Learn How to Series - Coils i.f & r.f

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How To Series: About - Coils I.F and R.F

Coils - I.F Transformers

The performance of a superheterodyne receiver is governed to a large extent by its I.F. system. Depending on the size and complexity of the radio, there will be one, two or three sets of I.F. coupling coils, known universally as I.F. transformers. Because I.F. amplifiers contribute a lot of amplification, their transformers generally have, with the infamous exception of the Philips V7A or Theatrette, individual shields in the form of cans and the range of styles, shapes and sizes is remarkable. Many manufacturers produced their own distinctive pattern, ranging from the large round cans of the early 1930's through the common square type to the Philips midget flat rectangular style of the 195O's. Atwater Kent used a neat round can with a lid, which could be removed to gain access to the trimmers.

General practice was to use two tuned circuits for each transformer, but exceptions will be found, particularly in imported receivers. This is not the place for heavy theory but it is important to realise that a pair of tuned circuits in close proximity and resonant at the same frequency have some very special characteristics and are used in the case of I.F. amplifiers to achieve a reasonably uniform response over their operating range but with a very steep sided selectivity curve thereafter. This cannot be done with single tuned circuits. Overall bandwidth of I.F. transformers is dependent on the spacing of the windings. Some designers used a combination of a double and a single tuned transformer to confer the required characteristics. The single winding unit was normally the second I.F. and this type will occasionally have a close-coupled untuned secondary. In some multi chassis Philips sets, two single tuned transformers were coupled by a low impedance link. By, in effect, splitting the transformer in two, it was possible to have considerable physical separation between the mixer and I.F. amplifier without instability or inefficiency.

Fortunately for the beginner, most receivers are less complicated and have a pair of the conventional transformers, but not so fortunately, there is no standardized operating frequency. European receivers with a long wave band often will be found with an I.F. frequency of around 125.kHz whilst the early Atwater Kents used 130.kHz. During the early 1930's, we almost had a standard of 175.kHz but the advent of shortwave receivers saw the frequency go up to around 460.kHz with a few trying out the 260.kHz region on the way. Of the higher frequencies, common examples are 450, 455, 456, 460 and 465.kHz. These are not the only ones to be found and, with charcterist1c independence, Atwater Kent settled on 472.5 kHz. Some Australian Stromberg-Carlson receivers used 392.kHz!

Various construction methods are to be found. Early patterns consisted of two pie windings on a wooden dowel supported horizontally beneath a block containing the mica dielectric trimmers. Soon the dowel was mounted vertically to reduce the diameter of the can, which commonly became square. Around the mid 1930's, iron dust slugs began to be used inside the former to increase efficiency. By 1950, adjustable trimmers had been superseded by fixed capacitors and adjustable slugs. Later practice was to mount the coils alongside each other, rather than on a common former. In the process of development, the size of I.F. transformers became steadily smaller, and during the last decade of valve receiver manufacture in New Zealand, the compact and very efficient types made by Philips were used extensively. It is worth noting too that during this period a large range of excellent I.F.s was produced by the Tauranga firm of Inductance Specialists, particularly' for replacement and home construction work. These units with the "Q" brand are often encountered as replacements.

This brings us to the servicing aspects of I.F. transformers. Many makes have proved to be most reliable, but some brands of receiver are notorious for failures. One example (but not the only one,) is the series made by Wells Gardner for the American Gulbransen receivers. It is very common to find open circuited coils or replacement units already in these chassis. What happens in cases like this is that the wire in a winding has corroded at a "green spot". Detection is easy in the case of anode connected coils. The receiver is generally very dead with no anode voltage on the mixer or an I.F. amplifier. Secondary winding faults can be less obvious"; but a resistance check should reveal a problem. It is impossible to give a firm figure but a measurement of more than 100 ohms should be suspect. Surprisingly, tests have shown that open strands of the Litzendraht wire commonly used for I.F. windings do not appreciably affect efficiency but even so the corrosion which caused the initial trouble could well be continuing. The transformers built for 175kHz were often wound with solid wire and prone to open windings.

Fortunately, the exact operating frequency of substitute I.F. transformers is not critical. . A transformer made for 450kHz. is likely to tune to 465kHz quite satisfactorily and iron and air cored types will interchange, but of course, a 450kHz. model can't be used as a replacement for 175kHz. Unfortunately, previous servicing efforts may have replaced an original transformer with one with an obviously different can. It is sometimes puzzling to find that new holes have been made to accommodate a replacement when it would have been simpler and quicker to transfer the new "innards" to the original can. The restorer can often do this, if necessary leaving the original trimmer assembly in position. If this is done, it is most important to have the windings in the correct phase relationship. If the grid and anode leads are to be the outer or finishing connections to the coils, then the coils should be wound in opposite directions.

I.F. trimmers can sometimes cause a frustrating fault. At intervals, the gain of the receiver will drop and the usual intermittent fault chasing is unsuccessful. This can be due to faulty soldering of a trimmer leaf. The result is for the effected. circuit to jump in and out of resonance erratically. If a transformer is dismantled at any time it is a good idea to check the trimmer soldering. This can also happen with fixed capacitors used in slug tuned I.F.’s but of course the problem cannot be picked up visually. Substitution of the suspect capacitor is probably the quickest way of locating the fault.
Transformers using adjustable slugs should be treated very carefully. Slugs incorporating slots or tool sockets are easily damaged. They should be adjusted only if this proves to be essential and then very carefully. Apart, from the slug fragility, the threads that they run in are frequently minimal and excess pressure can do a lot of damage. Often the slugs are sealed in wax. If it is essential to move the slug, the wax can often be softened with the aid of a hot screwdriver.

Coils - R.F

All radios have tuned circuits. They are fundamental to operation and the very early receivers had little else. Performance is closely related to the design and operation of these components and consequently their condition is of considerable importance. I.F. transformers, which are a specialized form of tuned circuit, were covered in the previous article. Tuning inductances, known universally as "coils", have undergone considerable change since the early period when monster cylinders several inches in diameter were used to achieve the maximum of efficiency. Around the mid 1930's single layer windings for broadcast band use gave way to multilayer "pies" but generally those for the intermediate and shortwave bands retained the single layer winding. If you are not familiar with these components, study of the underside of a typical multiband receiver is suggested. Connected to the wavechange switch will be the various coils. Those for the broadcast band will be seen to have bunched or pie windings whilst the shortwave coils will be single layer with spaced turns. The intermediate band coils may have a pie wound primary and a single layer secondary. Generally, there' will be two windings per coil, and in the shortwave coils they may be inter wound, the primary winding having the fewer turns of finer wire. Broadcast band aerial and R.F. coils usually have larger primary than secondary windings. This results in more even gain over the tuning range and avoids aerial changes upsetting tracking.

A special aerial coil is commonly found in older receivers that have low frequency I.F. systems but are without an R.F. stage. There is a second tuned winding with an associated tuning capacitor sect ion to improve image rejection. These receivers can be recognized by their having a three gang tuning capacitor and no R.F. amplifier valve. The aerial coil has only one primary or aerial winding, and two identical and separate secondaries.

Some aerial windings have a resistor of about 10,000 ohms connected across them. The reason for this is interesting. Aerial windings are normally self-resonant at a frequency just below the broadcast band. The exact frequency depends on the capacitance of the aerial but, in some designs, with little or no aerial connected, this coincides with that of the I.F. frequency. Coupling between the wiring of the I.F. amplifier and the aerial lead may cause instability in the form of a whistle behind each transmission. The resistor minimises this instability. Later patterns of R.F. coils generally have iron cores, often adjustable. Some receivers, especially car radios and at least one model of the humble Bell Colt, dispensed with the traditional tuning capacitor and used a mechanical linkage to the cores to tune the receiver.

The most likely coil fault is an open circuited winding. One common cause when radios had real aerials was burnt out primary windings after a thunderstorm! An outside aerial is capable of absorbing a lot of energy from a nearby lightning flash and this was one reason for the fitting of the traditional lightning arrester. Other windings can become open circuited from corrosion. Symptoms are poor performance or, in the case of a defective oscillator coil, no reception at all. Resistance checks will identify open windings, and, as with I.F. transformers, specific values of resistances cannot be given, but anything over 50 ohms should be suspect. It is not normally necessary to disconnect windings before measuring their resistances, but try to check right at the terminals. Be wary of wavechange switches as poor contacts can produce the same symptoms as open windings.

An elusive fault can occur in receivers with an R.F. stage. Symptoms are a general lack of performance with nothing much apparently wrong. Short circuiting the A.G.C. line to earth may improve the sensitivity considerably. R.F. coils with high impedance primaries require a few picofarads coupling the anode to the following grid. Often the capacitor is not obvious because it consists of a single turn of fine wire wound directly over the grid winding and, if covered with wax, it is hidden. As the primary is connected to the H.T. supply, and as the grid winding is likely to be connected to the A.G.C. line, any leakage between them will cause a positive voltage to appear on the A.G.C. line. Normal voltage checks will not reveal this voltages as the grids will act like clamping diodes, holding the A.G.C. line at cathode potential. However, the grids will draw current that reduces the gain of the receiver significantly. A similar problem can result from leakage across the wavechange switch contacts. A more precise way to check for this fault is pullout all -the valves except the rectifier. If any positive voltage is measured at the control grid terminal of the mixer, leakage is likely. Another fault that can produce similar symptoms is a leaking A.G.C. feed capacitor but this should be picked up in capacitor checks to be described in a later article.

As oscillators operate at a higher frequency than the received signal, their tuned windings have fewer turns than their associated R.F. coils. Oscillator coils are generally reliable but a fault will result in no signals being received on the affected band. A simple test can detect an inoperative oscillator. With the receiver tuned to the lower frequency end of the band, another receiver in close proximity can be used to pick up the oscillator. An unmodulated carrier will be found at the frequency of the dial reading plus the I.F. Check with a working receiver to get the idea. This method can also be used to ascertain an unknown I.F. frequency.

If an exact replacement coil is not available, the best approach is to use one with an adjustable slug. Universal replacement coils were made with just this feature, but often those receivers will be satisfactory provided that they are adjustable. Alignment methods will be given in a future article.




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