1993-06: IF amplifiers and their transformers
Contents
1. of frequencies will be passed between windings and severe sideband reduction occurs limiting audio bandwidth in receivers using nor mal intermediate frequencies The bandwidth and signal transfer increases as the windings are brought closer to gether until at the spacing called the critical coupling point the response curve is more broadly peaked and en ergy transfer is at a maximum Critical coupling is frequently used i in IF transformers as it gives a good com promise between selectivity and reason Fig 1 Valve IF transformers came in many shapes and sizes and many manufacturers had their own distinctive patterns in this group from the author s junk box there are cans made from aluminium copper and zinc 92 ELECTRONICS Australia June 1993 Fig 3 IF transformer contents varied as much as their cans The example at far left has a single tuned winding closely coupled to an untuned secondary the rest all have two separate tuned windings with either trimmers or slugs able quality If however coupling is fur ther increased an unusual situation oc curs A peak appears symmetrically either side of the centre frequency and as coupling is further increased these peaks shift further apart Fig 2 This character istic is used to good effect in broadening the IF response in high fidelity or wideband receivers No standard IF There is really no standard frequency for IF amplifiers The first untune2. primary and secondary is in creased and the tuning broadened Servicing IF s Electrically IF amplifiers are basically simple and apart from the usual valve resistor and capacitor faults the most common problem is open circuited or green spotted transformer windings Low frequency IF transformers gener ally wound with single strand wire were very prone to this type of fault Open primary windings are easily found there is no voltage at the anode of the associated valve and the receiver is very dead Faulty secondary windings may have sufficient leakage for the re ceiver to work after a fashion but perfor mance will be very substandard GENALEX BC _ 530 The easiest method of finding open windings is a resistance check a proce dure which should be carried out as a routine anyway Be suspicious of wind ings that have significantly higher resist ances than their companions This could indicate that there are broken strands of Litz wire resulting from a developing green corrosion spot Alternatively there may be a bad termination to the winding With each fine strand insulated with enamel Litz wire is notoriously hard to solder Various methods of removing the enamel have been suggested but the most reliable is very fine abrasive paper used very gently Unexpectedly tests have shown that the effect of a few bro ken strands is not serious Puzzling faults IF tuning capacitors can provide some p
3. the maker s instructions should be obtained if at all possible However in an emergency and provided that a signal generator is available and the set s intermediate frequency is known some alignment can be done Clip a high resistance digital voltmeter or a vacuum tube voltmeter to the AGC line or diode load resistor Inject a signal at the set s intermediate frequency into the control grid of the frequency con verter sufficient to give a reading on the meter of between 2V and 3V Now keep ing the injected signal always at a mini mum to give a meter reading peak each trimmer in succession Powdered iron cores are very brittle and are easy to damage irreparably Therefore never adjust them with a steel screwdriver but use a plastic alignment tool or a piece of sharpened plastic knit ting needle or even hardwood amp Collector s Corner Where readers display prized items of radios and other equipment from their collections and or seek information from other collectors Club members make Little General copies Rodney Champness who is publicity offi cer for the North East Vintage Radio Club based in Wangaratta Victoria tells me that this year the Club members set them selves the challenge of building replicas of the Little General receiver first described by John Moyle in the April 1940 issue of this magazine He says that the idea of doing so came from the articles we ran last year by
4. were usually mounted on a wooden dowel and attached to a block of insulating material containing adjustable trimmer capacitors of the mica dielectric variety Later the dowel was mounted vertically to permit a reduction in the diameter of the shield can Domestic receiver transformers using variable capacitance tuning invari ably used mica dielectric trimmer capac itors usually mounted in a ceramic or ELECTRONICS Australia June 1993 93 VINTAGE RADIO bakelite block Fixed capacitors were frequently used to provide additional ca pacitance In the mid 1930 s iron dust slug cores were introduced to raise efficiency by reducing the amount of wire needed Later the position of the iron cores was made adjustable for tuning rather than using trimmers These variable induc tance tuned transformer windings always have a fixed capacitor generally mica or ceramic dielectric with polystyrene found in some modern transformers Another development was to turn the coils through 90 so that they were alongside each other rather than axially on a common former Later still wind ings were enclosed in ferrite pots Some traps The position of iron cores in trans formers wound on a common former is important These transformers can be recognised by their having an adjustment screw at each end As can be seen in Fig 4 there are two positions of each core one either side of its winding where the inductance and tuni
5. Both are tuned to the same frequency but the IFT on the left s cores are in the incorrect inner position the frequency converter It is therefore usual for a receiver with 175kHz IF am plification to have a three gang tuning capacitor regardless of whether or not there is an RF amplifier stage Despite some efforts at standardisa tion there was no consistency in the choice of intermediate frequency A quick check through the Australian Offi cial Radio Service Manual for 1938 shows no less than 15 different frequen cies ranging from 175kHz to 470kHz in use for that year s models Most were in the range 446 to 470kHz with Stromberg Carlson using the very odd frequency of 392kHz As can be seen in Fig 3 many methods of IF transformer construction have been used but windings were invariably in separate pies with higher frequency transformers wound with stranded wire High frequency currents do not travel uniformly through conductors but are concentrated on the surface by skin effect which reduces the efficiency of coils This effect is minimised by the use of Litzendraht wire known universally as Litz wire Litz wire is a braided cable made up of strands of fine wire woven so that each strand passes from the centre to the out side at regular intervals This forces each strand to carry its share of the current and so lowers the RF resistance of the coil Early IF transformer windings
6. St Camperdown Electronics Sale Repairs Resurrection and Overhaul of valve radios audio gear instrumentation i 12 month warranty on all work i Parts sales circuit copies custom construction of valve gear VINTAGE M IRELESS RADIO CO NSW 2050 rhone Dol 221 2 READER INFO NO oe CRAAR RRA Aee a e a ata e a a e eeta a a a a e a d a oo oTe ar a aTe E e o a e a o o e a o T tate a atot POS PE EDE BE AAD D A E O E E E DO 0 AE PEE aE EE A a E at a e a a OO OO a
7. Vintage Radio by PETER LANKSHEAR IF amplifiers and their transformers Superheterodynes have such important advantages over simpler types of receiver that for the past half century their use has been practically universal This month we look at intermediate frequency or IF amplifiers the key to the successful performance of any superhet The supersonic heterodyne receiver to give it its full name had its origins in World War 1 There is evidence to show that the French pioneered the superhet erodyne concept but Major Edwin Arm strong of the American Army Signal Corps is generally credited with creating the first working examples and his work for General Electric enabled RCA to market the first domestic models Initially the problem that led to the invention of the superhet was that of ob taining any useful amplification at fre quencies around 1MHz and higher with the triode valves that were then avail able The solution embodied in the superhet was to use the principle of beating whereby the received signal was combined with a continuous locally generated signal to create a third lower frequency beat or heterodyne Musi cians have of course for centuries known about the production of a third note from two others by aural beating This new signal of much lower fre quency although still above the audio range i e supersonic could then be amplified and detected by conventional met
8. both Peter Lankshear and Neville Williams telling the story behind the design The set judged best was to be awarded the Club s annual Hellier Award named in honour of Mr Les Hellier who built the town s radio station 3WR and began broad casting on the 25th February 1925 it was apparenily the first licensed station in any of Australia s country towns Some 16 Club members accepted the challenge and set about scrounging parts making chassis and building cabinets When judging took place the winning set was a replica of the 1947 Little General built by Ralph Robertson of Kyabram and pic tured both in the front centre of our main shot above and at lower right The other sets in the main shot are top centre Rodney s own version of the 1957 model which won second prize left Noel Meagher s version of the 1961 model and right Bob Young s replica of the 1947 model The last two sets tied for third place At upper right is a shot of another Little General built by a Club member and housed in a cathedral style cabinet Rod ney says that the builder is a skilled wood worker and the cabinet is magnificently finished Rodney says that the NEVAC caters for anyone with an interest in early radio from technical and restoration work to collection of magazines and recordings etc Further information is available by writing to him at 17 Helms Court Benalla 3672 or phone 057 62 1454 E Australia
9. d trans formers favoured frequencies of around 50kHz but by 1930 175kHz tuned IF transformers had become popular and were used to a certain extent throughout the decade This frequency provided sta ble high gain and plenty of selectivity and may well have become the industry standard frequency But within a couple of years shortwave listening had be come popular and for multiband receiv Fig 2 The effect of coil spacing on the frequencies transferred by coupled tuned circuits as in an IF transformer A is optimum or critical coupling C shows under and B over coupling ers 175kHz was found to be too low for receiver RF circuits to separate funda mental signals from their heterodyning images or second spots Consequently shortwave stations appeared on two places on the dial To minimise this problem intermedi ate frequencies were raised generally to the region of 450 475kHz as high as was possible without encroaching on the broadcast band These frequencies are still used All wave European and English re ceivers had a problem because of Europe s longwave broadcasting band on 150 300kHz To simplify tracking many had an IF of only 110 125kHz and there was a serious image problem in these radios Even standard broadcast band images are a potential problem with IF systems operating below about 250kHz and in these cases two RF tuned circuits will frequently be found ahead of Fig 4
10. hods and Armstrong originally used a resistance coupled amplifier Known as the intermediate frequency amplifier and soon shortened to the IF this sec tion of a receiver has a major influence on receiver performance Initially the purpose of the IF ampli fier was simply to provide gain and re sistance capacity or untuned transformer coupling was adequate But by the late 1920 s it was realised that tuned circuits would confer considerable advantages in terms of higher gain and controlled se lectivity Depending on their size and complex ity domestic valve receivers had one and sometimes two stages of IF amplifica tion coupled by coupled tuned circuits known universally as IF transformers With rare exceptions these were con tained in individual shield cans with a remarkable range of styles shapes and sizes round square and rectangular with the older types usually having the largest cans Fig 1 A major advance in size reduction and efficiency was made in the late 1930 s with the introduction of powdered iron cores for the windings More recently ferrite cores have become universal Bandpass tuning Although variations will be found general practice was to use two tuned circuits for each transformer A pair of tuned circuits in close proximity and res onant at the same frequency display some very special characteristics With the windings well spaced only signals in a narrow range
11. ng will be correct providing four possible combi nations of core positions These positions can considerably influence the coupling between primary and secondary with significant changes to the bandwidth Individual transformers vary and if at all possible the manufacturer s align ment instructions should be obtained and carefully followed Transformers with in line windings and which can be recognised by their adjusting screws being at the sides of the cases do not MIXER 6G5 VISUAL TUNING INDICATOR INTERMEDIATE FREQUENCY 458 KC 94 ELECTRONICS Australia June 1993 have this problem and it was probably a major reason for their introduction Special types Most IF transformers have the conven tional pair of windings but occasionally other patterns will be encountered Sometimes only the primary winding will be tuned This type of transformer is easily recognised by the windings being much closer together than normal with only one adjustment or there may be only one winding Three winding IF transformers have been used occasionally Although tuned the third winding used to increase se lectivity has no external connections More common are transformers used in some large receivers with variable se lectivity a good example being the 1938 Tasma Genelex 580 As can be seen from Fig 5 a few switched turns of the sec ondary are wound over the primary winding When in circuit coupling be tween
12. r If an exact replacement is not available it is often possible to insert the internals of another make into the original can For tunately most transformers will operate satisfactorily over a range of frequencies and a unit intended for say 455kHz will usually operate quite well at any fre quency from 450 to 470kHz Never throw out a junked chassis without first salvaging the IF transformers When working with IF transformer windings it is essential to observe the sequence of connections Swapping the connections of a winding can reduce gain considerably Assuming that the windings are in the same direction if a grid or diode lead is connected to the start of one winding then the anode will be connected to the end of the other winding and vice versa A warning to beginners IF amplifiers are normally very stable so do not at tempt re alignment unless you sure that the receiver is otherwise working cor rectly and you are reasonably sure that settings have been tampered with Even after many years of use most IF adjust ments will be found to be correct Unfortunately though as any service man will tell you accessible IF align ment screws seem to hold a fatal attraction for non technical people at tempting to get a set working Many manufacturers sealed adjusting screws with wax and in these cases chances are that the alignment will not have been tampered with If it really is necessary to realign an IF amplifier
13. uzzling faults Defective soldering of the leaves of mica compression capaci tors can produce intermittent drops in level and rivets can provide unreliable contacts Moulded mica capacitors can have intermittent changes of capacitance too creating frustrating faults Ceramic capacitors used with slug tuned windings can occasionally change to a lower value and provide real traps The slug appears to go through reso nance as it passes through the centre of the coil but the overall gain remains low This condition can be recognised by there being only one peak whereas nor mally there are two Iron cores can become disconnected from their adjusting screws and move inside the former The symptoms are an inability to tune the offending winding and the gain of the receiver can vary if the chassis is tipped or up ended It pays IF AMP 2 DET AVC I AUDIO OUTPUT 6U7G 6G8G SPEAKER FIELD 2000 n RS 6V6G Fig 5 In the Tasma Genelex BC530 vari able selectivity was ob tained by winding afew turns of each IF transformer s second ary over its primary winding With the Switch in the upper po sition selectivity was normal coupling and bandwidth increased progressively in the sear two lower positions PLUG to investigate an IF transformer where a tuning control has no effect Fitting replacements Nothing stands out to the experienced eye like a foreign IF transforme
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