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&"TELEVISION TIMES" NEWN ES JANUARY 1951 PUBILICATI ON. Vol. 1 No. 8. Improving Video L.F. Response Mains Hum. Nature's IN THIS

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;ACTICAL TELEVISION, JANUARY, MI BOUT AERIALS & TELEVISION TIMES Vol. 1 No. 8 A NEWN ES JANUARY 1951 PUBILICATI ON Projection Television Making a Spot Wobbler Vpitgge- Multipliers. e;vicing- TV Receivers
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;ACTICAL TELEVISION, JANUARY, MI BOUT AERIALS & TELEVISION TIMES Vol. 1 No. 8 A NEWN ES JANUARY 1951 PUBILICATI ON Projection Television Making a Spot Wobbler Vpitgge- Multipliers. e;vicing- TV Receivers IN THIS ISSUE Making a Mains Transformer Improving Video L.F. Response Mains Hum Nature's PRACTICAL TELEVISION January, 1951 I hear you've bought a -Television set, old boy. Bought be blowed -1 built it! Why, I never knew you were one of these back -room boys -it's a pritty complicated job, isn't it? Not on your life! 'I didn't know the tlrst thing about television when I built my View Master, but the instructions are so simple that anyone who can read a diagram and use a soldering iron could malte it. But what kind of results do you get with it? Perfect performance. Not surprising really, because the View Master is a combined effort by eight leading firms. I built mine as 'VIEW MASTER' AERIAL vr ü (L to R) : Coaxial ; Balanced Twin Screened (U.K. Pat. No B) ; Twin Unscreened. ELEVISIdN p., CABLES LEA D-1% +N t`'0`'f We manufacture a complete range of Television Lead -in Cables for Local and Fringe Area service full details sent on request. In addition we make all types of Polythene and P.V.C. Cables for radio, radar and for all branches of H.F. work. Our long experience enables us to produce cables with specified characteristics and we shall, be pleased to put forward suggestions. W. T. Henley's Telegraph Works Co. Ltd., Hatton Gorden, E'.C.I a 12 Console, but I. had the choice of 9 or 12 and it could have been a table set if I'd preferred it. You've almost persuaded me to try my hand. How do I go about it Get a 5!- Constructor Envelope -it gives you eight full -size working drawings and a 36 -page book on building and operating. You'll find it dead easy, and most fascinating. ' Well, it's worth having a look at, anyway. up, and have ne on me! HOME -CONSTRUCTOR FOR EVERYONE Drink TELEVISION From wireless shops or 516 post free from View Master ' 10 Norfolk Street, London. W.C.2 e.d GI( Model TV '20' inc. 12 Tube and '21 valve oirnit operating on 200- ::50 volt A.C. mains with 'two main controls and SIX pre -set adjusters, completely enclosed E.H.T. unit and s, parate 10 1. - mdspeaker. Yes, Armstrong Television has been demonstrated to be absolutely efficient, particularly when used in the fringe areas., and all agree that the brilliant high definition in picture and excellent reproduction are worthy of the Armstrong tradition. Write now for complete specification. THE CHASSIS PEOPLE Armstrong Wireless Si Television Co., Ltd., Warlters Rid, Holloway, London, N.7. Tel.: NORth 321S i SOLE BIRMINGHAM AGENT : Hexes Company, 1, Alcestrr Road, Moseley, Birmingham, 'S. %12 SC21 lanusry, 1951 PRACTICAL TELEVISION 337 Telerision at 200 miles SC22 RETAIL PRICE LIST PRE -AMPLIFIER SC22,. L PRE- AMPLIFIER SC2I.. E PATTERN GENERATOR PGII... f SIGNAL GENERATOR SG12... L TRADE ENQUIRIES INVITED TELEVISION SIGNAL GENERATOR Frequency range Mks. Calibration chart for all Television Channels. Modulation on sound and vision optional. Sensitive meter fitted for use as grid dip oscillator. Ideal for service engineer and experimenter. Measures coil, aerial frequencies, etc. The only one of its kind on the market. Self -contained power supply v. A.C. 12 months' guarantee. Immediate delivery. TELEVISION PRE -AMPLIFIERS EUTTON COLDFIELD Two high -gain neutralised triodes with HIGH GAIN AND LOW NOISE. Customs built to the highest standards. Ample bandwidth for good definition. Ideal for the difficult, fringe and ultra -''ringe areas. Matches into any aeriai and receiver. Each pre -amplifier supplied guaranteed to have been air tested and to have received both vision and sound at 200 miles from Sutton Coldfield using a standard commercial superhet receiver of 50 micro - Model SC2I requires external power supply. [volts sensitivity. Model SC22 has self- contained metal rectifier power supply 203/250v. A.C. 12 months' guarantee. Immediate delivery. TELEVISION PATTER N Frequency range Mks. Calibration chart for all Tele. vision Channels. Modulation on both sound and vision. One horizontal and two vertical bars, full line and frame, etc., GENERATOR adjustments. Essential for the service engineer and serious experimenter. Seven valves. Power supply v. A.C. 12 months' guarantee. Immediate delivery. J. V. RADIO CO. 84 ÉgaROADr PLYMOUTH. PREMIER Long Range TELEVISOR KITS FOR LONDON AND BIRMINGHAM USING 9 OR 12 MAGNETIC C.R. TUBES (Carriage, etc., 15/ -) including all parts, valves and loud - speaker, but excluding C.R. TUBE The Vision Receiver, 4 R.F. stages (EF54s), Diode Detector and Noise Limiter (6H0) Video valve (EF54). Complet e Kit with valves, 53/16/0. Carriage 2/6, The Sound Receiver, 3 R.F. stages (08H7s), Doable Diode Triode (0Q7), which acts as Detector and L.F. Amplifier, Noise Limiter (EA50), output valve (6V6). Complete Kit with valves, 83/1/0. Carriage 2/6 The Time Bases, blocking oscillators on Line (6S117 and 807), and Freine (Vlt1a7 and 6V6).16.H.T. from Line Output. Transformer, 10in. P.M. Speaker, Sync. separators 6116 and OVO. Complete Kit with valves, 86/5/6. Carriage 5/ -. The Power Supply, double wound transformer isolating the receiver front the mains. Rectifier 5U4G. Complete Kit with valves, 84 /16/6. Carriage 5/ -. CONSTRUCTION BOOK 3/... USING VCR97 C.R. TUBE (Carriage, etc., is / -) Five Easy to Assemble Kits are supplied: Vision Receiver with valves, carriage 2/6.. =3/13,'6 Sound Receiver with valves, carriage 2/6 62/14/6 Time Base with valves, carriage 2/6.. 82/7/6 Power Supply Unit with valves, carriage 5/-.. 86/3/0 Tube Assembly, carriage and patting 2/6.. 82/18/8 This unit includes the VCR97 Tube, Tube Fittings and socket and a Gin. PM Moving Coil speaker with closed field for Television. The Instruction Book costs 2/6, but is credited if a kit for the complete Televisor is purchased. Any of these Kits may be purchased separately ; in fact, any single part can be supplied. A complete priced list parts of all will be found in the Instruction Book. 20 Valves are used, the coils are all wound and every part Is tested. All you need to build a complete Television Receiver are a screwdriver, a pair of a soldering iron and the ability to read a theoretical pliers, WORKING MODELS CAN BE SEEN DURING TRANS - MITTING HOURS A'l' OUR FLEET STREET AND EDGWARE ROAD BRANCHES. The foll5wing Sensitivity figures prove that the Premier Televisor Kits are capable of reception at greater distances- than any other Standard Commercial Kit or Receiver whether T.R.F. or Superhet. VISION RECEIVER. Sensitivity : 25 pv for 15v peak to peak measured at the Anode of the Video Valve. Sound Rejection Better than 40 db. Adjacent Sound Rejection : Midland Model. Better than 50 db. SOUND RECEIVER. Sensitivity : 20 pv. Vision Rejection : Better than 50 db. PREMIER RADIO CO. MORRIS î CO. (RADIO) LTD. ALL POST ORDERS TO OUR NEW FACTORY AT- 740, HIGH RD., TOTTENHAM, N.17. Tottenham CALLERS TO 152/3 FLEET ST., E.C.4. Central , EDGWARE RD., W.2. Ambassador This branch is open until 6 p.m. on Saturdays. 25, ; Dimensions 338 PRACTICAL TELEVISION January, 1951 Cap. in ;JF. CONDENSERS The abbreviated ranges of two popular types given here are representative of the wide variety of T.C.C. Condensers available. VISCONOL CATHODRAY CONDENSERS Cap. Range: '0005mfd. to I mfd. Voltage Range: 750 to 25,000 at 60'C. Max. Wkg. at 60 C. Dimer.s. (Overall) Length Dia. Type No. i. I! I ' j) in. '001 6,000 2;; in. in. E in. CP.57.HOO CP.55.Q0 '001 12,500 in., _ in. CP.56.V0 '01 6,000 3 in. Is'rin. CP.56.Q0 'I ; in. 2 in. CP.58.Q0 '25 5,000 5; in. 2; in. CP.59.M0 SUPER TROPICAL MINIATURE METALMITES lin Aluminium Tubes) Capacity '.P' Wkg. Volts D.C. at 71'C. at 100 C. Length Dia. Type No. ' ;in. '2 in. CPI IOS ' din. '2 in. CP110S '001 I 350 J ;in. '2 in. CPI ION ' ;in. '22in. CPI I IN ' ;;in. '22in. CPIIIH ' in. '34in. CPI I3N THE TELEGRAPH CONDENSER CO. ' LTD. Rodio Division : North Acton, London, W.3. Tel: Acorn 0061 By request At the request of many of our constructor friends, we give here full details of the famous range of Stentorian chassis. Type Cone dia. Flux Density (Gauss) Pole dia. Gap length Flux face Total Flux Speech coil Impedance (ohms) Handling Capacity (Watts) With Trans. E s. d. PRICES Without Trans. E s. d. *S 'S S. 507 *S. 610 S. 707 S. 810 S. 912 S S S ,; 3.f 5' 6 7' 8 9' 10 12 18 7, 7, 7,000.75 10,000.75 7,000 I 10,000 I 12,000 I 12,000 I 13, ' 14, .040 .040 .043 .043 .043 .043 .050 .0625 .093 5, 11, 14, 20, 27, 47, 47,400.25 106, 227, IO II I I 4 0' I 9 6 I * All chassis material is of Mazak 3 cxcept , cnd which are of Drawn Stec/ ei-st ß LOUDSPEAKER CHASSIS WHITELEY ELECTRICAL RADIO CO. LTD MANSFIELD ' NOTTS., TERAIIIM A & TELEVISION TIMES Editor : F. J. CAMM Editorial and Advertisement ()glees: Practical Television. George Newnes, Ltd., Tower Rouse, Southampton Street, Strand, W.C.2. 'Phone: Temple Bar Teregrams: Newnes, Rand, London. Registered at the C.P.O. for transmission by Canadian Magazine Post. Vol. 1. No. 8 EVERY MONTH JANUARY, 1951 Televiews Amateur TV *Broadcasts WE are glad to note that the P.M.G. has adopted one of the suggestións we made in a previous issue, by authorising the use of special frequency bands for amateur television broadcast experiments. As reported elsewhere in this issue the bands are 2,300 to 2,450, 5,650 to 5,850, and 10,000 to 10,500 Mc /s. These bands are extremely high, but no doubt the P.M.G. has in mind the possibilities of changing the present frequency as used by Alexandra Palace if experiments prove that the higher frequencies provide better results. We also made the suggestion, it will be remembered, that the B.B.C. should radiate special programmes for amateur experimenters, not, of course, on the existing frequency. This would enable many experi- menters who could not afford a complete transmitting and receiving system to build experimental receivers. We hope that a little later on he will see his way clear to do this. We also hope that once the amateurs have conducted vital experimental work the P.M.G. does not sequestrate the frequencies at present allocated to amateurs as was done in the case of radio. There are thousands of skilled technicians in this country keenly interested in television, and they can, without cost to the B.B.C., conduct experiments on a national scale which the B.B.C. itself could not possibly afford to do. The results will enable the B.B.C. to plot a television reception map of the British Isles which would be of enormous assistance to them when selecting the location of future stations, and the results would also be of use to their own designers. A large amount of credit for the excellence of our radio transmission is due to the amateur transmitter and experimenter. TV TIREDNESS? THE recent criticism by school -teachers that children were arriving at school in the morning tired -eyed through looking -in to television programmes at night does not reflect the intelligence which one should be able to associate with school -teachers. Presumably the scholars prefer the TV programmes to homework,' and it is nonsense to suggest that homework does not make them feel tired. Are we expected to believe that an entertaining TV pro- gramme tires them out, whereas homework does not? Or is the criticism intended to convey the impression that the programmes are so bad that they send people to sleep? U.H.F. TELEVISION TRANSMITTER A NEW type of transmitter that will aid in opening additional air lanes for television has been announced by Stanford Research Institution, California. It has been designed for transmitting signals in the ultra -high -frequency region of 475 to 899 Mc;s, recently authorised by the Federal Communications Commission for experimental television broadcast's. It is adapted to the present needs of the U.H.F. Experimental Station. at Longheath. As installed, the transmitter operates on 530 Mc1s and it radiates an entirely standard, amplitude modulated picture signal of high quality. At present it is radiating only 150 watts of power, but it is capable of being amplified by radio- frequency amplifiers to a much higher power. It is believed that this is the first time that phase -to- amplitude modulation has been applied to television. The heart of the transmitter is the phase modulator unit, which serves to advance the phase of one of the two signal channels by exactly the same amount that it retards the phase of the second channel. Another innovation is a vacuum tube which does the modulating work of several conventional tubes. The perfection of this transmitter is a step towards the opening of the U.H.F. region for commercial television programmes, which we hope one day to have over here. The Stanford Institute have also developed a converter for bringing U.H.F. signals to V.H.F., which is the standard for commercial receivers. BUILDING THE P.T. RECEIVER A REMINDER that readers who missed the details for building the television receiver designed and built in our laboratories may now obtain for 3s. 6d. a reprint of those articles in booklet form. Copies may be obtained from or through any newsagent, or direct from us for_ 3s. -9d: Orders shòuld he addressed to - the Publisher, Geo. Newnes, Ltd., Tower House, Southampton Street, Strand, W.C.2. F. J. C. PRACTICAL TELEVISION January, ÌJIXS HUM The Effects of Poor Smoothing in Modern Television Receivers By BERNARD BARNARD IT seems eery likely that every experimenter who embarks upon the thorny path of home television construction has, at some previous time, built at least one mains- operated radio receiver and, in this case, it is equally probable that he has had to deal with the problem of getting rid of mains hum. The problem of mains hum in television is one w hich underlines the fact that the human eye is one of the most C.P. Tute Light Ripple 'elfege Fig cycle ripple voltage and its effect on the picture. (Compare with Fig. 3.) intolerant of organs and whereas in sound radio an appreciable amount of hum is ignored by the ear, in vision only complete elimination is acceptable. The visual indication on the C.R. tube of the presence of 50 or 100 cycle ripple is not always as obvious and straightforward as is sometimes thought because, if only a small amount is present (which is usually the case), the effect may be to produce serious spoiling,of the picture which is suggest- ive of an entirely different fault. For instance, I have known two cases of hum which had been quite wrongly assessed as being due to shadow on ex- Government VCR97 and 517 tubes. It may be helpful to commence by enumerating the obvious effects of mains hum and then to analyse them and thus to suggest cures. The equipment -used for the various experiments made in the preparation of this article was a home -constructed outfit using a \'CR517 and the photographs were taken during actual transmissions ; they have not been faked in any way but the ripple effects that they illustrate were deliberately put on by removing relevant smoothing gear. The obvious effects of the presence. of mains ripple are as follow : (1) Dark bars across the picture. (2) A sine -wave edge to the picture. (3) Frame scan oscillator locking on the hum instead of the sync pulse. e Now let us examine these three effects in some detail in the order in which I have listed them. Firstly, dark bars across the picture. This is caused by the electron beam through the tube being modulated by the ripple voltage in much the same way as the signal voltage produces the light and dark parts of the picture ; since the frame sweep is occurring at 50 c.p.s. and the ripple voltage is alternating at either 50 or 100 cycles per second, the visual effect is a dark bar across the screen which will remain steady so long as the two frequencies remain the same.. If either frequency drifts, the bar will move slowly up or down the screen. The most likely electrode on the tube at which the mains ripple can modulate the electron beam is the grid.; but it is not the only one. A.C. ripple at the cathode, focusing amide or even the final anode can produce the same effect since the beam passes in twn through the field of all these electrodes. A Simple Test When considering the effect of this on a television picture, it is necessary first of all to modify the description dark bars across the screen. Figure 1' show s one cycle of 50 c.p.s. A.C. drawn in relation to the C.R. tube screen and it is readily appargpt that in addition to dark bars (when the ripple voltage goes negative) we also have light bars when the ripple goes positive. And it is very often. these light bars which have the most distressing effect on the picture because, if the brilliance control is adjusted to give reasonable detail in the dark areas, the light parts will be almost completely desoid of detail. This can be particularly noticeable in the Fig cycle ripple modulating the C.R. tube. (Compare with Fig. I.) Ripple has spoiled interlace and frame linearity -lines are spread out towards top of picture. January, 1951 PRACTICAL TELEVISION 341 fringe areas when the signal is weak and it is not readily apparent that the fault is due to hum. If you experience this trouble and want to check definitely whether it is due to mains ripple, the following is a simple and reliable test. Reverse the mains plug : this will reverse the phase of the ripple voltage with respect to the frame sweep and, if the trouble is due to hum, those parts which were dark before will now be light, and vice versa. This is perhaps a good point at which to stress that the amount of ripple voltage that is tolerable in a television receiver depends to a very large extent upon the strength of signal in the area in which the set is used. In the fringe areas it is quite possible for reception to be apparently hum -free during 90 per cent. of transmission time ; hum will only show up on those pictures which are represented by a low degree of modulation and therefore are weak at the C.R. tube. The hum bar will then show up on these shots and somewhat mysteriously disappear when the signal goes back to normal level. Another useful test which will give an indication of the actual hum level on any set is to adjust the brilliance control so that the raster is just visible, set the frame speed control to the usual position for reception and, with no picture transmission coming in, to notice the slight shadow which will be seen moving up or down the screen. This must be slight if the ripple is within acceptable limits and should disappear completely when the brilliance control is advanced. Ripple Edge Let us now turn to the second of the obvious effects mentioned above -the sine -wave edge to the picture. This is perhaps the most straightforward of ripple troubles. The cause is mains ripple getting into the time base H.T. supply and, in particular, on to the deflectbr plates which are associated with the line scan. We then have a state of affairs within the tube where the frame deflector plates are receiving a 50 cycle voltage from the frame oscillator and the lino plates, in addition to their normal function, are receiving a 100 cycle ripple voltage; the arrangement is exactly that of an oscilloscope giving a trace from the mains voltage and, if the line scan oscillator is disconnected, a single vertical line will be obtained showing two complete cycles of sine wave. The ripple frequency is almost certainly 100 in this case since the hum voltage is coming from the time base power pack which will be, in almost every case, a full -wave rectifier. To /OKil Fine/ Anode To Grid Fig. 2.- Additions to E.H.T. circuit wh :ch will help to remove 50 cycle ripple. It is necessary to look very carefully at the vertical edge of the raster to ascertain if a small amount of this type of hum is present. It can be quit
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