How to receive Continental television 

10 September 2018 tbs.pm/66122

HOW TO MODIFY YOUR SET TO RECEIVE FOREIGN TRANSMISSIONS

 

From Practical Television for January 1962

SOME time ago, interest was aroused by a report of reception of continental television transmissions by a Buckinghamshire service engineer. It was achieved with an ordinary modified domestic television receiver. Although doubtful as to whether such reception was possible in my area (Liverpool), it was thought worth investigating. So, some time later, a domestic receiver was modified. A matter of a day or so after the completion of the modifications, an outside broadcast from Portugal of a sports meeting was seen and, the followings day, test cards from Spain and Portugal were received at strength. Since then, test cards from Italy, Switzerland, Sweden, Spain, Portugal, Russia, and Czechoslovakia have been received and recorded, during periods of good reception, which never seem to be more than a few days apart during the summer months. Programmes too, have been received, and it is interesting to compare the type and quality of programmes from various countries with our own. This article gives details of how to carry out the necessary modifications for yourself and thus have your own “window on the world”!

Requirements

Although the modifications will only cost a matter of a few shillings, it is advisable to start off with the right kind of aerial and a reasonably good television receiver, or results may be disappointing.

Receiver

The receiver need not be ultra-sensitive. The receiver used by the author was a standard model (bought, by the way, for 30s. from a television dealer). Almost any 13-channel or 5-channel set will be adequate. It must of course be reasonably sensitive (a good test of this is to see whether any BBC stations other than the “local” can be received at all). A fringe model is ideal.

It is very important that the set should be fully tunable over the whole of the 5 channels in Band I, either by the fine tuner or by adjustment of the local oscillator frequency. The former is generally a feature of older receivers and for this reason, and the fact that they employ easily-modified group-boards, not printed circuits, they are more suitable for modification than modern receivers.

The television test pattern of Radiotelevisione Italiana.

Aerial

Various experiments proved that an elaborate aerial system is not essential. Naturally, a good aerial means good signal strength, but it must be remembered that signals may come in from any point of the compass, and that a multi-element array can only give a good signal strength from signals within a small angle. Thus, unless such an aerial may be rotated, it is more likely to be a hindrance than a help. A dipole or “H” aerial, at as high a point as possible, is advised. It should be “tuned” to the middle of the band i.e. a channel 3 aerial should be used if possible. If such an aerial is nor available, a simple dipole can be made and, provided it is erected in a high place, such as a loft or attic, and the aerial lead is not too long, very good results can be obtained. Using a similar aerial, hung from a light in an attic with the receiver a few yards away, many different test-cards have been received. To test such an aerial, tune to a weak BBC station (if possible) and then try moving the aerial for best results. (It should be mentioned here that all transmissions were received with the aerial vertical.)

Local Conditions

There is no need to worry too much about local conditions. If the reception is poor, it is probably due to an obstruction limiting reception in one direction. But as was mentioned earlier, signals may come from any direction, and it is reasonably certain that signals from a few countries should be able to weave their way through the hills and gasholders to your aerial!

If, on the other hand, poor reception of the BBC is caused simply by distance from the transmitter, then there is the advantage that weak continental signals will not be blotted out by the BBC signal.

Modifications

Because all Continental transmitters use a different standard from that of the BBC and ITV, British receivers will not produce a picture when supplied with a Continental signal, unless certain modifications are carried out. These modifications are not drastic, but are essential. They have been designed so that anyone who knows his way round a television can perform them, and the receiver can still be used for normal viewing after the changes have been made.

Most stations use the 625-line system with negative modulation and F.M. sound. Therefore, an unmodified receiver, when confronted with these signals would show sloping white lines (the sync pulses) on the screen. This is what to look for on an unmodified receiver to see whether signals are present.

The sound associated with the Continental video signal cannot, unfortunately, be received with the picture as the sound carriers are on the wrong side of the vision carrier for the I.F. in the receiver. If sound is required, another receiver must be employed or a separate tuner for the sound section of the receiver.

Fig. 1 – Typical video detector circuit.

The first step is to modify the video section for negatively modulated signals. This is achieved quite simply by reversing the video detector diode. The typical circuitry surrounding this stage is shown in Fig. 1, and the positive/negative switch circuit is shown in Fig. 2. D1 is the original diode.

A valve diode is shown, but matters would be unchanged if a crystal diode had been used. D2 is the extra diode used for the Continental stations. It will be noticed that this diode is the opposite way round to the original.

In some receivers, the interference limiter and video detector are both provided by a single double diode valve (e.g. HB91). If this is the case, and the limiter is not required, the limiter circuit may be removed and the spare diode used as D2.

The switch, SW1, should be placed on the chassis within two or three inches of the original diode. This is so that all wiring is kept short, to prevent oscillation, or losses which might cause poor sensitivity. The switch should be of the small toggle or slide type. After completion, slight readjustment of the last I.F. transformer may be necessary to restore the picture to its former strength. The effect of switching to “negative” when tuned to a British station will be to make the picture darker and negative. This will be accompanied by almost complete loss of sync.

Fig. 2 – The circuit of Fig. 1 modified by the addition of a switch and crystal diode to permit reception of signals with negatively modulated vision signals.

The next step is to convert to 625 lines. This is achieved quite simply by increasing the frequency of the line timebase from 10,125c/s to 15,625c/s. This can be done, in same cases, by readjustment of the line hold control. However, if the range is insufficient, the value of the series resistance is too big and should be replaced by a resistor of half the value. The line hold control may then need to be replaced by a 1M potentiometer to enable the lower frequency to be obtained as well. Setting the control is difficult unless equipped with an oscilloscope. (For those with a good ear, it is approximately the point where the line whistle becomes inaudible.)

The alternative, and certainly the most accurate way is to wait until signals are received, and then simply lock in a picture.

Unfortunately, the change in line frequency makes the readjustment of certain other controls necessary. Owing to the fact that the line output transformer is made to work most efficiently at 10,125c/s, on 15,625c/s the transformer works less efficiently and therefore the EHT voltage drops. Thus, the tube becomes more sensitive, the frame scanning has more effect and consequently the picture height doubles (approximately). The width would also increase were it not for the fact that the scanning coils do not respond as well to the higher frequency. Refocusing may also be found necessary.

It will be noticed when viewing a 625-line test-card, cramping on the left hand side will be evident. Readjustment of the horizontal linearity control should clear this, but will, of course upset the linearity on 405 lines.

The extent to which these faults occur will depend on the type of receiver used.

Reception

Because of the distance of the transmitters, continuous and reliable reception is impossible. Reception of signals from distant transmitters must be dependent on periods of good reception conditions. These come, usually in “batches”, probably governed by solar activity, etc., similar to short wave reception. These periods may last two or more hours, then fade. It is important to be able to recognise such periods and to be able to find the stations.

Signals

The first sign will be a buzz on channel 2, between channels 3 and 4, or on channel 3. The buzz (which is of course the vision signal on sound) will increase in volume as time goes on. The picture will be found by tuning the fine tuner, or oscillator core towards the low frequency end of the band (i.e. towards channel 1). If the picture is very close to the local BBC channel, the two sync pulses will tend to oppose each other, preventing a weak signal from locking. A strong signal, however, will suffer from no more than patterning.

Times

Although reception may be possible at any time of the day, signals seem to be strongest in the morning between 10 a.m. and 12 noon. Viewing at this time has the advantages that, in most countries, test-cards are being shown, so making identification easy.

Programmes

The best time in the afternoon is between 2 and 3 p.m. Test-cards are still being shown, but Portugal (or Spain) starts what seems to be a schools’ broadcast at about 2.30 p.m. (all times in BST).

At about 4 p.m., a film is shown on Portuguese television. Often this is a British television film with the translated dialogue superimposed. Test cards then reappear at about 6 p.m. At 8 p.m., the programmes restart on Portuguese and Spanish television.

Reception Patterns

Reception at night seems to be more frequent though not as strong as day time reception. The reception of continental stations should be logged, carefully recording times, test-cards, and strengths so that some pattern of reception can be deduced and the best viewing hours found for your particular locality.

Finally, it must be repeated that reception of continental stations is unreliable, inconsistent and, to some extent, unpredictable… the modifications described do not automatically provide another 7 or 8 programmes.

You Say

2 responses to this article

Joanne Gray 13 September 2018 at 4:47 pm

I discovered DXing rather late on in the early 90s during a rather hot summer in my bedroom on the North East coast of England. I was trying to watch something on Tyne Tees (my set was connected to the antenna on the roof) and was rather annoyed at the interference on the picture. I noticed the interference seemed to be a showing of Wheel of Fortune (which wasn’t showing in the UK at that particular day of the week and time of day). I tried tuning into this interfering signal, but couldn’t get a lock on it, so I plugged in my set top aerial and spent the next few hours excitedly twiddling and fine tuning to a commercial channel that I eventually ascertained was Denmark’s TV2. And not only that, I was able to receive two or three regional variations of TV2 because they transmitted local news bulletins (I recall that one of the regional transmitters was called Videbaek). Over the next few days, as the sun shone on, I found I could pick up several testcards from TV2 regional transmitters from about lunchtime in full colour, and get grainy black and white pictures from Sweden, Netherlands and Germany. No sound, which was a little disappointing, but I discovered plenty to keep me occupied for a few days. Interesting to note that the adverts were mostly the same as the ones on commercial channels in the UK. I regret that I did not discover the joys of DXing years before that. And now it is too late – digital multi media has taken the joy out of viewing what our continental cousins watch.

Nigel Stapley 13 September 2018 at 10:12 pm

Of course, there are times when no specialist equipment is needed.

I remember one August morning in about 1983 when I picked up the vision signal from ZDF in Germany on a 12″ Slovenian b&w portable with just one of those old V-shaped set-top aerials (no sound, presumably because the picture/audio frequency spacing was differen in Germany).

This was in north Wales, albeit about 850ft above sea level.

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