I’m always being asked… 

8 August 2022 tbs.pm/75621

 

Ralph W. Hallows, m.a., m.i.e.e., technical adviser to TV MIRROR, quotes and answers some typical questions he has received from readers

 

TV Mirror Annual 1956 cover

From the TV Mirror Annual 1956

We’ll start with a question put to me by four or five readers every week: Why should a loud sound (or in some cases a high note) be accompanied by dark horizontal bands across the picture?

When this happens sound is breaking through into the vision circuits and the set is said to be suffering from sound-on-vision. The commonest cause is a valve which has become microphonic. In that state it vibrates under the influence of strong sound waves and turns them (just as a microphone does) into electric impulses. These are fed to the cathode-ray tube and produce the objectionable stripes.

Sound-on-vision can also occur, though all the valves are in order, if the TV set is not quite properly aligned. Correct alignment keeps sound signals in their right place and makes sure that they are fed only to the loudspeaker through the sound amplifier.

There is a light vertical line (or it may be two or three dark vertical lines) near the left-hand side of my screen. The trouble there is known as ringing. In some cases a single white line cannot be entirely got rid of; but it should be possible to make it so faint that it is not noticeable. Ringing is responsible, too, for another frequent query: Why do dark objects have additional light outlines on the right? The cure in both cases is the same. The service man should attend to the alignment of the set. It may sometimes also be necessary to reduce slightly the HF response.

 

Robert Morley with his head stretched

These pictures illustrate four typical receiver faults. Here, the picture is too narrow. Adjustment of width control is necessary

 

A newsreader, in high contrast

Too much contrast here, producing a “soot and whitewash” effect. The contrast should be reduced to give a greater range of tones

 

The 'batwings' clock with shadowing

Two faults here. One is a ghost image caused by TV waves “bouncing” off some object. The picture also needs re-centring

 

Horizontal lines

It looks like an aerial view of the railway tracks. The picture can be brought back by adjusting the horizontal hold

 

Sometimes the picture disappears (or it may be both picture and sound as well, or only the sound) quite suddenly. I can get the set working again by switching a light on and off. Here the breakdown is caused by an intermittent fault, which is not in evidence when the set is cold, but develops as it warms up. If both sound and vision go, the fault is in a part of the set common to both; it is in a vision circuit if the sound is still there and in a sound circuit if the set becomes silent, though the picture remains.

Using a lighting switch causes a little “kick” to take place in the mains voltage. This gets into the TV set and may be sufficient to clear a small intermittent fault temporarily. What you should NOT do (though some readers write that they make a habit of it) is to get rid of an intermittent fault for the time being by switching the set off and then quickly on again. Here’s why it’s risky:

When you switch on a cold set it doesn’t instantly come into action. The electric currents and pressures increase more or less gradually and there is no sudden, violent strain. But switch on a hot TV set and matters are very different. Valves, CR tube and other delicate parts receive a terrific kick in the neck. You may get away with it time and again; but a good many readers have done it once too often and their letters make sad reading.

Intermittent faults can be very difficult to trace. It’s no use asking the dealer to cure one of them at your home; for it’s long odds that it won’t appear while he’s there. He should take the set to his place and run it until the fault does occur.

The picture has a vertical black border at the right. In “painting” the TV picture the line scan has to push the spot horizontally from left to right across the screen. If there is a vertical black border the line scan is not pushing it far enough. There are several possible reasons, such as a slight drop in the mains voltage, a defective line oscillator or output valve, or a faulty line-scan component.

A horizontal black border at the bottom, without the vertical one just mentioned, is a certain indication of valve or component defects in the frame scan, which is not pushing the spot far enough down the screen. The occurrence of both horizontal and vertical borders at the same time points to a “dud” HT rectifier, or to a biggish drop in the mains voltage. A really large drop may produce a shrunken picture with black borders all round it.

The picture often “rolls” and no adjustment of the frame (horizontal) hold will make it stay put. In this case the sync “kicks” are not able to lock the frame scan properly. A similar failure to lock the line scan may give jagged edges to vertical objects, cause the picture to be unsteady horizontally, or lead to its break-up into horizontal strips.

What the sync does, if it’s working properly, is to stop the outward travel of the spot and cause it to fly back to its starting point. You’ll see, then, that, if it’s to be effective, the “kick” must arrive while the spot is still travelling outwards. Suppose that the kick isn’t given until the spot has finished its journey: the sync can do nothing, for the spot has already started to fly back before it comes. Thus, if either the line or the frame scan is running a little too quickly, the sync cannot lock it.

That’s one possibility, but not the only one by a long way. The “kicks” are delivered by a valve called the sync separator. If the signals reaching it are weak, if the valve is past its best or if there is a fault in one of its circuits, the “kicks” may be of the wrong shape to do their job as they should.

Readers often tell me: The picture is pretty steady on studio programmes, but rolls (or breaks up) when films are transmitted. What that means is that the signal is on the weak side. The sync is just, but only just, able to take control on the studio programmes. Film scanners may produce rather smaller “kicks” and then the sync is too feeble to take charge. Those who suffer in this way are sometimes dwellers in fringe areas (remedies: careful alignment of the receiver and exact orientation of the aerial; or the use of a more elaborate aerial array, with or without a preamplifier). More often, they are users of indoor aerials in places much too far away from the transmitting station for these to be effective.

 

Cinemascope film - do not adjust your set

 

Objects on the left of my picture are far too narrow (or too broad); or the right side of the picture may be similarly affected. In this case the line linearity is poor. You can’t expect perfect linearity unless you are prepared to pay quite a lot more for your TV receiver, but it shouldn’t be noticeably bad. If there is a line-linearity control and if its use won’t improve matters, call in the service man. Should he be baffled, write direct to the makers.

A similar answer applies to the oft-recurring question: Why are people’s heads too long and their legs too short? In this instance it’s the frame linearity that is at fault.

Just one other stock query: What causes the picture to collapse, or fold up, into a narrow horizontal band across the middle of the screen? What’s happening there is that the frame scan has packed up. Only the line scan is working and the spot is travelling in that narrow light band 10,125 times a second from the left to the right of the screen. If it happens to you, switch off at once and don’t try to use the set again until the service man has put it right. The cause is a fault in the frame-scan circuit.

 

You Say

1 response to this article

Ronnie MacLennan Baird 8 August 2022 at 2:55 pm

“What you should NOT do (though some readers write that they make a habit of it) is to get rid of an intermittent fault for the time being by switching the set off and then quickly on again.”

How times change!

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Liverpool, Saturday 1 October 2022