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Old 24th Feb 2016, 11:11 am   #121
DonaldStott
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Default Re: Philips B3G63A

Still unclear about the significance of using 75Ω coax cable - if I use the loudspeaker cable I have it is screened so this should help with any potential RF interference from the two signal wires connecting the radio sockets to the dipole??

As I said in my previous post, the two horizontal legs of the dipole will be cut from 2.5mm mains twin and earth with the outer grey PVC covering removed.

Last edited by DonaldStott; 24th Feb 2016 at 11:17 am. Reason: Missed a sentence.
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Old 24th Feb 2016, 11:40 am   #122
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Default Re: Philips B3G63A

Loudspeaker cable will behave like balanced feeder rather than unbalanced cable like 75 ohm coax, though the impedence won't be the 300 ohm standard. I wouldn't expect interference pickup to be a significant problem.

If this is a lashup to get VHF working, then just try it - it'll probably be fine and there's nothing to lose. You can design things properly if you get to the stage of fitting a permanent aerial in the loft or outside.
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Old 24th Feb 2016, 1:59 pm   #123
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Default Re: Philips B3G63A

If it's effectively screened flex, it'll look pretty much like 75R twin feeder- technically a better match to a simple (rather than folded) dipole than either 75R coax or 300R twin feeder as long as the screen isn't connected to the dipole. If the set input is a simple floating coil coupling to the aerial tuned circuit it'll be good there too. Connect the screen to chassis or not- whichever works better.

What it will be is rather lossy compared to proper RF cable, but over a metre or two it simply won't matter.
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Old 25th Feb 2016, 10:32 am   #124
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Default Re: Philips B3G63A

Hi Donald,
Looking at the input circuit to the VHF section, I would think it's the continental standard (at the time) 300 ohm impedance balanced twin ribbon cable to a folded dipole aerial. More details here: https://en.wikipedia.org/wiki/Dipole_antenna The snag is that 300 ohm ribbon cable is getting as rare as hen's teeth. You may have to improvise with what is to hand, probably 75 ohm co-ax and a normal dipole. A mis-match, but probably the simplest answer. (A cut-down band one dipole is a quick solution if you can find an old one) Keep at it, Tony.
PS. my set did not work on long wave due to the ECH81 being low emission. Replacement cured the problem.
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Old 25th Feb 2016, 12:55 pm   #125
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Default Re: Philips B3G63A

Quote:
Originally Posted by boxdoctor View Post
The snag is that 300 ohm ribbon cable is getting as rare as hen's teeth.
What about this: -

http://www.maplin.co.uk/p/balanced-3...er-metre-xr31j

But does that then complicate matters as I would need a folded dipole aerial - if so more advice on cable lengths, construction methods etc. would be appreciated.

I note that although the "... folded dipole has a wider bandwidth than a single dipole" its " ... disadvantages are that it is more vulnerable to interference" - thanks to Wikipedia!

N.B. Note for Moderators - I'm not getting hung up on these things, just interested in expanding my knowledge.

Last edited by DonaldStott; 25th Feb 2016 at 1:05 pm. Reason: Missed out a couple of questions
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Old 25th Feb 2016, 12:59 pm   #126
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Default Re: Philips B3G63A

Don't bother about it at this stage. You really are looking for problems that don't exist. Just plug something into the dipole sockets and see what happens.
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Old 25th Feb 2016, 1:10 pm   #127
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Default Re: Philips B3G63A

One of these beasts:

http://www.ebay.co.uk/itm/Brand-NEW-...MAAOSwI-BWJkaS

is conveniently packaged. Just cut off the coax connector (why a 75ohm connector on a 300 ohm feeder is anybody's guess) and fit suitable plugs.
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Old 25th Feb 2016, 1:36 pm   #128
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Default Re: Philips B3G63A

Quote:
Originally Posted by paulsherwin View Post
You really are looking for problems that don't exist
As I said - just interested in expanding my knowledge!

Quote:
Originally Posted by Herald1360 View Post
One of these beasts
This looks like the business and won't break the bank - I can try one these beasts and also build a dipole to hear which sounds best?
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Old 25th Feb 2016, 1:54 pm   #129
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Default Re: Philips B3G63A

According to the Trader sheet it's 75 ohm balanced input.

Lawrence.
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Old 25th Feb 2016, 2:25 pm   #130
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Default Re: Philips B3G63A

Well spotted Lawrence ... so it's back to the dipole!
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Old 25th Feb 2016, 2:51 pm   #131
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Default Re: Philips B3G63A

As Chris said back in #123, your screened speaker cable is actually a good approximation of 75 ohm balanced feeder with a screen. This quite rare but happens to be a very good match for both a dipole and your radio's input. Use the inner wires to make the connection and connect the screen to the Earth socket.

No need to buy any fancy cable, at this stage anyway.
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Old 25th Feb 2016, 2:54 pm   #132
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Default Re: Philips B3G63A

Surely if you look at the circuit, it IS a balanced input, so does that not rule OUT a 75 ohm coax lead?
I think if you used a 75 ohm lead, it would be better to feed the inner to either half of the pair, with the earthy side to the actual earthed socket.
I for one WOULD just connect either way, or even just an odd AVO lead into one at this stage, but I would be interested in a considered response to my interpretation.
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Old 25th Feb 2016, 2:58 pm   #133
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Default Re: Philips B3G63A

Ah!, I see in crossed post that Paul has already sort of answered my point, or at any rate, half of it.
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Old 25th Feb 2016, 3:04 pm   #134
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Default Re: Philips B3G63A

Quote:
Originally Posted by DonaldStott View Post
Well spotted Lawrence ... so it's back to the dipole!
A dipole is a balanced antenna, if it's connected to an unbalanced line the dipole won't be balanced, you can connect the dipole to a 75 ohm ribbon line (balanced) but as I understand it ribbon line performance can be affected by rain and any proximity to earth, most antennas feeders used externally to a dipole are coaxial in conjunction with a balanced to unbalanced transformer at the antenna end and ditto at the receiver end if the receivers antenna input circuit is balanced.

In practice all kinds of lash ups might work, equally all kinds of lash ups might not work, it's all a trade off.

Lawrence.
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Old 26th Feb 2016, 3:25 pm   #135
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Default Re: Philips B3G63A

I'll get around to my Dipole aerial and 75Ω coax cable "lash up" at the weekend.

Enjoyed finding out (at last) about the difference between balanced and unbalanced inputs - also touched on baluns and found these: -

http://uk.rs-online.com/web/p/chip-baluns/8319305/

0.32 - Price Each (In a Bag of 5)

Would be interesting to find out if they make a difference although I probably don't need them and it's just me " ... looking for problems that don't exist"
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Old 26th Feb 2016, 6:16 pm   #136
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Default Re: Philips B3G63A

Interesting little gizmos, those.

Be a tossup between mismatch loss without and insertion loss with if you used one "top&bottom" to match a dipole to the set via coax feeder.

Simplest and cheapest aerial for this would be figure of eight cable (old lighting flex or bell wire) of suitable length simply split at one end to dipole length (maybe clamp a cabletie at the centre to stop further splitting) and plugged straight in with banana/wander plugs at the set.
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Old 28th Feb 2016, 5:15 pm   #137
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Default Re: Philips B3G63A

Bear in mind Donald, that you are receiving FM and not AM on this aerial. Once the signal strength is enough to operate the limiter action in the I.F.amplifiers, the volume of the output will not materially increase further, regardless of any further increase in input signal. The proof of the effectiveness of the aerial will ultimately be revealed on the improvement to weaker stations, which will show mainly as an improvement in signal-to-noise ratio, i.e. less background "hiss".
So the judgement should be made on the weaker signals you can receive.(This observation also applies to the use of EF183/4 valves in the VHF tuner, too.) Cheers, Tony..
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Last edited by boxdoctor; 28th Feb 2016 at 5:21 pm.
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Old 10th Mar 2016, 2:39 pm   #138
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Default Re: Philips B3G63A

Hopefully my last question before I put everything back together again...!

As I mentioned previously, the Tone is still a bit bright and I have to turn the knob all the way round before I get anything approaching "normal".

I looked again at Trader Sheet 1374 for that part of the circuit dealing with the Tone Control (R31) - see attached image. I have replaced C59 and C60 with modern equivalents but haven't done anything with C61 (470pF) which appears to be part of that circuit.

C61 is a strange looking thing (to me) and is the thin green component just left of top centre in the attached picture. All the black pitch type capacitors in the picture have been replaced as have the orange electrolytic capacitors. Does anyone know what C61 is and why it looks so different from the others? Would replacing C61 have any impact on the Tone or is there anything else obvious that could help in this area.

I have added an other extract from the Trader Sheet that should help explain how the Tone Circuit works?

As usual any response whatsoever would be welcomed.
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Last edited by DonaldStott; 10th Mar 2016 at 2:51 pm. Reason: Add another picture showing Text
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Old 10th Mar 2016, 3:29 pm   #139
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Default Re: Philips B3G63A

Tubular ceramic capacitor, IF shunt? Unlikely to be faulty.
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Old 10th Mar 2016, 5:06 pm   #140
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Default Re: Philips B3G63A

If you want to reduce the treble response you could try shunting a small value capacitor across C54 fixed, and a similar capacitor across C56 to give variable control then fit a suitable single component in their place when a satisfactory audio is found, (shunt means across a component in parallel). Do not touch C61, as Boater Sam says, it is very unlikely to be faulty it is a pF value to filter any IF carrier from the audio.
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