HE Radio Constructor AUDIO ELECTRONICS

RADIO TELEVISION -HE Radio Constructor AUDIO ELECTRONICS

VOLUME 18 NUMBER 3 A DATA PUBLICATION TWO SHILLINGS & THREEPENCE

October 1964

extra features for your

TAPE

RECORDER

Top Pocket Transistorised Simple M.W. Receiver * Transistor Mains Transistor Radio Calibrating Oscillator ^ for Beginners Intercom A NEW EDDYTONE MODEL THE RUGGED COMPACT `EC 10' TRANSISTORISED COMMUNICATIONS RECEIVER

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Five ranges give complete coverage from 550 kc/s to 30 Mc/s. Precision LIGHT anPORTABLE flywheel -loaded slow-motion drive with 110 to 1 reduction ratio. Uses GOOD thirteen semi -conductors, including REALLY stabilising Zener diode, resulting in PERFORMANCE excellent performance on all ranges. Internal speaker and panel jack for telephones. Self-contained battery unit. Aerial inputs for single wire aerial, balanced or unbalanced feeder, and for short rod or whip. Selective audio filter USEFUL IN CAR,CARAVAN, and SHACK for improved c.w. reception. Robust BOAT, LOUNGE construction and modern styling. Dimensions are 122 inches by 6$ S to 30 MC/S inches by 8 inches. Weight with 550 kc1 battery 141b. List Price £48

STRATTON ..& CO.LTD. BIRMINGHAM En:land THE NEW RADIO AND TELEVISION

by C. A. QUARRINGTON, A.M.BRIT.I.R.E.

RADIO AND TELEVISION is the most up-to-date work which deals with the principles, design considerations, applications, component parts, maintenance, adjustment and repair of radio and television. Essential for every radio and television engineer and enthusiast, these four magnificent volumes have been designed to meet the needs of all connected with this branch of engineering. By mastering everything in this work and using it for reference, you can join the ranks of men who are sought after because they know the trade from A to Z.

FREE INFORMATION BUREAU. To every purchaser we offer a free postal information service to keep him in touch with what is happening today, what happened yesterday, and even what may happen tomorrow.

SOME OF THE CONTENTS Waves in free space - Inductance - Capacitance - The tuned circuit SEND NOW FOR FULL INFORMATION - Detection - R.F. pentode - R.F. amplifier - The superheterodyne - Frequency changing stage .- A.G.C. and tuning indicators - R.F. coils, screens and switches - Output stage - Press -button tuning - To THE CAXTON PUBLISHING CO. LTD., Combined A.M. and F.M. receivers - Transistors and diodes - Power 1 Dept. 73/28, Gatehouse Industrial Estate, Aylesbury, Bucks supply and decoupling - High fidelity and stereo reproduction - Please send me full information about "Radio and Television" III Microphones and pre -amplification - Tape recording - Car radio - , Low -power transmission - Video signal - Waveform generator - , Synchronising circuits - Automatic gain control - Vision detection NAME (BLOCK LETTERS PLEASE) and amplification - Interference limiters - Colour television - , Aerials Fault diagnosis Workshop equipment Voltage and - - - ADDRESS. current testing - Testing basic components - Instability and motor - boating - Mains hum and background noise - Tracing distortion - , 1 Radio receiver alignment - Blank screen - Poor interlacing, television , - Pattern on screen - Television receiver alignment - Projection RT 73/28 111 television.

OCTOBER 1964 145 HI-FI AMPLIFIERS TUNERS RECORD PLAYERS ---w

S-33 S-99 AT -6 MA -12

HI-FI 6W STEREO AMPLIFIER. Model S-33. 3 watts per channel 0.3% GOLDRING LENCO TRANSCRIPTION PLAYER. Model distortion at 2.5W/chnl., 20dB N.F.B. Inputs for Radio (or Tape) and Gram, GL -58. With G-60 pick-up arm and Ronette 105 cartridge. £20.1.3 Stereo or Monaural, ganged controls. Kit £13.7.6 Assembled £18.18.0 GARRARD AUTO/RECORD PLAYER. Model AT -6. With DE LUXE STEREO AMPLIFIER. Model S -33H. De luxe version of the S-33 with two-tone grey perspex panel, and higher sensitivity necessary to R 105 cartridge. £13.12.1 With Decca Deram pick-up £14.6.1 accept the Decca Deram pick-up. Kit £15.17.6 Assembled £21.7.6 SUGDEN MOTOR UNIT "CONNOISSEUR CRAFTSMAN". HI-FI STEREO AMPLIFIER. Model S-99. 18W output. Ganged controls. Heavy duty motor operating at 334 and 45 r.p.m. Very heavy 12" Stereo/Mono gram., radio and tape inputs. Push-button selection. Printed turntable. Virtually no rumble. £17.2.11 circuit construction. Kit £27.19.6 Assembled £37.19.6 HI-FI MONO AMPLIFIER. Model MA-5. A general purpose 5W POWER SUPPLY UNIT. Model MGP-I. Input 100/120V, 200/250V. Amplifier, with inputs for Gram., Radio. Presentation similar to S-33. 40-60 c/s. Output 6.3V, 2.5A A.C. 200, 250, 270V, 120mA max. D.C. Kit £10.19.6 Assembled Kit £5.2.6 Assembled £6.12.6 £15.10.0 A wide range of American equipment available under direct mail order scheme. HI-FI MONO AMPLIFIER. Model MA -12. 12W output, wide Full details and catalogue 1/- post paid. freq. range, low distortion. Kit £11.18.0 Assembled £15.18.0 L50j Choose Heathki't mode/s for Performance FOR THE INSTRUMENTALIST TEST INSTRUMENTS PA AMPLIFIER PA -1. The T.V. ALIGNMENT GENERATOR. Model ideal compact unit for VOCAL- HFW-1. Offers max performance at lowest cost. ISTS, INSTRUMENTALISTS, Covers 3.6 to 220 Mc/s fundamentals. Elec- RECORDS, with 50 Watt out- tronic sweep oscillators. Built in marker put, 2 Heavy Duty Speakers. generators (5 Mc/s crystal) Variable TREMOLO. Elegant £30.15.0 Kit modern cabinet. Kit £54.15.0 £44.10.0 Assembled Assembled £74.0.0 HFW-1 5" GENERAL-PURPOSE LABORATORY OSCIL- Legs optional extra 17/6 set of 4 LOSCOPE. Model IO -12U. This outstanding oscil- POWER AMPLIFIER MA -50 loscope, with its professional specification and styling, 50W output. Kit £19.18.0 fulfills most laboratory and service requirements. Vertical frequency response 3 c/s to over 4.5Mc/s, sensitivity Assembled £27.18.0 IOmV r.m.s. per cm. at I kc/s.. T/B covers 10 c/s - 500 kc/s. Kit £32.12.6 Assembled £41.10.0 ELECTRONIC ORGAN 24" PORTABLE SERVICE 'SCOPE. Model OS -1. This (fully transistorised). Ideal for Soloists, Home use, Groups. FULL 20 is a light, compact oscilloscope, ideal for servicing, etc. WATTS VOLUME. £187.10.0 Matching bench £14.10.0 extra. uB Dimensions 5"x8"xl44" long. Wt. 1041b. -12U Kit £21.18.0 Assembled £29.8.0 IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIWIIInIIIMIIII110IIIIIIIIIIIIIIIIIIIIInIIMIIIIIIIIUIIIIIIIIIIIIIII11IIIIIIIIIIIIIIIIIIIII 111III1111II11IIWlllllllll1101lllllllllllllll11llllllllll IO ELECTRONIC SWITCH. Model S -3U (Oscilloscope TRANSISTOR RADIOS Trace Doubler). Enables a single beam oscilloscope to give simultaneous traces of two separate and independent "OXFORD" LUXURY PORTABLE. signals. Switching rates approx. 150, 500, 1,500, 5,000 Model UXR-2. Specially designed for use and 15,000 c/s. Sig. freq. response 0-100 kc/s. f 1dB. as a domestic, car or personal portable Separate gain controls and sync. output. Sig. input range receiver. Many features, including solid 0.1-1.8V r.m.s. Kit £12.18.0 Assembled £18.10.0 leather case. Kit £14.18.0 DE LUXE LARGE-SCALE VALVE VOLT- METER. Model IM -13U. Circuit and speci- TRANSISTOR PORTABLE. Model UXR- fication based on the well known model V -7A but with many worth -while refinements. 6" Ernest 1. Pre -aligned I.F. transformers, printed Turner meter. Unique gimbal bracket allows circuit. Covers L.W. and M.W. Has 7" operation of instrument in many positions. Modern x 4" loudspeaker. Real hide case. styling. Kit £18.18.0 Assembled £26.18.0 Kit £12.11.0 AUDIO SIGNAL GENERATOR. Model AG -9U. IM -13U 10 c/s to 100 kc/s, switch selected. Distortion less than TRANSISTOR PORTABLE RECEIVER. 0.1 °% 10V sine wave output metered in volts and dB's. Model RSW-1. In a ha:;dsome leather case, Kit £22.10.0 Assembled £30.10.0 it has retractable whip aerial and socket for car radio use. Covers Med., VALVE VOLTMETER. Model V-7A. 7 voltage ranges Trawler and d.c. volts to 1,500. A.c. to 1,500 r.m.s. and 4,000 peak two S wave bands. Kit £19.17.6 to peak. Resistance 0.10 to 1,000MG with internal battery. D.c. input impedance 11Mn. dB measurement, has JUNIOR TRANSISTOR RADIO. Model centre-zero scale. Complete with test prods, lead and UJR-1. Single transistor set. Excellent UJR-1 standardising battery. introduction to radio. Kit £2.7.6 Kit £13.18.6 Assembled £19.18.6 MULTIMETER. Model MM -1U. Ranges 0-1.5V to 1,500V a.c. and d.c.; 150µA to I5A d.c.; 0.20 to 20MO. Money -back Guarantee 44" 50µA meter. Kit £12.18.0 Assembled £18.11.6 Daystrom Limited unconditionally guarantees that each Heachkit R.F. SIGNAL GENERATOR. Model RF -1U. Up to product assembled in accordance with our easy -co -understand 100 Me/s fundamental and 200 Mc/s on harmonics. Up to RF -1U instruction manual must meet our published specifications 100 mV output. Kit £13.8.0 Assembled £19.18.0 for performance or the purchase price will be cheerfully refunded. A WIDE RANGE OF BOOKS ON ELECTRONICS AND RADIO AVAILABLE. SEND FOR LISTS OF PRICES

146 THE RADIO CONSTRUCTOR - *. TAPE DECKS CONTROL UNITS -- TAPE AMPLIFIERS

FM -4U AM/FM COLLARO D-93

Model FM -4U. Also available in two units. R.F. "STUDIO" TAPE DECK. The finest buy in its price HI-FI FM TUNER. COLLARO (£2.15.0 incl. P.T.) with I.F. output of 10.7 Mc/s, and amplifier Operating speeds: 31" and 74" p.s. Two tracks, "wow" tuning unit range. unit, with power supply and valves (£13.3.0). Total Kit £1$.18.0 and "flutter" not greater than 0.15% at 74" p.s. £17.10.0 TAPE RECORDING/PLAYBACK AMPLIFIER. Thermometer ' type recording indicators, press -button speed compensation and input selection. TAUVOX D93 TAPE DECKS. High quality stereo/mono tape Mono Model TA -1M, Kit £19.18.0 Assembled £28.18.0 decks. Stereo Model TA-IS, Kit £25.10.0 Assembled £35.18.0 4 track, £36.15.0 1393/4, 4 track, £36.15.0 D93/2, MONO CONTROL UNIT. Model UMC-1. Designed to work with the -I2 or similar amplifier requiring 0.25V or less for full output. 5 inputs. Available in two units MA HI-FI AM/FM TUNER. Model AFM-1. Baxandall type controls. 'Kit £8.12.6 Assembled £13.12.6 are sold separately. Tuning heart which, for your convenience, UNIT. Model USC -1. Push-button selection, incl. P.T.) and I.F. amplifier (AFM-Al- STEREO CONTROL (AFM-TI-£4.13.6 controls to ±IdB. Rumble and variable low- Printed circuit board, 8 valves. Covers L.W., M.W., accurately matched ganged £21.16.6). Printed circuit boards. Kit £19.10.0 Assembled £26.10.0 S.W., and F.M. Built-in power supply. Total Kit £26.10.0 pass filters.

1 1 1 1 1 1 11 1 111 1 1 1 1 1 1 1 1 1 1 1 1 111 111 1 1 111 1111 1 1 11 1 1 1 11 1 11111 111 11 1 11 11 11 1 1111 1 11 1 1 11 1 1 1 1 11 1 1 1 111 1 111 1 1 111 1 1 11 1 1 1 11 1 1 1 1 1 1 1 1 11 1 1 111 1 1 1 1 1 1 1 1 1 1 1 1 1 11 1 1 1 11 1 1 1 1 1 1 111 1 111 1 1 1 1 1 1 111 1 1 1111 1111 1 1 1 1 1 1 1 11 1 1 1 11 1 1 111 1 1 111 1 111 1 111 1 1 1 1 11 1 1 1 11 1 1 111 1 1 111 1 1 111 1 1 1111 1 1 11 1 1 1 11 1 1 1 1 1 1 1 1 1 11 1 1 1 111 11111 1 1 1111 1 1111 1 111 1 111 1 11111 1 111 1 1 11 1 1 1 1 1 1 1 1 1 111 1 1 111 1 1 1 111 1 1 1 1 1 11111 11111 1 11111 1 111 1 1 1111 1 1 1 11 1 1 1 11 1 1 111 1 1 1 111 1 1 1111 1 1 11111 11111 1 1 1 1 1 1 1111 1 1 11111 1 1 1 1 111 1 1 1 1 1 1 11111111111 1 1 1 111111 1 1 1 1 1 1 1111 ßeliability -The kitsets anyone can build "AMATEUR" EQUIPMENT SPEAKER SYSTEMS AMATEUR BANDS RECEIVER. Model COTSWOLD "MFS" SYSTEM. Specially RA -1. To cover all the Amateur Bands developed to give best possible results in from 160-10 metres. Many special features, small rooms. This minimum floor space including: half -lattice crystal filter; 8 valves; model is based on standard Cotswold. Size: signal strength "S" meter; tuned R.F. 36" high x 164" wide x 144" deep. Amp. stage. Kit £23.4.0 Assembled £30.15.0 Kit £39.6.6 Assembled £52.10.0 This is an acoustic- THE "MOHICAN" GENERAL COVER- RA-1 THE "COTSWOLD". GC -1U. With ally designed enclosure 26" x 23" x 151" AGE RECEIVER. Model speech 4 piezo-electric transfilters, variable tuned housing a 12" bass speaker with 2" B.F.O. and Zener diode stabiliser, this is coil, elliptical middle speaker together with a pressure unit to cover the full frequency an excellent fully doing purpose receiver for Amateur and Short range of 32-20,000 c/s. Capable of MFS wave listeners. Printed circuits, telescopic justice to the finest programme source, its aerials, tuning meter and large slide -rule dial. polar distribution makes it ideal for really Kit £37.17.6 Assembled £45.17.6 Hi-Fi Stereo. Kit £23.4.0 Assembled £30.15.0 160-10M TRANSMITTER. Model DX - 100U. Careful design has achieved high RG -1 performance and stability. Completely III -FI SPEAKER SYSTEM. Model SSU-1. self-contained. Ducted -port bass reflex cabinet "in the Two speakers. Vertical or horizontal Kit £79.10.0 Assembled £104.15.0 white". models with legs, Kit £11.12.0, without SSU-1 COMMUNICATIONS TYPE RECEIVER. Model legs, Kit £10.17.6. RG -1. A high performance, low cost receiver for the discriminating listener. Frequency coverage: 600 kc/s - 1.5 Mc/s and 1.7 Mc/s-32 Mc/s. Kit £39.16.0 Assembled £53.0.0 -OP HI-FI CABINETS A wide range of equipment cabinets SINGLE SIDEBAND ADAPTOR. Model SB -10U. are available to meet the differing May be used with most A.M. transmitters. Less than needs of enthusiasts. Designed for 3W R.F. input power required for 10W output. Operation or for or max. operating convenience SB -10U on 80, 40, 20, 15 and 10m bands on U.S.B., L.S.B. where room space is an overriding D.S.B. Kit £39.5.0 Assembled £54.18.0 consideration, this range includes Many other British `Amateur' models. Send for Catalogue. kits, ready assembled cabinets or fully finished cabinets, and has at least one model to suit your require- AMERICAN HEATHKIT "AMATEUR" EQUIPMENT ments. Send for full details. U.K. are limited, order as early as possible to avoid disappointment) Prices from £7.7.0 to £37.16.0 (As stocks in MALVERN 80-10M RECEIVER. Model SB -300E. This de luxe receiver offers unsurpassed value to the Radio Amateur. Of advanced design SEND FOR FREE BRITISH employing up-to-date construction techniques, its specification ensures unparalleled performance. HEATH KIT CATALOGUE Full details on request. Size 141" x 61" x 131". American catalogue also available 1/- post paid Kit £133.14.0 less speaker A fitting companion to this receiver is model obligation please send me (Tick here) SB -300E Without I SB -400E TRANSMITTER: Kit £165.4.0 FREE BRITISH HEATHKIT CATALOGUE. SEND FOR AMERICAN CATALOGUE. 1/- Post Paid FULL DETAILS OF MODEL(S) ...... Deferred Terms available in U.K. on all purchases over £10. (Please write in BLOCK CAPITALS) Full details available. NAME...... _._...... _. l ...... ADDRESS...... _...... _...... _.. IRC.10 DGLOUCESTER DEPT. DAYSTROM LTD L _

OCTOBER 1964 FOR THE FINEST RANGE OF LASICYS TRANSISTOR RECEIVERS We consider our construction parcels to be the finest value on the home constructor market. If on receipt you feel not competent to build the set, you may return it as received within 7 days, RADIO when the sum paid will be refunded, less postage. AND REALISTIC SEVEN THE SKYROVER THE SKYROVER DE LUXE * 7 -transistor Superhet. * 350 milliwatt * LONG WAVEBAND COVERAGE IS NOW output into 4" high flux speaker. * All com- AVAILABLE FOR THESE WELL-KNOWN SETS ponents mounted on a single printed circuit a board, size 5#" x A simple additional circuit provides coverage of the 1100/1950 M. 5#^ in one com- _. band (including 1500 M. Light programme). This is in addition to plete assembly. all existing Medium and Short wavebands. All necessary compon- Plastic cabinet wthwith con - Only 10 extra Post Free with carrying ents data. Y- handle, size 7" x This conversion is suitable for both models that have already 10" x 3#", available been constructed. in Blue/Grey or GENERAL SPECIFICATION. 7 transistor plus 2 diode super all Grey. * het, 6 waveband portable receiver. Operating from four 1.5V torch Easy to read Dial. batteries. The SKYROVER and SKYROVER DE LUXE cover the * External socket full medium waveband and short waveband 31-94 M, and also 4 for car aerial. separate switched band -spread ranges, 13 M, 16 M, 19 M and 25 M, * I.F. frequency with band -spread tuning for accurate station selection. The coil 470 kc/s. * Ferrite rod internal aerial. Oper- THE SKYROVER pack and tuning heart is completely factory assembled, wired and ates from PP9 or similar battery. * Full Controls: Waveband Selector, tested. The remaining assembly can be completed in under three comprehensive data supplied with each re- Volume Control with On/off hours from our easy to follow, stage by stage instructions. ceiver. * All coils and I.F.s, etc., fully wound Switch, Tuning Control. In plas- SPECIFICATION: Superhet, 470 kc/s. All Mullard transistors and ready for immediate assembly. tic cabinet, size 10" x 6#" x 3#", diode. Uses 4 U2 batteries. 5" Ceramic Magnet P.M. Speaker. An Outstanding Receiver. LASKY'S PRICE with metal trim and carrying Easy to read Dial Scale. Band -spread Tuning. 500mW Output. handle. for the complete parcel including Transistors, Telescopic Aerial and Ferrite Rod Aerial. Cabinet, Speaker, etc., and Full Construction Can now be built for H.P. WAVEBAND COVERAGE: 180-576 M, 31-94 M, and band - Data. Can be built for: Terms:- spread on 13, 16, 19 and 25 metre bands. 20/- dep.& All components available separately. Four U2 batteries 3/4 extra. P. and P. 4/6 £5.19.6 £8.19.6 11 months Data for each receiver, 2/6 PP9 Battery, 3/9. Data and instructions $ P. and P. 5/- extra at 16/6 extra, refunded if you purchase separately, 2/6. Refunded if you purchase the parcel. the parcel. THE SKYROVER DE LUXE REALISTIC De Tone Control circuit is incorporated, with separate Tone Control `Seven' Luxe in addition to Volume Control, Tuning Control and Waveband With the same specification as standard model Selector. In a wood cabinet, size I I#" x 64" x 3", covered with -PLUS a superior wood cabinet in contempor- a washable material, with plastic trim and carrying handle. Also ary styling. ALSO a full vision circular dial car aerial socket H.P. Terms: -25/- FOR ONLY r Con now be built for 19 6 deposit and II P. & P. es std. model E7 EXTRA P. and P. 5/- extra months at 20/- SUPER LAFAYETTE 12" FANE DUAL CONE Famous American Brand -Fully Guaran- SINCLAIR MINIATURES teed at record low prices. In sealed THE SINCLAIR SLIMLINE HI-FI SPEAKER cartons. The new 2 -transistor pocket radio, size only Type 122/í7A. Power handling 25 watts - 5in double play,'I,200ft. Mylar base ... 15/- 15 ohms imp. Flux density 17,000 gauss. 23/4" X 1 5/e" x 15/8". Micro alloy transistors 5in long play, 900ft. Acetate base ... Special and printed circuit. Easy to assemble. Anisotropie magnet. Listed at 12 gns. 5in standard play, 600ft. PVC base ... 18%6 CAN BE Limited stock. 51in long play, I,200ft. Mylar base ... 15- BUILT FOR 49/6 LASKY'S PRICE £6.19.6 P. & P. 5/ - 51in double play, 1,800ft. Mylar base... 22/6 51in long play, I,200ft. Acetate base ... 12/6 THE SINCLAIR MICRO -6 RECHARGEABLE NICKEL 51in standard play, 850ft. PVC base ... 11/6 DEAL 7in standard play, I,200ft. Mylar base... 12/6 Self-contained pocket radio. Size only 14/5' x CADMIUM CELLS long play, 13/10"x#" amazing performance. 7in 1,800ft. Mylar base ... 19/6 -truly Without Rating 1.22V 3.5 A.H. at 10 rate. 1,001 7in doubt the most advanced transistor hour double play, 2,400ft. Mylar base ... 25/- circuit uses for model makers, miniature electronic 7in long play, I,800ft. Acetate base ... ever offered to home constructors -yet may equipment, radios and tape recs., be in portable Sin message tape, 150ft ... 13/6 built an evening. Complete with earphone trans receivers, photo flash, etc. Hermetically Sin message tape, 225ft and detailed constructionp data. Mercury cell ...... 4/11 sealed. Size I 35/ BE 1/I1 3f" x f" x If". Listed at - Sin message tape, 300ft ...... 7/6 CAN extra (2 each. Sin BUILT FOR EO/6 required). LASKY'S PRICE each triple play, 450ft. Mylar base ... 12/6 15/- 4in triple play, 900ft. Mylar base ... 22/6 POST FREE. BANK of 9, 79/6. 5in triple play, 1,800ft. Mylar base ... 42/6 THE SINCLAIR TR750 51in triple play, 2,400ft. Mylar base ... 55/- Power amplifier -measures only 2" x 2". PICK-UP CARTRIDGES 7in triple play, 3,600ft. Mylar base. . 75/- Full 750mW transformerless output for 10mV Crystal P.U. Cartridges complete with P. & P. 1/- extra per reel; 4 reels and over Post Free. into 10kí1. Built-in vol. control and on/off styli -at lowest prices ever! switch. Ideal for use with Micro -6 or Slimline Mono C.T.I...... 4/11 receivers. Collaro Type C, stereo, 2 sapphires. :. 15/- HI-FI TAPE RECORDER HEADS CBAUIL EFOR 39/6 A5/. Col/el S.C.I. Stereo Diamond LP Sappl. High Impedance Record Play. Low impedance BUILT std...... 17/6 erase. Upper or lower track. All parts on the above sold sep. Sonotone 8T, stereo, 2 sapphires ... 19/11 LASKY'SiL Sonotone 2TA, mono, 2 sapphires ... 15/ - TRANSFILTERS by BRUSH CRYSTAL CO. PRICE 71 Post free per pair. State Postage I/- each extra. track required when ordering. TO -01B 465 kc/s ± 2 kc/s TO -01D 470 kc/s ± 2 kc/s TAPE DECK MOTORS MARRIOTT "X" Type } track heads. Record/ TO -02B 465 kc/s ± I kc/s 616 each Motors for the Collard Studio Play and Erase. TO -02D 470 kc/s I kc/s new and :t P. & P. 6d. Deck - unused. LASKY'S TF -01B 465 kc/s f 2 kc/s 200-150V A.C. Take-up and PRICE PAIR TF -01D 470 GNS. kc/s f 2 kc/s rewind. Listed at E2 5s. each. f -Track heads Record/Play and Erase 59/6 pair. LASKY'S PRICE 14/II each. GUITAR PICK-UP P. & P. 2/6. Crystal -high imp. Size only If" x1"x#". Clips State motor required when ordering. TRANSISTORS to fingerboard -no screws. Complete with New and cable. High quality tape deck capstan motor made by All Guaranteed LASKY'S E.M.I. Holland. Bi-directional. Size: 4" dia., x GET.SI, GET.S5, GET.S6-2/6; 837A, 874P- 1 5/1 1 P. & P. 1/ - 2" high. I" x f" spindle. 3/6; OC.45, OC.71, OC.8ID-4/6; OC.44, LASKY'S PRICE 19/11. P. & P. 3/6. OC.70, OC.76, OC.81 (matched pair 10/6)- TAPE POSITION INDICATOR 5/6; AF.I 17, OC.75, OC.171, OC.200-6/6; mi Open type -as used by most makers. With RECORDING TAPE SPLICER OC.42, OC.43, OC.73, OC.82D-7/6; OCP.71 re -set knob: By famous manufacturer -all metal construction. -9/6; OC.28, OC.201, OC.204-15/-; OC.205, 3 DIGIT 7/6; 4 DIGIT 10/6. P. & P. 9d. on each. Listed at 18/6 LASKY'S PRICE 14/6 Post Free OC.206-19/6; OC.28-24/6. 33 TOTTENHAM COURT ROAD W1 207 EDGWARE ROAD W2 152/3 FLEET STREET EC4 2 min.. Oxford St. Nearest Station: Goodge St. Few yards from Praed St. (ELECTRONICS Fleet St. Ltd) FLEet 2833. M USeum 2605 PADdington 3271/2 Open all day Thurs. Closed 1 p.m. Sat. Tottenham Ct. Rd. & Edgware Rd. addresses open all day Sat. Close I Thur. Please address all Mail Orders to Dept. W at above Edgware Road address

148 THE RADIO CONSTRUCTOR Salim

AS A CAREER OR AS A FASCINATING HOBBY...

THERE IS AN I C S COURSE FOR YOU! PRACTICAL INSTRUCTION! By taking an ICS specialised home study course you gain a sound theoretical and practical knowledge of valve and transistor circuits and servicing work while building your own 5 -valve receiver, transistor portable (or a.f. amplifier), signal generator and multi -test meter. All under expert tuition ! 1 ICS Construction Courses have been specially written and designed by experienced radio engineers to enable YOU to learn easily and quickly, Assembly and use of signal generator and multi -test meter in your own home, in your own time. (especially valuable in servicing work). ICS training will take you from basic principles to advanced applications of various circuits, all made up from the kits supplied. With the ICS Complete Radio and Electronics Construction Course you get SIX kits, including testing instruments, tools and first class components. And, what is most important, a personal library of Instruction Manuals. If you are making a career in the Radio Industry, you will be interested in the wide range of exami- Construction of 5 -valve 2-waveband AC/DC superheterodyne nation and specialist diploma courses offered by receiver, and a number of instructional experiments, using testing ICS. These include: instruments. Radio and Television Engineering, Industrial Television, Radio and Television Servicing, Colour Television, Electronic Technicians, Servomechanisms, Radio -Electronic Telemetry, Instrumentation, Principles of Automation, Calculus for Electronics, Digital and Analogue Computers. Specialised coaching for Grad.I.E.R.E., 3 C. &G. Telecommunication Technician's Cert. Radio &TV Servicing Cert.(R.T.E.B.), P.M.G. Certs. and Radio Exam. Construction of 6 -transistor (with semiconductor diode) 2 -waveband Amateur's portable, and a number of instructional experiments, including Examination students coached until successful a.f. amplifier with microphone pre -amplifier.

ICS (DEPT. 248) INTERTEXT HOUSE For full details PARKGATE ROAD, LONDON, S.W.11 ADDRESS of ICS Radio and NAME. ...., I

Electronics Courses ...... (

post coupon AGE 10.64 TODAY INTERNATIONAL CORRESPONDENCE SCHOOLS Member of the Association of British Correspondence Colleges

OCTOBER 1964 149 THE SMALLEST SET ON EARTH SINCLAIR MICRO -6 SIX STAGE RECEIVER

Over 8,000 built and in use in all parts of the world

THE SINCLAIR MICRO -6 continues unchallenged as the most remarkable receiver of its kind ever made available to the public anywhere in the world. It has special 6 -stage circuitry, and is, at the same time, the smallest set on earth. Everything except the lightweight earpiece is contained in the smart, minute white, gold and black case which is n I 14/5" x 13 appreciably smaller than a matchbox, as the illustration /5/10 /2" shows. With vernier -type tuning control, bandspread over the higher frequency end of the medium waveband WEIGHS UNDER 1 oz and powerful A.G.C. to ensure fade free reception of the most distant stations, the Micro -6 provides remarkable standards of performance. Quality of reproduction is outstandingly good, and again and again, the set is reported PLAYS IN CARS, BUSES, to give excellent results where other sets cannot be used CHASSIS TRAINS at all. The Micro -6 cannot be too highly recommended, VIEW both as an intriguing design to build, and a most practical BANDSPREAD TUNING FOR EASY radio to use. RECEPTION OF LUXEMBOURG Easily built in SINCLAIR TR750 POWER AMPLIFIER a single evening With built-in Using components never before Designed specially for use made available to the public, the volume Micro -6 is nevertheless easy to build. All parts including lightweight ear- control with the Sinclair Micro -6 piece and 8 -page instructions manual and come to THE TR750 (for building yourself or available switch ready built) measures only 2" x 2". It will provide powerful loudspeaker reproduction from the Micro -6 which can then be used as a car radio or domestic or portable loudspeaker set. The 5916 TR750 also has many other applications such as record reproducer, intercom or baby alarm. An "TRANSRISTA" black ny- output of 750 milliwatts for feeding into a Ion strap for wearing the standard 25-300 loudspeaker requires only a Micro -6 like a wrist -watch. 10mV input into 2kft. Frequency response 716 30-20,000 cis IdB. Power required -9 to 12 volts. t MALLORY MERCURY CELL ZM312 (2 ed) ' I All parstru with, Ready built and each in uctions tested with come to 39/6 instructions Handy pack of 6 cells 10/6

sinlair UNIQUE SINCLAIR GUARANTEE The following unconditional guarantee applies to everything you buy from Sinclair SINCLAIR RADIONICS LTD Radionics Ltd: If you are not completely satisfied with your purchase (we are confident you will be COMBERTON, CAMBRIDGE delighted) your full purchase price will be refunded instantly and without question. Telephone COMBERTON 682 * FULL SERVICE FACILITIES ALWAYS AVAILABLE TO SINCLAIR CUSTOMERS

150 THE RADIO CONSTRUCTOR SINCLAIR X-1 INTEGRATED 10 WATT HI-FI AMPLIFIER AND PRE -AMP

A radical A 100% departure British Design

A special feature of the X-10 is that from the 4 output transistors do not get hot even at full output because the circuit converts almost 100% of the power from battery or mains unit conventional into audio power for the loudspeaker. amplifier design * Number of transistors 11 * Overall size 6" x 3" x * Input Sensitivity 1mV SINCLAIR X-10 combined 10 will run for about 3 months with THE distortion >0.1% watt amplifier and pre -amplifier (Pats. ordinary use from two 4/- Ever * Total harmonic applied for) is so advanced in design that Ready 996 batteries) and great savings * Output power 10 watts amplifier ever in space AND COST. Furthermore, it outdates every type of * Frequency response 5-20,000 c/s made available to constructors, hi-fi the Sinclair X-10 is so designed that ± 0.5dB enthusiasts, experimenters and industrial the purchaser can select the tone Speaker impedance 150 users. Its unique eleven transistor control and input matching system * circuit solves once and for all problems appropriate to his requirements. A * Damping factor Greater than 100 is supplied inherent in conventional transistor fully descriptive manual * Quiescent consumption 75mA amplifier design so that users of the with each X-10 purchased complete Sinclair X-10 system enjoy far better or for building. This is truly the * Supply voltage 12 to 15 volts reproduction, true 10 watt output for amplifier of tomorrow-and it can less current consumption (the amplifier be yours today! Cost of all parts for building together with In- £5.19.6 p M ensures greatly improved struction Manual

standards of performance Ready Built and The Sinclair X-10 is the only amplifier in transistors, four of which are used in a new Tested together the world which enables constructors and type of output stage and P.W.M. plus many with Instruction £6.19.6 domestic users to enjoy the benefits of this other circuit refinements result in an amplifier Manual unique system. The use of P.W.M. ensures which is compact, rugged, stable and does not much better transient response-it is instantly require a heat sink-and it costs so little. noticeable the moment you hear it-no falling Used in pairs the X-10 brings new depths to Mains power sup- off in the higher audio -frequencies, no inter - stereo listening and there are no channel ply unit (A.C. £2.14.0 modulation distortion and a response curve matching problems. 200-240 volt) so flat you could draw it with a ruler! Eleven r MM IIIMIIUME31111111vMIll TO SINCLAIR RADIONICS LTD., COMBERTON, CAMBRIDGE. Comberton682 ÓPlease send items detailed below £ s d I NAME

I ADDRESS

I ' TOTAL £ For which I enclose cash/cheque/money order RC.10 - a - - - - - ME M - ...... - ...... ------151 OCTOBER 1964 ADD TAPE TO YOUR HI-FI WITH A MARTIN RECORDAIiIT Like famous Martin Audiokits, Martin Recordakits make building easy no matter how little experience you may have had. With a Martin Recordakit you can either build a complete tape recorder (with deck and portable -type case if desired) or assemble a pre - amp to connect the deck to existing amplifier system. There are Recordakits for two or four track decks and, when finished, you will enjoy performance and quality of wonderfully high standards more usually associated with far costlier equipment. Building is very easy and prefabricated sections are tested individually before despatch from our factory. Printed circuit board suctions supplied complete tested and with valves in position Money saving en Recordakits. and efficient too! Kits well packed in PRE -AMPS fitted cartons and 2 Track Pre -Amp Kit, 8 gns. 4 complete down to last screw and measured Track AND length of wire. Pre -Amp Kit, 9 gns. Kit with polished COMPLETE wood Cabinet and Deck from 17 gns. UNITS IN Full assembling and operation instructions. EASY -TO -BUILD MARTIN KIT FORM Build and save with Martin ELECTRONICS LTD. ' Details of Recordakits please TRADE ENQUIRIES INVITED your to IName ...... 154-155 HIGH STREET, BRENTFORD, MARTIN MIDDLESEX ' Address ...... ELECTRONICS LTD Telephone ISL 116112 ' RC.10.

LATEST HIGH QUALITY COMPONENTS FROM THE MAXI -Q RANGE .. . TESTED AND TRIED OVER 5 FREQUENCY RANGES

gru TRH NSISTOR TUNING COILS

Coils for transistor superhets or converters, with or without an R.F. stage and using 465 kc/s or 1.6 Mc/s I.F. * Noval B9A Based for Plug-in application-Screw threaded for Chassis application. * Formers moulded in low -loss polystyrene for best results. Each * coil is packed in an aluminium container which may be used as a screening can. * Brass threaded adjustable iron -dust cores. The following colour code identifies BLUE-Aerial coil the coils: with base input winding. YELLOW-Interstage R.F. RED-Oscillator coil for coil with couplings. 465 kc/s I.F. WHITE-Oscillator coil for 1.6 Mc/s I.F. PRICE 4/9 each. Coverages: Range 1T-.15/.4 Mc/s; 2T-.515/1.545 Mc/s; 3T-1.67/5.3 Mc/s; 4T-5/15 Mc/s; 5T-10.5/31.5 Mc/3. Full technical details now included in Technical Bulletin DTB.4-1/6d. IFT.13 Miniature 465 kc/s I.F. Transformer .. 7/6d. each GENERAL CATALOGUE covering IFT.14 Miniature 465 kc/s Last I.F. Transformer full IFT.16 Miniature 7/6d. each range of components send 1/6d. in stamps. 1.6 Mc/s I.F. Transformer .. 6/6d. each PLEASE IFT.17 Miniature 1.6 Mc/s SEND S.A.E. WITH ALL Last I.F. Transformer 6/6d. each OTHER ENQUIRIES. DENCO (CLACTON) LTD (DEPT. R.C.) 357/9 Old Road Clacton -on -Sea Essex

152 THE RADIO CONSTRUCTOR BON DACOUST Speaker Cab. Acous- 7 VALVE Volume Controls -5K-2 Meg - tic Wadding, AM/FM RADIOGRAM CHASSIS ohms, 3" Spindles superior grade, I" thick. 12" Morganite Midget wide, any length cut 1/6 per ft, 4/- per yd. Three Waveband & Switched Gram Valve line-up ECC85, ECH81 Type. 11" diam. Guar. I year. LOG EF89, EL84 ENAMELLED COPPER positions. Med. 200-550m. Long 1,000-"' - EABC80, or LIN ratios less Sw. 3/-. DP. Sw. WIRE- 2,000m. VHF/FM 88-95 Mc/s. Phillips ' EM81, EZ80 4/6. Twin Stereo less Sw. 6/6. D.P. fib reels, 14g -20g, 2/6; 22g -28g, 3/-; Continental Tuning insert with perme- Sw. 9/6 (100 k. to 2 Meg. only). 36g-38g, 4/3; 39g -40g, 4/6, etc. ability tuning on FM & combined AM/FM f Meg. VOL Controls D.P. Sw. TINNED COPPER WIRE -14-22g. IF -- 2/6 1 lb. transformers. 460 kc/s and 10.7 Mc/s. f" flatted spindle. Famous Mfrs. Dust core tuning all coils. Latest circuitry 4 for 10/- post free. PVC CONNECTING WIRE -10 including AVC & Neg. Feedback. 3 watt colours (for chassis wiring, etc.)-Single output. Sensitivity and reproduction of a or stranded conductor, per yd., 2d. very high standard. Chassis size 13}"x 6f". COAX 80 OHM CABLE Sleeving, Imm. and 2mm., 2d. yd., etc. Height 7f". Edge illuminated glass dial types: High grade low loss Cellular air KNOBS-Modern Continental I If" x3f", Vert. pointer Horiz. station Aligned Brown or Ivory with Gold Ring, I' dia., names. Gold on brown background. A.C. spaced Polythene - 1" diameter. 9d. each; and tested Corr. & I1", 1/- each; Brown or Ivory 200/250V operation. Magic-eye tuning. Stranded cond. Famous mfrs. Now with Gold ready f or Ins. 7/6. only 6d. per yard. Centre, I" dia., 10d. each; If', Circuit diag. now available, £13.10.0 1/3 each. LARGE SELECTION AVAILABLE. use. Bargain Prices-Special lengths: Comp. with 4 knobs-walnut or ivory to choice. Indoor FM aerial 3/6 ex. 3 S2 TRANSISTOR P.M. Speaker only required. 20 yds. 9/-. P. & P. 1/6. COMPONENTS Recommended Quality Speakers 10" Elac H/D, 30/-. Midget 13f" x 8" E.M.I. Fidelity, 37/6 12" R.A. with cone. Tweeter, 42/6. Carr. 2/6. 40 yds. 17/6. P. & P. 2/-. I.F.'s-465 kc/s ez" diam. 5/6 60 yds. 25/-. P. & P. 3/-. Osc. Coil-9s" diam. M/W. 5/3 Coax Plugs I/-. Sockets 1/-. Osc. coil M. & L.W. 5/9 Couplers 1/3. Outlet Boxes 4/6. Midget Driver Trans. 3.5:1 6/9 ANOTHER TAPE RECORDER BARGAIN Ditto 0/Put Push-pull 3 ohms 6/9 Manufacturers' end of production Surplus Offer Condensers-S/Mica all values 2pF to Elect. Condensers-Midget Type I5V I,000pF 6d. Ditto Ceramic 9d. each I mfd-50mfd, ea. 1/9. 100mfd. 2/-. A 24 gns. Tape Recorder offered at the .005, .01 and .1, etc., 1/-. Pape Ferrite Aerial-M. & L. W. with car bargain price of only 15 gns. plus 10/- carr. Tubular 450V .001 mfd to .01 mfd and aerial coupling coil, 9/3. Supplied in 3 Units already wired and tested. .1/350V 9d..02 -.I mfd 1/-,.25 mfd 1/6 Condensers -150V. wkg. .01 mfd. to A modern Circuit for quality recording 5 mfd 1/9. .04 mfd., 9d. .05 mfd., .1 mfd., from Mike Gram or Radio, using latest .25 mfd., 1/3. .5 mfd., 1/6, etc. Close Tol. 5pF-500pF B.S.R. Twin Track Monardeck Type TD2. S/Micas-10 % Tuning Condensers. J.B. "00" 208+ Valve line-up-EF86, ECL82, EM84, EZ80 8d. 600-5,000pF 1/-. I% 2pF-100pF I76pF, 8/6. 9d. 100pF-500pF lid. 575pF-5,000pF Ditto with trimmers, 9/6. and Silicon Diode. Send for detailed list - 365pF single, 7/6. Sub -min. 1" DILEMIN 1/6. Range 10 ohms 3d. stamp. Resistors-Full - 100pF, 300pF, 500pF, 7/-. IO megohms 20% 1 and fW 3d., ditto 10% 4d., 1W Sd. (Midget type modern Midget Vol. Control with edge control Completed Kit comprising items below knob, 5k12 with switch, 4/9, ditto less rating) I W 6d., 2W 9d. Hi -Stab 5% switch, 3/9. 1--}W 100 ohms I megohm 6d. Other BARGAIN PRICE ca Speakers P.M. -2" Plessey 75 ohms, 15 Gns. °/- values9d. I%fW 1/6. W/W Resistors 15/6. 25 ohms to IOK 1/6, 2f" Continental 8 ohms, 13/6. 2 -tone Cabinet and 8" x 5" Speaker. Size 14" 5W 1/3, IOW 15W 7" 4" Plessey 23/6. 2/-. Pre-set T/V Pots. W/W 25 x 35 ohm, x10#"x7}" ...... £3 10 0+5/ -Carr. Ear Wired Amplifier complete with 4 Valves, ohms -50K 3/-. 50K-2 Meg. (Carbon) Plug Phones-Min. Continental front Panel, Knobs, etc...... type, 3ft. lead, jack plug and socket. £5 12 6 + 3/6 Carr. High Imp. 8/-. Low Imp., 7/6. High B.S.R. Monardeck Type TD2 ...... £7 7 0 + 4/6 Carr. Accessories: Mike, Tape, empty Reel, screen- TRANSISTOR BARGAINS sensitivity M/coil 8-10 ohms, 12/6. ed Lead and Plugs, Instructions, etc...... £1 0 0 + 2/- Carr. Brand New-BVA Ist Grade Brand New. Mfrs. surplus 1st grade. 0C44 8/6 OC70 5/6 I 0C44 & 2 OC45, 15/6. I OCBID & 2 15/-. Jack Plugs. Standard 21" Igranic Soldering Irons. Mains 200/220V OC45 8/- OC71 6/- OC81, 00810 7/6 GEX34 2/9 All above and 0A81, 32/6, post free. Type, 2/6. Screened Ditto, 3/3. or 230/250V. Solon 25 watt Inst., Miniature scr. If", 2/3. Sub-min. 1/3. 2/OC81 15/6 OA70 2/9 22/6. Spare Elements, 4/6. Bits, 1/- GETI14 6/6 OA8I 2/9 JASON FM TUNER UNITS Jack Sockets. Open Igranic Moulded Designer-approved kit of paru: Type, 3/6. Closed 65 watt, 27/6 etc. OC72 7/6 GEXI3 2/9 Ditto, 4/-. Minia- 5 gns. 4 valves, 20/-. ture Closed Type, 1/6. Sub -min. (deaf Alumin. AFl l7 9/6 FMTI, Chassis. 18g. Plain FMT2, £7. 5 valves, 35/-. aid) ditto, 1/6. Stereo Jack Sockets, 3/6. Undrilled, folded 4 sides, 2" deep, JTV 10 Stereo Jack Plugs, 3/6. MERCURY gns. 6" x 4", 4/6, 8" x 6", 5/9, 10"x 7". 3 valves, 22/6. Phono Plugs, 9d. Phono Sockets 15/6. Goodmrs a.-3ns 3-}ß16/625" # E Rola 6/9, 12" x 6", 7/6, 12" x 8", 8/- etc, 15/6. 6" Elac 16/6. 7" x 4" Goodman JTV2 £13.19.6. 4 valves, 28/6. (open), 9d. Ditto (closed), 1/-. Twin NEW JASON FM HAND- Phono Sockets (open), 1/3. Alumin. Sheet. 18g. 6" x 6", 1/-, 15/6. 8" Rola 19/6. 10" R. x A. 25/-. 9" x 6" Goodmans 22/6. E.M.I. BOOK, 2/6. 48 hr. Alignment Grundig Continental. 3 p. or 5 p. 6"x9",1/6,6"x12",2/-,12"x12". Service 7/6. P. & P. 2/6. plug, 3/6. Sockets, 1/6. 4/6 etc. Tweeter 22/6. Speaker Fret-Expanded gilt ano- TRIMMERS, Ceramic (Compression dised metal 1" x 1" diamond mesh, Type)- 30pF, 50pF, 70pF, 9d.; I00pF, RECORDING TAPE Famous American Columbia (CBS) Premier 4/6 sq. ft., multiples of 6" cut. Max. 150pF, 1/3; 250pF, 1/6; 600pF, 1/9. quality tape at NEW REDUCED PRICES. A genuine recommended size, 4ft. x 3ft. 47/6. Carr. extra. Philips Concentric Type-2-10pF, 1/-; Quality Tape-TRY IT Brand new, boxed and fully guaranteed. Fitted with 3-30pF, If-. leader and stop foils. TYGAN FRET (c temp. pat.) 12" x METAL RECTIFIERS- STC Types- SPECIAL OFFER. 3" Mes- 12" 2/-, 12" x I8" 3/-, 12" x 24" 4/-, RM 4/9; Standard Double Play I, RM2, 6/3; RM3, 7/6; RM4, Play sage tape 150', 3/9; 3" L.P. 18" x 18" 4/6, etc. 16/-; RM5, 2l/-; RM4B, 17/6. 5" 600' 13/- 1,200' 31/6 900'g 17/6 225', 4/9; 3" D.P. 300', 6/6. 5f" 900' 10/- 1,800' 37/6 1,200' 19/6 P. & P. per reel 6d. 7" 1,200' 21/- 2,400' 47/6 1,800' 28/6 TAPE REELS. Mnfrs. sur- MULLARD "3-3" HI-FI AMPLIFIER plus 7", 2/3; 51", 2/-; 5", 2/-; Post & Package per reel, 1/- plus 6d. each for 3", 1/3; Plastic spool contain- 3 VALVES 3 WATT additional reels. ers, 5", 1/9; 5;", 2/-; 7", 2/3. 3 ohm and 15 ohm Output. New Reduced Electrolytics All Types New Stock A really first-class Amplifier Boxed VALVES Bargain Prices TUBULAR CAN TYPES giving Hi-Fi quality at a IT4 3/6 ECC83 7/- PCC84 8/- 2525V 19 8 +8/450V 4/6 reasonable cost. Mullard's I RS 6/- ECL82 10/- PCF80 8/- 50/12V 1/9 16+16/450V 5/6 155 6/- ECL80 PCL83 10/6 50/50V 2/- 32+32/275V 4/6 latest circuit. Valve line-up: 354 7/- EF80 7/6 PCL84 10/- 100/25V 2/- 50+50/350V 6/6 EF86, EL84, EZ81. Extra H.T. 3V4 7/- EF86 8/6 PL81 9/6 3 60 +250/275V 12/6 and L.T. available for Tuner DAF96 8/- EL84 7/- PL82 9/- 4/350V 2/3 100 +300/275V DF96 8/- EY51 9/- PL83 8/- 16+16/450V 5/6 12/6 Unit addition. This is the ideal DK96 8/- EY86 PY32 10/6 9/- 32+32/450V 6/6 2000+4000/6V companion Amplifier for FM DL96 8/- EZ8 I 79/6 PY8I 8/- 3/6 ECC81 7/- GZ32 PY82 7/- Ersin Multicore Solder 60/40, 4d. tuner units. ECC82 7/- EM84 U25 10/6 8/6 per yard. Cartons 2/6, etc. TECHNICAL SPECIFICATION-Freq. Response: ± IdE. 40 c/s-25 kc/s. Tone controls, max. treble cut 12dB at 10 kc/s. Max. Bass Boost 14dB at 80 c/s sensitivity: 100MV for 3W output. Output Power (at 400 c/s); 3W DE LUXE R/PLAYER KIT at 1% total harmonic distortion. Hum and Noise Level: At least 70dB below 3W. Incorporating 4 Speed Garrard Auto -Slim unit and COMPLETE KIT (incl. valves, all Bronze Escutcheon Panel, Print - Mullard latest 3 watt amplifier (ECL 86 and EZ 80), components, wiring diagram and ed Vol., Treble, Bass, On -Off, volume, bass and treble controls, with 8' X 5' 10,000 line speaker. special quality sectional Output Trans.) supplied with each kit. Contemporary styled BARGAIN PRICE £6.19.6 carr. 4/6. Recommended Speakers-R. Allen two-tone cabinet, charcoal grey and Complete wired and tested, 8 gns. 12" with tweeter 42/6, WBHF10-12 off-white with matching blue relief. Size: 17f" x 16" x 8". Wired power O/P socket and addi- £4.7.6, Goodmans Axiette £5.5.0, COMPLETE KIT Carr. & ins. 10/-. tional smoothing for Tuner Unit, Axiom IO £6.5.0, Audiom 51 E8.10.0. £13.1 9.6 10/6 extra. Carr. extra. Catalogue & construction details 2/6 (free with kit) STANDARD RECORD PLAYER KIT Send for detailed bargain lists, 3d. stamp. We manu facture all types Radio Mains Transf. Chokes, Quality Using BSR UA14 Unit, complete kit £11.10.0, 0/P Trans., etc. Enquiries invited for Specials, Prato carr. 7/6. Ready wired Amplifier, 7" x 4" types for small production runs. Quotation by return. quality Speaker and 0/P trans., £3.19.6, carr. 2/6. BSR UA14 Unit, £6.10.0, carr. & RADIO COMPONENT SPECIALISTS ins. 5/-. Rexine covered cabinet in two-tone 70 Brigstock Rd., Thornton Heath, Surrey maroon and cream, size 151" x x141" 8f" with THO 2188 Hours; 9 a.m.-6 p.m., 1 Wed. Terms all accessories p.m. plus uncut record player mounting C.W.O. or C.O.D. Post and Packing up to f lb. 9d., I lb. board 14" x 13", 59/6, carr. & ins. 5/-. 113, 31b. 2/3, 5 lb. 219, 8 lb. 3/6.

OCTOBER 1964 153 HOME RAD/0 of MITCHAM- DEPT. RC, 187 London -Road, Mitcham, Surrey. MIT 3282 WE ARE STOCKISTS SOME UNREPEATABLE BARGAINS!! OF THE WHOLE RANGE OF

Some exceptionally well made S.W. Tuning Condensers, silver plated MARTIN AUDIO KITS and split end vanes. I" spindle with Descriptive Leaflet extension for coupling two or more Send Today for together. Meters by Weston 2 inch scale Single gang size 14"x I*"x 11" Cap. SO Pf. 4/6 Worth 10/ - 500 Micro Amp full scale deflection. Scaled 0-15 and 0-600. Single gang size 1.1"x1*"X14" Cap. 60 Pf. 5/6 Worth 11/6 Price Twin gang size 2*" x14" x 11" Cap. 50 Pf. 8/6 Worth 15/ - 17/6

Also for S.W. Work: A high quality Output Transformer Air Tuner Condensers. 14" x 11" x 1" Cap. 20 Pf. 3/9 Primary 7000 Ohms. Secondary 3 Ohms. Worth 6/9 Suitable for Mullard ECL 86 Valve.

Air Trimmer Condensers. 14" x 1 }" x 1" Cap. 100 Pf. 2/6 for 3 Price 6/6 Worth 12/6 Worth 21/ - Air Dielectric High Efficiency Tuning Condensers with widely spaced Log -shaped Plates. 2" x 3" X 14" 20 Pf. 4/6 WE ARE STOCKISTS OF Worth 10/6 Similar Condenser to above, but plates not so widely spaced. RADIONICS KITS AND CIRCUITS 2" x 1 t" x 1 i" 160 Pf. 4/ - Worth 7/6 Send Today for Free Leaflet Please add 1/6 for Postage & Packing on all the above items HONE RADIO CATALOGUE New Edition! New Policy! With the arrival of the 10th Edition of the famous Home Radio Catalogue we announce a fresh policy. The new price of 5/- will just cover the cost of the paper and printing, but on the first page of the catalogue you will find 5 coupons, each worth 1/- if used as directed. Briefly the system will operate thus: for each complete £1 of your order you enclose one of the coupons and deduct 1/- from the money you send. There is no time limit so if you purchase £5 worth of components from us-even though spread over several years-your catalogue will have cost you nothing! r Please write your Name and Address in block capitals

We need only add that our new Catalogue i Name is bigger and better than ever ... so fill in the Coupon, enclose 6/- (5/- plus 1/- ' Address postage), and pop it in a letter box right away! I I

I I L------..1Home Radio Ltd. Dept. R.C., 187 London Road, Mitcham

154 THE RADIO CONSTRUCTOR Radio Constructor Incorporating THE RADIO AMATEUR

OCTOBER 1964

Top Pocket Transistor Radio, by G. Jeffries 156 Suggested Circuits No. 167: `Blowing Out" an Electric Light Bulb, by G. A. French 159 News and Comment 161 Vol. 18, No. 3 Transistorised Calibrating Oscillator, by D. H. Leslie 162 Published Monthly Inexpensive Output Meter, by E. S. Barker 164 (1st of month) Using the OCP71, by G. C. Dobbs, G3RJV 165 Simple M.W. Receiver for the Beginner, by E. Govier 166 Editorial and Ferroxcube Decoupling Beads, by M. J. Darby 169 Advertising Offices Understanding Radio, by W. G. Morley 170 W9 57 MAIDA VALE LONDON In Your Workshop 174 Experiences of an Early Amateur, Part 3, by C. H. Gardner 179 Telephone CUNningham 6141 Experimental 12AX7-12AU7 Gram Amplifier, by J. C. Noakes 182 Extra Features for Your Tape Recorder, by G. D. Howat 184 National TV and Radio Show Report 187 Telegrams Tubes, by J. B. Dance, M.Sc. 188 Databux, London Gas Filled Counting The New Dual-Standard TV Sets, Part 6, by Gordon J. King, Assoc.Brit.I.R.E., M.T.S., M.LP.R.E. 194 First Published The "Single -Span" Mains Driven Receiver, 1947 by Sir Douglas Hall, K.C.M.G., B.A.(Oxon) 197 Recent Publications 200 Simple Quality Oscilloscope, Part 1, by D. M. Williams, B.Sc. 201 Transistor Mains Intercom, by I. M. Rees 204 Radio Topics, by Recorder 208

© Data Publications Ltd., 1964. Contents may only be reproduced after obtaining prior permission from the Editor. Short abstracts or references are allowable provided acknowledgement of source is given.

Annual Subscription 33s. (U.S.A. and Canada $5) including postage. Remittances should be made payable to "Data Publications Ltd". Overseas readers please pay by cheque or International Money Order.

OCTOBER 1964 155 TOP POCKETRANSISTOR RADIO

By G. Jeffries

HIS LITTLE TRANSISTOR RADIO IS INTENDED FOR The collector of TRI is coupled via C4 to the fitting into the top pocket, and it was designed detector circuit given by the diodes DI and D2. after most of the bus and coach companies These allow the detected signal to appear at the base had barred the use of loudspeaker sets. of TRI which then carries out its secondary function The circuit employs the minimum number of of a.f. amplifier. The polarity of the two diodes is components to obtain the desired performance, and such that the base of TRI tends to go positive with its overall dimensions are only 41in long by 2- in increase in signal strength, with a result that a wide with a cabinet depth of approximately lin. rudimentary form of a.g.c. is obtained. The receiver incorporates two transistors, operates over the medium waveband, and drives a high Components List impedance earphone. It can also be used to feed (Fig. 1) a push-pull amplifier when operated at home, Resistors thereby giving loudspeaker reception. (All fixed resistors Ì watt 10%) R1 390k52 The Circuit R2 1k52 The circuit diagram of the two -transistor receiver R3 270kfZ appears in Fig. 1. In order to achieve the desired R4 5k52 potentiometer, log track, miniature, sensitivity with a ferrite rod whose length is limited rim adjust. (May be ganged with S1- to kin, the first stage is a reflexed r.f. and a.f. see text) amplifier with regeneration. The required signal is tuned in by the ferrite rod Capacitors aerial, L1, in conjunction with C1, and is fed to the C1 500pF variable, miniature, solid dielectric base of TRI via C3. An amplified r.f. signal appears C2 2-8pF concentric trimmer at the collector of TRI, and is prevented from being C3 0.01µF passed to the following a.f. stage by the r.f. choke, C4 500pF L2. A portion of the amplified signal is fed back C5 8µF electrolytic, 12V wkg. to the ferrite rod winding by C2 in order to provide C6 100µF electrolytic, 12V wkg. regeneration. Inductors L1 Ferrite rod aerial (see text) L2 R.F. choke, 2.5mH (Elpico)

Earphone Semiconductors BM C6 jack TR' 0C44 TR2 0071 DI 0A81 D2 0A81 Switch S1 s.p.s.t. on/off (see text)

TR2 Earphone 50052 to 1k52 impedance + Battery = D2 9V, type PP3 (EverReady) Miscellaneous Fig. 1. The circuit of the 2 -transistor receiver Groupboard, eyelets, cabinet, etc.

X56 THE RADIO CONSTRUCTOR Components List (Fig. 2)

Resistors (All resistors j watt 10%) R5 10052 R6 4.7kS1 R7 1052

Capacitor C7 0.1µF Transistors TR3 GET114 Matched pair TR4 GET114

Transformers Fig. 2. The optional power output stage. This enables T1 Driver transformer type LT44 (Henry's the receiver to operate a loudspeaker . Radio Ltd.) T2 Output transformer type L1'700 (Henry's from this diagram for ease of presentation, and it Radio Ltd.) connects between the centre tag of the volume control R4 and the base of TR2, the positive end Switch connecting to the transistor. S2 s.p.s.t. on/off switch The on/off switch, S1, is not shown in Fig. 3. This component is inserted between the negative Battery supply terminal on the groupboard and the negative 9 volt (see text) terminal of the battery. S1 may be a miniature slide type fitted to the side of the cabinet, or a switch Loudspeaker which is incorporated in R4. In the latter case, the 3S2 impedance wiring to R4 may vary from that illustrated in Fig. 3, which shows the connections applicable to a The amplified a.f. at the collector of TR1 passes potentiometer without a switch. through the r.f. choke L2, and is applied to the volume control R4. The slider of R4 taps off the The Ferrite Rod Aerial desired a.f. level, which is then fed to the base of Details of the ferrite rod aerial are given in Fig. 4. TR2. The latter functions as a high gain common emitter stage, driving the high impedance earphone. 27/16' A small amount of feedback is applied via R3. To C3 There is no stabilising circuit for TR2, which passes a relatively low current of about 1.5mA. In the writer's experience, and after a considerable period of use, there has been no evidence of thermal runaway using the circuit shown. Fig. 2 gives the circuit of the additional a.f. amplifier. This is quite optional and is only employed when loudspeaker reception is required. The circuit is quite conventional and consists of two transistors in push-pull offering an output of the order of 300mW. The amplifier couples to the receiver circuit of Fig. 1 by way of the terminal points indicated as A, B and C. 4 Construction A very convenient method of construction for the Fig. 1 circuit is given by the Paxolin groupboard technique, and the layout employed in the prototype is illustrated in Fig. 3. This diagram also shows the outside dimensions of the groupboard. The groupboard should initially be marked out and all the holes drilled. Holes required for connection points may then be fitted with eyelets. With To earphone the exception of R4, the components are all mounted Jack on one side of the board and take up the positions shown in the diagram. Capacitor C5 is omitted Fig. 3. The layout employed with the prototype

OCTOBER 1964 157 2l/2' required are a hole for the earphone jack, and an aperture for the on/off switch (if this is not ganged with the volume control). With the prototype, the groupboard was secured in the cabinet by a single nut fitted over the tuning capacitor bush. The final appearance of the completed receiver in its cabinet is shown in Fig. 6. 5turns 45turns Adjustment After the receiver has been completed and the Fig, 4. Details of the ferrite rod aerial. The total wiring carefully checked, the regeneration level has number of turns is 50 tapped at 5 turns, and the wire to be adjusted. is 36 s.w.g. enamelled rayon -covered wire The receiver should be switched on and the volume control set to maximum. A strong local The coil is home-wound and consists of 50 turns station is then tuned in and the receiver oriented for of 36 s.w.g. enamelled rayon -covered copper wire maximum pick-up by the ferrite rod aerial. Trimmer with a tap of 5 turns. The ferrite rod is 21-in long C2 is then adjusted until the receiver is just short of with a diameter of gin. The ferrite rod assembly going into oscillation. This procedure is then may be secured to the groupboard by clips or, more finally repeated at the high frequency end of the simply, by tying it in place with suitable thiead. band, working preferably with the Radio Luxem- It is important to ensure, when clips are used, that bourg signal. these do not cause an effective "shorted turn" to be formed around the rod. Tuning control The Cabinet The prototype cabinet was made from grin 'plume control Perspex, made up to the outside dimensions shown in Fig. 5. These allow a clearance of shin on all four sides of the groupboard. The depth of the Earphone cabinet, shown provisionally as lin, depends partly upon the clearance required on the non-component side of the board by the volume control, and on the size of the components around TR2. It is J necessary to fit the PP3 battery at the TR2 end of Phone jack the board, and the cabinet depth must be sufficient to enable it to be accommodated comfortably. Cabinet depth will also depend upon the manner in Fig. 6. The appearance of the completed receiver which the back is fitted, this point being left to the individual constructor's ideas. The cabinet is glued together by means of a Using the Additional Power Stage suitable Perspex adhesive, after which a hole has to If desired, the receiver may be employed from be drilled to take the tuning capacitor spindle, and time to time with the optional power output stage a slot cut for the rim of the volume control. Also shown in Fig. 2. In this instance, it is necessary to provide a simple means of interconnection between the two sections. It is also desirable to disconnect the small receiver battery and to employ a larger battery installed in the power amplifier unit. A simple means of interconnection for terminals A and C consists of fitting battery clips at the power amplifier, whereupon the battery leads in the receiver may be clipped to these. Connection to terminal B may be achieved by inserting a jack plug having a single wire soldered to its appropriate contact. In the writer's case, the additional power unit was built into an extension speaker cabinet, and fed to a 3i) loin loudspeaker.

Although, as stated by the writer, there was no evidence of thermal runaway in TR2 with the prototype, the theoretical risk of damage to TR2 is increased when the circuits of Fig. 1 and 2 are used in combination because the primary of Tt will have a much Fig. 5. The outside dimensions of the cabinet used lower resistance than is offered by the earphone. If this risk is with the prototype. The depth, shown here as considered sufficiently high it may be reduced by inserting a 47011 approximately lin, may vary according to the resistor between the negative supply line and the upper end of Tt primary, bypassing the junction with a capacitor of around 8p.F. components employed in the receiver -EDrroR.

158 THE RADIO CONSTRUCTOR The circuits presented in this series have been designed by G. A. French, specially for the enthusiast who needs only the circuit and essential data

1 6i suggested circuits

No. 167 "Blowing Out" an Electric Light Bulb

it Y THE TIME THIS ARTICLE APPEARS brilliance in some 20 seconds, after The current flowing through in print we will be well into the which it may be "blown out" once causes its temperature to rise and its autumn, and many of us will more. resistance to drop until, after a be turning our minds to the indoor To the uninitiated the effect is period, it reaches maximum tem- pastimes which are applicable at this quite puzzling. device the bulb glows at full time of the year and in the months may be made the focal point of a brilliance. If one blows on the to come. Typical of such pursuits game, the object of which is to thermistor, its temperature drops are parties, and in this respect the "blow out" the bulb with the and its resistance increases. By more technically minded members of minimum number of puffs. A game blowing vigorously three or four the family can often earn additional of this nature can be particularly times, the temperature of the ther- feathers for their caps by producing useful for children's parties. mistor can be sufficiently reduced, simple toys and games which not and its resistance sufficiently in- only amuse but mystify as well. The The Circuit creased, for the bulb to become device which is described in this The circuit of the device accom- completely extinguished. The ther- month's contribution to the "Sug- panies this article. As will be seen mistor commences to warm up as gested Circuit" series meets both it consists, quite simply, of a 6 volt soon as the blowing ceases and the these requirements and it employs low -consumption bulb, a thermistor, filament of the bulb starts to glow components which can be purchased a pre-set resistor, and a source of again after several seconds, its for several shillings only. So far as 27 volts. brilliance quickly increasing to the writer is aware, the particular When the on -off switch is closed, maximum. application involved has not been the 27 volt supply is applied to the It should be noted that, whilst the described before so far as the field of three components in series. The thermistor takes a relatively long "party gadgetry" is concerned. The thermistor is, at this time, cold and period to reach maximum tempera- circuit has a more serious use also, it offers a high initial resistance. ture immediately after switching on, since it can provide a convincing demonstration of the functioning of Pre-set a common electronic component. (see text) the device In its complete form, On -Off may be presented as a box on which is mounted a small electric light bulb shining at full brilliance. A parti- 6V O06A cipant is then asked to "blow out" T'1 in same manner as one the bulb the 27v blows out a candle. It will be found that, after three or four vigorous Thermistor CZIO i puffs, the bulb is completely extinguished, it becoming dimmer after each puff. Some moments after the lamp is extinguished it will com- The circuit of the device which allows an electric light bulb to be apparently mence to glow again, reaching full "blown out". The polarity of the 27 volt battery is unimportant

159 OCTOBER 1964 the time required for the lamp to The on -off switch may be any Alternative Combinations become illuminated again after it has rotary or toggle type, as desired. The writer checked the circuit been "blown out" is very short. The 27 volt supply is provided by using more than one bulb in series This is because, in use, the ther- a dry battery, the writer employing and a correspondingly higher voltage mistor works over a small tempera- three 9 volt transistor batteries in from the battery, but he found that ture range whose limits correspond series for the prototype. A drain of the larger change in resistance re- to full illumination and extinction of 60mA is rather high for dry batteries, quired of the thermistor was too the bulb. however, and alternative supplies great to be readily accomplished by The "secret" of the device lies, of are discussed later. blowing on it. Experiments with course, in the fact that it is the more than one thermistor were also thermistor which is blown upon and Setting Up unrewarding. not the bulb. In practice, the To set up the circuit it is necessary thermistor is mounted close to the to have available a milliameter Fairly good results were given by bulb, whereupon its very small size capable of giving reliable readings at using a single 6.3 volt 0.15 amp dial enables it to be rendered almost 60mA. Meters with full-scale deflec- lamp with a single thermistor, the completely inconspicuous. The ther- tions between 100 and 250mA will circuit running at 70mA. The lamp mistor specified is a wire -ended be satisfactory here. The meter is did not, of course, glow at full component having a length of -din inserted in series with one of the brilliance, but the circuit functioned and a maximum diameter of Ain battery leads. The pre-set resistor satisfactorily otherwise. only. Also, its body is black with is then adjusted to insert about 5011 By far the best results were given silvered ends, and the latter can be into circuit and the on -off switch with the single thermistor and the given a light coat of matt black closed. Initial current will be low, single 6 volt 0.06 amp lamp. enamel to prevent their reflecting the gradually increasing, after 40 seconds light bulb. from the As such, the or so, to about' 20 to 30mA, after Power may be easily which it will Supply thermistor `camou- commence to rise As flaged" by adjacent wiring or rapidly. If there is any indication of mentioned above, it may be decoration. It must be remembered, the current increasing above 60mA, considered that the 60mA drain however, that the thermistor requires more resistance must be immediately required by the circuit is rather heavy reasonable access to free air, so that inserted into circuit by the variable for dry batteries, and an alternative supply may be preferred. Although it may stabilise by losing heat by resistor. The variable resistor should not radiation and convection. be adjusted finally so that the current checked by the author, there is no reason why the circuit should not stabilises at 60mA. work The Components Circuit operation may next be equally well when powered by The two major components are the checked by blowing on the ther- a 50 c/s voltage whose r.m.s. value light mistor, and it is of the order of 27 volts. Such a bulb and the thermistor. The should be possible to voltage former is a Radio Spares m.e.s. pilot completely extinguish the bulb by could be obtained from the lamp having a rating of 6 volts at this means. The bulb should take secondary of a mains transformer. 0.06 amp. The latter is a Brimistor about 20 seconds to reach full Because of the risk of shock the a.c. supply must, of course, be isolated type CZ10, manufactured by Stan- brilliance again after it has been from dard Telephones and Cables Ltd. extinguished in this manner. the mains. The 60mA required by the bulb falls The on -off switch may next be It should be pointed out, here, comfortably within the 75mA maxi= opened and the thermistor allowed that the voltage required for power- mum operating current specified for to cool to room temperature. The ing the circuit is rather critical. If it the CZ10 thermistor. At the time switch should then be closed once is too low the thermistor will not of writing, the retail cost of the bulb more, whereupon it should be found pass enough current initially to warm is 9d. only, and that of the thermistor that about 60 to 70 seconds are up. If it is too high the resistance 1 s.. 6d. needed for the thermistor to reach change required in the thermistor The pre-set resistor should be a full operating temperature. increases. These points have to be wirewound type capable of being If desired, the value of resistance borne in mind when using power set to values of the order of 50 to inserted by the pre-set resistor may supplies other than the dry battery. 120t1. Any component having a be measured, whereupon it can be value of 150 to 50012 should be replaced by a fixed resistor of the With the a.c. supply, the circuit satisfactory here. The track of the same value. With the prototype may be set up to 60mA by inserting resistor should be capable of carrying circuit it was found that 8012 was an a.c. milliameter in series with one 60mA. Since the pre-set resistor is required with one particular ther- of the supply leads. Alternatively, only required to take up small mistor of the type specified and 7512 an a.c. voltmeter (provided it is a variations between individual ther- with a second. The milliameter may high resistance instrument) can be mistors, it can be replaced by a fixed finally be taken out of circuit, connected across the bulb, the pre- resistor of the appropriate value whereupon the device is complete set resistor being adjusted for a after initial setting up. and ready for use. reading in the meter of 6 volts.

E/do Telemetry Contract for EMI The Ministry of Aviation has recently extended EMI's responsibilities for the supply of telemetry equipment and services to Blue Streak, the first stage of the ELDO three -stage launcher, Europa I. These responsibilities now include the development of a new telemetry sender for use on the Blue Streak flight trials programme as from early 1965. Telemetry plays a very important part in the development programme. On the first Blue Streak firing at Woomera on 5th June, 1964, some 200 parameters were monitored and telemetered back to ground. This extensive and detailed information about the rocket's in-flight performance is essential so that each firing in the trials programme contributes in full to an orderly sequence of design proving tests.

160 THE RADIO CONSTRUCTOR NEWS AND COMMENT... Stereophonic Radio -1880 going to throttle radio, sound E.E.V. and A.E.I. Statement Shortly after these notes have broadcasting would be dead by Agreemebt has been reached for been written, judging for the Italia now. Yet consider these facts. A the English Electric Valve Co. Ltd. Prize 1964 will be taking place in recent research inquiry found only to purchase from Associated Elect- Genoa. The Italia Prize itself, is 4 per cent of the population declaring rical Industries Ltd. their Carholme for the best radio programme, and that they did not listen to radio Road factory at Lincoln, together the two B.B.C. entries are: "The at all, or had no radio sets. Over with the valve business being carried Flip Side" by Maxwell Charles the last six years, radio's average on there. Cohen, produced by H. B. Fortuin, daily audience in Britain has been The transfer of the factory will and "November Day" by Bill rising steadily, with marked increases enable the English Electric Valve Naughton, produced by Douglas each year. By the early weeks of Company to meet the rapid expan- Cleverdon. 1964, the figure had risen to a total sion of its business which is out- The B.B.C. has, from time to of 28 million people listening at growing the facilities now provided time, been criticised for not support- some time or other every day." by their factory at Chelmsford, ing sterophonic radio more en- From Sound Radio in the Tele- Essex. It is their intention to in- thusiastically, but it has an entry in vision Age, by Frank Gillard. crease still further the production this category in one of the associated of specialised electronic valves and events run in conjunction with the tubes used in television transmission Italia Prize. Tape Recorder in Court Some readers will have read of and telecommunications and event- The B.B.C. entry is "The Hearing ually to build up the labour force Aid", a programme not in fact yet the unauthorised use of a tape recorder at Bow at the Lincoln factory to some 1,000 heard, but due for transmission on Street Magistrates' people. the Third Network and TV sound Court recently. Learned The English Electric Valve Com- in October. This is a brief history Counsel interrupted a pany will lease to Associated Elect- of Stereophony, which, surprising policewoman witness to inform the Magistrate rical Industries a part of the factory, as it may seem, was the subject of that "there is a gentleman and A.E.I. are able to continue early experiment as long ago as the in court with an attaché case which, it has the development and manufacture of 1880s. A very early U.S. stereo been noticed, has been semi-conductors in the present pre- recording machine bore the incredible pointing at the witness box". mises. name of the Multiplex Graphophone The man had to leave the court Associated Electrical Industries, Grand. The programme' written by while the matter was looked into. one of the major manufacturers of Edward Greenfield and narrated The attaché case proved to have a semi -conductor devices in the United by Sir Adrian Boult includes early tape recorder inside and the Magis- Kingdom, will extend its operations tests and interviews with some of trate ordered it to be retained so in this field and semi -conductor the pioneer workers. The production that enquiries could be made as to production will continue at the is by Richard Keen. whether it was an offence or not Lincoln factory and at the A.E.I. to use a recorder in court. Rugby Works as at present; output Another Prize When the court was reconvened is to be stepped up. We referred in last month's it was announced that, although issue to the Radio Society of Great it was an offence to take a photo- A Real Constructor! Britain's annual International Radio graph in court, it was not an offence We would like to quote, part Communications Exhibition, to be to record the proceedings on a of a letter quote, in the August held again at the Seymour Hall, tape recorder. The owner of the issue of Mobile News, the excellent Marble Arch, London W.1, from recorder duly started it up again, monthly journal of the Amateur the 28th to the 31st of October unfortunately it then began to emit Radio Mobile Society. . I inclusive. Readers of The Radio a whining noise. The case was think we might cheerfully add that Constructor who attend are entitled again stopped and Mr. Babington, anyone who does not build his to enter a free competition, the the Magistrate, told the owner own motor car has no right to a prize for which is a £140 Hammer- you can play it to your driver's licence." lund HQ 170A Communication heart's content, but you must At what stage do we become, or Receiver. stop it squeaking." cease to become, radio constructors? Readers attending, who wish to The above true story is amusing, If we put together a kit, are we just enter the competition, should cut but it does raise important issues a radio assembler? If we save a out the advertisement for the for the owners of tape recorders. considerable amount of time by exhibition which appears on Page We have previously remarked on not winding our own coils, are we 212 and exchange it for the free invasions of privacy by transistor sort of cheating? entry form at the door. set owners, and we have heard of The foregoing reminded us of a friendship which was terminated another true story. A hobbyist Lecture Quotation because a private conversation was friend of a former member of our "Almost 13 million homes in recorded without the knowledge of editorial staff purchased a lathe. Britain now have television. Only the participants and then it was As far as is known he only ever about 2i million are left with played back to the embarrassment used it to make tools to use on the radio only. If television were of all concerned. lathe!

OCTOBER 1964 161 oR A DiÏORUE D DAm B R A ùuI10 O DE!L!AÙ O R By D. H. Leslie

NE OF THE CHIEF DIFFICULTIES FACING THE 100 kc/s calibrator occurred to the writer. The constructor of short wave receivers is the original valve model had been made up calibration of the on a small tuning dial. A very simple chassis, relying on the equipment under test for its method is to use a crystal controlled oscillator with power supply. The circuit a was of the Gouriet type fundamental frequency of 100 kc/s. The har- (Fig. 1) and gave harmonics up to monics of this about 15 Mc/s, oscillator provide markers at 100 kc/s but as the frequency increased these became very intervals which can be heard well into the shòrt wave weak and were often range, and masked by signals picked up the dial marked accordingly. Having on the short leads between the oscillator and the marked these points, one only requires to identify receiver. one particular frequency on each tuning range to In Fig. 1 a negative resistance proportional to have identified the corresponding 100 kc/s point. the gn, of the valve Standard appears at the terminals G frequency transmissions on 2.5, 5, 10, and E.* A tuned circuit connected at these points 15, 20 and 25 Mc/s and stations in the broadcast will oscillate provided the bands can negative resistance is be used to fix the 100 kc/s points. sufficient to overcome the loss resistance of the By plotting graphs of logging dial readings circuit. Small against adjustments in frequency may be marker points, intermediate frequencies can made by adjusting the trimmer CI. easily be found. Having once calibrated the receiver, the 100 kc/s marker is still useful as most An important point to be receivers, even noted with this type after initial warm-up, suffer from a of circuit is that if a triode is used the anode must slight variation in frequency from day to day or be at earth potential when with respect to r.f. Where a returning to one range from another. Thus pentode is used the screen is one can grid bypassed to earth mentally add or subtract the odd degree or and this acts as a virtual anode. A load can then so from one's normal calibration for a desired be placed in the anode circuit. frequency after checking the nearest 100 kc/s point. Using the basic circuit in Fig. 2, The experiments were transmitting amateur can use his v.f.o. near made using one of the new diffused alloy transistors, the band edges with complete peace of mind if he type MAT121. This has a very high knows precisely where current gain the band edge really is! and thus, by analogy, a high gm. As it is a "triode" the output was taken from the Gouriet emitter. At first the Oscillator circuit just would not oscillate. It was then realised Recently, whilst experimenting with transistor that the bias resistor R1 crystal was effectively connected oscillators, the idea of modernising his across the crystal and was probably causing too much damping. After a little "cut and try", com- HT+ ponent ' values were adjusted to give a reliable oscillator which "took off" each time the switch was operated, irrespective of the setting of the trimmer.

*A full mathematical analysis of the circuit, due to G. G. Gouriet, was published in Wireless Engineer, April 1950.

output T C2 a Fig. 1. The basic Gouriet oscillator Fig. 2. The basic transistor version

162 THE RADIO CONSTRUCTOR Base

Buse connections Fig. 3. The amplifier output waveform TRI TR2 R411Output Harmonics were strongly audible up to about 4 Mc/s but very weak beyond 9 Mc/s, even on a TR2 good communications receiver. What was required 2 was a distorting amplifier to enhance the harmonic IVVI output. This turned out to be the easiest part of the design. RZ

Adding An Amplifier T . Using a transistor (MAT100) with a high collector load and a high bias resistor to ensure bottoming on negative input peaks, and cut-off on positive Fig. 4. The final circuit of the calibrating oscillator peaks, an almost ideal (square) waveform was produced. (See Fig. 3.) The harmonics were now Components List found to be quite strong even at 32 Mc/s, which is (Fig. 4) as high as the author's receiver tunes; and this for a total battery consumption of about 0.5mA! Resistors The final circuit, shown in Fig. 4, was made up (All resistors iâ watt) in an old tobacco tin measuring approximately R1 2.2MS2 4 x 3 x lin. This gave ample room for all com- R2 10kS2 ponents, including a PP3 9V battery, without R3 1MS2 cramping. A ?din hole in the side allows a trimming R4 47ko2 tool to be inserted to adjust the concentric trimmer C1. A 15pF ceramic padding capacitor, C4, was Capacitors wired across C1 in order to allow the correct C1 5-35pF, air -spaced, concentric frequency to be obtained with the trimmer roughly C2 2,000pF in the mid -position. With other crystals a slightly C3 200pF different value may be required. In the interests of C4 15pF (see text) good frequency stability, mica compression types of C5 56pF trimmer are not advised. Transistors Oscillator Adjustment TR1 MAT121 To adjust the oscillator a short lead from the TR2 MAT100 output terminal can be placed near a long wave receiver tuned to the Droitwi'lh 200 kc/s Light way errors due to stray pick-up, image responses or Programme. C1 is adjusted until the low ripple is a missed marker can easily be seen and avoided. reduced to very slow beats. Alternatively a standard It is easier on the higher ranges to find the markers frequency transmission on 2.5, 5 or 10 Mc/s, etc., with the aid of the b.f.o. if the set has one, although may be used. Once adjusted, the oscillator will stay this should not be necessary if the set is reasonably within a few cycles per megacycle indefinitely. sensitive. If there is any doubt as to a signal being There is no point in trying to be too precise about a marker the oscillator can be switched off for a this adjustment. Five beats per second at 5 Mc/s moment. is an accuracy of one part in a million, and without Using lin graph paper with, say, 100 kc/s per inch, putting the crystal in a temperature controlled oven one can accommodate the calibration charts of a very conveniently in a one cannot expect anything better ! general coverage receiver To calibrate a receiver short twisted leads or "spiral" graph paper notebook. Parts of the band screened cable are used to reduce stray pick-up, and of special interest, e.g. a broadcast band, can be the receiver should have been switched on for at plotted using a larger scale. least half an hour to avoid any drifting during The value of this simple and reliable instrument warm-up. is out of all proportion to the moderate cost and A graph should be plotted of dial readings effort involved in its construction, and it is recom- against markers as the calibration proceeds. In this mended to any short wave listener or constructor. Corrections In the September issue, several diagrams were inadvertently transposed. On pages 94 and 95, Fig. 5 should be in the Fig. 4 position, Fig. 6 in the Fig. 5 position and Fig. 4 in the Fig. 6 position. Similarly, on pages 127 and 128, Fig. 18 should be in the Fig. 16 position, Fig. 16 in the Fig. 17 position and Fig. 17 in the Fig. 18 position.

OCTOBER 1964 163 The Circuit The circuit is shown in the accompanying diagram and was built around a thermoammeter with £££££££££££££££££ a full-scale deflection of 1 amp, and an internal resistance of 0.3552. ££ INEXPENSIVE The two resistors R1 and R2 were wound from 28 s.w.g. nichrome wire £££££££££££££££££ OUTPUT METER on to a ceramic former as used in electric fire pencil elements. R1 has By E. S. BARKER a value of 2.655 and consists of 10.2in of the wire. R2 has a value of 1252 and was given by 46.4in of wire. * The switch serves the dual purpose of changing the impedance of the circuit, and also the value for Whilst a number of different types of thermoammeter are currently the full scale power. available at low cost on the surplus market, these instruments do In the prototype it was not felt not readily lend themselves to applications outside transmitting work. necessary to recalibrate the instru- dial this is a They can, however, be employed as audio output meters by the ment but matter of personal choice. simple addition of one or more series resistors. Our contributor The Table shows the relationship describes a typical output meter built around a thermoammeter between meter indication and power. having a resistance of 0.3511 and an fs.d. of 1 amp. Provided the If a copy of this Table is kept handy, any power can be immediately read internal resistance is less than 351, and the is 1 amp, any other fs.d. off against its appropriate value of thermoammeter may be employed in the same manner as that discussed current. The thermoammeter scale here. -Editor was cramped towards the lower end, but values of current could be read off accurately down to a minimum of 0.1 amp. Since this corresponds AVERY USEFUL INSTRUMENT TO moving coil meter and a bridge to a value of 0.03 watts (on a 352 have around the workshop is rectifier. output) the cramping was not felt to is, be a great disadvantage. an audio output meter. It Another method of calculating 4 when aligning power is measure the for instance, useful to current If a 0-1 amp thermoammeter having radio receivers with a signal genera- flowing in an accurately known an internal resistance other than 0.350 tor, whereupon it is connected resistor and then apply the formula is employed, the value of R1 may be altered across terminals in place W (where W in watts, to suit. To obtain the results shown in the the output =I2R =power Table, R1 plus the internal meter resistance of the normal loudspeaker. In this I=current in amps and R=load must equal 30. Ill. should be remembered way output variations can be resistance in ohms). This is the that R1 dissipates nearly 3 watts at f.s.d. degree and that R2 dissipates 12 watts. Suitable observed with a much greater method decided upon for the present resistance wire is available from Post of accuracy than is given by aural design, as it is cheaper and a suitable Radio Supplies, 33 Bourne Gardens, methods, and without discomfort. meter was to hand. London, E.4.

Measuring Output Power TABLE The more usual method of deter- power of an mining the output Meter Power amplifier is by measuring the voltage Meter Power which the amplifier develops across Reading (Watts) Reading (Watts) a known load resistor, this voltage being measured by means of a 352 1552 3Q 1552 0.1 0.03 0.15 0.6 1.08 5.4 0.15 0.067 0.34 0.65 1.27 6.35

Thermaammeter 0.2 0.12 0.6 0.7 1.47 7.35 F.S.D.= IA Int.res -0.550 0.25 0.18 0.9 0.75 1.7 8.5

R1 2.65n 0.3 0.27 1.35 0.8 1.92 9.6

150 0.35 0.37 1.9 0.85 2.17 10.9 R2 0.4 0.48 2.4 0.9 2.4 12.0 12n 0.45 0.61 3.05 0.95 2.7 13.5

The circuit of the output meter. 0.5 0.75 3.75 1.0 3.0 15.0 The value of R1 may need changing to suit meters having 0.55 0.91 4.55 different internal resistances

164 THE RADIO CONSTRUCTOR heavy current passed by the OCP71, when illuminated, is used to energise a small relay. The relay is connected in series with the collector TNF as shown in Fig. 2, and should be able to close et16 when passing a current of 1 to 3mA. A relay that is very suitable can be found in the U.S. Army surplus Beacon Receivers type BC -357-L. Alternatively, the ex-government relay type P.27258 can be adjusted to close at about 2mA. Failing this, a normal G.P.O. type 200 relay, with a coil °CT\ resistance of 5k12 and one contact set should close with about 3mA passing through the coil. The diode across the relay coil is a desirable G. C. DOBBS G3RJV addition to the circuit. If the relay coil is passing about 3mA and this is suddenly reduced to a few microamperes, self-induction within the coil may to be produced. This Some simple applications for this readily available cause a large back e.m.f. 40 volts, and will be in phototransistor e.m.f. may be as large as the reverse polarity to the normal transistor supply, but it could still damage the phototransistor. THE OCP71 IS A NORMAL P.N.P. TRANSISTOR IN which the photo -electric effect is used. It is similar in construction to the 0071 except that it does not have the latter's opaque coating and its clear plastic capsule is filled with silicon grease to diffuse the light. In the basic circuit of Fig. 1 the collector current, with the transistor in darkness, is in the order of microamperes. When light reaches the sensitive area of the transistor the collector current increases, and may rise as high as 4mA. This change is large enough to operate a small relay without the use of an amplifier. In effect the phototransistor is a photodiode with a built-in amplifier, and is a very useful com ionent. Fig. 2. An OCP71 can control a sensitive relay Exposure Meter directly, without an intermediate amplifier By using the circuit of Fig. 1, a very simple exposure meter can be made. A 1mA meter is The diode, which can be just a normal germanium used to measure the collector current and a Mallory type such as the 0A71, prevents the formation of cell type RM -1 can be used as a voltage source. the back e.m.f. The 10kí2 potentiometer functions It may be possible to mount the components as a sensitivity control, but changing tappings within the meter case, with a small hole to enable on the battery would have the same effect.* light to reach the OCP71. The dark current is The applications of the circuit are numerous: so low that a switch may not be necessary, but one garage door opener, liquid level control, article could be added in series with the battery if desired. counter, smoke detector, and so on. When .setting up the circuit of Fig. 2, or any similar circuit, the Light Switch OCP71 collector current must not be allowed to exceed its limiting maximum rating of 20mA. It may be added, incidentally, that the The usual application of the OCP71 is that of OCP71 will function in the applications shown here with its base a light switch. In this type of circuit, the relatively open-circuit.-EDrroR. Lamp OCP71 ImA FS.D.

Light beam

Lens I Lens 2

Fig. 3. A simple burglar alarm. The OCP71 is continuously illuminated by the lamp unless an Fig. 1. Using an OCP71 as an exposure meter intruder breaks the light beam

165 OCTOBER 1964 Sensitive area

c e

Fig. 5. Illustrating the lead-out connections and the Fig. 4. An experimental "light oscillator" sensitive area of the OCP71 Burglar Alarm beam will release the relay, which can then switch With the aid of two lenses a simple burglar on any warning device. alarm can be constructed. The lenses used by the author were taken from sixpenny plastic magnifying Light Oscillator glasses. The focal length can be found, during A novel circuit is shown in Fig. 4, in which daylight, by focusing the image of the window a lamp is illuminated by the relay contact when the frames of the room on to a piece of paper, and relay is de-energised. If the lamp is brought close measuring the distance between the lens and the to the OCP71 it causes the transistor to conduct paper. The light source and the OCP71 should and switch the lamp off. This results in the relay be placed in the same position as the paper behind falling out and switching the lamp back on again, their respective lenses. Both should be enclosed and the whole cycle is repeated. The lamp, therefore, in light-tight boxes with a simple lens shield. will flash on and off giving us a novel form of They can be arranged as shown in Fig. 3, so that oscillator. The uses of this circuit are doubtful the relay is energised. An object crossing the light but it is fun, nevertheless! Simple M.W. Receiver for the' Beginner E. GOV/ER

ASIMPLE INEXPENSIVE AND EASY -TO -CONSTRUCT Circuit receiver for medium wave coverage is often From the diagram it will be seen that the circuit required by the beginner who feels that he consists of a detector stage constructed around would like to produce such a radio for the work- the 6SL7 octal based high -mµ double -triode, shop, the kitchen or for use as a bedside radio. the second half of the valve being utilised as the For most beginners the writer has found that first audio amplifier stage. For those readers who medium wave coverage only is required. happen to have on hand a 6SN7 double -triode, Apart from simplicity of design, the question this may be employed instead of the 6SL7 type of cost is also a factor which must be taken into specified without any changes being made to the account. Not all readers-beginners or old timers valveholder wiring, since both valves have the same alike-can afford multi -valve designs. A superhet base connections. The 6SN7 will, however, produce design, whilst more efficient than the "straight" less audio gain than the 6SL7. type of receiver, is more expensive and complex, The output stage uses the well-known 6V6GT and so the latter category of receiver was chosen. beam tetrode. The "metal" 6V6 may also be The actual prototype was constructed by the employed. With the "metal" valve, pin 1 should writer for an old age pensioner with rather limited be connected to chassis. means and, although many of the components The rectifier is a 5Y3GT operating in a conven- were available in the junk box, two of the valves tional full -wave circuit. Full isolation from the had to be purchased. Those most cheaply obtain- mains supply is given by the inclusion of- a small able on the surplus market were therefore included mains transformer.. In the interests of safety, in the design. especially where old persons or children are con -

166 THE RADIO CONSTRUCTOR Output transformer L.F Choke .e000 250V TcuT 30V 220V

Speaker

Black

Gree

3 V2 63V Centre tap twin green Green 6 On/Off switch __/ /4 ganged Aerial with R7 :oil

AC Mains

5Y3GT Chassis 6SL7 6V6GT

Components List Resistors Capacitors (All watt 20% unless otherwise specified) C1 500pF variable, Cat. No. 2093 (Jackson R1 2MS2 Bros.) C2 As for C1 R3 47kS2 C3 200pF silver mica R4 47k52 C4 81.LF, electrolytic, 350V wkg. R5 470k12 C5 0.01µF, paper, 359V wkg. R6 1k12 C6 25µF, electrolytic, 12V wkg. R7 1M52 pot. log track with d.p.s.t. switch C7 0.01µF, paper, 350V wkg. R8 30052 Cs 25µF, electrolytic, 25V wkg. Rg 50k52 pot. log track C9 0.1µF, tubular, 350V wkg. *Cio 16µF, electrolytic, 350V wkg. Valves *G11 8µF, electrolytic, 350V wkg. V1 6SL7 (see text) *Contained in single can V2 6V6GT V3 5Y3GT Aerial Coil Type QR11 (Osmor Radio Ltd.) Speaker 312 impedance 5in round (or other convenient size), Mains Transformer Secondary voltages: 250 250V, 60mA; 5V, Choke 2A; 6.3V, 2A. Type MT161 (Ellison) 10H, 60mA (see text) (H. L. Smith & Co. Ltd.)

Valveholders Output Transformer International Octal (3) 45:1 ratio (6V6 matching or multi-ratio type) Miscellaneous Chassis and Panel Knobs (4); panel -Signs Set No. 6; wire, nuts and 6 x 8 x 24ín. (H. L. Smith & Co. Ltd) bolts, grommets, etc. Panel, aluminium cemed, the writer prefers complete isolation from QR11 (see Components List), the third (grid) the mains even though the small mains transformer winding being tuned by the variable capacitor C2. required is slightly more expensive. Reaction is provided in the normal manner A short aerial, some twenty feet in length, is by the variable capacitor C1, this being connected connected to the primary winding of the coil type between the second winding and chassis. The

OCTOBER 1964 167 other end of the reaction winding is connected loudspeaker. One tag of the speaker is connected to anode (pin 2) of the first triode of the 6SL7. direct to chassis (it does not matter which tag) The grid components, C3 and R1, are connected and the speaker itself is mounted on the. front at one end to the grid (pin 1), the other end of, panel. resistor R1 being connected to the cathode (pin 3) The capacitor C9 and the potentiometer R9 and therefore direct to chassis. The other end of provide the tone control components. If, however, C3 is connected direct to the junction of the third this control is not required or the added cost coil winding and C2. of the components is not considered worthwhile, Both the variable capacitors C1 and C2 are C9 and R9 may be omitted. An alternative simple mounted directly to the front panel of the receiver tone correction circuit would be given by connecting as also are the ganged volume control and switch, a 0.002í.F fixed value capacitor directly across and the tone control. the primary winding of the output transformer. The anode of the first triode is taken to h.t. The full -wave rectifier circuit provides adequate positive via the resistors R2 and R3. The de - h.t. and the smoothing components C10, C11 coupling capacitor C4 is important in that it should and the 1.f. choke ensure that the h.t. potential be of good quality if efficient and smooth reaction is virtually ripple -free. The choke was a 10H is to be achieved. The detected output from the 60mA type that was already to hand in the junk first triode is now fed into the following stage box, but this could be replaced, where such an (the second triode of the 6SL7) via the coupling 1.f. choke is not to hand, by a 4kí2 3 watt wire - capacitor C5, this being connected to the grid wound resistor-this being much cheaper than the (pin 4) as also is the grid resistor R5. choke specified. The components C6 and R6 provide cathode bias for this stage, and beginners should note the The rectifier heaters are supplied from the 5V polarity markings for the lead -outs of C6 before heater winding of the mains transformer whilst wiring up and ensure that these are connected those of V1 and V2 are supplied from a separate correctly. The other electrolytic capacitors used 6.3V winding on the same transformer. Pin 7 in the circuit must also be connected with correct of V1 and pin 2 of V2 are connected direct to polarity. chassis as also is one side of the 6.3V heater winding The anode of the second triode (pin 5) is taken, of the mains transformer. No connection is made via the resistor R4, to the junction of the components to the 6.3V centre -tap, which should be taped up R2, R3 and C4. The amplified audio signal is now out of the way. fed to the output stage via the coupling capacitor The whole receiver was constructed on a chassis C7 and the volume control latter compo- measuring 6 x 8 x 2 -j -in, the metal panel (aluminium) nent also includes the on/off switch shown in the being secured to the chassis 3,v the controls CI a.c. mains input lines to the primary winding of (reaction), R7 (volume) and R9 (tone); these the mains transformer. controls being mounted under the chassis and The output stage grid (pin 5) is connected to direct to the front panel. The tuning capacitor the centre tag of the volume control and a short C2 was mounted above the chassis, directly to length of screened (braided) wire should be used the panel and in a central position. The tuning for this connection, the braiding being soldered scale used with this latter control was that available to the chassis tag of the volume control. This will from Panel-Signs Set No. 6. greatly assist in minimising undesirable a.c. lum Once constructed, the receiver may be easily pick up. housed in a small wooden (or hardboard with The components Rs and Cs provide cathode wood bracing) cabinet. Alternatively it could, bias voltage for this stage. Pin 4 of the valveholder like the prototype, be housed in a bookshelf; is taken direct to h.t. positive. Pin 3 of the valve - in which case (provided it is fitted at one end of a holder (the anode) is connected to h.t. positive shelf) only an additional side piece will need to via the primary winding of the output transformer, be secured to the chassis side-that between the the secondary winding being connected to the radio and the books. Scout Amateur Radio Club The inaugural meeting of the Baden-Powell House Scout Amateur Radio Group was held on 23rd July at Baden- Powell House. The Group's aims include the organising of GB3BPH for Jamboree on the Air (17th/18th October) and the interest of Scouting in Amateur Radio and vice versa. - The establishment of a permanent amateur radio Club Station, G3TGS at Baden-Powell House will demonstrate amateur radio to Scouts visiting the House and encourage other Scouts who are licensed amateurs to contact each other. The officers elected were, chairman Eric White G3KSO, secretary Alf Watts G3FXC, and treasurer Reg Flower. Meetings are held at 7.15 p.m. on the third Thursday in each month at Baden-Powell House, Queen's Gate, S.W.7. The group would like to hear from other amateurs and listeners who are members of the Boy Scouts Association.

168 THE RADIO CONSTRUCTOR order to obtain a considerable increase in impedance. Types of Bead Beads with a single hole for threading on to a wire can be Ferroxcu be obtained in two different materials, as shown in the Table. The type FX 1115, which is made from grade Al Ferroxcube, is most satisfactory DecouplingI for a frequency range of about 3 to 16 Mc/s. Above this frequency range the type FX 1242, made of grade B2 Ferroxcube, provides a higher im- Beads pedance. Each bead of type FX 1115 provides an impedance of about 10û By M. J. Darby at 3 Mc/s, 2052 at 15 Mc/s, rising to a maximum of about 3252 at 50 Mc/s. An FX 1242 bead provides an impedance of about 452 at 3 Mc/s, 200 at 15 Mc/s and 50n at 100 Mc/s. THE PROBLEM OF DECOUPLING a special magnetic material (a ferrite) A twin heater supply is convenient- high frequency r.f. and i.f. made by Mullard Ltd. ly decoupled by a somewhat larger amplifiers is a difficult one, Although Ferroxcube is normally bead with two holes (type FX 1516). especially if a high gain is required, used as a core material at frequencies This bead is made only in grade B2 but is nevertheless important owing such that the losses are low, these Ferroxcube. Bare wires can be used to the widespread use of such losses increase rapidly with fre- owing to the high resistivity of this amplifiers in TV, f.m. and radar quency. This property, which would material. receivers. At high frequencies normally be undesirable, can be Another type of bead consisting of undesired coupling between stages made to serve a useful purpose. The grade B2 material (type FX 1898) can occur via the h.t., heater, a.g.c. bead, which is threaded on to the has six holes through which the wire or any other leads which feed supply lead, acts as an inductor of to be decoupled may be threaded. several stages, and may result in poor Q; it therefore absorbs r.f. A comparatively large impedance oscillation. The use of decoupling power and prevents it from passing may be introduced if a single coil of capacitors does not always give along the wire. 2i turns or two coils of 1 f turns each satisfactory results owing to the are wound using one of these beads. inductance of these components, Some Advantages The number of beads required and although feedthrough capacitors The beads are small (see Table 1) the most suitable type of bead for (which have a very low inductance) and are so light in weight that they any particular purpose are best found may provide a satisfactory solution. require no soldering or fixing. They experimentally, as the calculations Similarly, chokes may not be effective can, therefore, be easily used in are rather awkward and not very owing to their self-capacitance. places where the available space is reliable. An extremely simple method of very limited. The passage of d.c. or The normal method of illustrating introducing the necessary impedance low frequencies along the decou1led the use of a Ferroxcube bead in a into the supply lead consists of using wire is virtually unaffected by the use theoretical circuit is shown in the one or more beads made of Ferrox- of the beads, and no d.c. voltage drop accompanying diagram. This shows cube which are threaded on to the occurs. Several beads can be used to the ferrite core alongside the wire. lead to be decoupled. Ferroxcube is increase the impedance presented to Acknowledgements are due to high frequency currents. Alterna- Mullard Ltd., for making available tively the wire may be threaded details of their range of Ferroxcube through the bead more than once in materials.

TABLE The Mullard Range of Ferroxcube Decoupling Beads

Outside Type Length diameter Hole size No. of Material No. (inches) (inches) (inches) holes

FX 1115 0.197 0.157 0.079 1 Al

To heater FX 1242 0.216 0.159 0.059 1 B2 supply FX 1516 0.472 0.220 For 0.026in 2 B2 Ferroxcube beads are depicted diam. wire in theoretical circuits by dashed (23 s.w.g.) lines (to represent a ferrite core) drawn alongside the wire on to FX 1898 0.394 0.236 For 0.020in 6 B2 which they are threaded. In this diam. wire diagram there are two Ferroxcube (25 s.w.g.) beads, one on each heater lead

OCTOBER 1964 169 understanding radio

By W. G. MMORLEY RtE

A further article in our series which, starting from first principles, discusses the basic theory and practice of radio

IN LAST MONTH'S CONTRIBUTION TO THIS SERIES The ring magnet assembly is shown again in we discussed loudspeaker resonance and im- Fig. 235 (b), but in this case the magnet is significantly pedance, examining these points with respect shorter than occurs in Fig. 235 (b). to generally encountered moving -coil loudspeakers. Fig. 235 (e) depicts a slug magnet assembly After briefly dealing with the desirability of express- with pole -pieces, the slug magnet being illustrated, ing loudspeaker response curves against a logarith- on its own, in Fig. 235 (f). As will be noted, the mic frequency scale, we next discussed restoring assembly is rather similar to that of Fig. 235 (a), force and speech coil displacement. We shall with the exception that the magnet does not extend now conclude on the subject of moving-coil loud- right into the gap. Instead, a small central pole - speakers by dealing with magnet types and strengths. piece extends the field from the upper surface of the magnet into the gap. Loudspeaker Magnets The magnet constructions shown in Figs. 235 (a), When we introduced the moving-coil loudspeaker' (b) and (d) represent those which are most likely we used, for purposes of illustration, the magnet to be encountered at present or in recent years. assembly shown in Fig. 235 (a). This assembly The ring magnet construction shown in Fig. 235 (b) employs a cylindrical rod magnet at the centre, is used very extensively. together with pole -pieces which allow the pole In all the types shown, with the exception of at the lower end of the magnet to be effectively Fig. 235 (d), the material employed for the magnets shifted to the gap. In consequence, a strong mag- normally consists of one of the metallic alloys netic field appears across the gap. specifically intended for this application, such as The assembly illustrated in Fig. 235 (a) represents "Alcomax" or "Alnico".2 Ceramic magnetic only one of a number of methods of providing materials for loudspeakers have recently been a magnetic field in the gap. An alternative construc- introduced, and they are particularly suitable for tion is shown in Fig. 235 (b). In this instance a the ring magnet assembly. They may be produced ring magnet is employed, this having the 'shape with a comparatively short length, with the result illustrated in Fig. 235 (c). The upper flat surface that the overall dimensions of the loudspeaker of the ring magnet offers one pole and the lower become reduced. The construction shown in flat surface the other pole. The pole-pieces shown in Fig. 235 (b) then cause these poles to be 2 "Alcomax" is a trade name for an alloy consisting basically of nickel, iron and aluminium, and "Alnico" is a trade name for an effectively shifted to the gap. alloy consisting basically of aluminium, nickel, cobalt and iron. Different grades of these materials have different proportions of the basic metals together with, in some cases, further metals. Thus, some 'In "Understanding Radio", part 32, May 1964 issue. of the Alnico grades include a small proportion of copper.

170 THE RADIO CONSTRUCTOR

il Fig. 235 (d) would, for instance, normally use a Cylindrical rod magnet Pole -pieces ceramic ring magnet. This type of magnet assembly is very useful for small radio receivers and the like, where space is at a premium. The magnet assembly shown in Fig. 235 (a) has the advantage that the external magnetic field (i.e. the field radiating outside the assembly) is weak. As a result this construction is useful for loudspeakers fitted to television receivers. In such receivers, stray magnetic fields are undesirable Pole -piece assembly Ring magnet as they may distort the shape of the picture reproduced by the cathode ray tube.3 (a) (b) The slug magnet assembly of Fig. 235 (e) was commonly used in loudspeakers in the past, al- Pole -pieces though it has now been largely superseded by the ring magnet assembly. In earlier years, when the production of reliable permanent magnets at low cost was difficult to achieve, many moving-coil loudspeakers employed "energising coils" to provide the magnetic field Ring magnet in the gap. An energising coil was fitted into the pole -piece assembly in the manner shown in Fig. (c) (d) 236 whereupon, when a direct current was passed through its turns, the lines of force produced about it flowed through the metal of the pole -pieces and gave a strong magnetic field in the gap. Despite the cost of the copper wire employed for the coil and the disadvantage of having to provide the j direct current, "energised" loudspeakers still proved, at the time when they were produced, to be more economic for many applications than permanent Slug magnet magnet types. As the development of permanent (e) (f) magnet materials progressed, however, it was the energised loudspeaker which became the more Fig. 235. The basic types of magnet assembly costly. It is hardly ever encouraged nowadays employed in moving -coil loudspeakers. In each except in equipment of early design and manufacture. assembly diagram the magnet is shown shaded. A centre rod magnet assembly is illustrated in (a) and a ring magnet assembly in (b). The shape of the Magnet Strength ring magnet is illustrated in (c) whilst (d) shows the As is to be expected, the strength of the permanent reduced depth which is possible with ceramic ring magnet in a moving -coil loudspeaker has an im- magnets. An axample of the slug magnet assembly portant bearing on performance. If, without is given in (e), and a view of the slug magnet on its altering the dimensions of the pole -pieces, the own in (f). The slug magnet shown in (e) is narrower air gap or the speech coil, the strength of a loud- than the central pole -piece above it, but in many speaker magnet is increased, the loudspeaker loudspeakers using this type of assembly it may be wider than will offer a greater sound output for a given electrical the pole -piece input signal. There may also, in practice, be a slight change in the frequency response of the 1 gauss being equal to 1 maxwell per square centi- loudspeaker. metre. However, the strength of the magnet does not Let us aseume that we have two magnets of provide a .complete measure of loudspeaker equal flux density and that these are fitted to efficiency as there are a number of other factors two loudspeaker assemblies both having the same which have to be taken into account. One of these air gap length (i.e. the same distance between is the resistance and number of turns in the speech the outside edge of the centre pole -piece and the coil, and another is the area of the air gap. Inside edge of the outer pole -piece). However, It is usual to refer to loudspeaker magnet :he diameter of the centre pole-piece of one loud- strength in gauss, this being a unit of magnetic speaker assembly is twice that of the other. It flux density. Since the term "gauss" !efers to a will be possible to put a larger speech coil, having density, it follows that it applies to the amount more winding wire, into the magnet assembly of flux in a unit area and not to an overall amount with the larger centre pole-piece with the result of flux. The latter may be measured in maxwells, that, apart from considerations of sensitivity, the loudspeaker will be capable of handling a greater power. Because of factors such as this, the flux s If the magnetic field passes into the cathode ray tube, it causes the density of the magnet does not, on its own, provide moving electron beam which builds up the picture to be deflected from its proper route. sufficient information concerning the performance

OCTOBER 1964 171 Energising coil Pole-piece assembly assembly. An armature made of magnetic material i is pivoted at its centre, and it remains centrally positioned between two pole -pieces of the shape f' shown. The pole -pieces effectively bring the poles of a permanent horseshoe magnet close to the ends of the armature. Fitted around the armature is a stationary coil. This is shown in the diagram as having a few turns only, but in practice it would Fig. 236. Cross-section through an energised be wound with a large number of turns. The loudspeaker magnet assembly, as used in earlier centre of the coil is sufficiently large to provide models. The outer part of the pole-piece assembly clearance for armature movement. completely encircles the energising coil. The two coil In Fig. 237 (b) we cause a current to flow through lead -outs are taken out through holes in this outer section the coil, thereby energising the armature. Let us assume that the left hand pole of the permanent of a loudspeaker. We need further information magnet is its north pole, and that the current flows covering the dimensions of the air gap as well. in the direction which causes the upper end of the In practice, this further information is provided armature to become a north pole also. The upper by specifying the "total flux" of a loudspeaker end of the armature is, therefore, repelled by the at the gap. This total flux is expressed in maxwells north pole of the magnet and attracted to its south and is proportional to the flux density at the gap pole. At the same time its lower end, now a south multiplied by the average area of the gap (i.e. pole, is repelled by the south pole of the magnet the area given by averaging the area on the inside and attracted to the north pole. The result is edge of the outer pole-piece and the corresponding that the armature moves over to the position area on the outside of the centre pole -piece). The illustrated in Fig. 237 (b). total flux figure for a loudspeaker still does not In Fig. 237 (e) we reverse the current through offer a complete guide to its performance because the coil, with the result that the upper end of the other points (such as speech coil resistance, etc.) armature becomes a south pole and the lower have to be considered, but it offers a more repre- end a north pole. The consequent repulsions sentative idea than does the figure for flux density and attractions then cause the armature to move on its own.4 over in the opposite direction to that of Fig. 237 (b). Practical loudspeakers may have total flux It may be seen that, if an alternating current figures lying between some 6,000 to 200,000 (or more) maxwells. High total flux figures usually appear in the more expensive loudspeakers which are intended for high fidelity reproduction, and they are obtained by employing relatively costly magnet systems. Before concluding on this subject, two other terms of measurement need to be mentioned. The first of these is the weber. This is another unit of magnetic flux, 1 weber being equal to 108 maxwells. Similarly, 1 weber per square metre equals 104 gauss. The flux density cif loudspeakers may sometimes be quoted in webers per square metre. The second unit is the line (or line of flux), (b) 1 line being equal to 1 maxwell. However, there (a) seems to be a tendency to use this term somewhat loosely, and one occasionally encounters references to loudspeaker specifications where "line" is employed as though it were synonymous with "gauss". Whenever the term "line" appears with reference to a loudspeaker specification, it should be confirmed that it does in fact apply to the total flux. The Balanced Armature Loudspeaker We have now concluded our examination of (c) the moving-coil loudspeaker, and shall carry on Fig. 237 (a). The-basic construction of the moving next to a brief consideration of other types. The armature loudspeaker. The coil is stationary, and first of these is the balanced armature loudspeaker. does not move with the armature Fig. 237 (a) illustrates the basic balanced armature (b). Passing a current through the coil causes the armature to be displaced as shown here 4 The flux density in the gap may sometimes be referred to, in (c). Reversing the current in the coil causes the gauss, as the "gap flux". armature to be displaced in the opposite direction

172 THE RADIO CONSTRUCTOR coil, the armature will is caused to flow through the Ribbon securing points move back and forth at the same frequency as that of the current. Further, if a restoring force Magnet is applied to the armature causing it to return to its central position in the absence of current, Pole -piece the amplitude of the armature movement will be assembly proportional to the current flowing through the coil. By coupling an extension from the armature to a cone (as in Fig. 237 (a)) the latter may be made to move back and forth in sympathy with the frequency and amplitude of the current in the coil. We have, therefore, a loudspeaker; and the cone Ribbon will reproduce, as sound, any electrical signal which has been derived from a microphone or similar device. The balanced armature loudspeaker was employed fairly extensively in the early days of radio, being Magnet superseded, around the '3O's, by the moving -coil loudspeaker. Nevertheless, the balanced armature technique may still be occasionally encountered Fig. 238. Front view and side view (partly cut away) (in, for instance, the balanced armature earphones of the ribbon loudspeaker. The inner sections of the used by the Services) and in other applications. pole -pieces taper inwards as they approach the Also, the principle employed is of interest, and ribbon. Current is passed through the ribbon by way merits a short description here. of the two securing points The Ribbon Loudspeaker flowing through it, it may be used to reproduce, The ribbon loudspeaker has. the basic construction as sound, a sound -derived electrical signal. How- shown in Fig. 238. In this diagram a springy ever, the surface area of the ribbon is small, and metal ribbon having corrugations along its length it is necessary to fit a horn to it to obtain an adequate is positioned between the pole -pieces of a powerful sound output. The horn has a rectangular cross- magnet system. The ribbon is secured at its ends. section, its throat being close to one of the surfaces When a current is passed through the ribbon it of the ribbon. is subjected to the same force as occurred with the Ribbon loudspeakers offer a useful response single conductor in a magnetic field which we at the higher audio frequencies and may be employed examined in Part 32 of this series.s. If the current as tweeters. Due to the necessarily wide gap, is passed in one direction the ribbon will tend to a powerful magnet has to be employed to provide move forwards. If the current is passed in the other an adequate total flux. The impedance of the direction it will move backwards. Since the ends device is very low, as may be imagined by comparing of the ribbon are secured, the centre will suffer it with a moving -coil loudspeaker. The ribbon the greatest displacement, the corrugations in is roughly analogous to a single turn of the speech the strip allowing this displacement to take place. coil in the latter. The restoring force is provided by the springiness of the metal itself. Next Month Since the ribbon is capable of moving in sympathy In next month's article we shall examine further with the frequency and amplitude of the current types of loudspeaker, these including the electro- static loudspeaker and, if space - permits, the

5 "Understandin% Television" Part 32, May 1964 issue. lonophone.

New Switch Adjusts Lighting - Saves Power and Bulbs too! A new lighting control switch has been introduced by Regentone which enables you to adjust room lighting to any level required for reading, TV viewing, cine shows or just decorative effect. Called the Varilite, the new control replaces the normal on/off wall switch. Unlike previous lighting control units the Varilite uses a new kind of rectifier which enables it to control up to 300 watts of lighting without generating wasteful heat and without taking up too much space. The Varilite uses no power itself and cuts electricity bills since you pay only for the light you select. It also prolongs bulb life to a surprising degree: by reducing the light only 17 per cent you double the life of your lamps. The Varilite costs 61 gns., fits any standard wall switch box and can be installed in a few minutes with the aid of a screwdriver. It is designed for filament bulb lighting only and is not suitable for controlling fluorescent tubes.

_OCTOBER 1964 173 This month, Smithy the Serviceman, aided as always by his able assistant Dick, deals with the latest batch of hints contributed by readers.

B"T LIMEY," SAID DICK, "another Smithy sighed. When there was then stay firmly in place whilst pill?" plenty to do, Dick took up his you cut and slot it to suit. You Startled at the voice which time with pleas for assistance can also give the resulting grub had unexpectedly materialised behind When there was nothing to do, screw a little touch round the top him, Smithy swallowed involuntarily. Dick took up his time with requests with a file to clean off the rough He immediately gave vent to a for something to occupy himself edges given by the cutting and strangled gasp, followed by a with. In general, Smithy preferred slotting operations." . violent bout of coughing. After the former state of affairs. "That's a good idea," commented some moments, he spat out a large Looking around at the racks Dick approvingly. "With jobs like green capsule which dropped to the and then at his own bench, Smithy that I usually find myself trying to floor and rolled under his bench. realised with a start that he, too, put lock -nuts on the bolt and The Serviceman fell back on to would very shortly have nothing holding these in the vice!" his stool and, whilst he regained to do. He had just completed a "Well, you know a better way his breath, glared through bloodshot repair on the only chassis which was now," said Smithy, turning to eyes at his assistant. outstanding and it now merely another letter. "Now let's have a Eventually, he arrived at a con- remained for this to lie fitted into look at this one! In this instance dition where speech was possible. its cabinet. we have two hints from one reader. "What do you mean," he snarled, A thought struck him and he The first describes a method of "by creeping up on me like that? turned to a drawer in his bench. making connections to a transistor I nearly choked myself over that "How about a sesh," he asked, without applying any heat. You blasted pill." "with the readers' hints, then?" obtain three short lengths of rubber "It was because of the pill," "That would be fine," responded covered or p.v.c. covered flex, replied Dick, "that I made the Dick enthusiastically, "we haven't using different colours for base, remark." had a go with those for quite a long emitter and collector. You next "And what," snorted Smithy, time." pull three or four strands out of "is wrong with taking a pill?" "Fair enough," said Smithy, ex- each piece of flex and solder the "You've taken three already this tracting a sheaf of letters from other ends into position in the morning." the drawer. "I'll start off with the circuit. The lead -outs of the tran- The Serviceman drew himself up. first one right away." sistor may then be slipped into "If," he remarked, with dignity, Smithy thumbed through the the open ends of the flex. They "I choose to take advantage of letters and quickly extracted one. slip in quite easily.. because of the the preparations which modern "Ah, here we are," he said. space left by the strands which medical science has made available, "Now, this is an idea which will have been removed, and you get a I would suggest that that is a be of assistance when you want nice reliable contact. Apart from matter which concerns nobody else to make up grub screws from the fact that no heat is applied to but me." ordinary bolts. All that the gadget the transistor there is the further "Fair enough," commented Dick comprises are two pieces of metal advantage that it may be used equably, "but if you carry on like bolted together. (Fig. 1). The many times in different circuits, you have been this morning you'll bottom piece is plain, whilst the being `plugged in' to the bits of be rattling away like a box of top piece has a number of holes flex in the same way as a valve dried peas by the time you get drilled and tapped to suit the grub can be plugged into different valve- home!" screws it is anticipated will be holders." required. I would guess, myself, "I should imagine," commented Readers' Hints that three holes, tapped 2BA, Dick, "that the best flex to use "That," repeated Smithy, "is 4BA and 6BA, should meet most for this job would be the type my business. In any case, how is cases. When you want to make employed for mains lighting and it that you've got the time to watch up a grub screw you simply insert similar jobs." what I'm doing anyway?" a bolt into the appropriate hole "I would think so," agreed "That's easy," replied Dick. "I've and screw it down hard against Smithy. "The second hint in this got nothing to do." the lower piece of metal. It will letter is concerned with valves

THE RADIO CONSTRUCTOR whose top caps have come off and difficult job to wind on a few extra "Ah, there it is!" he exclaimed which only have a small stub of turns and bring the heater windings triumphantly. wire projecting from the glass. At up to 6.3 volts. You then have an Under the horrified eye of his tempts at soldering may, in some excellent mains transformer at very assistant, Smithy picked up a 12 cases, ruin the otherwise undamaged low cost, just by the expenditure inch steel rule, leaned under his valve. With the present dodge of a little time and trouble. The bench, and flicked out the green you take about 3 inches of replace- extra turns can be wound on the capsule which had earlier caused ment wire and apply solder to one outside of the existing winding and him so much discomfort. He end so that a large blob forms. they are, of course, connected in stooped down and picked it up. The wire is held at the other end series with the 4 volt windings "You surely aren't going to with pliers and is taken to the valve already fitted. You'll need an a.c. swallow that thing now, are you?" so that the* blob is above, and just voltmeter to find out how many gasped Dick. "Dash it all, it's touching, the projecting stub of turns are required and to get your all wet and it's been running around wire. You next apply the soldering phasing right. I should guess that, all over the floor." iron to the wire mid -way between in many cases, you won't have to "Of course I'm going to swallow the pliers and the solder blob, and remove the laminations at all. it," replied Smithy indignantly. allow the heat to travel along it. You could just poke the additional "It's my vitamin C tablet." After a short while the solder at wire through the space between the "But it's all dirty." the end will begin to run, whereupon existing winding and the inside "Nonsense," snorted Smithy. "It you remove the iron. If everything edge of the laminations." just needs swilling under the tap, is held steady the two wire ends "You could save quite a few that's all." will join and a good joint will be bob with that idea," interjected With Dick's fascinated gaze on made. You then replace the top Dick. "Added to which is the him, Smithy walked over to the cap, soldering this to the new bit fact that many of those old trans- sink, held his pill under the tap, of wire you've added." formers with 4 volt windings give then swallowed it. "That's quite a novel way," quite hefty currents from the h.t. "What do you want vitamin C commented Dick, "of obtaining a secondary." tablets for, anyway?" asked Dick. solder joint in places which are "True enough," said Smithy. "Are you coming down with scurvy awkwardly positioned, or where "The second hint in this letter or something?" too much heat may damage insula- deals with something entirely dif- "Of course I'm not," snapped tion. I should imagine that this ferent. You sometimes have to Smithy, walking back from the sink. idea has quite a few other applica- mount polystyrene coil formers "It's just that vitamin C helps to tions, apart from the valve top in very awkward positions, with protect you from colds and things. cap idea." the result that it's extremely difficult There are a lot of colds and things "Very probably," said Smithy. to fit the nut on to the fixing bolt around at this time of the year." "I would suggest, incidentally, that even with taper -nosed pliers. So By this time he had returned a little flux should be applied what do you do ?" to his bench, whereupon he reached to the projecting stub of wire before "I don't know," said Dick. you start the operation. However, "What do you do?" To receiver circuits want on to another "You put a thread on the holes I now to pass 15OpF two hints. As in the previous case. in the former," replied Smithy. Ferrite rod both of these appear in a single "You start off by selecting a bolt letter." which is slightly too large for the Smithy paused and cleared his hole, and you heat it just enough throat. to melt the plastic. You then push 1'V IIuIILN! "The first of these," he continued, it into the mounting hole. If "is of a very simple nature. It it's at the right temperature it 2-turn Tuned coil seems that mains transformers with will slip in quite easily. Take care windings 4 volt heater windings are so little the bolt isn't too hot or it will distort needed these days that it is possible the entire mounting lug. The to pick them up, if you shop around screw is then allowed to cool in a (a) for several shillings each. vertical position, after which it carefully, To 2-turn windings At the same time, it isn't at all a may be removed. However, it will have to be unscrewed because Tapped holes the polystyrene will have now I formed a thread around it. Repeat the process for the other hole, and you have two threaded holes in the former which make mounting in awkward positions considerably easier." Fig. 2 (a). Two station switching. Capsule Recovery The switch short-circuits either "That's a knobby idea, too," of the 2 -turn windings, the commented Dick. "You've certainly latter being moved as required by the Fig. 1. A simple piece of brought out some useful hints this along the coil tuned equipment which simplifies the Smithy." 1 50pF fixed capacitor time, is making of grub screws. Two But the Serviceman had suddenly (b). If a 3 -way switch of the 2 -turn pieces of metal are bolted remembered something. He placed employed, neither in together, the lower piece being his sheaf of letters on the surface windings are short-circuited plain and the upper piece of the bench and was looking its central position. Three be selected having a number of tapped holes carefully at the floor. stations may then

OCTOBER 1964 175 have If necessary, insert resistance "It depends on the transformer," here, or here said Smithy. "The easiest way of finding the required value is by experiment, and our correspondent suggests that it will, in most cases, need to be of the order of 20052 and that it should have a dissipation of 2 or 3 watts. One method of finding the required Fig. 3. Obtaining a full-wave output value which springs to my mind H.T. secondary consists of AC. from a mains transformer with a obtaining the same half-wave rectified voltage, at half load mains Primary Reservoir secondary. If the same current, voltage does not appear across the reservoir capacitor for capacitor across the primary and the h.t. secondary, either of the windings. That is resistance is inserted, as indicated, to say, you first connect the lower in series with the winding having the voltage winding to one anode of the higher voltage rectifier and check the voltage across the capacitor. You then disconnect this, apply the other winding, and adjust the series resistor for behind his the same voltage. Another, and oscilloscope and pro- "That's something new," said perhaps simpler, duced a tin. He next extracted a method would Dick. "I'll have a bash at it with consist of setting up the complete large tablet, which he proceeded the next small to transistor set I circuit with full load, and of adjusting munch with great vigour. knock up." the "What's that, a purple heart?" resistor so that the same a.c. "There's a bit more to come voltage with respect to chassis "It's vitamin D," said Smithy, yet," said Smithy. "If you use ceasing his a appears on each anode of the chewing for a moment. switch which has a central position, rectifier. "It's good for thickening the blood. Yet another idea, which such as a miniature slide switch, assumes that a 'scope is available, Stops nose -bleeding, for instance." the central position leaves the two "Well, I don't know," would be given by connecting up remarked additional windings open. (Fig. the full circuit and adjusting the Dick helplessly. "I think the 2 (b) ). Under this best condition you resistor until the alternate charging thing we can do is to get back can set up the coil on the rod so peaks to some more of these across the reservoir capacitor readers' that this picks up a third station. all have the same A hints." So size. further you've then got three station point to watch out for is that the Smithy swallowed the last frag- switching!" of two windings must, of course, ment his anti -nose -bleeding tablet, "Better and better," remarked be and picked up his sheaf series -aiding. You should get of letters. Dick. "The sliding winding idea about twice the "As you like," he said serenely. mains voltage is certainly cheaper than using across the rectifier anodes as measur- "Here goes with the next one! switched trimmers." Now, this is a novel idea for ed with an a.c. voltmeter. If one obtain- "Definitely," agreed Smithy. "I of the windings is ing station selection without a connected wrong should add that the third station way round, then you'll get zero tuning capacitor. When you make offered by the scheme a will have or very nearly zero volts across the simple miniature receiver, the to have a longer wavelength than two largest and most expensive compo- anodes." the other two. Still, that's obvious "Any more hints?" queried Dick. nent is usually the tuning capacitor. enough anyway." It is, however, "Plenty!" replied Smithy. "Here possible to elimin- are two more ate this by using a switch and in a single letter. Full -Wave Rectification Both of these are improvisations taking advantage of the tuning Smithy drew another letter characteristics from from cycle wheel spokes, which of the ferrite rod the pile. seem to aerial. A small -value fixed capacitor have quite a variety of "This is a hint," he pronounced unexpected applications. The first (say 150pF) is connected across the after a few moments' coil. Two perusal, hint is a useful little tip for use small windings, each "which enables you to get a full - with tape having a couple of turns of wire, recorders. When it is wave output from a mains trans- necessary to fit new driving belts are then placed around the coil, former having a half and -wave secondary. to certain models of tape recorder, are connected to a single -pole (Fig. 3). The idea is quite simple two-way switch which the top plate has to be raised so, selects either but it does necessitate having a that the new of two stations by short-circuiting belts can be slipped live chassis. The transformer re- over the flywheel spindle and thence the appropriate winding. (Fig. 2 (a) ). quired should have an h.t. The secondary round the flywheel and capstans. short-circuited turns cause a voltage which is the same, or very station to be selected by altering nearly the same, the as the primary inductance of the aerial. It voltage, whereupon you simply is very easy to tune in the chosen connect together one end of each stations, this being done by sliding winding and apply the other ends the windings along the coil. The to the anodes of a full -wave rectifier. components required are inexpensive If the h.t. secondary doesn't give and take up little space, and the precisely the same voltage idea as the Fig. 4. A simple hook, fabricated has been successfully used by primary you insert a resistor in the reader for a `radio jack' series with the from a rycle wheel spoke, which winding which can be of intended for plugging into a tape offers the higher voltage." considerable assistance recorder." "What value when fitting tape recorder drive should the resistor belts

176 THE RADIO CONSTRUCTOR "Vitamin B," said Smithy, "with "Very well," said Smithy, "if you a dash of vitamin E." insist." are He leaned over and once more "What a mixture! What picked up the letters. (a) they for?" after said Smithy, "This one he announced "The vitamin B," a moment, "offers a method of Screwdriver "is to buck you up, and the vitamin _Hook it worthwhile bucking obtaining a miniature substitution E is to make resistor assembly. The assembly is yourself up." very small and you can wire or "Well, I just don't know," said "What started clip it into compact layouts directly. Dich helplessly. Once you've done that, it offers you on this pill jag, anyway?" of ten different fixed By now Smithy had once more you a choice where- resistance values." reached the Workshop sink, "That interesting," re- upon he swallowed his four pills sounds draughts of marked Dick. "How is it done?" with the aid of copious "You start off," said Smithy, water. "with two of the Bakelite pin "I've been feeling rather low protectors which fit on to B9A lately," he said, after all the pills valves. These types have ten small (b) had been safely dispatched, "and it to the I've been talking about Resistor ends linked at my club. They've Fig. 5. A method of retensioning chaps down together all been feeling low too, and they're switch contacts. The hook, over it. So gently the all taking pills to get shown in (a), pulls out a selection of the of the contact upwards I'm trying centre different pills which each one has as illustrated in (b), a screwdriver me to take." ì being held against the contact advised tip "But dash it all," protested Dick. "you aren't supposed to take 89A valve pin All this is rather a fiddling operation, them all at the same time." protector Lead "I don't see why not," replied and it can be made a lot easier (a) by using two hooks made from Smithy. "So long as I spread the cycle wheel spokes. (Fig. 4). The vitamins and things out a bit. hooks are used to handle the Over the last hour, for instance, 89A valve pin 68A solder tog belts, and with their aid it is possible I've had a nicely balanced mixture protector to position these very easily, raising, of vitamins B, .C, D and E. I lowering and stretching them as to locate anything Vo(i required. The belts can be guided with vitamin A in it yet, but, when into their correct positions without I do, I should have pretty near a having to dismantle more than the full set so far as the vitamins are minimum of parts." concerned. In the meantime I'm o "That's something else I'll have building up on the minerals as Resistor ends bent to knock up," said Dick. "We well, and I've got some tablets over and filed smooth could do with a couple of those in which contain iron, copper and the Workshop ourselves." cobalt." (b) be very useful," "Blimey," said Dick, "take enough "They'd certainly 10 resistors ,Pin protectors asserted Smithy. "The second hint of those and you'll be pointing in this letter also has to do with a permanently to the North. Your hook made from cy.;le wheel spokes. medicine cabinet at home must be In this case the hook is used for bulging at the seams by now." retensioning contacts on wafer "I must confess," said Smithy, End (a) switches and similar types. The with a note of pride in his voice, gadget is hooked through the hole "that I have acquired a pretty in the contact, or is hooked under- wide-ranging collection of medicants neath if there isn't a hole, and is over the last day or so." then gently pulled outwards whilst "An you swallow all of them?" the point of a screwdriver is pressed "Of course I don't," replied on to the contact tip. (Fig. 5). Smithy. "Not every medical pre- (c) In this way the contact is bent paration is intended to be taken by Fig. At one end of the so that it applies more pressure." way of the mouth." 6 (a). "I beg your pardon?" miniature substitution resistor Substitution Resistor Assembly "Some preparations," explained assembly discussed by Dick and Smithy glanced at the clock, Smithy, "have to be squirted up Smithy, the resistor ends are all put down the letters he was holding the hooter. And they're very good connected together and reached across the bench. for clearing out the old nasal (b). At the other end of the Pushing his testmeter to one side passages, too." assembly, a solder tag is free he extracted a large bottle from "Charming," said Dick hastily, to rotate and make contact which he gravely counted out four "I think the less I hear about that to any of the resistor lead -outs pills. alchemist's hoard of yours the (c). The complete assembly. Dick, "is getting more unblighted my young life is The ten resistors have values "This," gasped particular beyond belief. What are those going to be! Let's get back to extending over the pills ?" the hints." range required by the constructor

OCTOBER 1964 177 the neon makes contact to this Neon Window Resistor Ball pen case the case of, say} a fin Whitworth via a spring. The series resistor nut, and larger, it might be worth comes next, after which there is a trying a thin shim of brass or rod to take the circuit back to the even tin if the paper doesn't do the metal pocket -clip. If the case of trick. Apparently, this idea is the pen is opaque a window may as old as the drilling dodge. I be cut in it at the same level as the think it's still worth passing on, neon." though." spring Washers Metal rod "Is the resistor a wire-ended "Definitely," agreed Dick. job?" "And now," said Smithy, "we Fig. 7. Assembling the "Oh no," replied Smithy. "The come to the final idea for this components of a neon screwdriver resistors fitted in these neon screw- session, after which we'll have to in a ball pen case. The metal drivers have two brass caps and call it a day. This hint comes, rod from the series resistor are similar in shape to a cartridge incidentally, from the same reader connects to the pocket clip fuse. The value is usually around who sent in the previous one It's I Ma" a simple but very effective little holes in them. You drill a 6BA dodge for keeping ferrite rod aerial clear hole through the centre of coils in place during and after one of the protectors and bolt a Easing Nuts alignment. All you do is slip a solder tag to it. The solder tag "I see," said Dick. "Any more small rubber band on to the former has to be able to rotate, so you hints?" and then `double' it over so that don't do the bolt up too tightly, "There are," replied Smithy, "just one turn of the band stays on the and you fit a pair of lock -nuts two left. The next idea is a modifica- former and the other stays on the on the side opposite to the bolt head. tion of an old dodge which is rod." You then take ten or watt often employed when drilling holes. "The rod," interjected Dick, "being resistors of suitable value and size If you're making a hole with a the latter?" and pass their wires through the power drill, it occasionally happens "Don't confuse me!" said Smithy holes in the protectors. At one end that you need the hole to be a sternly. "This rubber band dodge of the assembly you solder all the `thou' or two larger than is given is a jolly good idea and it enables lead -out wires together and connect by a particular size drill. The coil adjustment to be made quite a short flexible lead with a clip to trouble is that the next size drill easily. At the same time, it holds them. (Fig. 6 (a) ). At the other would make a hole that's too large." the coil firmly in place after it's end of the assembly you cut the "I've bumped into that one, been set up, and there's no need resistor wires back so that they myself," said Dick. "What I to use any other means of securing, protrude by about *in, bend them do is to drill the hole initially, such as wax." over, and file them smooth. (Fig. slip one or two thicknesses of 6 (b) The tag is next paper hole; Future Policy adjusted so that it can be rotated apply the drill once more. The Smithy put the letter he had been and is capable of resting on each paper gets chewed up as soon as reading down on the bench beside lead -out wire in turn. Solder the drill enters the hole but it still him. a second short clip -lead to a con- provides a little bulk and causes "And that," he said with an air venient point on the solder tag the sides of the drill to press more of finality, "is that." and the assembly is complete. tightly against the sides of the "We've certainly had some interest- (Fig. 6 (c) ). You just rotate the hole. The result is that the hole is ing hints this time," remarked solder tag so that whatever resistance a few `thou' wider than occurs Dick. "I enjoy these little sessions." value you need is brought into without the paper." "They're very pleasant," assented circuit." "That's the idea," agreed Smithy, Smithy, "and they make a nice "Well, I'm jiggered," said Dick. "and it is, of course, an old dodge. break from the usual run of things. "I've got quite a few of those pin A good point in its favour is that We'll have another one as soon as protectors knocking round, and the trick can be used even when we can." I've never liked to chuck them out. the work being drilled is a thick "Good show," said Dick. "In- I can now put them to use and piece of metal. Also, the sides of cidentally, isn't it time for another make myself a neat little device the hole are just as smooth and of your pills?" in the process as well!" straight as are given by the drill "Very shortly," agreed Smithy. "It's surprising," commented on its own. However, let's now "I'm just about due for the odd Smithy, "how all these odds and get down to the actual hint! An spot of riboflavin." ends seem to have applications idea similar to the paper dodge which their designers never intended can be used when one is faced them to have! I've got another with the problem of getting a nut one here which takes advantage on to a tight screw. The tip is of discarded ball point pens. Our to cut a piece of paper into the Nut correspondent broke the blade on shape of a triangle with a sharp his neon screwdriver and decided angle, the sharp angle then being to reassemble the bits into a ball inserted into the nut. (Fig. 8). (a) (b) pen which had sufficient internal After this a tap of the appropriate diameter to take the neon and its size is put into the nut and run Fig. 8. Easing a tight nut and series 7). resistor. (Fig. In this through. The nut should then bolt assembly. A piece of thin particular instance, the two halves screw on to the bolt quite easily. of paper is cut to the shape shown the pen screwed together, but It is best to start off with a thin in (a), the sharp angle being the idea is applicable to other bit of paper since it is very easy inserted into types. the nut, as in (b). The brass tip forms the to take too much out of the nut, A tap is then p..t into the nut business end of the assembly and whereupon it becomes useless. In and run through

178 THE RADIO CONSTRUCTOR The Serviceman stood up and "You might suddenly," said Dick, reached behind his signal generator stretched himself. "go like Dr. Jekyll and Mr. Hyde. for yet another means of defence "Do you know, Dick," he re- During the day you'll be fixing in his battle against the ravages of marked, "I only started taking tellys and, during the night, you'll Time. these vitamins and things this turn into an 'orrid monster and go morning and yet I feel better already. around bashing them all up again !" The hints described in this month's In a couple of weeks I should be a "That's a possibility," commented episode of "In Your Workshop" were I find contributed (in the order in which they new man!" Smithy musingly, "which appear) by E. E. Meachen, B. Barker, "You'd better be careful," warned rather attractive. If these pills C. J. Collister, T. E. Millsom, J. F. Reed, Dick, "that you don't accidentally continue to buck me up, I'll give S. E. Stratton, R. Achilles, B. H. Wynn and G. M. Watson. take the wrong combination of it serious consideration." Further hints for this feature are welcomed, pills." And with this sinister threat and payment is made for all that are pub- "Why's that?" for the future, the Serviceman lished-EnrroR.

Experiences of an

Early Amateur

The Third of a Five -part Series by C. H. Gardner

TO UNDERTAKE SOMETHING NEW AND ORIGINAL have to be small enough to rest between the in radio these days would almost certainly operator's feet. I was to be the operator. be the result of lengthy technical discussions, followed by laboratory work by a team of engineers Racing Car Transmitter and considerable final development before trial. Searching through my spares, I was lucky enough Forty years ago there were many interesting things to come across a sparking coil and spark gap which had not been attempted either because from an early aircraft transmitter. This, together no one had thought of them or because, in some with a small capacity 16 volt accumulator, would cases, the difficulties had appeared too great. seem to supply the necessary "prime mover". I do not know into which category one would A tuning helix was embodied in this transmitter place the idea of transmitting messages from a and the whole outfit could be fitted into a wooden racing car in 1922, but the difficulties certainly box less than one foot square. The transmitter appeared after the idea entered the head of Mr. was, however, too tall to allow a lid to be fitted S. F. Edge, the racing motorist, the day before so the aerial and earth terminals were well and he was due to try to break the World's Record truly exposed-a matter which was to cause some for the double twelve hours. In my last article I concern a little later in the proceedings. The explained how the job was put fairly and squarely morse key was screwed down on the edge of the on my own shoulders with the provision that the box and I estimated that the whole outfit could installation would have to be made during a refuel- sit on the floor of the car between my feet. They ling stop of less than two minutes and that the key would have to be operated by passing my equipment would have to be available in rather hand underneath my right leg, a little foresight less than 12 hours after I had first heard of the causing me to realise that my left hand would be idea. I also mentioned that valves were too fragile fully engaged in hanging on to a bar provided for to use for the job. Also, the available space in this 1,urpose for the mechanic. the car was such that the whole transmitter would The Brooklands track, where the record attempt

179 OCTOBER 1964 was to be made, consisted of a concrete oval banked Receiving Equipment at each end and providing a distance of approximate- Whilst these experiments with aerials had been ly two and a half miles per lap. In appearance taking place the necessary it was receiving equipment deceptively smooth but, from experience, had been erected. The receiver was, as I knew that this was far described from being the case when in the Press at that time, a "detector with reaction one really got going. There was one particular and two valve note on magnifier". The equipment bump what was known as the home banking was erected on what was known as Test which could cause one Hill, this to become literally airborne. providing an elevated site to assist in reception At very moderate or very high speeds this bump when the car was at the more could be dodged distant part of the but; over the upper middle range, track. The fact that any message received had it coincided with the position it was necessary to then to be sent to the take up on the refuelling depot on the side banking. I also surmised that of the track by "runner" only goes to verify that Edge, being engaged on a long distance record, the undertaking was more in the would be travelling nature of an experi- within this upper middle ment than a useful means of rapid communication. speed range. Hence the unsuitability of valves A much more certain and rapid and the necessity of means of passing having a free hand to hang on messages was evolved by the writer when a painful with. emergency The terminated the transmissions. aerial would have to trail behind, and a In the meantime, a rushed trip had fair length of wire would be been made needed to tune to the to the local butchers from whom a good strong 440 metres intended for the experiment. Aircraft bladder, such as is can trail their normally used for sausage aerials beneath them (a normal skins, had been obtained. It was hoped practice at that time) but the car that this would have might be instrumental in lifting the aerial from to trail its aerial above it. This indicated some the track, and form it had at any rate the merit of being of kite which would stand up to the speed non -lethal should any further and not materially reduce the speed of dive-bombing in- the car. cidents take place. At this critical juncture we Luckily, Vickers had an aircraft factory on the side were informed of the track that the transmitting equipment and, on arrival at Brooklands, the should be installed in the record problem was put to them. Edge had breaking car thoughtfully during its next refuelling stop in a few minutes provided a fast touring car for preliminary trials time. and the idea of designing a suitable "kite" was Immediately the car carte to a sheer joy to the Vickers staff who, within an standstill the hour, transmitter was lifted into it and placed on the had produced a most lethal-looking miniature floor in front of the glider which they mechanic's seat. The earth had tested in their wind tunnel connection was connected to the and which they guaranteed to "fly metal body of beautifully" the car and the aerial laid out behind it and con- at the necessary speed. Its lethal appearance was nected to the due to appropriate terminal on the trans- the wings having been constructed from mitter. By the time I had jumped aluminium and having very sharp into the mechanic's leading edges. seat the car was already on the move again. Just The nose end of the "fuselage" consisted of a as we a hefty started large fish basket was handed to me lump of lead and these two features suggested with a shouted injunction that the glider would that it contained Edge's knock me out even if it "lunch" and would I please feed him didn't decapitate me, should its flight as we circled become the track. This additional piece of equipment was, erratic. in fact, the First basic cause of all the trouble to come. things had to come first, and a preliminary It had to sit on my knee trial was made. The aerial and, unbeknown to me was coiled up on the at the time, contained grapes, pears seat of the car, one end being attached to the and other car juicy fruits, all equally fragile and as susceptible and the other to a hook thoughfully provided in to bumps and jars the correct as the valves that I had discarded position on the glider. Speed was then as unsuitable for transportation. worked up until "flying speed" was indicated on After the we had circled the track a few times Edge speedometer, whereupon the glider was launched suggested that I should into space. But we had transmit a message asking forgotten the turbulence for an air cushion to be provided for behind the car. This resulted in a most devasta«ng him at the next fuel stop. Disentangling myself from the display of aerobatics in which dive-bombing fruit, I managed to played a major part find the key and to my joy and in which the lead nose found that a long dash showed and razor-sharp leading edges of the wings a radiation of whistled some 300 milliamps. I was a little dubious as to past my head with only inches to spare. So far intelligibility I of the morse that I was transmitting as was concerned I could duck at the approach as the jolting of the of the glider, but the car was not conducive to driver was blissfully ignorant accuracy but a few laps later a of the goings-on immediately behind him, and I message on the blackboard at the fuelling depot showed "message pictured that at any minute his stunned body received". would slump over the wheel and that we would This encouraged Edge to request the follow the example of many others who had vanished of transmission into further messages with similar results, but tiring space over the top of the banking. My howls of this game he decided of anguish for an that it was time for lunch immediate stop eventually and demanded to be fed. It was penetrated and the glider experiment then that I dis- terminated. covered that the jolting had reduced much of 180 THE RADIO CONSTRUCTOR the I fruit was holding to pulp and that a damp in order to be able to speak over a distance of a sticky juice was trickling down my overall legs. mile. It may be noted that no attempt had been It was not long before this somewhat sticky waterfall made to make the communication two-way. had reached the lower extremities of my trousers. At about the same time Edge altered course a little Reception On A Train taking us well and truly over the famous home I suppose there was something about those early banking bump. The jolt of the car on landing days which inspired one to undertake something after this bump was sufficient to cause the complete which had not been attempted before and it was transmitter to become airborne, with the result perhaps natural that, having tried mobile experi- that the key made contact. At the same time the ments with a car, one's mind should turn to the bottom of the leg of my overall hit the spark possibility of wireless reception on trains. gap and the major portion of the discharge found It is quite astonishing how often, acting on an its way up my overall leg via the fruit juice and impulse and whilst full of enthusiasm, one finds returned to earth via that portion of my trousers quite unexpected co-operation and enthusiasm separating me from my metal seat. As this occurred from unexpected sources. Within half an hour of once each lap at intervals of less than two minutes thinking of the idea in the spring of 1924, I found and as I knew I was doomed to sit in that seat myself in the central offices of the Great for at least Western another hour I felt justified in feeling Railway and actually telling an official what I that the success of the experiment had been proved wanted to do; whereupon, less than an hour later, and I managed to disconnect the low tension I received a promise of a suitable van to be placed supply. For subsequent messages I brought into in a siding close to my home. After I had installed use my reserve method which consisted of writing the necessary equipment this van would these then be on slips of paper and hurling them discreetly attached to a main line express running between out of the car as we passed the pits. Birmingham and Paddington. A visit to view the van on its arrival indicated Two World Records two major matters. Firstly, it was constructed Nevertheless the experiment had been successful, from wood and this would ease the aerial problem. and we had made history. Edge so kindly wrote Secondly, it had very recently been engaged in in a letter its received a few days later "it had been normal duties of fish transportation; and some nice to be instrumental in breaking two World's treatment would be necessary before installation Records at the same time." Although the whole commenced! episode was great fun and there was some sense _ The van was a good length and provided plenty of achievement in being the first to transmit from of space for an internal aerial, which was soon a car actually engaged in record breaking, there erected in the form of a twin inverted L suitable was little to be proud of on the technical side other for reception of 2L0. The design of the receiver than the satisfaction of overcoming the impossible required a little forethought. The screen -grid time elements. valve had not at that time made its The début and the episode made me anxious to see what might superhet was still some distance away, so a limit be possible with a little more time for preparation. was imposed on the level of r.f. amplification Having obtained permission from the G.P.O. available. The mutual conductance of the r.f. and being given a special call sign for the job, valves of that time was less than unity it was and the decided to attempt the transmission of amplification factor something under 20. With speech from a car in motion. 180 metres was the the type of valve and circuit information available, wavelength allowed and a limit of 10 watts was and taking into consideration the frequency that imposed. A standard A.C. open two-seater tourer had to be received, two stages of r.f. amplification was used for the experiments and this was fitted with tuned circuits were about the most up with a that was six foot wooden mast at front and rear possible. A detector valve with reaction and a carrying a twin T aerial. The transmitter used a single stage transformer -coupled a.f. valve com- suitably adjusted grid controlled circuit, the h.t. pleted the receiver. On looking back, I note that and l.t. supply being provided by dry batteries. we fed the output from this receiver into a separate The first experiments were again carried out two valve a.f. amplifier. This may strike readers on the Brooklands track using the same receiving as curious until it is realised that no form of de - equipment as before, and the results were entirely coupling was used, and if too many stages were successful over the range of the mile or so which run from a single set of batteries, weird was and wonder- available. As The Motor reported at the time: ful effects resulted. The aerial tuning was by "It is quite possible technically, but impossible variometer, the two tuned -anode r.f. amplifiers legally, to transmit telephony from a moving car".* each being tuned by its own variable capacitor. My own view was that, without considerable As there was, in addition, an adjustable reaction development of the equipment, no sane motorist coupling control and each valve had a separate would want to be saddled with a couple of wooden adjustable resistance in its filament masts, circuit, setting a box full of batteries, and a transmitter up for maximum sensitivity at a particular frequency which took up the whole of the passenger's seat was quite a tricky business. This receiver proved to be surprisingly efficient The Motor, 20th February, 1923. when put on test with the aerial in the fish van.

OCTOBER 1964 181 It seemed reasonable to suppose that reception minutes delay, my log for this part of the journey of broadcasts would be possible on most of the showed a short blank whilst I endeavoured to journey so, after a telephone call to G.W.R. head- retain my balance and keep the equipment intact quarters, it was arranged to tack the van on to at the same time. one of their crack two hour expresses running On arrival at Paddington we were met by station between Birmingham and Paddington. The next officials who informed us that senior officials morning an engine collected us and in due course of the line would accompany us on the return backed us on to the express as it stood in Birming- journey and that for their use the van would be ham Snow Hill station. The shunting operation furnished with easy chairs. They also told us took a little time with the result that we left some the time of departure and this immediately intro- ten minutes late, a matter for some concern later duced difficulty, as no broadcasting was due to in the proceedings. take place at that time. The complete log of the journey appeared in A telephone call to the B.B.C. resulted, a little the Great Western Railway magazine of the period surprisingly, in their agreeing to fill the gap for but unfortunately my copy went adrift during the us. This is something which must seem quite last war. Loudspeaker reception was obtained incredible to readers who have not experienced both on the outward and home journeys, this the enthusiasm evinced by everyone concerned only being blotted out when passing through with radio in those early days. tunnels, or weakened when passing through deep I gathered that, although interested in participat- cuttings. A little difficulty was experienced in ing in the experiment, the senior railway officials keeping the equipment on the tables provided were not displeased that their return journey when negotiating the somewhat twisty piece of would be in more salubrious and comfortable line around High Wycombe. Being "Tail -End conditions. Charlie" with an engine driver determined to arrive at Paddington on time in spite of the ten (To be continued)

from the half-wave secondary of Experimental thw mains transformer, T2i and is rectified by any suitable rectifier capable of operating at the secondary voltage and at a current of 20mA. The rectified voltage is applied to the reservoir capacitor C7, and is smoothed by LI and C6. It is possible for the choke to be replaced Gram Amplifier by a resistor having a value of 2.7kg2 and a rating of 2 watts, J. C. Noakes but this was not tried by the author. It will be noted that bias for Vi(a) and V2 is provided by a dry cell Our contributor describes a simple a.f. amplifier which incorporates and dry battery respectively. This a number of unusual features. We present the unit as an experimental method of biasing was employed project for readers who are interested in as it was felt that the cost was less somewhat unconventional than would be given with normal circuits cathode bias components and because, in the case of V2, the valve received a higher effective h.t. voltage. As no current is drawn HE ACCOMPANYING CIRCUIT DIA - correction circuit offered by RI, from the dry cell and battery, their gram illustrates a simple gram R2 and C1, and to the volume control life is equivalent to their shelf amplifier which employs two R3. The a.f. tapped off by the life. If desired, however, it would inexpensive double -triodes. The pro- slider of this control is then fed be possible to dispense with the totype has given good results and to the grid of the voltage amplifier 1.5 volt cell biasing and was Vi(a) designed to operate with a Vi(a). The anode of Vi(a) couples insert a 2.7k C1 resistor in the cathode minimum of components. A feature directly to the grid of the phase- lead instead. This could then be by- of the circuit is that the low h.t. splitter triode, VI(b). Out -of-phase passed by a 25µF electrolytic capaci- consumption enables a "converter" voltages then appear at the anode tor if it was found that gain was mains transformer to be employed, and cathode of VI(b) and are too low or that hum was introduced with the result that complete isola- applied, via C3 and Ca, to the into the cathode circuit. If the tion from the mains supply is grids of the 12AU7, V2. The cathode bias resistor is fitted, the achieved at low cost. anodes of V2 finally connect to a lower ends of R2 and R3, and the push-pull output transformer, Ti, lower terminal of the pick-up, are The Circuit capacitor C5 providing a level of returned to chassis. In the circuit diagram, a crystal top -cut. The 100k preset potentiometer, pick-up connects to the tone- The h.t. supply is obtained R5, offers a convenient method of

182 THE RADIO CONSTRUCTOR setting V1(b) to its optimum working L MR1 condition with respect to bias, and is adjusted at various volume levels to obtain the best performance. The output transformer employed in the prototype was a component offering 8k12 impedance anode-to - anode (i.e. 26+26:1 for a 312 T2 speaker), but it is probable that Zoo -a better results would be given by a 225Va, transformer offering 12 to 14kû o anode -to -anode impedance. These impedances correspond, for a 312 Crystal speaker, to a ratio of the order of pick-up 33 +33 :1. i R

Components List Resistors okC.o (All fixed resistors 20 % watt unless mains otherwise stated) RI 220kû R2 100kû R3 1MÛ potentiometer, log 5v track, ganged with Si. R4 100kû The circuit of the experimental two valve gram amplifier. R5 100kû pre-set potentiometer, linear track V1, 12AX7, V2, 12AU7 R6,7 100k0 matched 2% T2 Mains transformer. Secon- Switch R8,9 IM12 10% daries: 200-225 volts at Si d.p.s.t. on -off. Ganged with 20mA (minimum), 6.3 volts R3 Capacitors at 0.6A (minimum) C1 0.01µF paper L1 Smoothing choke (See text) C2 8µF electrolytic, 250V wkg. Batteries C3,4 0.05µF paper Valves B1 1.5 volt cell C5 0.001µF paper V1 12AX7 B2 9 volt battery. C6,7 32µF electrolytic, 250V V2 (Ever Ready wkg. 12AU7 PP4 would be suitable) Inductors Rectifier T1 Push-pull output transfor- MR1 Metal or contact -cooled h.t. mer (See text) Pick-up rectifier, 20mA minimum Crystal pick-up

ITA Orders Two Reserve Transmitting JUST ANNOUNCED . . . Aerials Independent Television Authority is taking steps to reduce to a minimum the time that an ITV programme would be off the air if a transmitting aerial 'nere seriously damaged. Contracts have been placed with EMI Electronics Ltd. to supply a reserve mast and two aerials-one vertically and one horizontally polarised. Subcontractor for for the supply of the mast is Access Equipment Ltd. This is the first time that precautions of this kind have been taken by a broadcasting organisation in the United Kingdom. Both aerials will be stored at the ITA's station at Lichfield, near the centre of England, for speedy transportation to any transmitting station in this country of whichever aerial is required. The mast can be erected in eight hours by a team of seven men, ready for the appropriate aerial to be erected and powered. The mast is 250ft tall. Both aerials are omnidirectional, Sinclair Radionics Ltd. have just announced the release with facilities for giving a figure -eight diagram if required, of their new 10 watt integrated amplifier. The Sinclair and will transmit on Band Ili. Each is of 20ft vertical "X-10" is a high fidelity integrated power amplifier and aperture, giving a gain of approximately 4 times. pre -amp using 11 transistors and having a transformer - The vertically polarised aerial consists of four stacks of less output of 10 watts for feeding into a 1512 loud- dipole clusters in a quadrature feed arrangement. The speaker system. The overall size of the unit is horizontally polarised aerial consists of four stacks of 6 x 3 x iin and the whole compact arrangement is super-turnstile elements similarly fed. shown in the illustration above.

OCTOBER 1964 183 ape Kec r er

y(a.Q.F11SWP1

Our contributor describes some simple modifications which, when carried out on a low-cost tape recorder, may considerably extend the facilities which it provides. This article is aimed at the reader who has sufficient experience and ability to carry out the simple changes described

DURING THE PAST TEN YEARS A GREAT NUMBER circuit must be broken at the point X. For technical of tape recorders have become available in reasons it is not possible simply to open -circuit the this. country. Most are in the £20-£100 price leads to the erase head because, if this were done, range and have become very popular with the the changing load on the oscillator circuits could public: there are nearly a million privately owned cause serious trouble. It is, tape however, a simple recorders in Britain today. Although many matter to overcome this difficulty and all that is recorders are excellent machines of high quality, needed is to switch the oscillator leads from the there are often certain special features lacking. For erase head to a resistor of equivalent value, as shown instance, input mixing or superimposing are avail- in Fig. 2. In position 1 the circuit functions nor- able on some models but not on others. Facilities mally as an erase unit, but in position 2 the oscillator such as these greatly enlarge the range of uses to output is connected across the resistor R. This recorder can be put, and recorders not simple circuit, requiring one switch and one resistor, having them can usually be converted at a very is all that is needed to provide superimpose facilities! reasonable cost. It is the aim of this article to The value of the resistor R should be of the same describe how these modifications can be effected. order as the impedance of the erase head at the oscillator frequency.1 ` The writer had a Adding machine "Superimpose" whose erase head impedance was 500 to 60052 and As an example of a typical modification a simple a lk52 resistor was used for R. The switch can be alteration will be discussed first-the addition of a an ordinary toggle or rotary type, to suit individual "superimpose" facility. When a tape recorder is requirements. switched to "record" the tape first passes over the erase head, which removes any existing recording Input Mixing and leaves the tape blank. It then moves past the A rather useful. feature is mixing of inputs. This record head where the signal to be recorded is enables two inputs to be varied in amplitude applied to it. In order to superimpose a fresh signal independently of each other. Many recorders over the existing one, it is necessary to stop the erase claiming to have mixing facilities do not, in fact, head from working. have them, and although these machines can record The block diagram of a normal recording chain from two inputs at once, they cannot alter their is shown in Fig. 1. In order to prevent erasure, the level separately.

Directions of tape 1 In a typical modern commercial recorder using this technique, Erase Record the load hen resistive switched into circuit for superimpose has a rating of } watt only. It is probable, however, that higher wattage ratings will be required with earlier machines, and this point will have to be determined by experiment.-Enrroa.

I Input to 2 11. Erase erase head 1111 head

Fig. 1. Simplified diagram showing the basic recording Fig. 2. Obtaining "superimpose" by means of an system. (The bias circuit is omitted) additional switch and resistor

184 THE RADIO CONSTRUCTOR -EF86 grid I 2 mic in ut

Fig. 5. Fitting a gain control after the microphone input socket initial microphone input circuit of Fig. 4 and mixing the output of the EF86 with the "gram" input as in Fig. 3. An external mixer circuit Fig. 7. This is really incorporating the mixer of Fig. 3 into the amplifier. There are two pos .ible ways of adding mixers to a recorder. Either ai external mixer can be used or Monitoring the internal circuits of the record pre-amplifier can Some recorders have a system whereby it is be altered. Mixers vary from simple potentio- possible to use the internal loudspeaker and playback meters to complex electronic types using several amplifier to listen to the recording signal as it is valves. being recorded. This is known as "monitoring". A simple external mixer is shown in Fig. 3. This It is accomplished by tapping off a portion of the circuit will work tolerably well provided that recorded signal, usually just after the pre -amplifier perfection is not sought by the user. Fig. 4 shows stage, and applying it to the audio output stage, the basic simplified circuit found at the input in usually via the playback volume control. In some many of the cheaper tape recorders. It is seen that recorders the audio output valve operates as the the microphone input is amplified by a low -noise oscillator when recording and, in these cases, amplifier and applied, together with the "gram". monitoring is impossible. Most modern recorders input, to the input gain control. Clearly, the two incorporate monitoring facilities when it is possible inputs cannot be varied independently by this to do so but older models may not. The addition arrangement. There is another snag in this system. of this feature is an easy matter if the audio output If the input to the microphone stage is excessively stage is available when recording and it is not being high, it may overload the EF86 and cause serious used for any other function. All that is needed is distortion. As there is no way of reducing the input a link between the signal (i.e. non -earthed) end of to this stage nothing can be done about it with the the volume control, and the input to the record circuit as it stands. However, the circuit can be main amplifier. If the record amplifier doubles as a modified by replacing R1 in Fig. 4 by a 2MSl playback pre-amplifier a switch should be inserted potentiometer, as in Fig. 5. This provides indepen- in the monitor lead so that it can be disconnected dent mixing of the inputs and also prevents the on playback, otherwise feedback may occur, possibility of overloading. If this system is adopted leading to violent oscillation. The monitor link care must be taken to screen all wiring and the metal should be made with screened lead to . prevent the potentiometer case, as these circuits are very introduction of hum.2 susceptible to hum. In addition, a modification must be made z We would advise a careful check of the tape recorder circuit to prevent the "gram" input control diagram before carrying out this modification, as it may not be from varying the microphone gain-the fully applicable to a number of models. It will be desirable, also, to modified is Fig. 6. ensure that a means exists for cutting out the monitor facilities circuit shown in The writer has when the recorder microphone is close to the recorder loudspeaker, used this arrangement in practice and found it to otherwise, acoustic feedback may occur. If there is a possibility work very well. of the link upsetting bias conditions in either of the circuits joined, or if there is a difference in d.c. potential between the two circuits, An alternative mixer is formed by retaining the an 0.01µF capacitor should be inserted in series.-Enrrox.

HT+ HT+ 1-- i E F86

ampM`

RI 2Mn 2'211ìi. mic 2.2M 1 n gram mit input (reT . input o output gain input g?input input inpu T 0 0 mi grnnai

Fig. 4. A typical tape recorder microphone input Fig. 6. Providing microphone and "gram" mixing stage facilities

OCTOBER 1964 185 EF86 no 3watt anode micinput gain Mn input to Erase erase kw it head

Main ram amp input gram gain Fig. 9. An experimental method of obtaining an erase -fade facility Fig. 7. An alternative method of mixing microphone damage to the output transformer and associated and "gram" inputs components. External Loudspeaker Erase -Fade Control Another item fitted on most modern recorders but The modifications just described can be in- absent from some earlier types is an external loud- corporated in many commercial tape recorders. In speaker socket. Due to the acoustic limitations of conclusion, a few. notes will now be given concerning a recorder cabinet an external loudspeaker is usually changes which may be made with slightly more necessary to obtain maximum fidelity. Fitting a difficulty. The first concerns the superimpose unit jack socket is of course a perfectly simple matter described previously. Instead of having a simple and, in its simplest form, the external speaker is superimpose on/off switch it is possible to use a connected in parallel across the internal unit as in potentiometer .to give an erase-fade control. This Fig. 8 (a). However, this presents the wrong is useful in adding endings to recordings having a impedance to the circuit as the two speakers are in sudden finish, and for other uses. The only difficulty parallel,- and the effective impedance is halved. in fitting this control lies in the fact that a 3 -watt Also the available power must be shared between potentiometer must be used, and some trouble could both speakers, so that there is a reduction in the arise in fitting such a large component in a congested power available at each. To prevent this a special cabinet. The circuit for the erase -fade unit is shown jack socket can be used which switches out the in Fig. 9.3 internal speaker when the jack plug for the external one is inserted. Such jack sockets are readily Experimental Echo available and are connected as shown in Fig. 8 (b). A circuit which has recently caught the writer's internal attention is an echo unit working on the following L.S. principle. The signal from the playback head after output amplification is returned to the erase head trans- and re-` former II recorded there (this is quite feasible). After a time lag dependent on the tape speed, the signal again new o.p. reaches the playback head and the cycle is repeated. jack Theoretically the number of echoes is infinite, but (a) internal in practice the inherent distortion in the circuits spea produces a badly distorted output after more than output II trans- about eight repeats. The writer took a signal from forme the loudspeaker output jack via a 1:10 matching transformer back to the erase head. Bias was switched output supplied from an external 50 kc/s oscillator and lack mixed with the returned signal. This arrangement worked fairly well but the writer cannot guarantee (b) the success of any similar experiments. With the exception of this last example, all these Fig. 8 (a). Adding an external speaker jack socket modifications can be incorporated in most com- (b). A more satisfactory external speaker circuit mercial tape recorders and will greatly extend the operating capabilities of the machines. If desired, a fixed resistor may be used in place of the internal speaker. A point of importance here is 3 The potentiometer shown in Fig. 9 has a value of 2U), but this may be considerably higher than the impedance of some erase that a recorder should never be operated without a heads, and would put this modification in the experimental category. speaker (i.e. with an open -circuit output transformer There is a slight risk, also, that the varying load presented to the erase oscillator when the potentiometer is adjusted may have an secondary). Operation in this manner can cause effect on bias amplitude.-EDITOR.

1964 R.S.G.B. EXHIBITION This Exhibition will again be held at the Seymour Hall, Seymour Place, Marble Arch, London, from 28th to 31st October. The hall will be open daily from 10 a.m. until 9 p.m. and the admission fee will be 3s. See "News and Comment" on page 161 of this issue for further details.

186 THE RADIO CONSTRUCTOR THE 1964 TELEVISION AND RADIO SHOW

THE 1964 TELEVISION AND RADIO amplifier giving 5 watts per channel tuners follow the standard two - Show at Earls Court was open and a gram deck. The latter has a triode line-up, the first triode acting to the general public from 26th special lightweight pick-up having a as r.f. amplifier and the second as August to 5th September, 24th and diamond stylus and a pressure of mixer/oscillator. Lecher lines with 25th August being given over to the only 2 grams. continuously variable tuning are Press and the trade. Transistorised record players and standard practice, a tuned line being It was a little disappointing to find radiograms were also featured by employed instead of aperiodic tuning in this, the first Radio Show for two H.M.V. and Ferguson. in the aerial stage. R.G.D., how- years, that only the ground floor at Apart from the introduction of ever, exhibited sets employing a Earls Court was occupied. In the band -spreading at the high frequency transistorised u.h.f. tuner, it being more ambitious Radio Shows of end of the medium wave band-to claimed that this could work with earlier years emphasis has been facilitate the selection of Radio 50 per cent less signal, for equivalent placed on such things as colour tele- Luxembourg and the pirate stations noise level, as compared with a valve vision, the 405/625 line controversy, which appear at these wavelengths- tuner. A demonstration was pro- the introduction of transistors, and and the gradual introduction of vided, by means of which a direct spectacular stagings of radio and complementary Class B output stages comparison between a valve and a TV personalities, but nothing of this -in which one p.n.p. output tran- transistor tuner could be made nature was evident at the present sistor handles the negative half of under fringe conditions by touching exhibition. Nevertheless, despite what the signal and one n.p.n. transistor a button. the bugler would have described as handles the positive half-transistor its "reduced circumstances", the radios tended to exhibit few out- The Pye stand included a very Show still had plenty to offer that standing changes. well -attended demonstration of final Two portables test and was of interest to both the radio from Ekco had push-button pre- alignment, under factory enthusiast and the layman. tuned selection of Radio Luxem- conditions, of u.h.f. tuners. The bourg in addition to manual tuning tuner under test was swept across Radio and Audio over the medium and long wave the u.h.f. band, the response being In the sound reproducing field, a bands. shown on a large receiver -type number of manufacturers exhibited cathode ray tube. Demonstrations single -cabinet stereo equipment. One innovation was, however, of this type tend to attract a crowd, R.G.D., for instance, showed their evident on the Perdio stand. This and a thoughtfully provided closed- RGD 211 stereogram. This offers an consisted of a communications -type circuit camera and monitors enabled output of 6 watts per channel fed to portable receiver known as the people on the outskirts to see, in four loudspeakers, and "Marco Polo", which covers 1.6 to close-up, the actual process of incorporates 30 Mc/s a 5 -band radio (long, medium, 2 ae well as medium and long alignment. short waves and v.h.f./f.m.) together waves and v.h.f./f.m. A b.f.o. is with a Garrard AT6 record changer. included in the design, and the circuit employs 16 transistors. Featured in the design is a switch Other Exhibits which offers bass boost at low listen- In the past, Roberts Radio have exhibited portable transistor receivers In addition to the domestic radio ing levels. The cabinet is 5ft long and TV stands, there were compre- and includes a space which may be in diamond -studded and solid gold cases. (In the latter, the gold was so hensive displays by the Services. The used for record storage or the in- Royal Air Force presentation installation of a tape recorder. solid that the receiver had to have an external telescopic aerial.) This cluded a full-scale replica of an Air Pye introduced their Model Traffic Control Tower, whilst the "Double One Double One" receiver, year the tradition was maintained, and Roberts presented a portable Royal Navy exhibited a Wessex i which reproduces stereo broadcasts HAS1 helicopter with its covers over two speakers, one of which may covered in real leopard skin at 120 guineas. This price includes the opened so that all internal equip- be moved to any position. This unit ment could be examined. is capable of working with the cur- batteries! rent B.B.C. test transmissions, and The home -constructor was amply sockets allow stereo records and Television catered for,. both by Daystrom and tapes to be played through the radio. Ferguson exhibited their new by Mullard. The Daystrom stand The "Double One Double One" is "900 Series" TV chassis, which has carried an impressive array of com- completely transistorised, as were the feature of cool running due to plete Heathkit equipment, and the four new stereograms demonstrated the use of an autotransformer for Mullard home -constructor stand was by Pye. Also shown by Pye was the the heaters instead of the usual fully staffed to cater for amateur "Achoic" Stereo Projection System. dropper resistor. There is only a enquiries. Mullard home -construc- In this, a cluster ,pf six speakers single circuit board, this being tor designs on view included an allows twin-channel sound to be mounted at the bottom of the cabinet. electronic organ, a baby alarm. a reflected from the walls of the room. All television receivers were 405/ burglar alarm, an electric fence unit The "Achoic" unit is only 22in wide, 625 line models, having separate and transistorised high fidelity am- but it incorporates a transistorised tuners for v.h.f. and u.h.f. U.H.F. plifiers.

OCTOBER 1964 187 GAS FILLED COUNTING TUBES

By J. B. Dance, M.Sc.

HENEVER ONE VISITS AN EXHI- one complete revolution. An output Cold Cathodes bition of modern electronic pulse is provided each time the glow In common with all other types of equipment, one can almost In a tube moves from position 9 to cold cathode tube, gas filled decade always see apparatus containing position 0. tubes have characteristic striking and tubes in which a rather fascinating Counting circuits are known as maintaining voltages. They must red glow is rotating. The speed of always be used in series with a rotation can have any value up to scalers, since each decade divides or scales down the number of input resistive load. They have a very long some hundreds per second, at which life when correctly used, but will be speed the discharge appears as a pulses by a factor of ten. Gas filled decade tubes are often known as quickly damaged by excessive cur- circle of red light. rents. The life of the tubes is pro- Such tubes are employed in one "Dekatrons", but this term is a registered trade mark for the gas longed if care is taken to ensure that of the most common types of circuit the discharge does not remain at any for counting electrical impulses. filled decade tubes made by Ericsson Telephones Ltd., and should not be one cathode for a period which is The first tubes of this type were longer than a week or so. designed about 1950.1 The applied to tubes of other manu- tube on facturers. the right-hand side of the display Double Pulse Operation indicates the number of units Pulse counting equipment is a very Many tubes operate on the double counted, the tube next to it indicates important part of modern electronic pulse principle. A double pulse tube the number of tens, the tube next to instrumentation. Apart from its use consists of a circular anode with that the number of hundreds, and so in radioisotope work, counting cir- thirty rod -shaped electrodes placed on. The glow in the units tube cuitry is much used in the automation around it as shown in Fig. 1. All of rotates at ten times the speed of the processes of modern factories for the rod -shaped electrodes are identi- glow in the tens tube. Decade tubes such applications as counting shaft cal. Ten are main cathodes and the divide the number of input pulses by rotations, or for counting the other twenty are known as guide or ten so that the output pulses from number of articles coming off a transfer electrodes. The guide elec- any tube may (after amplification production line and automatically trodes serve to transfer the discharge and phase inversion) be used to placing the desired number in each from one main cathode to the operate the succeeding tube. Ten batch for sale. The pulses need not succeeding main cathode. They pulses must be fed into the equip- necessarily be evenly spaced in time. normally receive a positive bias and ment to cause the units tube to make the discharge will therefore not rest Some types of gas filled decade at a guide electrode for any appre- tube can count at speeds up to 4 kc/s, but ciable time, since the anode to main a few types are limited to about cathode potential is greater than the 1 kc/s. Counting tubes of these types anode to guide potential. are very useful for use with Geiger - The ten guide electrodes which are Müller tubes for radioisotope work. on the clockwise side of the adjacent Faster gas filled decade tubes have main cathodes are all connected also been developed for counting at together and are known as first speeds up to 10 kc/s, 20 kc/s, 50 kc/s, guides. (Fig. 1.) The guides on the 100 kc/s and 1 Mc/s. They do not anti-clockwise side of the adjacent all function on the same principle, main cathodes are also connected although certain aspects of the together and are known as second principles of operation of all gas guides. filled decade tubes are very similar. If the glow discharge is resting at The colour of the discharge is usually the zero position, Ko, an input pulse red, but some of the higher speed must cause it to be transferred to the tubes (such as the GC1OD and the next main cathode, Kt, for a count EZ1OB) are filled with a different gas to be registered. This involves three mixture and emit a bluish glow. separate steps. Initially the input High vacuum decade tubes (the pulse, after suitable amplification E1T and Beam Switching Tubes) are and shaping, is applied as a negative - Fig. 1. The structure of a double also available, but do not, of course, going rectangular pulse to the first pulse selector tube come within the scope of this article. guides so that the potential of these

188 THE RADIO CONSTRUCTOR guides falls to a value which is well below the earth potential. The discharge therefore moves from the main cathode K. to the succeeding first guide, since the anode to first guide voltage is greater than the anode to main cathode voltage. It does not move to any other first guide, since only the first guide succeeding the main cathode which is glowing is "primed" by the ions in the gas formed by the discharge. The ions only travel a short distance in the gas and therefore the priming effect is not very great except between neighbouring cathodes and guides. When the discharge has rested at the first guide for a short time (about 75 microseconds), a negative -going pulse is applied to the second guides and the first guide pulse terminates Fig. 2. The basic integrator input circuit for double pulse shortly afterwards. When the first tubes guide potential returns to its quies- simple type of circuit is known as for the GS1OH and 12 -way tubes cent positive value, the discharge an integrator circuit, since a graph types GS12D and GC12/4B. It will moves to the second guide which is of the second guide voltage against count up to one hundred, but further on the clockwise side of the pre- time is (for short times) approxi- decades similar to the second decade viously glowing first guide. This it mately the waveform which would shown may be added so that the the only second guide which is have been obtained by mathemati- circuit can count up to any desired strongly primed. At the end of the cally integrating the voltage/time number. pulse the second guides return to waveform applied to the first guides. The input pulses applied to this their positive bias potential and the circuit should be positive -going and discharge moves to the succeeding Outputs have an amplitude of at least 20 main cathode which is strongly 'An output pulse can be obtained volts. Vi(a) and Vi(b) form a mono - primed, namely Ki A count has from any cathode by inserting a stable multivibrator. In the quiescent now been registered. suitable resistor in that cathode state Vi(b) is saturated, since its grid Double pulse tubes derive their circuit. Normally an output is is returned to the h.t. positive line,. name from the two guide pulses required from the zero cathode of whilst V1(a) is cut off by the bias which must be applied to them to each tube for the operation of the developed by the V1(b) anode current cause one count to be registered. succeeding tube, as shown in Fig. 2. flowing through the common cathode The two pulses must overlap slightly In this case the tube may have all of resistor. A positive -going input in time. If the second guide pulse is the main cathodes from 1 to 9 pulse causes Vi(a) to conduct. The applied before the first guide pulse, inclusive joined to a common base anode voltage of Vi(a) therefore falls the glow will move in an anti- pin, only the zero cathode having a and V1(b) is cut off. The circuit clockwise direction and the counting separate external connection. Such remains in this state whilst the will be in reverse (that is, subtraction tubes are sometimes known as capacitor C is charging through the will take place). Double pulse tubes "counter" tubes to distinguish them grid resistor of V1(b). As soon as are symmetrical in each direction and from "selector" tubes which have V10)) commences to conduct again, it is only the order in which the each main cathode returned to a the circuit quickly switches itself pulses are applied to the two sets of separate base.pin. An output pulse back to its quiescent state in which guides which determines the direction can be obtained from any main in which the counting will occur. cathode of a selector tube. In practice the two guide pulses The output pulses obtained from must be derived from a single input gas filled decade tubes are positive - pulse. The usual type of circuit going. These positive -going pulses which is used to derive the two over- must be changed in phase before lapping pulses from a single input they can operate a succeeding tube. pulse is shown in Fig. 2. The- square In addition the output pulse voltage brackets inside the tube show that is only about thirty volts and must there is more than one first guide and be amplified to about 120 volts more than one second guide, but before it is suitable for the operation only one of each is shown for of a succeeding tube. Some form of simplicity. When a negative -going coupling amplifier is therefore re- pulse is applied to the circuit, the quired. capacitor C maintains the second guide voltage constant for a short 4 kc/s Double Pulse Tube Circuit time. Similarly at the end of the A typical circuit designed by the input pulse C holds the voltage of Ericsson Company for use with their the second guides constant for a double pulse tubes is shown in Dekatron short time. Thus the pulse at the Fig. 3.2 This circuit is suitable for A modern miniature second guides is effectively later in the 4 kc/s tubes type GC1OB, -the Ericsson GS1OH (Ericsson time than the first guide pulse. This GC10B/S, GC10/4B, GS10C/S, and Telephones Ltd)

OCTOBER 1964 189 475±25V

15okn

03/3

V10 Vlb V20 E 12AT7 I/2 12AT71F O02yF 71.n 4.7kn IMn

INPUT CIRCUIT DECADE SECOND DECADE Fig. 3. A two -decade circuit for counting at speeds up to 4 kc/s. V3 and V4 may be GS10H, GC10B, GC10B/S, GC10/4B, GC12/4B, GSI OC/S or GS121) Vi(a) is cut off. Thus the amplitude at the and junction of the 10kí3 and lished.2 Mullard duration of the pulse at the 150kí2 resistors. circuits for driving anode of Vi(a) are independent of their tubes from valves,3'4 trigger pulse characteristics. The The zero cathode of each counter tubes,5,6 and transistors7 are also GEX55/1 diode in the input circuit tube is returned to a -20 volt supply available. American manufacturers prevents the negative trailing edge of so that larger output pulses can be have designed rather different types the input pulse from reaching Vi(a) obtained from the tube. The output of circuits.8.9 and causing this tube to be cut off pulses are fed to the coupling valve prematurely. The duration of the V2(b). This valve is normally cut off Asymmetrical Tubes pulse at the anode of Vi(a) can be by the negative voltage applied to Double pulse tubes are sym- controlled by choosing an appro- the output cathode, but when the metrical with respect to the clockwise priate value of C. In the circuit discharge rests at the output and anti -clockwise directions, but cathode, two shown the pulse fed from Vi(a) to V2(b) conducts. The positive-going guide electrodes are required V2(a) has a duration of about 80 pulse from V3 is thus amplified and between each two main cathodes to microseconds. The cathode follower phase-inverted so that it is suitable ensure that counting occurs only in stage, V2(0, provides a low im- for feeding to V4. The capacitor the desired direction. Other types pedance output suitable for the coupling ensures that the pulse of tube are available in which the operation of the counting tube, V3. applied to V4 does not have an direction of counting is determined A portion of the pulse from V2(a) excessive duration even if V2p,) is by the asymmetrical structure of is fed to the first guides of V3, whilst conducting for a long time. either the guide electrodes or of the the second guide pulse is obtained Ericsson main cathodes or both. Such tubes by means circuits using trigger employ only of the normal integrator tubes and also circuits one set of guide elec- circuit. The guide using tran- trodes. Examples are the bias supply is sistors instead of valves for coupling Elesta obtained from the point marked E, double EZ1OB10 and the S.T.C. G10/241E pulse tubes have been pub - "Nomotron".11 If the Counter Tube Equivalents discharge is resting at the and Near Equivalents zero cathode of one of these tubes, an input pulse applied to all of the American CV Code Ericsson Mullard guide electrodes will cause the discharge to move to the guide 6476 CV2325 GS10C/S following the zero main cathode. 6482 Z502S At CV2271 GC10B/S Z303C the end of the input pulse the 6802 CV1739 GC10/4B discharge moves to the main cathode CV6100 GC10/4B/L following the zero cathode, since the CV6044 GC10B/L guides receive a positive bias. The CV5143 GC1OD shape of the guides and/or 6879 of the GC10/2P (Maintenance main cathodes ensures that the type only) discharge can move only in a clockwise direction. These tubes cannot

190 THE RADIO CONSTRUCTOR be used to count in the reverse direction. The GC10D Tube Another type of tube is the Ericsson single pulse Dekatron type GC10D12. The construction of the GC10D is similar to that of the double pulse tube shown in Fig. 1, but forty electrodes are employed around the central anode instead of thirty. Ten of the electrodes are main cathodes whilst the remaining thirty are first, second and third guide electrodes. There are three guide electrodes between each two main cathodes. All of the first guides are joined together and are brought out to a common base pin. Similarly all of the second guides are brought out to one external connection. The third guide preceding the zero cathode is connected to its own base pin, whilst the remaining third guides are connected to a common base pin. The GC1OD can be used in the type of circuit shown in Fig. 4 for counting at frequencies up to 20 kc/s.2 Normally a GC10D tube is only employed in the first decade, The Ekco automatic scaler type N610A for nucleonic work. The upper row since a slower and slightly cheaper of tubes performs the counting operation on the input pulses, whilst the lower 4 kc/s tube is perfectly satisfactory row performs the automatic timing operations which enable the scaler for use in the second decade. The to switch itself off after a pre-set time. The two tubes on the top right input circuit of Fig. 4 is very similar hand side of this scaler are not counting tubes; they are tubes for indicating to that of Fig. 3, but the capacitor the state of the count in two high speed valve counting circuits. (Ekco which is connected between the Electronics Ltd) anodes of Vita) and V1(b) has been reduced in value from 470pF to The rinput pulses are applied current flows through the first guide 100pF so that the pulses provided simultaneously to the first and second resistor, the first guide potential is by the input circuit are shorter. The guides of the GC10D (V3). The raised and the discharge quickly GC10D can operate with input discharge moves first to the primed steps to the second guide. At the pulses of about 25 microseconds first guide, but when the anode end of the input pulse, the discharge duration. +475±25V

Fig. 4. A GC10D 20 kcls circuit with coupling to a succeeding 4 kc/s tube (V4)

OCTOBER 1964 191 moves to the third guide, but the crease so that the glow will not step The Use of current flowing through the third back to this third Decade Tubes With E-M guide resistor guide. Counters causes the third guide The output pulses from the potential to be increased until the The electro-magnetic counter13 coupling tube, V2(b), in Fig. 4 are can normally count discharge moves to the succeeding used to operate a at speeds not main cathode. normal 4 kc/s exceeding about 25 pulses per decade tube. Any further decades second, but has If the third guide preceding the which may be the advantage that zero main required may be one small, compact and reasonably cathode were connected similar to the second decade shown directly to earth, the discharge in cheap unit can indicate up to at least the circuit of Fig. 3. six digits. Each decade might step back from the main Although there are tube can only cathode to this four separate indicate a single digit. If it is guide when the main stepping movements of the discharge cathode became positive due to the in required to count pulses at frequen- the operation of the GC1OD, two cies up to I flow of the tube current through the of these occur kc/s, it is common zero main cathode automatically. The practice to employ two cascaded resistor. The overall operating speed can therefore tubes third guide preceding the zero main be followed by an electro- greater than that of the simple magnetic counter. The cathode (shown on the right of the double pulse tubes in decade tubes tube symbol) is which three each divide the pulse rate by ten, so therefore returned stepping operations occur for each through a 220k0 resistor to the zero that the electro-magnetic counter pulse which is counted. Double will not be main cathode. This ensures that as pulse required to count at a the tubes are available for opera- speed greater than ten main cathode potential increases tion at 10 kc/s, 50 kc/s counts per when and 100 kc/s; second. This method is the anode current flows the input pulses fed more through the to these tubes economical than if decade tubes cathode resistor, the have a shorter duration than those third guide potential will also in- alone are employed to display six or required by 4 kc/s tubes. eight figure counts. The output Table of Gas pulses from the decade Filled Counting Tubes tube do not have nearly enough power for them to be able to operate Anode Max Remarks an electro-magnetic counter directly. Operating Base Current In addition their duration is not (Micro- P. usually Freq. Double Pulse) suitable. A circuit such as amp) that shown in Fig. 5 may be used to Ericsson: amplify and shape the pulses so that GC1OB they are suitable for the operation of GC10B/S an electro-magnetic counter.14 The input circuit which operates GC10B/L 4 kc/s Octal 250-550 GC10/4B ' D.P. the decade tube, VI, may be similar GC10/4B/L to that of Fig. 3. The positive -going . output pulses GC12/4B .. 4 kc/s from VI of Fig. 5 are Octal 250-550 D.P.; 12 -way short-circuited to earth by the GC1OD 20 kc/s Octal 700-1,200 GCAIOG l Single pulse decade 0A85 diode. The negative going GSAlOG f 10 kc/s B27A 500-900 D.P.; Ten auxil: trailing edges are used to trigger the anodes monostable multivibrator circuit of GS10C/S .. 4 kc/s B12E 250-550 GS12D .. D.P. V2 which shapes the pulses into a 4 kc/s B12E 190-350 D.P.; 12 -way form suitable for GS1OD .. 10 kc/s the operation of B12E 700-900 D.P. the electro-magnetic counter in the GS10E .. 10 kc/s B12E 700-900 GS1OH .. D.P. anode circuit of V2(b). The voltage 5 kc/s B13B 250-550 D.P. dependent resistor, VDR820B, GS10J .. 1 kc/s short- B17A 350 D.P.; low voltage. circuits any voltage peaks which are tube developed as the current in the coil Mallard: of the counter is cut off. The value of R can be adjusted Z303C .. 4 kc/s Octal 250-550 Z502S D.P. until each pulse has the desired .. 4 kc/s B12E 250-550 D.P. duration. If the value Z504S .. 5 of R is 120k02, kc/s B13B 250-550 D.P. the duration of the Z505S .. 50 kc/s pulses of current B13B 600-1,000 D.P. through the coil of the electro- American: magnetic counter will be about 20 milliseconds. 6476 .. .. 4 kc/s B12E 300-600 6802 D.P. .. .. 4 kc/s Octal 300-600 D.P. Reset 6879 .. .. 5 kc/s B7G 300-600 7978 .. D.P. In most circuits it is desirable to. .. 5 kc/s 13 pin 300-600 D.P. be able to reset 6910 .. .. 100 the decade tubes to kc/s B12E 600-800 D.P. zero. This can be carried 6909 .. .. 100 kc/s Octal out in the 600-800 D.P. circuits of Figs. 3, 4 and 5 by 7155 .. .. 100 kc/s B12E 600-800 D.P. returning the cathodes 1 to 9 of all S.T.C.: tubes to a common negative reset line separate from the G10/241E .. 20 kc/s B12E earth line. 2,400-5,000 Asymmetrical tube (See Fig. 6.) This negative line is Elesta: connected to earth via a resistor (R) which is normally EZ1OB .. 1 Mc/s 13 short-circuited pin 1,200-1,900 Asymmetrical tube by a switch. During the resetting ECT100 .. 1 Mc/s Special Reversible decade operation the switch is opened so tube that all of the main cathodes except the zero cathodes rise in potential. 192 THE RADIO CONSTRUCTOR The glow in each tube therefore moves to the zero cathode. The value of the resistor joining the +475 t4Ovolts common return line to earth varies with the number of decade tubes em- +35Ovolts ployed, but should be chosen so that the cathodes (other than the zero cathodes) of the decade tubes rise in /E -M VDR8208 potential by about 100 volts during counter l\oltage the resetting operation. 82kn dependent The switch S of Fig. 5 must be resistor closed during the resetting operation to prevent a spurious pulse being recorded by the electro-magnetic counter. Normally this is carried out by contacts on the relay or switch which is used for reset. The switch S should open after the decade tube resetting contacts have returned to their normal position. Accurate Timing Decade tubes are often useful when an operation must be accurately timed. For example, in welding L operations it is often required to apply the power for a certain time and decade tubes can be used for Fig. 5. Using a decade tube to drive an electro-magnetic counter. V1 is a this purpose.15.16 They may also be 4 kc/s Dekatron used to provide small marks on an oscilloscope screen which correspond decade tubes so that the second ling Circuits for the Z504S Stepping to very short time intervals.'7 Thus decade tube provides one output Tube". Mullard Applications Lab. the X axis is calibrated in time units pulse per second. If another three Report, ARL594. directly. decade tubes are added, the final 7. G. C. Chappell and G. F. In many Geiger counting systems tube will provide one pulse each Jeynes. "Transistorised Coupling it is desirable to have an automatic thousand seconds. This pulse may Circuits for the Z504S Stepping timer which will stop the counting be used to cut off a valve and stop Tube". Mullard Applications Lab. the counting process in a separate Report, ARL612. counting circuit. 8. Sylvania Counter Tube Hand- Alternatively a scale of twelve tubes book. combined with a decade tube used 9. "Operation of Decade Counter as a scale of five may be used to Tubes". Raytheon Publication No. divide the 1 c/s pulses by 60 in order SP1097 (1963). to obtain 1 pulse per minute. 10. "Die Zählrohre EZ1OB in Circuits have been published which Transistorschaltungen". Elesta Tech- enable decade tubes to divide the nische Mitteilungen, Nr. 18, April incoming pulse rate by any number 1962. up to nine.9 11. "G10/241E Nomotron". S.T.C. Valve Division Application To next stage References Report, No. MS/114. 1. R. C. Bacon and J. R. Pollard. 12. J. R. , Acton. "The Single Reset line "The Dekatron", Electronic Eng., Pulse Dekatron". Electronic Eng., from other May 1950. February 1952. 2. The Ericsson Tube 13. tubes Technical J. B. Dan `Electro- Handbook. (Available on a sub- magnetic Counter The Radio scription basis from Ericsson Tele- Constructor, October 1963, p. 190. phones Ltd., Beeston, Nottingham.) 14. "Dekatron-Tube Circuit for 3. G. F. Jeynes. "Decade Step- Reducing the Frequency of Impulses ping Tubes and Their Operation". for Operating TCe Counters". -20 volts Mullard Technical Communications, Sodeco Information Sheet E48. Vol. 4, No. 37 (February 1959). 15. T. W. Brady. " "The Use of Fig. 6. A circuit which enables (Also reprinted as a separate booklet Cold Cathode Counting Tubes for the tubes to be reset to zero of the same title.) the Control of Resistance Welding". 4. "Stepping Tubes", Mullard Electronic Eng., February 1956. Outlook, March 1959. 16. G. F. Jeynes and M. Veeraju. after a pre-set time. This may be 5. G. F. Jeynes and S. Zilkha. "Synchronous Resistance Welder achieved by using decade tubes to "Trigger, Tube Coupling Circuits for Controller for 1 to 130 cycles". divide the mains frequency a number Counting Tubes". Mullard Technical Mullard Technical Communications, of times. The 50 c/s mains may be Communications, Vol. 7, No. 64, p. Vol. 6, No. 54, p. 172. full wave rectified so that 100 c/s 184 (August 1963). 17. J. H. L. McAuslan. "A pulses are obtained. These may be 6. G. F. Jeynes and S. Zilkha. Dekatron C.R.O. Time Marker". divided in frequency by two cascaded "Cold Cathode Trigger Tube Coup - Electronic Eng., December 1952.

OCTOBER 1964 193 THE NEW DUAL-STANDARD TV SETS

By Gordon J. King, Assoc. Brit. I.R.E., M.T.S., M.I.P.R.E. PART 6

This the article, sixth in our series on 405-625 line receivers, deals with a.c. and d.c. couplings, high-level contrast controls and the extraction of the intercarrier signal

LAST MONTH IT WAS SHOWN HOW A.C. COUPLING the annoyance of aircraft flutter it also, between the vision unfortunate- detector and the video ly, causes the black level of the picture to vary amplifier is sometimes employed with the 625 - in accordance with the mean of the line signal so as to picture content. permit the video amplifier valve Of recent years the effect has been to be biased towards class A further ag- conditions on both gravated by the use of mean -level a.g.c. systems standards. in the In the vision channel in which, as we saw last early days of television, the importance month, the control bias depends not of retaining the d.c. only upon the component of the picture amplitude of the actual signal but also upon the signal was stressed in technical articles and books video modulation itself. dealing with TV reception. Where it was necessary It has been said that the sum effect is now to break the d.c. connection in the chain from the deleterious to the vision detector to picture display. This can, in the tube grid or cathode in fact, be true of low key scenes, for when the those early models a small diode was picture often used is mainly dark the resulting rise in the black level to restore the d.c. component. The diode was produces the caused to appearance of a sooty grey background. conduct on the sync pulse tips and the Lack of d.c. in the picture signal resulting voltage across its load also shows up had the missing as streaking from captions, a symptom which is component present on almost all recent models. Keyed a.g.c. systems, in which the true signal Black Level level at the porches of the As time passed and picture waveform is designers had more field sampled and converted to an a.g.c. bias, avoid experience to draw upon, it was found undesirable the deletion of the d.c. under certain component, but whether conditions to retain the full value or not a correctly stablised black of the d.c. of the vision level picture signal. While full value results also upon the other factors referred to d.c. stabilises the black level to an optimum degree, above. ' it also tends to emphasise the flutter effect associated We have already noted that a.c. with passing aircraft and other coupling between like media. the vision detector and the video amplifier on Consequently, d.c. attenuation was adopted 625 lines assists from (and is the aspect of video amplifier in use still) between the video amplifier biasing. There is, however, anode and the picture tube a further aspect in cathode. This is ac- this respect when mean -level a.g.c. is used on the complished by an RC parallel combination, as negative picture modulation of shown in Fig. 20. Although 625 lines. We d.c. attenuation reduces have seen that on the positive picture modulation HT+ of 405 lines the a.c. component of the video signal is held constant. The same applies on negative modulation but since here the tips of the sync Picture tube pulses correspond to 100 % modulation, and the black level to about 77 %,. over -emphasis of the black level can occur on low key scenes, resulting in black going "blacker than black". This can be Cl avoided by switching over to a.c. coupling from the video detector to the video amplifier on 625 Video lines. amplifier Some dual -standard models use a.c. coupling and a restoring diode, others use a.c. and d.c. coupling and others d.c. coupling throughout, Fig. 20. The d.c. component of the picture signal is depending upon the exact design of the receiver. attenuated by passing it through an RC combination When gated a.g.c. systems are used, complications to the tube cathode. Ri effectively attenuates the d.c. can arise with regard to the operation while C1 passes the a.c. of the manual component of the video signal contrast control. With this control in the normal

194 THE RADIO CONSTRUCTOR position, a reversal in the sense of the control HT -F action can take place. This, then, demands contrast Black level potential control switching on changing from one standard R5 to the other. 12mA R3 Contrast High -Level Contrast Control These last difficulties may be overcome by the use of so-called "high-level" contrast controls. A typical -41 To tube circuit is shown in Fig. 21, and this uses one section cathode of the new double-pentóde valve, type PLF200. The other pentode section is used as an intercarrier Video To tube sound amplifier, and the valve is mounted on the grid j new decal (10 -pin) base. From vision --3 amplifier detector The pentode video section of this valve is of Brightness the frame grid construction and thus has a very Part PLF 800 high gm and can handle high peak anode currents (Double pentode) without distress. To give extended video response the anode load, R1, is only 2.7k2, a value which, although small in comparison with conventional video circuits, is made possible by the high gm Fig. 21. High level contrast control. The video signal of the valve. Moreover, the shunt capacitances across the load R1 is fed to the contrast control and of the contrast control network in the anode would the required signal level tapped off for application to the tube. The circuit also compensates for black level impair the top video response if a larger, more and avoids this changing as the controls are operated conventional, value were used. The gain is retained, (Note: For PLF800 read PLF200) of course, with the low value load because of the high gm. The video signal across the load is reflected 405 across the contrast control and this control taps Detector off the requisite amplitude of signal to fully drive the picture tube. This, then, means that the r.f./i.f. gain of the vision channel can be retained at a constant level, and this is facilitated by the a.g.c. Video system. Note also that by keeping a constant out level r.f./i.f. signal the sync separator operating conditions remain similarly constant, which is not so when the contrast is varied gain -wise in the signal channel proper. On 625 lines, the same reasoning applies to the intercarrier signal which, as we shall see later, is itself derived from the vision detector. The video amplifier valve thus needs to continuously handle the full video signal amplitude. It will be seen that the brightness control is also fed from the contrast control network, and this ensures that alteration of the brightness is Video and intercarrier generally unnecessary each time the contrast signal in control is adjusted. A "black level potential" is also obtained due to the fact that resistor R2 is connected to a circuit Intercarrier s_ which causes a current of 12mA to flow through signal out it, and the final i.f. amplifier may be used for this purpose. The current is drawn through R3 from To vision the upper end of R1 and about 40 volts develops AG.C. line across R3, which is the black level potential. This tends to hold the black level constant-or reasonably so-over the range of the contrast control. Fig. 22. In this circuit two vision detectors are used, The network provides for de -emphasis of the one for 405 and the other for 625 lines. S1 selects the d.c. gain of the video amplifier as the result of the detectors and S2 selects the video signal from the relatively appropriate load. On 625 lines the intercarrier signal high impedance of the power supply is developed across the primary of T1 and is taken off circuits at the lower and d.c. signal components. from its low impedance secondary winding. L1 and L2 There are various schemes available for high-level are the 625 video i.f. filters while L3 and L4 are similar contrast control, but most of them approximate filters for the 405 i.f. R1 is the 625 detector load and that shown in Fig. 21. Video correction is usually R2 the 405 load, the former coupling to the a.g.c. line, provided over the entire range of the contrast as has been discussed in the text

OCTOBER 1964 195 HT+ The vision detector acts, in fact, as a kind of modulator due to its non-linear characteristics. Video It thus produces the demodulated picture signal I out l plus the intercarrier signal, the former rising to Intercarrier about 5.5 Mc/s from a little above d.c. and the signal out latter appearing at 6 Mc/s (i.e. the frequency difference between the 625 -line sound and vision signals). Video The intercarrier signal can be filtered out either Video and amplifier valve at the output of the vision detector (Fig. 22) or intercarrier from a suitable point in signal in the video amplifier stage. In Fig. 19 (published last month), for instance, the signal is tuned at the amplifier cathode and taken via a capacitor (C54) to the intercarrier - amplifier. Alternatively, the signal may be filtered Fig. 23. In this circuit the intercarrier signal is out at the video amplifier anode circuit, as shown extracted by T1 in the video amplifier anode circuit. The primary, with in Fig. 23. C1, is tuned to 6 Mc/s and the The secondary gives a low impedance link to the intercarrier filter usually takes the form of intercarrier channel a parallel tuned LC circuit or transformer. In Fig. 22 two semiconductor diodes are used in the control. Ordinary video amplifier valves, however, vision detector, one for the 405 signal and the have the disadvantage that they are working towards other for the 625 signal. The circuits are switched the limit of their peak current capabilities when by S1 and S2. In the "625" position, the detector used in this way, but the new valve mentioned- responds to both the vision signal and the 6 Mc/s and others like it-is well able to stand up to this sound -vision difference signal, and the intercarrier method of contrast control. signal is developed across the primary of T1. This Another mode of contrast control features a transformer also acts as a 6 Mc/s rejector and screen -grid potentiometer, adjustment of the video prevents the intercarrier signal from gaining ad- amplifier valve screen -grid voltage varying the mittance to the video amplifier stage. amplitude of the video signal across the anode A coupling winding on T1 permits the intercarrier load. An extension of high-level contrast control signal to be extracted at low impedance, matching circuits takes the form of a light dependent resistor being accomplished in the input of the intercarrier in a potential -divider circuit, with a bulb whose amplifier by a similar type of transformer, but intensity is adjusted by a variable resistor, the this time with a low impedance primary. former changing the value of the l.d.r. and the latter In Fig. 23 the intercarrier signal is purposely functioning as the contrast control. In this way allowed to get into the video amplifier stage, where- contrast control stray capacitances are minimised upon a 6 Mc/s transformer in the anode circuit and the response of the video amplifier is enhanced. extracts the signal and passes it back to the infer - carrier amplifier. The transformer also acts as a Intercarrier Signal rejector, preventing the 6 Mc/s signal from getting In earlier parts we saw that the overall response to the picture tube. of the vision i.f. channel is adjusted on 625 lines One advantage of passing the intercarrier signal so that the response to the sound signal falls about through the video amplifier is that less subsequent 30dB below the maximum vision response. This gain is needed in the intercarrier amplifier, compared permits the passage of some sound signal as well with taking the signal direct from the detector as the correctly tailored vision signal through the output. There are, however, several disadvantages channel. At the vision detector the two signals in terms of linearity problems and beat effects beat and a signal equal to the frequency difference which can spoil colour reception. between them is produced. This is called the "intercarrier signal". (To be continued)

Building & Mast Contracts for BBC's Scarborough TV & VHF Sound Relay Station The BBC has placed contracts with Messrs. Jaram & Son of Scarborough for the construction of the building, and with the Cornubian Construction Co. Ltd., of Danbury, Essex, for the erection of the 200ft aerial tower for the Scar- borough television and v.h.f. sound relay station. The new station, which is expected to be completed in early 1965, will be built at Rowbrow, about 2 miles west of Scarborough. It will radiate BBC -1 television and provide improved reception in the Scarborough and Filey areas, in parts of which reception is unsatisfactory and at times subject to interference from Continental television stations. The new station will also radiate the BBC's three sound programmes on v.h.f. to the Scarborough area. The v.h.f. sound transmissions will provide reception which is much less susceptible to interference than are the medium and long wave services and, with suitable receivers, can give much better sound quality. The BBC's site and mast at Rowbrow will also be used by the ITA for its projected television relay station to serve the same area.

196 THE RADIO CONSTRUCTOR THE "SINGLE SPAN" MAINS DRIVEN RECEIVER By Sir Douglas Hall, K.C.M.G., B.A. (Oxon)

The ability to tune continuously from 150 to 2,100 metres without range switching is only one of the ingenious features of this unconventional receiver design. Also employed is an adaptation of the author's "Spontaflex" single -transistor reflex circuit, this feeding directly into a 4 watt audio output stage

OME CONSTRUCTORS MAY HAVE NOTICED THAT drum of 1 -fin diameter fixed to its spindle. Along- a coil which is designed to receive the long side this, the coil is mounted in a vertical position, waveband with a ferrite rod as a core will cover being fixed to the tuner's baseboard by a small plug the medium waveband if the core is removed. It of wood which is cut to fit the coil sleeve and which occurred to the author that it should be possible is secured into a hole in the baseboard. A piece of to arrange a tuner which combined variation of -gin ferrite rod, 4in long, has a loop of drive cord inductance with variation of capacitance, thus fixed to one end by means of a rubber grommet. bringing about a total variation of inductance A length of drive cord is tied to this loop and is multiplied by capacitance of well over 100 to 1. passed over a pulley which is mounted above the This theory worked, and the tuner to be described coil at a height that allows all but about 4-in of the will tune from 150 metres to 2,100 metres in a rod to be withdrawn from the coil. The other end circuit in which stray capacitances are kept down of the cord passes round the drive drum and is fixed to a reasonably low level. to it so that, with the tuning capacitor fully closed, the rod just rests on the peg which fixes the coil to Tuning Assembly the board. This peg is inserted #in into the coil Fig. 1 shows the arrangement. A 500pF variable former, at which point the winding starts. With capacitor is mounted on a bracket and has a drive the tuning capacitor at minimum capacitance position (all vanes out) the rod will have been lifted Pul ley- Loop of cord held about 21.in. As the winding is approximately tin by grommet long the rod will be -in4 clear of it, whilst still being held steady by the top of the sleeve. A terminal with a groove can be used as the pulley, it being 4`x 3i? Ferrite rod bolted loosely to the 8in length of wood behind the coil. 8`high wooden Details of the coil are shown in Fig. 2. The support for pulley sleeve is made of a piece of contact or similar paper - Capacitor backed adhesive plastic, 4in by 3in in size. The spindle sleeve should be 4in long and is made by wrapping Coll sleeve the plastic round the rod. About half the paper - backing is left in place, and half cut away. The

Ferrite rod _Coil windings

Fixing peg 1;i81111111>11111194111VilliillÌl

`I /2li 2approx. II/2 approx.

Fig. 2. Details of the coil. The inner coil has 225 Capacitor Variable Base board turns of 38 s.w.g. enamelled wire close wound, and the bracket capacitor outer coil has 40 turns *of the same wire spaced to cover the inner coil. Details of winding are given in Fig. 1. The "single span" tuning assembly the text

OCTOBER 1964 197 paper -backed part is wrapped on first and the sleeve transistor circuits. But the unit must be vertical in held together by the sticky half as the wrapping is order to operate properly, and the coil will not completed. It should be made a nice easy sliding therefore provide satisfactory pick-up without an fit without being sloppy, since the rod has to be aerial. In consequence, the unit is not suitable for able to drop down it by its own weight and should a portable, as against a transportable receiver. If, not wobble in the process. as seems advisable, it is to be built into a static 225 turns of 38 s.w.g. enamelled, wire are close - receiver, it would follow that the apparatus could wound on the sleeve, starting +in from one end. be mains powered. When this winding is completed it is covered with After trying out a number of circuits, using both a further piece of adhesive plastic, which should be valves and transistors, the author found that the a tight, and not a sliding, fit, and should be about most satisfactory and sensitive arrangement, bearing 2in long. It is made in the same way as the first in mind the limitation of one tuned circuit, was to sleeve. On this second sleeve a coil is wound use a MAT101 transistor employing the "Sponta- consisting of 40 turns of the same wire, wound in flex" principle, this feeding into a high slope output the same direction, and spaced out so as to cover pentode, and deriving its modest power require- the whole of the first winding. Sellotape can be ments from the "waste product" flowing in the bias used to secure this second winding. resistor of the pentode. It is necessary for the tuning capacitor to have a The Spontaflex circuit has previously been little natural stiffness in its movement so that it is described in this journal.* In brief, it consists of a not closed by the weight of the rod. The Jackson common collector radio frequency amplifier feeding Bros. "Dilecon" component is very suitable. into a crystal diode and followed by a common base A 6 to 1 reduction drive is useful, but tuning will audio frequency amplifier. As it is a voltage not be found difficult despite the large span covered. amplifier at audio frequencies it is very suitable as It will be seen that for the lower (wavelength) part a driver for a pentode valve, which is a voltage of the medium waveband, where stations are most operated amplifying device; and as the input of the crowded, inductance is constant and only capacitance varies. It will be found in practice that the first 50 See the article by Sir Douglas Hall in the June 1964 issue, and degrees of swing will tune from about 150 to 300 our note on page 56 of the August 1964 issue. metres. 300 to 550 metres will take a further 40 degrees. The next 30 degrees will be occupied by Components List the intermediate section from 550 to 1,000 metres, (Fig. 3) and the long waveband will be covered by the last 60 degrees of movement. Resistors (All fixed resistors + watt 10% unless otherwise Receiver Circuit specified) The coil, with its two windings in ratio of about R1 22kû 5+ to 1, is suitable for use in a number of valve and R2 470kû R3 1801) 1 watt HTt R4 4700 250V VR1 5kû potentiometer, linear track Capacitors Cl 3,000pF TR1 V C2 21.LF electrolytic 12V wkg. MAT 101 C3 1,000pF EL 41 C4 100µF electrolytic 12V wkg. C5 0.01µF 500V wkg. VCI 500pF variable "Dilecon" capacitor (Jack- son Bros.) VCI Inductors L1, L2 See Text 3 63V T1 Intervalve transformer type LF38 (Elston) C2t= T2 Output transformer, 50:1, 4 40mA D Valve V1 EL41

4 Semiconductors TR1 MATIO1 D1 OA81

Fig. 3. Circuit of the receiver. This employs a Loudspeaker MAT1 01 transistor in a "Spontaflex" circuit 3û impedance

198 THE RADIO CONSTRUCTOR Components List (Fig. 4) D L3 T + +250V Capacitors C6 275V wkg. 32µF electrolytic 200- C7 32 or 64µF electrolytic 275V wkg. 250V Mains A.C. Inductors L3 Smoothing choke 10 henry 50mA T3 Mains transformer. Primary: 200-250 volt. 6.3V Secondaries: 250 volt at 50mA; 6.3 volt at 1.5 amp (0.7 amp minimum) Diode D2 Metal rectifier 250 volt 50mA Earth socket of mains plug

Switch Fig. 4. A suitable power supply circuit Si 2-pole mains on -off switch (may be ganged with VR1) Hum Cancellation The small winding (primary) of Ti is used to Note cancel out hum by negative feedback. The base of The above power pack components may be obtained, as a kit TR1 should be at chassis potential' but, due to the from R.S.C. Ltd., 5 County Arcade, Leeds 1. ferrite rod in L1, this coil picks up some hum from the mains transformer, especially when tuned to the longer wavelengths. The hum is amplified by TR1, pentode has an impedance approaching infinity, the acting as a very inefficient common emitter amplifier. output load of the transistor is kept very high, As the base should be at earth potential, negative which results in large amplification of voltage feedback can be applied to it without affecting the provided the collector circuit is supplied with a amplification of wanted signals. However, if R1 is suitable load. In fact, with its high impedance made too small in value in order to increase negative input at radio frequencies, and its high impedance feedback of hum, there will be loss of amplification output at audio frequencies, the Spontaflex functions of wanted signals. This will occur not because of in a similar way to a pentode detector, except that negative feedback but because R1 is effectively in the Spontaflex circuit provides true radio frequency parallel with the primary of T1 and consequently amplification in addition to gain through re- reduces, by reflection, the impedance of the second- generation. ary of T1, which is, of course, the collector load. Fig. 3 shows the circuit adopted. C1 is reduced For this reason R1, and consequently R2, are kept in value from the 0.01µF specified in previous large. The value chosen for Rt reflects a resistance versions, in order to slightly reduce reaction effects of over 2001a2 across the secondary of T1, and this, on the long waveband. With switched coils it can together with a further small damping effect pro- be arranged that the long wave reaction coil is duced by R2, will only reduce amplification by a smaller in relation to the main winding than is the small and quite acceptable amount. At the same case with the medium wave coils. But with the time, R1, in conjunction with C2, produces sufficient "single span" arrangement it has been necessary to feedback to render hum nearly inaudible. Inci- adopt a compromise in the relationship between dentally, C2 cannot readily be reduced in value reaction and main windings. from the specified 2µF in an attempt to increase The secondary (large winding) of T1 is the negative feedback of hum, because this would collector load of TR1, and must have a very large reduce the efficiency of TR1 as a common base inductance of several hundred henries to obtain amplifier which, as already pointed out, requires the maximum gain. There is virtually no damping of base to be at chassis potential. this inductance by the input of V1, and if a small C5 is essential for the maintenance of stability as inductance is used there will be a serious loss of well as to compensate for a tendency in V1 to over- bass. The author strongly advises the use of an emphasise the top notes. T2 should have a ratio Elstone transformer type LF38, to which connections of about 50:1 and be capable of handling 40mA. are made as shown in Fig. 3. There will only be a R4 is necessary to prevent instability at the small drop of less than 1 volt through the large frequency at which the secondary of T1 and C3 form winding, and direct coupling is therefore possible a tuned circuit. With R4 in circuit there is a useful from the collector of TR1 to the grid of Vl. It will small bass boost, with complete stability. be noted that the voltage swing available for TR1 The power pack can be quite simple and may be is approximately the same as the grid bias available bought, very economically, as a kit. The circuit is for V1, so. that TRi can load V1 fully before becoming shown in Fig. 4. The only precaution to be taken overloaded itself. The value of R3 is slightly larger is to keep the axis of the mains transformer windings than is normal for an EL41 to compensate for the at right angles to the axis of L1 and L2, and to keep fact that the grid is a little positive as regards chassis. these components as far from each other as possible.

OCTOBER 1964 199 stations may interfere with each other. Thirdly, there may be interference from powerful short wave To top To aerial stations at the low wavelength end of the tuning of L1 terminal scale. The use of a series aerial capacitor, by itself, is not very satisfactory unless it is a variable panel control. This is because it will almost certainly (a) provide too loose coupling for the long waveband and will have little effect on short wave stations. A device which will do much to cure all three forms of interference is shown in Fig. 5. L4 consists of 50 turns of 38 s.w.g. enamelled wire, pile -wound, To top To aerial and tuned by a trimmer to suit the local station. of L1 terminal Broadly speaking, C8 should be 100pF for wavelengths up to 290 metres, 250pF for stations between 290 and 400 metres, and 500pF for those above 400 L4 Ferrite rod metres. A wave trap is formed which is adjusted, once and for all, (3/8`x II/21 to tame the local station. L5, which is close -wound with the same wire (b) and in the same direction, has 40 turns, and will suppress all stations below 150 Fig. 5 (a). A coil assembly which counteracts metres in wavelength. L6 has 200 turns interference on medium and long waves of the same wire, pile -wound in the opposite direction (b). The practical appearance of the assembly. from L4 and L5. This coil offers a high impedance Winding details, together with suitable values for C8 to all stations in the medium waveband, but very and C9 are discussed in the text little to long wave stations. It is shunted by C9 which is adjusted to provide suitable selectivity on Performance the medium waveband. All coils are wound on a This receiver will give very good results on a few 1 }in length of din ferrite rod. The wave trap formed feet of wire as an aerial. Earthing is automatic, by L6 and C9 does not affect performance, as it will through the mains. If the receiver is used with a tune to a wavelength of about 800 to 900 metres, long outside aetial its sensitivity will outstrip its which is not required. selectivity unless precautions are taken. Interference With this device in circuit the prototype gives in these circumstances may be of three kinds. excellent results from many stations on a long aerial Firstly, the local medium wave station will be found in South Devon. About 4 watts output is available to have a big spread. Secondly, other medium wave from the local station. Recent Publications AMATEUR RADIO. By F. G. Rayer. 192 pages, 5i x 81in. Published by Arco Publications. Price 30s. It is of interest to note that amateur radio has now become a sufficiently popular hobby to justify producing a book designed specifically to introduce the subject to those contemplating taking it up. Amateur radio has its own literature for those already participating but the interested "outsider" is not so well catered for and this book therefore more than fills his need. The book covers both amateur radio transmitting and short wave listening. It explains the need for an examination prior to obtaining a transmitting licence and outlines the manner in which the necessary studies should be tackled. The morse code test is similarly dealt with and ideas and code practice oscillators for learning the code are described. Sufficient basic theory is given in Chapters 2 and 3. Chapter 4 deals with both t.r.f. and superhet receivers. Oscillators, c.w. transmitters, microphones, amplifiers and modulators, phone transmitters and power supplies are covered in Chapters 5 and 6, whilst Chapter 7 deals with aerials and their associated equipment. The final two chapters cover radio waves and their propagation, station operating, instruments, test equipment, etc. This book is a very good introduction to the fascinating hobby of amateur radio and can be confidently recommended to those contemplating taking up this interest.

SHORT WAVE AMATEUR RADIO. By J. Schaap, PAOHH. 160 pages plus illustrations, 51- x Bin. Published by Philips Technical Library. Distributed in U.K. by Cleaver -Hume Press Ltd. Translated by G. du Cloux. Price 21s. This book, originally published in Holland, is intended to cater for Dutch readers in a similar manner to that of F. G. Rayer's book, reviewed above, for English readers. The approach is somewhat different and it makes interesting reading when particularly compared with Rayer's Amateur Radio. The Preface explains that the book is not "an extensive practical manual", but is "to present the newcomer with a concise reference book which will introduce him to amateur radio". This the author does well. The treatment is comprehensive, covering components, receivers, transmitters, modulation systems, aerials, measuring equipment, station planning, building and operating. A number of useful appendices are included, such as bandspread calculations, coil calculations, countries lists, Q and amateur codes, and a comprehensive list for further reading in Appendix 12 entitled "For Those who Want to Know More". There are some excellent illustrations and fold -out circuit diagrams of typical amateur radio equipment. Again-good reading for those about to take up amateur radio.

200 THE RADIO CONSTRUCTOR ########### ########### 1:13Simple Quality Oscilloscope ########### ########## ########### PART 1 ## By ## D. M. Williams ## ##########B.Sc. ##

This article, the first of a two-part series, introduces an efficient and carefully designed oscilloscope which has many attractions for the home constructor and experimenter. A particularly useful feature is the fact that the design offers two alternative amplifiers, one being a.c. coupled with a range of 5 c/s to 1 Mc/s and the other d.c. coupled with a bandwidth of 250 kc/s

HE AUTHOR HAS LONG FELT THE NEED FOR A phase with the suppressor voltage which produces versatile but not over -elaborate instrument for it. If, therefore, the screen is RC coupled to the radio work and home experimentation. The suppressor, any infinitesimal voltage change on the instrument described in these articles was designed suppressor will precipitate a change in the screen for such a purpose, and uses easily obtainable and suppressor voltage in the same direction as the components. initial impulse. The eventual displacement of the electrodes is limited only when the valve has been Design Considerations forced into such a state that the suppressor has very The cathode ray tube used, the Mullard DG7-32, little influence on the screen. The circuit, therefore, has comparatively low extra high tension require- is unstable and will take up one of its two limiting ments, and hence no special mains transformer is conditions. These conditions are: (a) when the required. This tube is also highly sensitive, enabling suppressor voltage is positive, so that the anode low values of h.t. volts to be used. To minimise the current is large and the screen current small; and problem of ripple volts in the supply a balanced (b) when the suppressor is negative, so that the cathode -coupled amplifier is used in the Y circuits, anode current is small and the screen current large. the gain being controlled by the application of The switching of the circuit is extremely rapid, negative feedback. A switched input attenuator is so that the charge on the capacitor C1 has no time also fitted. Two alternative Y circuits are described, to change. After the valve has switched, the voltage one being a.c. coupled and the other a hybrid d.c. on this capacitor moves towards the value it would coupled amplifier. In both cases the cathode ray reach were the circuit stable. However, before it tube is fed from cathode follower stages. The d.c. can reach this value the valve is again in the state amplifier provides a gain of about 200, with a HT+ bandwidth of 250 kc/s, and the a.c. version a gain of 70 with a bandwidth of 5 c/s to 1 Me/s. The timebase is a straightforward Miller- Transitron designed around an EF91 valve, with an associated ECC81 providing the sync amplifier and phase inverter. The frequency of the timebase is continuously variable, from 12 c/s to 50 kc/s in four overlapping ranges. A method of calibrating the timebase at any point is included, although it must be remembered that this is only approximate as the sync control affects the repetition frequency. The power supply is a simple tapped voltage R9 doubler circuit using selenium rectifiers. The Timebase Consider first the transitron relaxation oscillator shown in Fig. 1. It can be shown that the screen voltage in a transitron-connected pentode is in Fig. 1. The transitron relaxation oscillator

OCTOBER 1964 201 Timebase out To C 4(fig.7) X in To C8, and X X gain control mim From Yamp 3a3V

R36 R38 C31 41 R45 Int. timebase R35 switch MR2 R43 R4 37 Int. 0A81 RV7 3 eR32 C32 C33 Sync To C10 sockets C2111 C25 1C28122- =291 and X° Ext. MI Ufa» Sync C23 T j control C27 I I 4 T3O MI C34

I Sta 5 S2b

Vla R33 R65 2 Vlb 2 R39 7 7 V. 3 . 2 52-Coarse frequency 3 Fine frequency 4,5 control control R34 a b a MRI= R42 p31 0A81

i. 1/2 ECC 81 EF 91 112 ECC 81

Fig. 2. The sync amplifier and timebase of the oscilloscope

Components List (Fig. 2)

Resistors C23 3,O0OpF silver mica (All fixed values 10% + watt) C24 47OpF silver mica R31 500kû C25 5OpF silver mica R32 82kû C26 2OpF silver mica R33 270kû C27 0.03µF paper, 25OV wkg. R34 10kû C28 0.005µF paper, 25OV wkg. R35 27kû C29 5O0pF silver mica R36 12kû C30 56pF silver mica R37 100kû C31 0.2511F paper, 25OV wkg. R38 10kû C32 33pF silver mica. R39 220kû C33 33pF silver mica R40 68kû C34 0.05µF paper, 25OV wkg. R41 100kû C35 1,000pF paper, 35OV wkg. R42 10Mû R43 1Mû R44 1.2Mû Valves and Diodes R45 100kû V1 ECC81, 12AT7 R65 47kû V2 EF91, Z77 RV6 250k12 linear, carbon track MR1 0A81 RV7 25kû linear, carbon track MR2 0A81 RV8 1Mû linear, carbon track Switches Capacitors S2 2-pole, 4 -way C22 0.002µF paper, 25OV wkg. S3 1 -pole, 1 -way toggle where the suppressor controls the screen, and the the output across R1 is a square wave with a circuit switches to its other limiting condition. Thus repetition frequency controlled by the time constant

202 THE RADIO CONSTRUCTOR, of the coupling components. Flyback period Now, it can be shown that an external signal applied to the control grid can trigger the circuit Va A from one state to the other at a faster rate than the 1i natural repetition frequency. In the Miller-transitron timebase the charge across a capacitor is Anode arranged to fulfil this function. It is also obvious volts that a signal applied to the suppressor grid can trigger the circuit, and this method is used to synchronise the timebase frequency with the work Linear sweep voltage frequency. V9 Assume that the transitron is in the condition where the screen current is very large, and the anode Grid volts current very small. (See Fig. 2.) In this state one of the capacitors C27 to C30 charges via the anode load resistors, RV7 and R38, until the transitron 1 -t switches to its opposite limiting condition, i.e., small screen current and large anode current. Vsc However, in this case, when the anode current begins to flow the potential across RV7 and R38 Screen is added to that already across the capacitors volts C27 to C30, so that the voltage on the control grid becomes negative with respect to the cathode. This .t action limits the anode current, and as the valve Vsupp is operating above the knee of its Va-Ia characteristic with approximately constant screen potential, the anode current is substantially constant. The Suppressor capacitor C27 to C30 discharges itself at constant volts current through the valve. Thus, the voltage at the anode runs down linearly from nearly full h.t. potential to the point where the anode potential is the same as that on the screen. The transitron then switches to cut off the anode current. The capacitor Fig. 3. The timebase waveforms given by V2 C27 to C30 begins to charge again via the anode load resistor and the above cycle repeats itself. duration of the flyback. The grid of the c.r.t. is The potential divider chain including RV8 places coupled to the transitron screen by two 0.25p.F a known positive voltage on the control grid, which capacitors in series (C13 and C14 in Fig. 7). This the sum of the voltages across the capacitor C27 - is a precaution against damaging the tube. If a C30 and the anode load resistor RV7, R38, minus single capacitor were employed and it short- the h.t. voltage (i.e., VC27 to C30 VRv7, R38 circuited, a large positive voltage would be applied -Vh,t.) must exceed, in order that the control grid to the grid which could completely destroy the shall become negative. This voltage (VC27-C30 c.r.t. The diode MR2 (Fig. 2) ensures that only VRV7, R38 - Vh.t. - VRVs) determines the negative -going pulses are applied to the tube grid. anode current during run down, and hence the rate of discharge of the capacitor. It is therefore obvious Synchronisation that RV8 -controls the repetition frequency of the Synchronisation is accomplished by the injection timebase. of a sync signal to the suppressor of the transitron. In practice, as shown in Fig. 2, various values of The signal is obtained by feeding a pulse amplifier, the capacitor are switched into the circuit to give Vi(a), from the Y amplifier, differentiating the an extended frequency range. The capacitor C23 output via capacitor C35, and selecting a positive - to C26 is also altered to give constant drive on going pulse with diode MR1. This pulse is applied each range. Table I gives frequency ranges for each position TABLE I of S2. Position of S2 Timebase Frequency Trace Blanking The various waveforms obtainable from the 1 12 c/s -140 c/s device are shown in Fig. 3. From these it can be seen that a negative-going pulse, with a width equal 2 80 c/s -1 kc/s to the flyback time, can be obtained from the screen. This pulse is used (as will be shown in 3 800 c/s -12 kc/s Fig. 7, to be published in Part 2) to obtain flyback suppression by its application to the grid of the 4 10 kc/s-55 kc/s cathode ray tube, thus cutting off the trace for the

OCTOBER 1964 203

í the X plates. This signal must, however, be of the From time- order of 20 volts for a readable deflection, as the base output phase inverter has unity gain.

A method of timebase calibration can be in- _ t2 corporated whereby the sharp pulse A (Fig. 3) on the timebase anode waveform is used to shock- excite a tuned circuit. The damped wave train thus produced (an example is shown in Fig. 4) is fed to the input of the oscilloscope. Since the tuned circuit is excited each time the timebase fires, a stationary pict- Fig. 4. Employing a resonant circuit for timebase calibration. In ure of the wave train appears on the screen. The fre- this diagram the period of the wave is quency of these 1 oscillations is determined only by t2 =1Q6 µS. But f- so t2=2aß/ x 109Sf the resonant 2aß/ Q' et frequency of the tuned circuit, when coupled to the oscilloscope input. Hence the period to the suppressor of V2. In normal use the of the oscillations can be used as a time standard. "Internal Sync" and "External Sync" sockets are In actual fact, this method is only approximate connected by a short link but, if required, syn- since the work voltage applied to the synchronising chronisation can be obtained by the application of circuits has a slight effect on the timebase frequency. an external source to the "External Sync" socket. Nevertheless, this method of calibration has proved A control of X gain (i.e., trace expansion) is given very satisfactory, provided large values of capacitor by the potentiometer RV7. The lead from the relative to the oscilloscope input capacitance are anode of V2 to the lower terminal of RV7 should used in the tuned circuit. be screened. From this potentiometer the sawtooth Any reader who is especially interested in an wave is fed directly to one of the X plates and into analysis of the above or more sophisticated timebase a phase inverter which provides the signal for the circuits is referred to Ref. 1. opposite X plate. Provision is made for the extraction of the timebase Reference voltage before the phase 1. Puckle, O. S., Time Bases. Chapman and Hall. inverter. When switch S3 is open the output socket also allows the application of an external signal to (To be concluded)

.rr.... -.-... r..r .ÿ:::i _,..... ' . v. l::: .'#:w:äJ.1i:Sl: -, '::jjti . 'l.. .

THE NEED AROSE IN THE WRITER'S HOUSEHOLD FOR Higher quiescent currents can be tolerated in the a general purpose intercommunications ampli- output stage when battery economy is of no concern. fier which would also be suitable for use as a This naturally leads to the use of the cheaper and baby alarm or call set at the sick bed. It had to very much simpler circuitry given by a single -ended meet the following specifications : output stage working in Class A. In the present (1) It should have sufficient sensitivity and output circuit an output of approximately 100mW is to ensure that normal breathing could be obtained. heard with the microphone several feet from By reference to Fig. 1 it can be seen that the the subject. amplifier employs a straightforward resistance - (2) Building cost should be low. capacitance coupled circuit. Partial stabilisation of (3) It should provide economical and reliable TRI and TR2 is given by obtaining their respective running over long periods. bias currentz via their collector circuits, while TR3 (4) Both installation and operation should be has the more conventional potential divider network. simple. The input matching transformer, T1, enables (5) Two-way communication should be provided. cheap unscreened twin flex to provide the connection (6)'The equipment should be compact and between master and slave stations. The use of this portable. transformer avoids the capacitance losses given by The intercom amplifier system described in this long lengths of screened lead, as would be used on article meets all these requirements. a high impedance line, with the inherent top -cut effects. Circuit The power requirements of this amplifier, at Although transistor operation was chosen it was approximately 20mA, are very small by valve not thought desirable to work from batteries. standards, with the result that a simple half -wave

204 THE RADIO CONSTRUCTOR. 4 5V;

S1 Positions T - talk L- listen

BBC

IIV

TRI TR3 CI nil

(3) R C5 +

+ve line

t QOQ9' T f'

Si Jack Slave T L A V Lines

Line terminals Internal L.S.

resistance winding) Fig. 1. The circuit of the transistorised intercom. T1 is connected such that its primary (the high couples to R1

Components List

Resistors Transformers (All fixed resistors 4 watt 20%) T1 Input transformer, 18:1, type D167 1 R1 5kû potentiometer, log track (Ardente) R2 390kû T2 Output transformer, 18:1, type D167 R3 10kû (Ardente) R4 220kû T3 Mains transformer, 6.3 volts 50mA mini- R5 4.7kû mum. (Heater or bell transformer) R6 lkû R7 3.3kû Transistors R8 lkû TR1 OC71 R9 68û TR2 OC71 Rto 22û TR3 0072 with heat sink. (See Fig. 3) Capacitors paper Switches C1 0.1µF, toggle switch C2 10µF, electrolytic, 6V wkg. S1 d.p.d.t. S2 d.p.s.t. toggle switch C3 501.LF, electrolytic, 10V wkg. C4 10µF, electrolytic, 6V wkg. C5 50µF, electrolytic, 6V wkg. Rectifier C6 1,000µF, electrolytic, 12V wkg. MR1 L.T. rectifier, 50mA minimum. (Prototype C7 1,000µF, electrolytic, 12V wkg. employed Sen-Ter-Cel rectifier type H440)

OCTOBER 1964 205 Loudspeakers unit is in the positioning of the unscreened trans- Master station: 24in, 3f1 impedance formers T1, T2 and T3. Slave station: 6in, 3f) impedance The input transformer, T1, must be placed as far from T2 and T3 as possible, the minimum distance Miscellaneous being 6in. This avoids both audio instability and Jack. Closed-circuit type-see Figs. 1 and 2. 50 c/s hum pick-up due to inductive couplings. The (Igranic) positioning of T2 and T3 is not quite so critical Paxolin panel although it might be wise if the design allows their Line terminals cores to be mounted at right angles to one another. Grommet, etc., etc. The writer employed screens in the form of mild steel brackets fitted to T1 and T2, although Mumetal mains supply is used. In spite of its simple nature or soft iron might be preferable. Fig. 4 shows the this gives an adequately ripple -free output. method of construction and fixing. In the case of For compactness, a small 2-fin speaker unit is T2 the screening bracket also serves as a mount (as used in the master station. For most applications shown in Fig. 4) for C6 and C7. this is found to be adequate. However, extra Due to the high gain provided by the circuit a volume as a speaker and sensitivity as a microphone separate mains on -off switch had to be fitted instead (the loudspeakers at both slave and master stations of using a combined potentiometer and switch. double as microphones according to whether S1 is This enables the live mains wiring to be kept well in the "Talk" or the "Listen" position) can be away from the signal circuits. obtained by connecting a larger speaker in circuit by means of the jack socket. This action dis- Cabinet connects the small internal speaker. Fig. 2 is merely a guide to the constructor as far The slave station simply consists of a 6in speaker as cabinet dimensions are concerned. Components fitted to a suitable baffle or cabinet. A smaller may vary in size and fixings. speaker should not be used here. Connection is The method of construction used in the prototype taken direct from the master line terminals to the is very simple. Top, bottom and sides are made speech coil at the slave station. from in plywood, and the front panel is of single faced hardboard. Construction All the materials are cut to the required sizes as The layout and wiring employed in the prototype approximately indicated in Fig. 2. The main are shown in Figs. 2 and 3. Constructors may components are mounted later on the front panel attempt to miniaturise this design layout, but it by means of countersunk screw fixings. The should be pointed out that an earlier model, which required holes are drilled and the speaker aperture was very similar suffered from numerous troubles cut out. The cabinet parts are then all assembled which were only successfully resolved by re -designing together and held with glue and panel pins. Com- the layout to its present form. ponents T3, T2, T1 and the speaker are fitted to the The main trouble associated with the amplifier front panel, the loudspeaker having an expanded ls'

Line terminals

Red +ve

Grommet A Mains

Retaining knot (from C6)

Fig. 2. The wiring and components fitted to the master station cabinet. Cabinet depth is tin

206 THE RADIO CONSTRUCTOR metal grille behind its aperture. Suitable holes are C6 and C7 (fitted with sleeves Screen drilled to take MRI, the grommet, the line terminals and the handle (if required). All countersunk screw holes are filled with putty or wood filler and made flush with sandpaper. The edges of the cabinet are then smoothed and rounded with the sandpaper. If desired, the whole case can now be covered carefully with a suitable covering material. S1, S2, R1, MRI, the grommet, the jack, the handle and the line terminals may next be assembled in their appropriate positions. Finally the two wooden blocks which support the component panel are FRONT PANEL glued inside the cabinet. These blocks should have just sufficient height to allow the component board, Fig. 4. Detail showing the manner in which C6 and when fitted, to clear Si and R1. C7 are fitted to the screen for T2. T1 has a similar screen, but no components are fitted to it Component Panel The component panel employed in the prototype three transistors should have a heat shunt on their is a piece of din Paxolin cut to 24 by 3*in. Twelve leads during soldering. 6BA clearance holes are drilled at the points indi- Before soldering up a point ensure that no other cated in Fig. 3. To those in positions C, D, F, G, connection has to be made there. J, K, L and M are assembled eight 6BA single ended After the wiring and assembly is completed a solder tags. The remaining 6BA tags at B, H, I visual check against the circuit in Fig. 1 should be and N are secured by means of the wood screws made. Special attention should be given to the fixing the panel to the spacing blocks. Note that polarity of the electrolytic capacitors and transistor the heat sink for TR3 is secured, along with an extra connections. It should also be noted that wide- tag, at B. spread circuit damage would result from a wrongly connected rectifier. Wiring All the usual precautions must be observed with Operation regard to transistor circuit wiring. Just sufficient After the constructor has satisfied himself that the heat must be applied, and for the shortest possible circuit is wired correctly the unit may be given a time, to the miniature component lead-outs. The functional test. Connect the slave station to the master via the Heat sink To earth tag Ä line terminals. If violent acoustical feedback is to be avoided the slave loudspeakers should be positioned some distance from the master, preferably outside the room. Connect the unit to the mains supply and switch on. With the volume control set half-way, and the function switch in the "Talk" position, tap on the master speaker grille. This should be loudly reproduced at the slave end. In the "Listen" position the reverse will apply. With volume set at a suitable level the voice should be clearly reproduced free from hum. As an aid to the constructor Fig. 1 shows the voltage readings obtained on the prototype. These will be found a great help in the case of a unit which, although wired correctly, refuses to work due to a faulty component. All readings are

RIO/C6

Fig. 3. Components on the Paxolin component board. This is *in thick and measures 2 by 31in. It is Fig. 5. A general view of the complete intercom secured to the wooden blocks shown in Fig. 2 equipment

OCTOBER 1964 207 between the points indicated and the positive line. One last word of warning. This unit is of no use The writer used a 1,000 ohms -per -volt meter on its to those who are of weak constitution. If in the 20 volt range to ensure that the current drawn by still of the night the infant decides to make his or the instrument was small. her presence known, the 100mW of power available The system is capable of working on cable lengths seems like a public address amplifier, and will jangle of over 50 yards. you out of the deepest sleep into violent action!

RADIO TOPICS . . by Recorder

NE OF THE INCIDENTAL ADVAN- from the line output stages of see your next-door neighbour adding tages of 625 line TV reception neighbouring TV sets. The 20th a u.h.f. aerial to his collection on the is that the line frequency is harmonic of 10,125 c/s is 202.5 kc/s, chimney, you may be able to tell 15,625 c/s instead of the 10,125 c/s with the result that a 2.5 kc/s beat whether he is viewing B.B.C.-2 by which occurs with 405 lines. Self- is formed with the 200 kc/s Light tuning in to the Light Programme generated line output transformer Programme carrier. The effect is on your radio. If the whistle stays whistle is, therefore, removed to a usually worst if a wire aerial (offering at 2.5 kc/s all the you frequency at evening will which the ear is less a capacitive pick-up) is used with the know that his new aerial is just for sensitive, and should be less irritating radio receiver, and it appears to be show, and that he's still using the in the television receivers of the less evident when a ferrite frame is same old TV that he's had for the future. employed. past decade! How troublesome is r.f. radiation Another Form of Whistle from line output stages likely to be In practice, it is extremely difficult There is, however, another form on 625 lines? The line frequency to entirely eliminate r.f. radiation of line output stage interference here is 15,625 c/s, whereupon the from a television chassis, although which also results in a whistle-only, 13th harmonic turns up at 203.125 manufacturers go to great lengths to in this case, the whistle is reproduced kc/s. So any possible whistle which keep such radiation to a very low by another receiver. If, like me, you may occur with the Light Pro- level. For complete freedom from live in an area where the B.B.C. gramme will have a frequency of radiation it isn't only necessary to Light Programme transmission on 3.125 kc/s. As the 625 line inter- screen the line output transformer 200 kc/s (1,500 metres) is fairly weak, fering signal is a 13th instead of a and the valves and components you are quite liable to hear a 20th harmonic it may, all else being which immediately connect to it, as continual background whistle, par- equal, be somewhat stronger than has been standard practice for many ticularly in the evenings. This that given by a 405 line timebase. years now. It is also necessary to whistle is the result of r.f. radiation One incidental point is that, if you screen the deflector coil assembly as well, an undertaking which, with very wide-angle components, is by no means as simple. And, of course, the whistle only becomes troublesome in areas where the Light Pro- gramme signal is at low level. Another point that occurs to me is that if your neighbour on one side is switched to B.B.C.-1 or to I.T.V., and your neighbour on the other side to B.B.C.-2, Light Programme reception on 200 kc/s will be accom- panied by whistles on 2.5 kc/s and 3.125 kc/s. Plus a beat note, between them, of 625 c/s! At the Dagenham factory of the Ford Motor Company, an Moment of Truth engineer checks the Ocrasionally, family commitments open circuit voltage force me to sit through the B.B.C. of a regulator on a television programme "Compact". Ford Consul Corsair During these sessions I find it im- using an AvoMeter possible to concentrate on the plot Model 12. Avo is a as I am so completely fascinated by member of the M.I. the incredibly large number of Group. people who are needed to publish

208 THE RADIO CONSTRUCTOR one small magazine. Until very the Council, making a grand total of x=y recently, this single fact has caused thirty-four. .'. x2=xy me to assume that the world of It was at that moment that the .'. X2 --y2 =xy -y2 "Compact" is one of pure fantasy. truth dawned upon me. The pro- .'.(x+Y) (x-y)=Y (x-y) A few days ago, however, I gramme "Compact" is based on .x+y=y chanced to look at the index page Which? magazine. :.2x=x of the latest issue of Which?, the .'.2=1 monthly journal published by Con- Two Equals One The error, as most of you will sumers' Association, to find that its Finally, a word about the "proof" probably have spotted, lies in the executive staff numbers at least I gave last month that 2 equals 1. third line, because subtracting y2 eight, and that they are backed up You may recall that we started off makes both sides equal to zero. All by no less than twelve Honorary by assuming that x is equal to y. the third line says is that zero is Vice-Presidents of the Association The "proof" then proceeded in the equal to zero, after which everything together with fourteen Members of following manner. else that follows is just nonsense!

SHELL FIND MANY USES FORD CLOSED-CIRCUIT TV

Members of the staff of Shell International Petroleum Company Limited, at all levels, can now be coached for speaking on television. A closed-circuit TV studio at Shell Centre, London, has recently been designed for this and other purposes and the cameras and associated equipment were supplied by EMI Electronics Ltd. When it becomes apparent that some subject in which the company is interested-a new oil refinery, or pesticides, for example-is coming into the news, specialists in the subject are trained in television techniques. Then, should the BBC or ITV ask for an interview or for a speaker to take part in a discussion, there is an expert ready who can face the cameras with confidence. Public relations executives from Shell companies overseas are also TV -trained while attending general courses in London. Two EMI type 6 cameras are used in the studio for live programmes. In the control room, a third camera televises titles and captions, while a fourth forms part of a simple telecine arrangement for transmitting 16mm and 35mm films. Fades, super-impositions and other facilities are available. A battery of monitors gives the production team complete control of all camera outputs, and an off-line receiver indicates the quality of picture and sound being received throughout the Shell Centre offices, conference rooms, lecture halls, theatre and lounges. There are many other ways in which members of Shell's public relations department propose to use this versatile installation-for instance, trying out TV scripts before sending them to overseas companies for use on local television programmes. A script that may look good on paper will not always make a good programme, but a trial run on the closed-circuit will help to evaluate the script and indicate any necessary amendments. They can also mock up part of a film without entailing heavy production costs, to see if the treatment is on the right lines. It is always useful, too, to see how a colour film, which might be used on TV, comes over in black and white. Shell Film Unit uses the TV system to check the visual content and perspective of film sets. Other departments are finding uses for it too. Language teaching, secretarial and supervisory courses are increasingly finding it an invaluable aid in their programmes.

Queries. We regret that we are unable to answer queries other than those arising from articles appearing in this magazine nor can we advise on modifications to equipment described. Queries should be submitted in writing.

Correspondence should be addressed to the Editor, Advertising Manager, Subscription Manager or the Publishers, as appropriate.

Opinions expressed by contributors are not necessarily those of the Editor or proprietors.

Contributions on constructional matters are invited, especially when they describe the building of particular items of equipment. Articles should be written on one side of the sheet only and should preferably be typewritten, diagrams being on separate sheets. Typewritten articles should have maximum spacing between lines. In handwritten articles, lines should be double-spaced. Diagrams need not be large or perfectly drawn, as our draughtsmen will re -draw in most cases, but all relevant information should be included. Sharp and clear photographs are helpful, where applicable. If negatives are sent, we usually work from these rather than from prints. Colour transparencies normally reproduce badly-black and white photographs are very much better. Details of topical ideas and techniques are also welcomed and, if the contributor so wishes, will be re -written by our staff into article form. All contributions must be accom- panied by a stamped addressed envelope for return, if necessary, and should bear the sender's name and address. Payment is made for all material published.

Production.-Letterpress/contact litho.

Published in Great Britain by the Proprietors and Publishers Data Publications Ltd. 57 Maida Vale London W9 Printed by A. Quick & Co. (Printers) Ltd. Oxford Road Clacton -on -Sea England Obtainable abroad through the following Collets Subscription Service Continental Publishers & Distributors Ltd. William Dawson & Sons Ltd. Australia and New Zealand Gordon & Gotch Ltd. Electronics Publications (Australia) South Africa Central News Agency Holland "Radio electronica" ANE//lIMACT/C,4L PAW Of UNOEZTAND/NG

RADIO . TELEVISION ELECTRONICS

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210 THE RADIO CONSTRUCTOR SMALL ADVERTISEMENTS BENTLEY ACOUSTIC Rate: 9d. per word. Minimum charge 12/-. CORPORATION LTD Box No. 2/- extra. 38 CHALCOT ROAD CHALK FARM LONDON NW1 THE VALVE SPECIALISTS Telephone PRIMROSE 9090 IL4 2/3 9BW6 9/6 DK40 15/6 ECH84 9/6 EY88 8/9 PY88 7/9 I R5 4/- 10C1 8/9 DK92 6/9 ECL80 6/- EZ40 5/3 PY800 6/- Advertisements must be prepaid and all copy must 154 5/- 10C2 12/3 DK96 6/3 ECL82 7/- EZ4I 6/- PY801 6/3 be received by 1S5 3/6 IOF1 10/- DL96 5/9 ECL83 9/6 EZ80 3/9 TY86F 11/8 the 4th of the month for insertion IT4 2/3 10F18 9/9 DM70 5/- ECL86 8/9 EZ8I 4/- U24 12/6 in the following month's issue. 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(Replies to Box 3V4 5/3 12AU6 5/9 EI8OF 19/6 EF85 4/6 PC86 10/3 U329 9/- numbers 5Y3 4/3 12AV6 6/6 EABC80 5/6 EF86 6/- PC88 14/7 U404 ó/- should be addressed to: Box No. -, The 5Z4 7/- 12BA6 5/9 EAC9I 3/3 EF89 4/- PC95 11/8 U801 16/3 Radio Constructor, 57 Maida Vale, London, W.9.) 6AQ5 5/9 128E6 4/9 EAF42 7/6 EF9I 3/- PC97 7/3 UABC80 5/- 6AT6 3/9 12BH7 6/- EB41 4/9 EF92 2/6 PCC84 5/6 UAF42 7/- 6AU6 5/3 I9AQ5 7/3 EB9I 2/3 EF97 11/8 PCC85 6/9 UBC41 6/3 6AV6 5/6 20D1 10/- EBC41 6/6 EF98 10/- PCC88 10/6 UBC8I 6/3 6BA6 4/6 20F2 11/6 EBC8I 5/9 EF183 7/- PCC89 7/9 UBF80 5/9 DIRECT TV REPLACEMENTS LTD. Largest stockists 613E6 4/9 20L1 12/6 EBF80 5/6 EFI84 7/- PCC I89 10/6 UBF89 6/9 of TV components in the U.K. Line output trans- 6BH6 5/3 20P1 12/6 EBF83 7/3 EL33 6/9 PCF80 5/6 UC92 6/3 6836 5/6 20P3 12/- EBF89 6/3 EL34 8/6 PCF82 6/3 UCC84 8/9 formers, Frame output transformers, Deflector coils 6807A 7/6 20P4 13/6 EC70 4/9 EL36 8/9 PCF84 8/6 UCC85 5/6 for most makes. 6BR7 8/3 20P5 12/3 EC8I 27/6 EL41 7/3 PCF86 7/9 UCF80 8/9 Official sole suppliers for many 6BRB 8/- 30C15 9/- EC92 6/6 EL42 7/6 PCL82 6/6 UCH42 7/- set makers. Same day despatch service. Terms 6BW6 6/9 30C18 10/6 ECC40 7/- EL83 6/9 PCL83 7/9 UCH81 6/6 6CD6G 18/- 30F5 5/9 ECC8I 3/6 EL84 4/6 PCL84 7/- UCL82 7/9 C.O.D. or C.W.O. Send S.A.E. for quotes. Day and 6CH6 5/- 30L15 9/3 ECC82 4/6 EL85 7/6 PCL85 7/6 UCL83 8/6 night telephone: Gipsy Hill 6166.-126 6F1 9/6 30P4 12/3 ECC83 4/6 EL86 7/3 PCL86 8/9 UF41 6/9 Hamilton 6Fí3 4/9 30P12 7/6 ECC84 5/9 EL95 5/6 PL36 8/- UF42 4/6 Road, West Norwood, London, S.E.27. 617G 4/6 30P19 12/3 ECC85 6/3 EM34 11/6 PL38 16/- UF80 6/3 6K7G 1/3 30PLI 8/6 ECC88 8/9 EM7I 13/6 PL8I 6/9 UF85 6/9 6K8G 3/3 30PLI3 9/6 ECC189 11/6 EM80 6/3 PL82 5/3 UF86 9/- THE INCORPORATED PRACTITIONERS IN 6L1 I0/- 35W4 4/9 ECC804 16/4 EM8I 7/- PL83 5/3 UF89 6/- RADIO AND ELECTRONICS 6L18 I0/- 85A2 6/6 ECF80 6/- EM84 6/9 PL84 5/- UL4I 6/9 (I.P.R.E.) LTD. 6LD20 5/6 5763 7/6 ECF82 6/3 EM85 8/5 PM84 9/3 UL46 8/6 Membership conditions booklet ls. Sample copy of 6Q7G 4/- AZ3I 6/6 ECF86 11/6 EM87 7/6 PY33 8/9 UL84 6/- I.P.R.E. 6V6G 3/9 AZ4I 6/6 ECH35 6/3 EY5I 5/6 PY80 5/- UM80 8/3 Official Journal 2s. post free.-Dept. D, 6X4 3/9 DAF96 5/9 ECH42 7/6 EY83 9/3 PY81 4/9 UY4I 4/6 Secretary, 32 Kidmore Road, Caversham, Reading, 6X5 4/6 DF96 5/9 ECH81 5/9 EY84 9/6 PY82 4/9 UY85 5/- Berkshire. 6/30L2 8/3 DF97 10/- ECH83 6/6 EY86 5/6 PY83 5/9 X78 20/6 Terms of business: Cash with order or C.O.D. only. Post/packing 6d. per SERVICE item. Orders over E3 post free. C.O.D. 3/6 extra. All orders despatched SHEETS, 1925-1964. From Is. Catalogue same day. C.O.D. orders by telephone accepted for immediate despatch 6,000 models, Is. 6d. S.A.E. enquiries.-Hamilton until 3.30 p.m. Any parcel insured against damage in transit for 6d. extra. Radio, 13 Western Road, St. Leonards, Sussex. We are open for personal shoppers 8.30-5.30. Saturdays 8.30 a.m.-1 p.m. CATALOGUE No. 15. Government surplus electrical and radio equipment. Hundreds of items at bargain prices for the experimenter and research engineer, 2s. 6d. post free. Catalogue cost refunded on purchase of 50s.-Arthur Sal is Radio Control Ltd., 93 North Road, Brighton. pitman HIGH QUALITY `BRAND FIVE" RECORDING TAPE at reduced prices. Acetate. 5in, 900ft, 13s.; AN 5iin, 1,200ft, 16s. 6d.; 7in, 1,800ft, 24s. 6d.; Mylar ELECTRONIC 5in, 1,200ft, 26s. All new. Money back guarantee. All enquiries S.A.E.-Spectron, 6 Bingham Road, ORGAN Radcliffe -on -Trent, Nottingham. FOR THE S.E.S. SERVICE SHEETS for all Television, Radio, HOME etc. Lists Is. 6d. plus S.A.E.-Sun Electrical Services, 38 St. George's Road, Hastings, Sussex. CONSTRUCTOR METALWORK. All types cabinets, chassis, racks, etc., Alan Douglas, Sen. Mem. I.E.E.E. to your specifications.-Philpott's Metalworks Ltd., Chapman Street, Loughborough. A very popular "how to make it" book. FIND TV SET TROUBLES IN MINUTES from that Shows exactly what is involved in work great book The Principles of TV Receiver Servicing, and materials (a list of sources of supply 10s. 6d., all book houses and radio wholesalers. If not is included) in constructing an inexpens- in stock, from-Dept. D, Secretary, 32 Kidmore ive instrument for use in the home. Road, Caversham, Reading, Berkshire. For this new edition, an alternative arrange- HAMMER FINISH PAINT. The modern finish for ment of continuous tuning has been electronics. Can be brushed or sprayed. Blue or added to a revised and up-to-date text. silver. 2i oz. tins 3s. 6d., pint 7s. 6d., 1 pint 15s. Post 6d. on any order. Trade supplied.-Finnigan Second edition 20s net Speciality Paints, (RC), Mickley Square, Stocksfield, Northumberland. from all booksellers Pitman Parker London continued on page 213 St WC2

OCTOBER 1964 211. RADIO SOCIETY OF GREAT BRITAIN

International RADIO COMMUNICATIONS EXHIBITION SEYMOUR HALL, SEYMOUR PLACE MARBLE ARCH W.I OCTOBER 28th to 31st (WEDNESDAY TO SATURDAY ) lO a.m. to 9 p.m.

FIFTH ANNIVERSARY FLEET AIR ARM DISPLAY OF AIRCRAFT CARRIER RADIO CONTROL ROOM DISPLAY & DEMONSTRATIONS BY ROYAL AIR FORCE, ARMY AND POST OFFICE ENGINEERING COMPETITIONS OF HOME CONSTRUCTION EQUIPMENT LATEST RECEIVING & TRANSMITTING SETS NEW TV & V.H.F. AERIALS and MASTS EDUCATIONAL & BOOKSHOPS DISPLAY. MOBILE EQUIPMENT Win £140 Hammarlund HQ 170A Latest Communication Receiver

(EXCHANGE THIS ADVERTISEMENT FOR ENTRY FORM AT DOOR)

ADMISSION 3/-

212 THE RADIO CONSTRUCTOR RETURN - OF - POST SMALL ADVERTISEMENTS ON CASH OR C.O.D. ORDERSHirse LATEST TEST METERS Cash Price Deposit Mthly Pmts. AVO Model B, Mark III ... f24. 0.0 £4.16.0 12 of £1.15. 2 AVO Model 7, Mark 11 ...... £21. 0.0 £4. 4.0 12 of £1.10.10 AVO Multiminor Mark IV ... £9.10.0 £1.18.0 12 of 14/4 continued from page 211 T.M.K. TPIO ...... £3.19.6 £1. 3.6 3 of £l. 2. 0 T.M.K. TP5S ...... £5.19.6 £1.15.6 3 of £1.11. 4 T.M.K. Model 500 ...... £8.19.6 £1.15.6 12 of 13/8 TAYLOR Model I27A .. £10.10.0 £2. 2.0 12 of 15/8 Full details of any of the above supplied free on request. The AVO Models CONVERT ANY TV SET INTO AN OSCILLO- 7 and 8 are both latest models from current production -not to be confused SCOPE. Instructions and diagrams, 12s. 6d.- with Government surplus. Redmond, 26 St. John's Road, Hove, Sussex. GRAMOPHONE EQUIPMENT ALL LATEST MODELS Hire Purchase ALL POST FREE Cash Price Deposit Mthly Pmts. RECORD CHANGERS NEW, UNOPENED CRATES. 5BKP31 oscilloscope GARRARD AUTOSLIM ... £7.19.6 £1.10.6 6 of £1. 4. 0 tubes. Data available. Cost over £25. Few at (Mono PU in head) GARRARD ATSug £10.8.2 £2. 4.2 12 of 15/4 £4. 19s. 6d., carriage paid. Also lists, valves, tubes, (Mono PU plug in head) Road, New Barnet, GARRARD AUTOSLIM etc.-Bearman, 43 Leicester (Mono PU) £6.17.6 £1.12.6 6 of E1. 0. 0 Herts. Telephone: BARnet 1934. GARRARD AUTOSLIM De-luxe AT6 (Mono PU) £11. 9.0 £2. 6.0 I2 of 16/11 GARRARD AUTOSLIM testing and inspection. AT6 (Stereo/Mono PU) f12. 5.4 £2. 9.4 12 of 18 /- SUB -ASSEMBLY, wiring, B.S.R. ÚA15 (TC8 Mono PU) £7.15.0 £1.18.0 6 of El. 2. 0 Small unit service work. Layout of components, B.S.R. UAI5 (TC8S Stereo/LP/78) £8.15.0 £2. 6.0 6 of LI. 4. 0 Many of the above can be supplied for stereo working. See our Gramo- parts, etc. Collection and delivery if necessary. phone Equipment List for details. Write or telephone: Mersea Marine Services Co., AMPLIFIER KITS High Street, Burnham -on -Crouch, Essex. Telephone: We have full stocks of all components for the Mallard 510, Mallard 3-3, Mallard 2 and 3 Valve Pre -Amp, Mallard Stereo, Mallard Mixer, Fully 3335. detailed list on any of these sent upon request. Instruction Manual: All Mallard Audio Circuits in "Circuits for Audio Amplifiers", 9/5. Post Free. FOR SALE. Heathkit valve voltmeter model AV -3U. ILLUSTRATED LISTS Unused. £15 o.n.o.-Box No. F212. Illustrated lists are available on LOUDSPEAKERS, TAPE DECKS, TEST GEAR, GRAMOPHONE EQUIPMENT, AMPLIFIERS. Any will be sent free upon request. MID -FI DISPOSAL. Jason F.M., record player, 10 watt amp., loudspeaker £10. Tape deck and WATTS RADIO (ORDER) LTD pre -amp., ribbon mike, 6,000 ft. tape £15.-10 Over - 54 CHURCH STREET WEYBRIDGE SURREY stone Gardens, Shirley, Surrey. Telephone Weybridge 47556 Please note: Postal business only from this address Callers welcome by appointment OFFERS for Philips 3 -speed playing deck with auto change and 5 watt amplifier. -Box No. F216.

WANTED. October 1961 to March 1962 copies of The Radio Constructor. -Sear, Lovatts Cottage, The MODERN Green, Croxley Green, Herts.

U.S.A.F. SURPLUS. 400 c.p.s. 3kVA regulated £50. CAR RADIO RECEIVERS 0-50V d.c. 150 amps. £60. 6 ft. racks £5. 24V relays. 13 in our Data Book Series. This book -Box No. F219. No. contains three designs, covering Medium and FOR SALE. Newnes Radio & Television Servicing Long Waves, and each being fully described 7 volumes, 1955-1961. Heathkit R.C. Bridge, model with point-to-point wiring. The Jason - C -3U as new. Offers? -Box No. F220. designed "Everest 6" and "Everest 7" employ and both now contain FOR SALE. Commando II S.S.B. transmitter. New. transistors throughout Owner going v.h.f. £,110. -Box No. F221. the latest modifications, together with extra illustrations and step-by-step instructions. A TELEPRINTERS. Creeds No. 3 £10. Matching rectifiers, complete section, "Tackling Car Radio Westinghouse full voltage control for motor supply Interference" is also included. £5. New spares for machines available. Carriage paid. Limited supply. -Box No. F222. Fully illustrated with photographs and circuit diagrams, this Data Book should prove of interest to all car COLLECTOR interested in purchasing really early owners requiring a car radio. wireless valves-First World War or earlier. Has for disposal numerous mid -1920s types. Also for sale 48 pages, with varnished board cover several early crystal detectors. -Enquiries to Box No. F223. Price 3/6 postage 4d. FOR SALE. Decca "1000" projection model, complete Available from your usual supplier, or direct from with custon built triple speaker enclosure housing separate amplifier and retractable 4' x 3' projection screen. Nearest £50.-8 Melville Street, Wombwell, DATA PUBLICATIONS LTD Barnsley, Yorks. Telephone: Wombwell 2391. 57 Maida Vale London W9 continued on page 215

OCTOBER 1964 213 THE MODERN BOOK CO

Guide to Broadcasting Stations. Com- Hi Fi Year Book, 1964. 10s. 6d. Postage piled by "WW". 5s. Postage 6d. 1 s. A Beginner's Guide to Radio. Pub. by Test Gear for Amateurs. Illustrated. Newnes. 8s .6d. Postage 9d. 3s. 6d. Postage 6d. Worked Radio Calculations. By A. T. The A.R.R.L. Antenna . Book. 18s. Witts. 15s. Postage 9d. Postage 1s. Frequency Radio and Line Transmission. By G. L. Divider Organs for the Constructor. By A. Douglas. 25s. Post- Denielson. Vol. 1 21s. Vol. 2 22s. 6d. age 1 s. Postage 1 s. each. Radio, Television and Electrical Television Servicing Handbook. By Repair. By R. C. Norris. 50s. G. J. King. 35s. Postage 1s. 3d. Postage 2s. Radio Servicing. By G. N. Patchett and Telecommunications. By W. Fraser. B. Fozard. 1 vol. ed. 27s. 6d. Postage 65s. Postage 1s. 6d. is. Practice Problems in Electronics Telecommunications Cal- Principles in culations. By A. And rews. 30s. Postage M.K.S. Units. By R. N. Renton. 45s. 1s. Postage 1 s. 6d. Radio and Television Reference Data. Radio Servicing Problems. By W. A. L. Compiled by J. P. Hawker. 10s. 6d. Smith. 5s. Postage 6d. Postage 1s. Complete Catalogue 1s. We have the Finest Selection of British and American Radio Books in the Country 19-21 PRAED STREET (Dept RC) LONDON W2 Telephone PADdington 4185

Patron: His Royal Highness The Prince Philip, Duke of Edinburgh, K.G. THE SAIL TRAINING ASSOCIATION 48 WELBECK STREET, LONDON, W.1 Tel. WELbeck 2634 Special Appeal Committee for The Sail Training Ship Chairman: The Earl St. Aldwyn, P.C., T.D. The Sail Training Association was founded in 1955 to help bring together the youth of different nations in friendly competition and character training. These aims have been achieved by such events as the "Tall Ships Race" in which half of each of the crews must be youths with little experience of the sea. A National appeal was recently launched to build a three -masted top -sail schooner, which will give character training to about six hundred boys a year. This 300 ton vessel has been designed by Camper and Nicholson's and her keel is to be laid in November, her builders being Richard Dunston of Hessle on the Humber. The Association has relied on voluntary subscriptions to date, but the Council of the Association has now decided to offer Ordinary Membership to anyone who is interested enough to subscribe two guineas or more per year. Also Cadet Membership is offered to young people between 15-21 years of age who subscribe 7/6 per year. All such members, are entitled to wear the S.T.A.'s lapel badge and will receive S.T.A. literature as it is published. Earl St. Aldwyn, President of the Special Appeal Committee announced recently that the keel of the 300 ton topsail schooner will be laid on November 21st next. Donations should be sent to The Sail Training Association, c/o Hoare & Co., Bankers, 37 Fleet Street, London, E.C.4, and information about the activities of the S.T.A. and the endowment of berths can be obtained from Lt. -Commander P. S. Boyle, R.N.R., The Sail Training Association, 48 Welbeck Street, London, W.1.

214 THE RADIO CONSTRUCTOR. SMALL ADVERTISEMENTS TO DESIGNERS & CHIEF ENGINEERS

continued from page 213 FOR SALE. Quantity of new accumulators. Willard OSMOR Radio -20-2, 2 volt. Size: 5in high x 3in x 2in., 5s. each plus carriage. -Box No. F224. SPECIALISES IN WINDING MINIATURE COILS AND TRANSFORMERS OF INTEREST TO BULK INTERESTED IN AMATEUR RADIO TELEPRINT- MANUFACTURERS IN THE INDUSTRIAL FIELD. ING? Then join the British Amateur Radio Tele- printer Group. Details from Hon. Sec., G2UK, East Keal, Romany Road, Oulton Broad, Suffolk. MANUFACTURERS USING UP TO 50 S.W.G. WOULD FIND OUR SERVICE ECONOMICAL JOIN THE INTERNATIONAL S.W. LEAGUE. Free AND RELIABLE. ENQUIRIES APPRECIATED. Services to members including Q.S.L. Bureau. Amateur and Broadcast Translation. Technical and Identification Dept. -both Broadcast and Fixed LARGE CAPACITY AVAILABLE Stations, DX Certificates, contests and activities for the SWL and transmitting members. Monthy magazine, Monitor, containing articles of general interest PLEASE TELEPHONE CRO 5148 to Broadcast and Amateur SWLs, Transmitter FOR SWIFT ACTION Section and League affairs, etc. League supplies such as badges, headed notepaper and envelopes. QSL cards, etc., are available at reasonable cost. OSMOR RADIO PRODUCTS Send for League particulars. Membership including monthly magazine, etc., 21s. per annum. -Secretary, 540 PURLEY WAY (,FACILITI S) ISWL, 12 Gladwell Road, London, N.8. CROYDON CRO 5148/9 THE INTERNATIONAL HAM HOP CLUB is a non -profit -making organisation open to RADIO AMATEURS AND SHORT WAVE LISTENERS. OBJECT: to improve international relationships through an organised system of hospitality. MEMBERS offer overnight hospitality to visiting members, subscription TESTED TRANSISTORS 10s. per annum. AssoCIATE MEMBERS invite radio amateurs to visit their stations. Associate membership 1/- each Red or White Spots. OA81, 0A202. 5s. per annum. FAMILY EXCHANGE holidays arranged, also FRIENDSHIP LINKS between radio clubs. The 2/- each XA101, XA102, XA111, XA112, XB103, Club's official journal is free to both Full and Associate NKT122, 0A90, 0C430. Members. -Hon. Gen. Secretary: G. A. Partridge, 17 Broadstairs, Kent. G3CED, Ethel Road, 3/- each 0C44, OC45, 0081, OC81D, 0C200, GET16. SITUATIONS VACANT 4/- each AF114, AF115, AF117, OC170, OC171 YOUNG MAN required over 18 years, to work in busy radio shop and mail order dept. Some previous ex- SX658, XV611. perience necessary. For appointment telephone Paddington 1008. Henry's Radio Ltd., 303 Edgware 5/- each 0072, 0C139 OC140, 0C204, GET8, Road, London, W.2. ORP60, BY100.

ELECTRONIC TECHNICIANS 10/- each 0C35, 0C26, 0C28, AFY19, 2S013, ORP12, GET571. Cambridge Works Limited, a member of the Pye (Cambridge) Group Companies, require technical staff for their Limited of 33 volt watt, 3/6 each, Test Engineering Department. If you are seeking an interesting ZENER DIODES 4.7 volt to f job covering a wide field in electronics this may be it. The work 1.5 watt, 5/- each, 7 watt, 6/- each. consists of the construction and maintenance of special test equipment for telecommunications also the repair of standard items of test equipment made by top line manufacturers. Send 3d. stamp for Full Price List, and Free Basic knowledge of AC and DC theory, ability to read circuit Equivalent Chart diagrams, plus common sense in good measure are the qualifications required. These are staff appointments for technicians wishing to expand their experience and progress to design and development. B. W. CURSONS Please apply to Personnel Manager, Cambridge Works Limited, 78 BROAD STREET, CANTERBURY, KENT Haig Road, Cambridge.

OCTOBER 1964 215 CHASSIS 287/9 Edgware Road BLANK CHASSIS-Same Day Service London W2 Of over 20 different forms made up to YOUR SIZE. and at Order EXACT SIZE you require to nearest 1/16th. TELEPHONE (Maximum length 35", depth 4".) by -E D-C'i:llGFItRE _ ROi4 D Specials dealt with promptly. CASES PADdingto.1 5891/7595 SEND FOR ILLUSTRATED LEAFLETS or order straight away, working out total area of material required and referring to table below which is CASES Y for Type W -Type four-sided chassis in 18 s.w.g. (for 16 s.w.g. add ALUMINIUM, SILVER HAMMERED FINISH 1/6th). 48 sq. in. 4/- 176 sq. in. 8/- 304 sq. in. Type Size Price Type Size Price Type Z 12/- 80 sq. in. 5/- 208 sq. in. 9/- 336 sq. in. 13/- U 4x4x4" 9/6 Y 8x6x6" 25/- 112 sq. in. '6/- 240 sq. in. 10/- 368 sq. in. 141-- U 5}x4}x41" 14/6 Y 12x7x7" 39/- /Type U 144 sq. in. 7/- 272 sq. in. 11/- and pro rata U Bx6x6" 20/- Y 44/- 13x7x9" I P. & P. 2/6 P. & P. 2/9 P. & P. U 15 x9 x9" 3/- 42/6 Y 15x9x7" 46/- - Discounts for quantities. Finishes W 19/6 arranged for 8x6x6" Z 17x10x9" 63/- quantities of 25 or W 12x7 x7" 32/6 Z over. 19x10x8i" 67/6 FLANGES (I" or a"), 6d. per bend. W 15x9x8" 42/- Height STRENGTHENED CORNERS, 1/- each corner. Type Z has removable back and front panels. PANELS Type Y all -screwed construction. Any size up to 3ft at 4/6 sq. ft. 18 s.w.g. (16 s.w.g. 5/3). °lus Postage and packing (orders over E2 Post Free).

BRASS COPPER DURAL ALUMINIUM BRONZE STAINLESS STEEL 20 SUGGESTED CIRCUITS by G. A. FRENCH ROD, BAR, SHEET, TUBE, STRIP, WIRE. 3,000 STANDARD STOCK SIZES No Quantity too small List on application No. 15 in our Data Book Series. This book contains the first 20 circuits to be published in the very popular series of the same H. ROLLET & CO LTD name appearing in The Radio Constructor. HOWIE ST. LONDON SW11 BATtersea 7872 The contents include: simple and inexpensive two -valve capacity Also at Liverpool, Birmingham, Manchester, Leeds, Glasgow bridge, short wave regeneration preselector, one -valve speech operated switching circuit, transformer ratio analyser, series. noise limiter, receiver remote mains on -off control, and many others; covering subjects ranging from electronic laboratory equipment the unique P A N L black crackle paint to a simple 2 -Stage Gram Amplifier. PRICE 3s. 4/-d. per 1/8 pt. Can 6d. Postage 4d. 48 pages. (We regret we can only supply on cash with order basis) Available from your usual supplier, or direct from BRUCE MILLER LTD., DATA PUBLICATIONS LTD 249 Coastal Chambers, Buckingham Palace Road, S.W.1. 57 Maida Vale London W9

For Discerning Enthusiasts YOUR CAREER IN Quality Transistors and Components RADIO ? UHF Big opportunities and big money await the qualified man in Amplifier Transistors (BBC.2, etc.): AF139, 36/9; T2028 every field (2N2398), 53/9; GMO290, 31/6. of Electronics today-both in the U.K. and through- Tunnel Diodes: 1N3712 (TD -1) out the world. We offer the finest home study training for all 1mA, Ipk 2,300 Mc/s, 50/3; 1N3720 (TD -5) 22mA, Ipk 1,600 Mc/s, subjects in radio, television, etc., especially for the CITY 6 25/-. VHF Power Transistors: 2N3053 60V, 5W, fT=100 Mc/s, GUILDS EXAMS. (Technicians' Certificates); the Grad. Brit. silicon planar, 15/-; AUY10 70V, 4.5W, fi=120 Mc/s, germanium I.R.E. Exam.; the RADIO AMATEUR'S LICENCE; P.M.G. alloy -diffused, Certificates; the R.T.E.B. Servicing Certificates; etc. Also 53/-; BUY10, silicon npn epitaxial planar, 40V, 10W, courses fT=90 Mc/s (typ.), 45/-. in Television; Transistors; Radar; Computers; Servo- Audio Transistors, matched pairs: mechanisms; Mathematics and Practical Transistor Radio course 0081 Z, 16/- pr.; AD140, 50/- pr.; OC35, 36/- pr. with equipment. We have OVER 20 YEARS' experience in Pi -mode Amplifier-printed circuit boards, 12/6 mono, 21/- stereo. teaching radio subjects and an unbroken record of exam. Mains transformers and all components available. Also circuit (6d. successes. We are the only privately run British home study stamp). Low noise metal oxide resistors from 2/3 each. Institute specialising in electronic subjects only. Fullest details will be gladly sent without any obligation. PROMPT SERVICE. C.W.O. PLEASE. POSTAGE 6d. SEND FOR FREE BROCHURE TO: PLEASE SEND 1/- (STAMPS) FOR REFERENCE CATALOGUE 208 GLEY BRITISH NATIONAL RADIO J. (C), SCHOOL WILLIAMS & CO. BIRMINGHAMA 016D DEPT 4, RADIO HOUSE READING BERKSHIRE

THE RADIO CONSTRUCTOR - BOUND VOLUMES Now available Volume 17, August 1963 to July 1964 Price £1 10s Od Postage 3s Od Where all issues are returned : Price E1 Postage 3s Od Limited number also available of:- Volume 13, August 1959 to July 1960 Price f1 Ss Od Postage 2s 6d Volume 16, August 1962 to July 1963 Price E1 10s Od Postage 3s Od Where all issues are returned : Price E1 Postage 3s Od We regret Volumes 14 and 15 are now sold out Send for latest brochure containing details of our other publications DATA PUBLICATIONS LTD 57 MAIDA VALE LONDON W9

216 THE RADIO CONSTRUCTOR en INNN Y_ `MIeMI` S _ __fl_f flfe_NN.MfleNNI``z MULTI -METERS TRANSISTORS, CRYSTALS, NOMBREX TEST EQUIPMENT + _ VALVES AND COMPONENTS LEAFLETS OF THE ABOVE ON Multi -range test THE LARGEST RANGE REQUEST meters featuring easy ITEMS IN THE COUNTRY to read scales and FULLY provided with full We stock far too many types to list GUARAN- operating instruc- here. See our Catalogue for full range TEED tions, lead and bat- * New 8 -page Transistor, Rectifier, All transistor teries. Suitable for List Free on amateurs, designers, Valve and Quartz Crystal portable units repair shops, all request supplied with LET US QUOTE FOR PARTS FOR YOU domestic uses. Full full instruc- RETURN OF POST SERVICE details and specifications. tion in our catalogue. A SELECTION FROM OUR STOCKS * 50 kc/s to 350 Mc/s generator. RF See Catalogue for full details Mod., AF, 8 ranges. Leads, batt., E1.19.6 Sinclair Micro -6 (to build) ... 59/6 instructions, 09.10.0. * Ml. 2kS2 V/V f2. 9.6 Sinclair TR750 Amplifier (to build) ... 39/6 * Resistance/Capacitance Bridge, E8.5.0. * THL33, 2k02/V (illus) E3.15.0 Sinclair Micro Injector (to build) ... 27/6 * Transistorised Power Supply, 06.10.0. £3.19.6 Tape Jack Tuner (Full Med. Wave) ... 29/6 * Audio Generator, El 6.15.0. * TI- 20kS2/VV* ... E4. 2.6 10-Watt Horn Tweeter ...... 27/6 * Inductance Bridge, f18. E5.19.6 20 -Watt Super Tweeter ...... 69/6 * EP30K, 30kó2/V* E5.10.0 Two-channel Mixer for Mics/Pu's ... 15/- 4 -CHANNEL TRANSISTOR * 500, 30k02/V* 08.19.6 1 to 200 Mc/s Field/str. Meter ... 59/6 MIXER * EPSOK, 50kS2/V* £8.10.0 5 -hole Chassis Punch Kit ...... 49/6 * EP100K, 100kí2/V 09.19.6 Headphones 2k12 12/6; 4k52 ... 14/6 (*Leather Cases Available) Stereo Stethoscope Headset, 8+8 arm 25/ - Resistor Substitution Box ... 3216 4 -inputs, suitable for Crystal Mono Stethoscope Headset .. 10/6 Mics, Pick -Ups, Tuners, BUILD A QUALITY Magnetic Mono Stethoscope Headset... 12/6 etc. Built-in Battery. TAPE RECORDER Transistor Signal Injector ... 35/ - 49/6. P.P. 1/6 Fully Disappearing Car Aerial ...... 35/ - * Pre Built Test Lead Kit with Pouch ...... 8/6 Equipment -6 Capacitor Substitution Box . 22/6 junction TRANSISTOR PORTABLE Valves-Col faro De Luxe Stereo Headsets with WALKIE-TALKIES Studio Decks- £6 Portable PP3 9V Batt. Eliminator 18/6 Transistor Telephone Amplifier ... 59/6 9 -Transistor Portable Trans -receivers. Sup- Cabinets with plied complete with leather cases, batteries, 8" x 5" Speakers 8+80 Stereo Headphones ...... 62/6 Crystal Lapel Microphone .. ... 12/6 aerials, etc., in presentation case. Range US) Magnetic Lapel Microphone ... 86 up to 5 miles in favourable conditions. Crystal Contact Microphone ...... 12%6 Sizes 6}" x 3" x 1=,". Superhet crystal and receive. 2kSO } track Tape Head 15/- controlled on transmit Cost TWO TRACK Total £26 2k52 1" diam. Mag. Mic. Insert...... 15/- Price f21 Per Pair. P.P. 3/6 P.P. 8/6 MCI Crystal Mic. Insert ... 5/- OR Deck 10 gns. P.P. 5/-; Amplifier Magnetic ... 5/- POCKET RADIOS Earphones, Crystal or _ 11 gns. P.P. 3/6. Cabinet with Speaker Complete with Jack and Socket. 5 gns. P.P. 3/6. Telephone Recording Attachment ... 9/6 50-0-50µA Edgewise Meter ...... 25/- All complete with battery, Total Cost FOUR TRACK £30 6/12V Min. 212w Relay 12/6 earphone, carry case and P.P. 8/6 Transistor Coil Set With Tuner, fully guaranteed. 6 and 7 OR Deck 013.19.6. P.P. 5/-; Amplifier ... 20/- Ferrite Aerial and Circuit ... transistor superhet designs. 12 gns. P.P. 5/6. And Cabinet with Complete Test Lead Kit .. ... 8/6 Speaker 5 gns. P.P. 3/6. 30W Pocket Mains Solder Iron ... 14/6 All complete in every detail. Illustrated 8" Hi-Fi 15/16 ohm Speaker ... 69/6 * 6 -Transistor Med. Wave, 69/6. P.P. 2/- leaflets on request. Ideal for portable 100 kc/s Quartz Crystal ... 15/- * 7 -Transistor Med. Wave, 79/6. P.P. 2/- or domestic equipment. Twin Crystal 1 Mc/s and 100 kc/s ... 22/6 * 6 -Transistor Med./Long Wave, 5 gns. P.P. 2/6 * 7 -Transistor MW/LW/SW, GARRARD 2 -TRACK BATTERY RECORD/REPLAY/ERASE £12 P.P. 3/6 TAPE DECK TAPE HEADS

9V operated, 2 track, 2 MAINS AND BATTERY speed. Brand new, fitted RECORD DECKS heads, etc. Supplied complete with cassette. Brand ALL COMPLETE with ARM and CARTRIDGE New. £11.15.0 P.P. 3/6 * BSR UA14 4 -speed ...... £5.19.6 * BSR UA15 4 -speed .. .. £6.19.6 Garrard Autoslim 4-speed ... £6.10.0 FM TUNING HEART * VHF * Unplugable Head Version ... £6.19.6 Printed circuit construction, fully tunable * Stereo Version ...... £7.19.6 y * 6-9 volt version £8.19.6 85 to 105 Mc/s. Completely screened. Marriot Type R/RP/3 # track record -replay R.F. * SRP10 4 -speed or BSRGU7... f5. 0.0 Requires UCC85 (8/6), stage and osc./ head and Mul ard type TS erase head with with mixer stage. With circuit. * Garrard AT6 Stereo mounting pos . Ideal for transistor circuits. 4 -speed .. ... 010.19.6 Brand new in maker's cartons 15/-. P.P. 1/ - Cartridge, Bargain Offer 15/- Pair. P.P. 1/- * AT6 with Mono Head 610.10.0 * 7005 IMPEDANCE * P. & P. 3/6 any type above TRANSISTOR * AT6 with EV26 or Sonotone £11.19.6 SUPERHET TUNER CRYSTAL MICROPHONES * 9V Star, 45 r.p.m. ... 01.19.6 * 45 and 33 r.p.m. Star, 9V ... £4.12.6 (See Catalogue for details and for * 45 r.p.m. Garrard 6V £1.15.0 Dynamic Mics) * 4 -speed B.S.R., 9V or S.R.P.10 Acos 39-1 Stick Microphone ... 32/6 9V .. ... E5. 5.0 Acos 40 Desk Microphone ... 15/- P. & P. 2/6 any of above Acos 45 Hand Microphone ... 19/6 Lapel/Hand Microphone ... 12/6 Acos Stick Microphone ... 15/- HENRY'S RADIO LTD 100C Stick with Stand ...... 32/6 9V medium wave 3 -transistor and diode BM3 Stick with Stand ...... 49/6 303 EDGWARE ROAD LONDON W2 + superhet radio tuner. Ideal for all amplifiers Lapel/Hand Magnetic ...... 8/6 PADdington 1008/9 + Price with circuit 67/6. P.P. 1/6. MM71 2k Mag. Mic ...... 12/6 Open Monday to Sat. 9-6. Thurs. 1 o'clock -iAlso mW/sW 4 -transistor E7.9.6. Trade and Export Enquiries Invited PLEASE TURN TO BACK PAGE dlull.luussis.s.II.II.Iu I1.11.11.11.11.11311.Iu1u11.Iu1u

3S Components and Equipment Specialists for more than 20 years

=g. Transistor High Fidelity PUSH-BUTTON = Equipment CO N VAI R PORTABLE/CAR RADIO: a FOR MAINS OR BATTERY OPERATION -" * New printed circuit 6 -transis- - tor superhet design. Over g - MONO 700mW push-pull output. g. FULL FUNCTION -- * Push-button selection of g g ` - MW and LW with full tuning PRE-AMPLIFIER/CONTROL ',1a e.. G UNIT on both wavebands. - _ * A new design with amazing PRICE £5.10.0 2'P' = -,- performance at a realistic OR KIT 99/9 P.P. 2/- price, -'_ _ : New attractively Dark brown and _\1_? ° * designed * gold CTO. portable cabinet with hori- 1 front panel plate 8 /6 TgFBIE éasa - zontal tuning and all stations E.SFFor use with one or two mVOWME marked. Size: 10 x 7" x 31" transistor e power amplifiers as below or valve Total Cost of amplifiers-hum free-high sensitivity -8 inputs -- (Batteries 6/- extra) between 1.5mV and ALL Parts £7.19.6/ 3/6P. = c 300mV, with full equalisation. For all pick-ups-microphones, tuners ALL PARTS SOLD SEPARATELY it;\_ -tape replay. Separate treble and bass with cut and boost -4 posi- FOLLOWING A LINE OF tion low pass filter. Volume, 40 c/s and 20 kc/s, etc. Output 250mV. QUALITY AND VALUE IN PORTABLES ! Overall size 9" x 2k" x 2". Battery operated (21mA) or from power supply. Signal to noise ratio 70dB at 10 watts with power amplifier. -= 20 watts VHF FM TRANSISTOR TUNER 2 output when used with two power amplifiers. Supplied built and ready to use. * Mullard 5 -Transistor. * 4 Diodes. s I INTEGRATED STEREO PRE -AMPLIFIER * Full coverage 87 to 105 Mc/s. * AFC and AVC. 'TRANSISTOR STEREO HI-FI Io ] _J For use with Valve or * Over IV output. Transistor Equipment Printed circuit superhet design of high sensitivity and quality. Output for valve or transistor amplifiers. Size 3k" x 2f" x 3". Simplified Two channel version of above building with addition of instructions. Operates from 9V balance and 9-10mA. All parts sold separately. mono/stereo controls. Over- Complete all size 9" with dark brown and TOTAL .07.16.61;./:. x 3}" x 11" deep. gold panel plate. Designed to feed two 10 COST watt power amplifiers as (Cabinet assembly 20/- extra) below. Supplied built and ready to use. Detailed 5mA current. Batter./ %crated (5mA) or from mains unit. Circuit Book on Request Dark PRICE £1 0.1 .7.6 P.P. 3/6 * brown and gold front panel plate 12/6 Minigram } WATT Transistor 4 -TRANSISTOR s MULTI -INPUT PRE -AMPLIFIER Portable Record Player AMPLIFIER For use with one or two transistor power * Made by well known amplifiers or as a general purpose pre- British manufacturer. Fea- amplifier. A simplified version of the tures ready built 4 transis- S full function pre -amp. 8 inputs from 1.5mV tor printed circuit, 1 watt to 300mV. Input selector, tone and amplifier, elliptical speak- volume controls. Size 5" x 2k" x 2". er and volume control. Ideal for public address systems, for Low current Starr microphones and 1ck-ups. constant speed 45 PRICE P.P. 1/6 * Dark brown and gold r.p.m.turntable with 6b/- front panel plate, 6/6 crystal pick-up. Strong moulded two- 10 WATT POWER colour cabinet with 1k watt peak handle. Plays anywhere on output. Built AMPLIFIERS long life 9 volt battery. Requires less than half an hour's work to connect up using ±3dB 70 c/s 49/6 = to 12 * 150 10 watt 40V supply. ready built units and easy instructions. kc/s. P.P. 1/6 TOTAL P.P. Output to 3 £5.19.6 COST 79/ 6 5/- (Battery 3/9) ohm speaker P.P. 2/6 9V operated. Or Amplifier with volume control and 5" Or Kit OR KIT x 3" Speaker, 4 -Transistor, 35/-. P.P. 2/- (5.10.0 P.P. 2/6 9V Starr Player with Pick-up 39/6. P.P. 2/6 Details on 39/6 And Two-Tone Case 5/-. P.P. 2/- request. P.P. 1/6 * 35210 watt 24V supply. £5.10.0 4 peak 7 -TRANSISTOR RECORD PLAYER OR KIT 99/6 P.P. 2/6 P.P. 2/6 * watt output * Full Treble and Bass boost PORTABLE AMPLIFIER Direct coupled transformerless design. and cut Sensitivity 100mV from 10k52 or less source for 10 watts 40 c/s to 20 kc/s ±3dB output. Response ±1dB 40 c/s to 10 c/s. 0.25% total distortion at * 10 watts. Push-pull class B output. 60dB feedback. Two versions * Inputs for Pick-ups, Radio available for 30 co 40 and 150 to 1652 output. Battery or mains Tuners, Microphones, Mixers supply operation. For use with mono or stereo preamplifiers as above. Supplied built and ready to use. The 24V version is suitable *9-12V mains or battery for 12V operation with an output of 3/4 watts. Also 4/5/8 or 15 ohm operation for 3-50 speakers speakers. (mains unit 49/6 extra) * Size only 6" x 2k" x 2". Built DETAILS WITH CIRCUITS OF ABOVE Ideal for mains or battery, Ready UNITS To Use E5.19.6 P.P. 2/ FREE ON REQUEST portable or domestic record - player, grams, etc., or car. * New Improved High Gain Version * _ We can supply from stock most LATEST CATALOGUE of the components and items RADIO HENRY'S LTD 86 PAGES, 10" x 7f" i PLEASE specified on circuits published 3 in this and other magazines. 303 EDGWARE ROAD LONDON W2 Now with new valve, transistor TURN Quality parts and crystal supplement. at realistic prices. PADdington 1008/9 Fully PAGE Let us quote for your circuit. detailed and illustrated. Open Monday to Sat. 9-6. Thurs. 1 o'clock Price 2/6 post paid.

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