerial and went to earth, I assume this was connected
to the screen at the aerial end, inside the house the aerial was terminated using 2amp 3pin plugs and sockets, we could then move the set into another room.
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Old 2nd Mar 2005, 10:42 pm   #12
Mick Mearman
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Default Re: Philips TV and balanced aerial feed

I've found a good pic of this set, I don't know if anyone else has seen it
I think the speaker fret may have been changed, has I remember it was
light brown with a large diamond pattened gold inlay?
http://members.lycos.nl/patstvmuseum/english385.htm
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