"1946 amateur junior" receiver
Contents
1. ALL NECESSARY All these accesscries add up to quite an impfessive array of controls They are all desirable for convenience in amateur working and are not included just to make the receiver panel look impressive One could talk at great length about the electrical features essential to an amateur or communications receiver of the mechanical rigidity which is so necessary to maintain frequency sta bility and of the need to use the best possible components and a smooth dial movement However further details along these lines will emerge aS we go on from here to describe our new receiver Setting about the deslgn of the re ceiver it was a pleasant change to plan it all on the drawing board and know that a chassis could be made to exact specifications The layout is Continued on Page 23 3 BAND COIL DIA LENGTH PRI SEC Yet another requirement for the re OO OOS S eS se 32 18 Mc s Aer in in 2 63 9 4 16 7m Ose in in b 6 19 9 6 Mc s Aer tin lin 2 9 15 8 31 3m Osc Ilin lin 4 gt 11 5 5 8 Mc s Aer Lin I Zin 3 7 26 52m Ose Itin din 5 16 6 2 3 2 Mc s Aer Itin in 5 27 48 94m Ose Idin Zin 7 25 through the IF channel The beat oscillator is conventionally fitted to larger amateur and communications receivers but it normally entails the use of an extra valve Our new set Coils for 3 6 Mc band wound entirely with 22 B amp S DCC All other primaries w2. UO bracket to carry the oscillator valve and cscillator coil This bracket was made up from scrap aluminium with two smal flanges to permit rigid bolting to the chassis Le NCEE ne EE Dek Two valve socket holes are required in this bracket one to carry the con verter yalve horizontally and the other to carry the vertically mounted oscil lator coil These holes are best cut and the mounting holes drilled before the bracket is bent Make sure that the valve holes are far enough apart so that the socket lugs will not foul when they are finally mounted in place Use the best sockets you can get for We have not had the time or opportunity to test this re ceiver on the 50 54 m c band We intend to do some work with it dur ing the next few weeks and expect it to behave quite well in that region Look for results in Radio and Hob bies for May a the tuning coils and the converter valve A steatite or amphenol type is a good selection but beware of any socket which has the appearance of having been impregnated with flux You will find that the leads between the converter valve socket and coil pins are less than an inch long in most cases Make sure that the bracket fits the chassis then wire it all up before mounting in place Inside the bracket you will have to tuck away the oscil lator grid condenser grid resistor cathode resistor and bypass screen by pass and oscillator anode
3. 25 Shelves You can f Make Lam ps Chemi a Experiments From Pattern to Casting Build Ycur Own J Auto Trailer Techniaue of Using Drills and Wood Bits Running a Metal Turning Lathe Wood Turning Weaving and Re f caning Skipper 14 foct Outbeard Run about House Wiring Action Tovs Elec f trorlating Plain and Novelty Wood Fin J ishing Small Sailboats Spray Paintine J Extra Rooms in Your Attic Automobile Kinks How to Build Your Own 14 foot Sailing Dinghy Concrete Garden Fur niture Serving Trays Lawn and Gar den Novelties Mirror Silvering ROBERTSON and MULLENS LTD 107 109 111 113 Elizabeth St MELBOURNE CI PAGE TWENTY FIVE FERGUSON RADIO Dear Mr Radioman A MULTIPURPOSE TEST INSTRUMENT Is that what you ve longed for Of course it is Well that s the type of instrument you have at your finger tips when you use a Cathode Ray Oscillograph The number of different tests that an amplifier can withstand and still come through with flying colours determines its final performance Practically every major race that a HI Fl Amplifier has to run is made with a Cathode Ray Oscillograph in the judge s box Ferguson s Radio of Willoughby are now manufacturing a 3 inch cathode ray oscillograph equally suitable for design laboratory or ham shack Just look at these performance figures DEFLECTION AMPLIFIERS FREQ RESPONSE Flat
4. OSCILLATOR ception of Morse transmissions is a beat oscillator This can be covered by arranging for portion of the set to oscillate generally the detector Alternatively one may provide a separate beat fre quency oscillator stage to heterodyne the incoming signal as it passes neat connection for phones and tone control uses a regenerative second detector of Simple and proved design Coming to the detector and audio stages the requirements in regard to fidelity and high power are less rigid than with conventional broadcast re ceivers In fact high fidelity may be an undesirable feature As often as not interest centres in weak signals which are heard through a solid background of noise The essential thing is to obtain the great est clarity of speech or alternatively the clearest Morse tone possible TONE CONTROL By deliberately attenuating the bass response or the treble response or even both together a lot of the noise can be cut out while still preserving the middle register where the vital speech or tone frequencies are centred So one can expect a tone control arrangement in a communications superhet quite distinct from the type found in broad cast receivers On top of that are other essential details like provision for earphones COIL WINDING DATA with an earphone loudspeaker switch Perhaps a standby switch to render the receiver inoperative while keeping the valve heaters alight
5. Page Aerials s Aer Coil Cd Osc Rer Or Randsets F a Drawn one quarter full size these sketches show the main holes in the chassis and front panel and also in dotted out line the position of the aerial terminal strip tuning con densers coils and converter valve assembly A ready puached steel chassis should be available immediately through normal supply houses but the condenser mounting and converter valve brackets will still need te be made from scrap metal PAGE TWENTY THREG OHN MARTIN announces Limited supplies are beginning to arrive Call or write for full particulars to 116 118 Clarence Street Sydney Telephone BW3109 3 lines Telegrams Jonmar PAGE TWENTY FOUR 54 RADIO E ELECTRICAL Sydney Continued from Previous Page ing of these condensers will make them particularly difficult to adjust as they are normally fitted with a direct drive knob The main oscillator tuning conden ser is driven direct from the dial to minimise the effects of backlash which may develop in the coupling between the tuning condensers Jut in passing a fibre bush was used instead of the usual brass bush between the dial collar and the tuning condenser spindle The chance of noise originat ing in the dial mechanism is thus mini mised by avoiding possible noisy metal to meta contact OSCILLATOR TUNING The layout arrangement calls for a
6. U H F Radio Sim plified a ee 26 2 3 Nil on and Horning Prac tical Radio Communication 42 1 Ghirardi Radio Physics Course 40 1 6 Ghira2 i Modern Radio Ser vicing x 40 1 6 Cooke Mathematics f o r Electricians and Radio men ee eae 28 I Cocking Wireless Service Manual 12 6 A Almstead and Tushill Radio Material Guide W4 4d De Forest Television To day and Tomorrow 27 4d Everett Communication En gineering 35 9d Rider The Meter at l Work 1 ee W o 3d Admiralty Handbook of Wireless Telegraphy Vol ume L 7 3 gd Admiralty Handbook of Wire less Telegraphy Volume IL 141 6d ELECTRICAL Vingeomb Flecttivity To day ea ae ee eee 79 8d Sniith Testing Dynamos and E Motors 15 6 a MacFarlane Electricity in the House 5 sd Muller Gatman and Drory Experimental Electron ies saraa ae BRD 6d Dawes Industrial Electri i city Pt L Le ey 1575 fd Dawes Industrial Electri city Pt H ga ae 19 3 6d Covne Electrician s Handbook 23 6 4 i Braymer and Roe Rewind ing Small Motors 17 6 6d Braymer and Roe Rewind ing and Connecting Al i ternating Current Mctors 2 6 64 Amick Fluorescent Light ing Manual Z1 5d Little Library of Useful In l formation Price 1 9 wed amp Tool and Saw Sharpening Cleansing Preparations Small Tables Book Cases Turned Novelties
7. about a quarter inch from the bottom of the former drilling a small hole for the wire to pass through Drill another hole for the grid end of the winding the speci fied distance from the first so that the overall length of the coil will automatically be right Connect one end of the wire through to the base pin in the coil former unroll the amount of wire you antici pate will be necessary to wind the coil and then clamp the reel in the vice Keeping the wire taut begin your winding moving towards to the reel as the wire is taken up Space the winding slightly as you go put on the required number of turns cut the wire and finally push the end through hole and solder to the base pin If the wire has been put on tightly and without kinking it should now be possible to move it along with the fin gers until a nice even spacing is achieved along the full length of the coil PRIMARY POSITION Note the position the primary will now occupy and drill holes for the top and bottom to drill the top hole you will need to part the heavy wires slightly Using the fine wire specified or similar wind on the specified prim ary turns keeping the wire central between the turns of the secondary For the aerial coils on each of the higher frequency bands the primary winding is started near the bottom of the secondary and interwound turn for turn But _ there is an important difference in the oscillator coils in that the prim
8. bypass Leads coming out include two for the heater and one each for the Oscil lator B supply screen supply and mixer plate These pass down through a hole in the chassis to their respec tive destinations Double check your wiring and re vord the chosen coil connections before you mount this unit in place for it is not a pleasing job to have to dis ronnect it all to correct some suspected fault One lead will need to come out the side of the oscillator bracket to the stator plates of the tuning condenser It is also a good idea to cross connect he earth points of the condenser and oscillator assembly and run another UNDER CHASSIS VIEW OF SET Note mounting and extension shafts for band set condensers Other points include shielded leads and neat layout mounting for tuning dial earth wire through to the bandset con denser But be particularly careful to avoid unnecessary stray capacitance effects between the oscillator grid circuit and chassis Allow a bit of air space around the condenser resistor and grid circuit leads Coming to the converter valve itself We decided to use the well tried 6J8 G Despite temporary shortages and talk of other valves the 6J8 G is likely to remain the most readily available con verter for many months to come and it was therefore the logical choice Experimental work with other convert ers may follow at a later date I F EXPERIMENTS The operating conditions a
9. reduces the hum level to the usua low limit How VIEW ABOVE THE SET CHASSIS REPAIRS CALSTAN EXPERTS The new Repair Department at Zenith is the answer to a long felt want Having secured the services All the main components above the chassis are visible here The well grouped of a highly qualified instrument leads between valve coils and condensers is quite evident Note terminals for maker and technician with 35 doublet aerial connection and the angle bracket supporting the dial years experience Zenith can now effect repairs and adjustments to all graphic recording and standard and substandard electrical instru ments For dependable repairs consult Zenith ever if you are fussy on this score and requirements will be clear from it is a simple matter to add an extra what has already been said and from filter choke and condenser the diagrams and pictures Once the Most of the constructiona methods Continued on Page 61 CLOSE UP OF TUNING SECTION CAlibrated to STANdard MULTIMETER AND OUTPUT METER MODEL 511 AC DC wide volfage current and resis tance ranges Portable attractive and completely reliable CALSTAN D C MULTIMETER Guaranteed within 2 per cent accuracy In sturdy black case and complete with test leads CALSTAN MODELS 223A AND 223AV Analyser and valve checker ae ee eg eee ne en Ne 6 ee ee RE A eR 7 RL AEE stn RE aa amea ne weno tte oe E QUALITY PRODUCTS O
10. within 2db 10 20 000 e s t i INPUT Continuously variable impe dance 1 0 meg SENSITIVITY 0 5 volts R M S for one inch deflection low distor tion DIRECT PLATE INPUT INPUT 2 meg impedance SENSITIVITY 50 volts per approx LINEAR TIME BASE RANGE 25 to 30 000 c s inch THANKING YOU FERGUSON S RADIO PAGE TWENTY SIX o mmea a eee the same neat trick which was utilised on our Communications Five and other receivers of a few years ago It involves connecting a small in ductance in series with the detector cathode to earth return and shunting it with a wire wound potentiometer The latter may have a value of be tween 2500 and 5000 ohms and the inductance may be wound easily by hand The genera idea is to wind on just enough turns to ensure reliable oscil lation with the control well advanced Too many turns will make the regen eration ploppy in action FEEDBACK COIL The feedback coil in the Communi cations Five had about 100 turns of about 30 gauge wire jumble wound on a lin diameter former A somewhat neater coil as used in this Set con sists Of 125 turns of 30 gauge wire Jumble wound on a iin bakelite bush between two small bakelite cheeks These figures will be a good guide but experiment for yourself and get the best results In the original set the reaction was very smooth with the IF gain at maxi mum as would be the case when se
11. A new receiver for the Now Cluu itwuy Here is a new receiver for short wave and amateur use just the thing for the new ham or the old hand who wants to build a new station Special attention to layout and design combine to make it close to the ideal in medium sized sets It works well on 10 metres and provides con tinuous coverage to 100 metres It may well be considered our standard medium sized short waver for some time OW that amateur activity is well under way we decided to spend some time answering an oft repeated question When will you describe a set suitable for use on 10 metres Mine is OK on 20 but not so hot on higher frequencies The time appears to be a good one for the production of a standard medium sized set which would be good for 10 metres and above At the same time a review of the main design points concerning SW sets in general seemed to be in order In this article we have attempted to do both these things GENERAL PROBLEMS The problems involved in designing and building a shortwave receiver are quite different from those which apply Chassis Tiin x lOin x 24in Metal or masonite panel I4in x 9 in 180 degree nameplates Selector 5 plate midget cond 35 mmfd 9 plate midget cond 70 mmfd Flexible eondenser couplings Vernier dial to suit 465ke lron cored F trans 80 or 100 mA power trans Valve shields Potentiome
12. EAKER To match single 6V6 G 2000 ohm field coil VALVES 6J8 G 1 6U7 G I 6J7 G 6V6 G 5Y3 G SOCKETS 5 octal 4 pin Also 2 sockets to suit plug in coil formers SUNDRIES Screws nuts washers hook up and shielded wire spaghetti 3 small grid clips Three terminals 2 red black and mounting strip Pillars for coil socket and in dia rod for extension shafts Coil formers I4in dia two for each band Short length of din dia coil former Scrap aluminium or steel for mount ing brackets amp c Winding wire as specified in coil data Ww by W N Williams separate them In any case it is most difficult to record their positions on the dial for future reference When a receiver is designed for operation only on the shortwave bands tne need for a large tuning gang con Genser say 0004 mfd per section disappears Instead one can very con veniently tune the signal and oscil zator circuits with variable conden sers having no more than a small frac tion of this capacitance say 0001 mfd or less By so doing the frequency coverage of tne particular coil condenser com bination is naturally reduced Instead of tuning from sav 16 to 51 metres in sone sweep of the dial one may tune from sav 16 to only 22 metres for tme same dial pointer movement The exact Sgure of course depends on the constants of the tuning circuit but it is not difficult to see just how Stations are spread o
13. F This close up illustrates in detail many of the points in the top picture It also gives an excellent idea of the 10 metre coils mounted within valve bases 131 133 Palmer Street off William St Sydne 1 5 p PAGE TWENTY SEVEN THE 1946 AMATEUR JUNIOR Continued from Page 27 wiring has been completed and checked there remains the last big job of winding up the various sets of coils The performance of the receiver will largely be governed by the coils so that any amount of trouble is justified in making the best possible job of them Cleanliness rigidity and accur lacy are the points to watch Four distinct connections have to lbe made to each aerial and oscillator coil so that four pinned formers would suffice However an extra pin or two is handy for possible elaboration at later date Also it is a good idea to have a different number of pins on the aerial and oscillator coils to avoid possible confusion between them Thus a set of 5 and 6 pin formers is a good choice We used 6 and 7 pin formers because they happened to be on hand It is not particularly important just how you bring the coil endings to the respective pins THe main point is tc see when the former is ultimately plugged into the socket that the two wind ings are conneced correctly into cir cuit with the shortest possible leads CONNECTION TO PINS All windings must go on in the same direction Referring to the aer
14. ake the lead even more direct The oscillator tuning condenser is mounted immediately behind the dial in front of the aerial condenser and the space occupied by the shafts and coupling brings the oscillator stator plates right alongside the base pins of the valve Here again the leads are much shorter than would be pos sible had conventional layout been used BANDSET CONDENSERS The aerial and oscillator or bandset condensers are mounted below the chassis but in such a way that they are eaSily connected in parallel with the ganged tuning units A quarter inch hole is drilled through the chassis and the respective sets of stator plates connected by a short busbar which passes straight up through each hole with a minimum capacitance effect to chassis All four tuning condensers need to be mounted on angle brackets which in the case of the ganged units must be about two inches high to allow the spindles to fit conveniently to a stand ard type of dial In ganging the main tuning con densers great care is necessary to see that the spindles are exactly in line before the coupling is installed Mis alignment caused by bad mounting or even unequal pressure in the coupling grub screws will cause the condensers to weave badly as the dial is rotated The same remarks apply for the bandset condensers which need to he operated from the front panel through extension shafts Weaving and spring Continued on Next
15. arching for weak signals With the IF gain backed off a slight ploppiness is evident but this is unimportant in practice Note that the lead from cathode to the re generation control should be shielded Use of a leaky grid detector and regeneration com plicates the addi tion of automatic volume control al though it is still quite feasible How ever it was felt that an AVC Sys tem and switching could be added if desired at a later date A U C CIRCUIT If you are keen to add AVC at some time we suggest that you purchase a 6B8 G and use it in place of the 6J7 G detector It will be just as effective in this service and the diodes are available for future use Altern atively an EBF2 G could be installed as IF amplifier leaving the diodes un used for the time being There is a nice spot in the front panel for the AVC switch The layout of the chassis up to this point follows logical sequence and al lows for short leads and freedom from feedback paths _ N Output from the IF amplifier feeds into a leaky grid detector This type was chosen both for its selectivity and for its adaptability to regeneration As already explained regeneration is necessary to produce a beat note with unmodulated morse carriers The only alternative would be a separate beat oscillator stage which requires another valve and quite a few extra bits And in any case regeneration offers a useful increase in gain below the point
16. aries are only part inter wound Thus in the 6 11 megacycle coil the bottom turn is wound close under the bottom of the secondary with the other four turns interwound And in the 9 19 megacycle coil two of the four turns are around the bottom of the secondary and the other two inter wound COILS FOR 18 32 M C For the 18 32 megacycle coils no attempt was made to use the standard in formers because of the poorly shaped coil which would result The coils were wound instea on short lerfgths of gin diameter paper bake lite and then mounted inside two suit able valve bases In the oscillator coil three of the primary turns are around the bottom of the secondary and three interwound If your coils are wound exactly to specifications and stray capacitances in the receiver kept to a minimum band coverage figures should be very close to those obtained in the original set If a check shows all to be in order in this respect it is a good idea to coat the coils with trolitul or other good insulating varnish or with smer lac or pure paraffin wax This will hold all turns firm and help to main tain accurate calibration But enough for the present Next month we hope to have more to say about the operation of the 1946 Amateur Junior PAGE SIXTY ONB
17. ere ultimately installed and subsequent testing has confirmed the wisdom of this chcice for our particular circuit require ments The IF amplifier valve is an ordin ary 6U7 G With a variable cathode resistor to permit control of IF gain A voltage divider supplies the screen oltage for this and the converter valve K is noteworthy that although both screens are supplied from the same point two separate bypass con Gensers are used one on each of the sockets REGENERATION Regeneration is something of a prob lem with conventional IF transformers which have no tertiary feedback wind ing But the problem was solved by Continued on Next Page ine eee teens erent ne MULLENS Technical Books 6d RADIO Price Post Bernard s Manvual of Short wave Technique and In structional Broadcast Re cepticn oe 4g et 3 2 BY Bernard s Radio Coil and Transformer Manual 3 2 22d Bernard s Radio Pocket Brok ne 04 ree 71 Bernard s Cathode Ray Oscelioscope Manual 8 2 22d Bernard s Manual of Direct Disc Recording 3 2 2d Bernard s Bulgin Radio Service Manual 4 24d Bernard s Radio Valve Man f ual F 5 6 24d Terman Radio Engineers Hariibook 42f e 1 3 Tucker Introduction to Practical Radio 20 3 Gd Radio Amateurs Handbook Defence Edition 3 6 4 Simcn Radio Service Trade Kinks a 21 4e 8d Kiver
18. ery nice but they cost a lot of money Seeking to strike a happy medium we decided that our first postwar short wave set should be a superhet would use standard parts throughout and have five valves in all no more no less Time enough later for larger and smaller varieties to fit sundry re quirements and budgets Pre supposing five standard Aus tralian valve types the line up more or less automatically worked out as follows Converter IF Amplifier Detector Output valve and Rectifier Much the same line up as any ordin ary 4 5 super but there the resem blance ceases PLUG IN COILS Realising that low capacitance tun ing condensers will be used several sets of tuning coils are necessary to cover the desired short wave bands Home wound plug in coils are used for simplicity and for flexibility There being no tuned RF stage an aerial and an oscillator coil are necessary for each band The choice of converter valves is limited but the intermediate frequency channel provides some food for thought A long process of development has resulted in the evolution of efficient 455 ke IF transformers which are now more or less standard equipment in broadcast and dual wave_ receivers But a 455 kc IF channel has certain disadvantages for a purely short wave receiver INTERMEDIATE FREQUENCY One disadvantage is that only 910 kc 455 x 2 separates a signal from its image position Again the re quired bea
19. ial coils the connections must be such that the top of the secondary further away from the pins goes to grid and the bottom to earth The top of the small primary winding must connect to the main aerial terminal and the bottom to the second aerial terminal which is normally grounded Top of all oscillator secondaries the heavy winding goes to the oscillator grid condenser and the bottom to earth Top of all oscillator primaries goes to B plus and the bottom to oscillator plate i The coil for the 3 to 6 megacycle band is the simplest to wind and is best tackled first Drill a small hole through the former about in from the bottom for the bottom of the grid winding and another a little less than an inch above it for the top of the winding Then close wind the stated number of turns using 22g DCC covered or some similar gauge SPACING The primary can then be fitted in the space below the grid winding and comprising the stated number of turns for the aerial and oscillator coils Space the windings about 1 16th inch in the case of the oscillator coil and about 1 8th inch in the aerial coil Connect the windings through to the base pins plug the coils into the sockets and check to see that the wind ings connect into circuit as required The secondaries of the remaining six coils are all wound with 18 gauge enamel or similar wire the windings being spaced out according to the tabulated data Start all secondaries
20. of os cillation Ea From the detector plate across to the E got so darned interested in this grand little set that the allotted space ran out too However all the constructional de tails are here allowing you to go ahead with the job we ll give you more about operation band coverage and other matters of a more general nature volume control tone control and out put grid the leads are much longer However only audio voltages are in volved and shielding takes care of this without ill effects With an IF gain and a reaction con trol the audio gain potentiometer may not appear essential Perhaps it isn t but it is very handy in limiting the audio power in the brrrps and squeals for which the short wave bands are famous Very useful too is the tone control switch which is rather unusual in its action In the centre position it gives normal frequency response on one side the bass is cut severely and on the other both bass and treble are at tenuated leaving only the middle fre quencies Proper use of this control can make a world of difference to the clarity of signals either phone or Morse OUTPUT STAGE The output stage is quite straight forward except for a special network in the plate circuit to allow the use of phones when desired The output plate load is provided by the normal loudspeaker transformer in series with a 400 ohm resistor at the B plus end A two way toggle switch cuts
21. ound with 30 B amp S DSC and secondaries with 18 B amp S enamel All windings in the same direction See text for further details PAGE TWENTY ONE such that it meets present require ments very nicely and leaves room for future elaboration of the circuit A distinctive feature of the layout is the position of the converter valve which lies horizontally across the chassis This arrangement is techni cally sound and makes possible very short leads in both the signal and os cillator circuits as the tuning conden sers bandset condensers and tuning coils are grouped within an inch or so of each other AERIAL CONNECTION Two aerial terminals are mounted on a small bakelite panel above the rear edge of the chassis These con nect to the respective ends of the aerial coil primary and permit the use of twisted or transposed aerial feeders When a single aerial lead in is used the terminal connecting to the lower end of the primary must be bridged across to the earth terminal which is mounted on the rear of the chassis The socket for the plug in aerial coil is mounted on pillars just near the aerial terminals so that the intercon necting leads are quite short So also is the lead from the grid pin of the coil to the grid cap of the con verter valve and to the stator plates of the aerial tuning condenser In the original set a small solder lug was mounted under the rear stator assembly bolt to m
22. out one or the other as required When the loudspeaker is required the switch simply Shorts out the re sistor and the set Operates in the normal fash ion With the switch in the alternative position the loud speaker transfor mer is shorted out and an audio volt age developed across the resistor Owing to its com paratively low d c resistance the d c Plate voltage of the output valve is not greatly affected The audio voltage is fed to one side of the phone jack through a coupling condenser the other terminal being earthed A resistor across the jack keeps the condenser at earth potential at all times so that there is no heavy plop as the phones are plugged in while the set is operating Use a good condenser and you can rest assured that the phones will always be at earth poten tia as far as d c is concerned The amplitude of signal in the phones can be varied by altering the value of the resistor but 400 ohms gives a nice balance between the output from loudspeaker and phones It is pos sible to switch from one to the other without having to alter the gain con trol setting The switching operation could actu ally be carried out by the jack itself if it happens to be one with an insu lated set of leaves giving a SPDT switch soon Next month action Apart from a stand by switch in the transformer high tension circuit the power supply is quite conventional The single section filter
23. re quite conventional The idea of oscillator plate tuning was considered but at this stage there did not appear to be sound reasons for adopting the un orthodox The plate lead of the converter passes down through the chassis to the first IF transformer And here we recall previous discussion Knowing the advantages of a higher frequency IF channel and the scar city of suitable transformers the set was tried out with standard iron4 cored RF coils in place of the con ventional IF transformers Tuned with small postage stamp trimmers these were found to resonate very nicely at a little over 2000 Kc s By including an RF coil with reaction in the last position a very neat set up promised A practical test showed Also cut out and gain but fair overall inadequate selec tivity for our purpose We gained the impression that two IF stages with reaction using these coils WOuld be practicable but as we desired only one IF stage we changed to 465kc intermediates I900K C INTERMEDIATES aoe aS Coil manufacturers at our request have since intimated their intention of resuming production of IF trans formers for 1500 or 1900 kc s but at the time of writing their technica fea tures are unknown As gain and selec tivity are likely to be poorer than with existing 455 kc s units a two stage IF channel may also be necessary using them With all this in mind standard iron cored IF transformers w
24. scillator immediately behind the power transformer PAGE NINETEEN CIRCUIT OF THE 1946 AMATEUR JUNIOR RECEIVER 240 VAC 1946 AMATEUR JUNIOR 5000 sf PHONES The circuit reveals many well tried features as well as some new ones The set has every essential control and a particularly Continued from Page 19 carrier frequency The percentage dif ference is actually not very great and depending on the exact circuit arrange ment there is a tendency for the local oscillator to lock in with the in coming carrier and thus produce no output at intermediate frequency All these difficulties are minimised by increasing the intermediate fre quency and in fact this practice is commonly followed in short wave Superheterodyne receivers Gain control of the IF and other pre detector stages can be automatic AVC or manual according to the degree of circuit complexity which can be tolerated Automatic volume control is a handy feature when listen ing to fading phone stations but it is undesirable for most morse transmis sions In many cases under these con ditions the AVC action causes the back ground noise to intrude in a most dis concerting fashion between each break in the signal In other words AVC is not a feature to include haphazardly in a short wave set Its effectiveness and time constant must be carefully considered and means provided to render it inoperative when necessary BEAT
25. t trequency can be produced in the IF channel by a wanted signal and by another unwanted carrier which happens to be 910 kc awav from it And at high signal frequencies the signal tuned circuits may not be selec tive enough to discriminate against the unwanted carrier or image GAIN CONTROLS Then there is the matter of oscil lator pulling which begins to show up at frequencies about 30 megacycles To function normally the local oscillator must operate on a frequency 455 ke removed from the incoming Continued on Next Page FRONT VIEW OF CHASSIS A tront view ot the completed receiver From left to right the controls along the bottom are standby switch regeneration oscillator band set tuning aerial bandset audio volume phone jack The switch above the phone jack selects phone or loudspeaker output To the left of the dial is the I F gain control with the tone switch on the right The dial can be chosen to meet indivi dual taste but it should have a smooth positive action REAR VIEW OF RECEIVER Ww This rear view shows how the asrial terminals coil wiring tuning condenser and converter valve assembly afe all supported above the chassis in the interests of short leads The busbar from the aerial tuning condenser can be seen running straight down through the chassis to the aerial bandset condenser The oscillator condensers are similarly connected There is useful space for a beat o
26. ters 0 5 meg ohms 2500 ohms 3 position S P rotary switch S P S T toggle switch S P D T toggle switch Knobs 2 pointer knobs Phone jack or terminals 25 000 ohm voltage divider 8 mfd electrolytic cond 600 P V 16 mfd electro cond 525 P V IO mfd electro cond 40 P V 0 1 mfd tubular cond 05 mfd tubular cond Ol mfd tubular cond Ol mfd mica cond Osc bypass 00 amp mfd mica cond WAI N W NNN 10 000 PAGE EIGHTEEN PARTS LIST for an ordinary duai wave e7 Some concession to shortwave verformance may be made in the latter Dut the economics of production and the re quirements for domestic broadcast re ception always remain the gorerzing factors TUNING RATIOS For example a large tuning eoz denser gang is necessary to cover tne broadcast band and this same con denser is used for short waves The resultant shortwave band coverage i broad enough to constitute a goo selling feature but the tuning from station is necessarily very sharp Stations in the various international broadcast bands likewise the amateur bands are packed tightly _ together and the greatest care is necessary to 00025 mfd mica cond 000 mfd mica cond 00005 mfd mica cond 1 0 meg resistor 25 meg resistor 50 000 ohm resistor 20 000 ahm resistor 5000 ohm resistor 400 ohm resistor W W 300 ohm resistors W W 250 ohm resistor W W SP
27. ut To cover the whole shortwave spec trum therefore one may need several distinct sets of coils for as many bands But the shortwave enthusiast is not greatly worried over this be cause it enables him to separate and record the many stations to be heard Furthermore he knows that the small condensers and relatively large coils improve the electrical efficiency of the tuning circuits BAND SREAD Receivers for purely amateur band work go one step further The amateur is interested primar ily in identifying stations in the allotted bands Other shortwave stations are of secondary interest Most amateur receivers are there fore designed in such a way that the relatively narrow band of frequencies is spread over a goodly part of the dial scale What appears to be a meaningless jumble of ham stations over a quarter inch of the usual dual wave scale is resolved into a much more orderly array by a good amateur re ceiver More concrete discussion of this prob lem will follow when we come to dis cuss this month s receiver in detail Now a word about the size of an ama teur set It is possible to achieve bandspread characteristics quite simply on a smal regenerative set but generally speak ing something more pretentious is required for serious amateur or short wave listening Many amateurs in deed operate receivers with anything up to a dozen odd valves and bristling with special features V
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