The Tuning Signal, Test Card C, and Tracing Faults
17 Apr 2017 3 comments. tbs.pm/11675
From A Beginner’s Guide to Television by FJ Camm, published by George Newnes Ltd in 1958, 1959 and 1963, with additional images by Dave Jeffrey and Russ J Graham
At the commencement of each TV programme a standard tuning signal is radiated (see below) It is intended as a guide to receiver adjustment, so that this can be carried out before the actual programme commences.
The controls should be adjusted so that the central ring is circular and the segments around the circle should appear as depicted. This will ensure correct values of black and white, and correct linearity, and the Brilliance and Contrast controls should be set so that the various tones are as illustrated. In general, the Contrast control will govern the lighter shades, and the Brilliance control the lower shades. This tuning signal is used to check the performance of the receiver, but it must also be remembered that it will also indicate defects in the transmission itself, such as ghost images.
The controls on television receivers vary according to make, and so it is impossible to give specific instructions. In general, the controls which set Brightness and Contrast are the most important and they are inter-related. If this adjustment is not correct, the brightness of the picture will change in accordance with the intensity and position of the highlights in the studio.
The BBC recommends that the receiver should be switched on 15 minutes before the start of the programmes so that the circuits may have enough time to reach their normal working temperature. Next, turn the Contrast and Brightness controls fully down, then turn the Brightness control slowly up until a very faint glow is just visible on the screen. Next, turn it down slightly until the glow just vanishes. Now turn the Contrast control up until the topmost shapes on each side of the circle on the tuning card are white, and the shapes below them are light grey. Then, readjust the Brightness control so that the bottom shapes are black and those above them dark grey. Make a slight adjustment to the Contrast control to get the best contrast between the white and pale grey shapes.
Finally, adjust the Focus control to give the clearest definition to the vertical lines in the centre of the circle. On modern sets there are only two controls, and the major adjustments have been carried out by the makers.
Test Card C
A special test pattern is included in the morning television transmissions on weekdays, and at certain other convenient times, and it is shown below. It is designed to give an immediate indication of the performance of the whole transmitting and receiving chain. As the performance of the transmitting equipment is maintained in accordance with agreed standards, during the normal periods of radiation for test purposes, Test Card C serves as a check on propagation and the performance of the receiving apparatus.
The card, which bears the identification letter C, incorporates a number of patterns each designed to assess one particular characteristic, and they are listed as follows.
◯ Aspect Ratio
Concentric black and white circles surrounding the five-frequency gratings will appear truly circular when the width and height of the picture are adjusted to the standard aspect ratio of 4:3. The BBC is, however, experimenting with an aspect ratio of 7:4, a system which they term widescan. It is unlikely to be adopted for some time.
◯ Resolution and Bandwidth
Within the circles there are two groups of frequency gratings, each consisting of five gratings having black and white stripes corresponding to fundamental frequencies of 1.0, 1.5, 2.0, 2.5, and 3.0 Mc/s. In the left-hand group the 1.0 Mc/s grating is at the top, the frequency increasing towards the bottom, and in the right-hand group the order is reversed. The response of the whole system is required to be uniform to 2.7 Mc/s, so that the 2.5 Mc/s gratings should be clearly reproduced, but the 3 Mc/s gratings may be blurred. The picture must fill the viewing aperture during the test, with the black and white border visible.
A five-step contrast wedge appears in the centre of the test card. The top square is white, corresponding to 100 per cent, modulation, and the lowest square is black, corresponding to 30 per cent, modulation. The three intermediate squares should be reproduced as pale, middle and dark grey.
◯ Scanning Linearity
The background of the test card is a middle grey, bearing a graticule of white lines. The areas enclosed between the lines should be reproduced in all parts of the picture as equal squares.
◯ Synchronisation Separation
The border consists of alternate black and white rectangles, which facilitate recognising interference between the picture signals and synchronisation.
◯ Low-frequency Response
A black rectangle within a white rectangle is provided at the top and in a perfect system it would be reproduced as a rectangle of uniform blackness on a clean white background.
At present, imperfections in the transmitting system result in a slight streaking at the right-hand side of black area, even with a perfect receiver, but by experience it is possible to judge whether the reproduction is abnormal.
Reflections, which occur in propagation or in the receiving installation, are indicated by two single vertical bars, which should be reproduced without positive or negative images at their right-hand sides. The width of these bars represents a pulse of 0.25 μs.
◯ Uniformity of Focus
These are four diagonally disposed areas of black and white stripes corresponding to a fundamental frequency of about 1 Mc/s, and all four should be resolved uniformly throughout.
The size of the tuning signal and of Test Card C will, of course, vary according to the size of the tube. Each must completely fill the screen.
Of course, troubles develop in course of time, and readjustments are usually carried out by the dealer who is agent for that particular make of receiver. When a new receiver is installed all that it should be necessary to do is turn on the picture. Just as one would turn up the sound volume on a normal broadcasting receiver. In course of time, however, valve deterioration takes place, or components may vary in value. These will call for additional adjustments and these adjustments are made by resetting the pre-set controls at the back of the receiver.
Some of the elementary defects can be corrected by use of a simple system, but for regular servicing a very expensive and comprehensive range of instruments is required, and these include an oscilloscope, an all-purpose test meter for measuring the A.C. and D.C. voltages, and a special meter for measuring EHT, with readings up to 25 kW. This is the voltage used in most projection receivers. A valve tester, and a capacity bridge are other instruments which are necessary.
As I have already said, however, a simple universal or general-purpose meter will suffice for simple adjustments. It must cover D.C. voltages up to 500, A.C. voltages up to 250, and a low reading scale for testing heater circuits. The EHT voltages can be calculated without direct measurement.
We have seen that the cathode-ray tube is fed from the timebases in such manner that even without a signal a rectangular area of light known as the picture area and of the correct aspect ratio of 4:3 appears on the end of the tube.
We have seen that this area of light is modulated by the output from a visual receiver, and so produce a picture. The loudspeaker is fed from the sound receiver to produce the speech or music. From this it can be deduced that if there is a raster (picture area) and a signal is obtainable from a loudspeaker, the fault lies in the vision or video part of the receiver. On the other hand, a picture, unaccompanied by sound indicates that the sound receiver is faulty, and absence of picture or raster with a sound signal indicates trouble either in the tube or in the timebases.
The horizontal and vertical timebases are independent, and it is extremely unlikely that both will fail together, and thus absence of raster will indicate a fault in the tube or in the power supply to both timebases. A horizontal line only on the tube indicates failure of the frame timebase, and a vertical line indicates failure of the line timebase.
Once the functions of the receiver are understood, irrespective of make or design, a little reasoning will quickly indicate the path of the circuit in which to look for faults.