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Aviation History
1961
1961 - 1434.PDF
Above, the automatic crystal switching unit of the AD 360 compared with the earlier mechanical unit on its right. Left, sealed modular construc- tion of the AD 260 VHF navigation radio Aviation Electronics . . . the rating is used in the transistorized circuit an improvement inlife of thirty times will be obtained. A good example of the benefits derived from low voltageoperation, even in a valve equipment, is the Marconi AD 722 sub- miniature radio compass. This set uses standard valves but they arerun at 28V HT. From a component rating point of view the set is very much like a transistorized one, though it still has the hightemperatures of a valve equipment. The AD 722 has shown excep- tional reliability in service, resulting undoubtedly to a considerableextent from the reduction in electrical load on the components. On the subject of heat, it should be remembered that in a valveequipment, even when the average temperature is not very high, there can be intense local hot spots. In a valve equipment the heatis generated inside the box, whereas in a transistor set the only function of cooling is to keep external heat from getting in. A further advantage of transistors is that they allow muchmore consistent performance to be achieved in navigation equip- ment, because internally stabilized supplies eliminate all errorscaused by aircraft supply variations. In the Sixty Series special care has also been taken to minimize drifts due to time and temper-ature variations, which has resulted in greatly improved accuracies in both VOR and ILS. Such an improvement is particularlyimportant when the equipment is part of an autolanding system. Sealed modules The remaining environmental faults are either contact troubles or deterioration of components caused by the environment. Ex- amples of contact troubles are intermittencies of relays, plugs and sockets due to vibration, and the contamination of switch contacts. Contact problems can best be dealt with by eliminating the contacts, and much can be done by replacing relays and switches by elec- tronic circuits using transistors and diodes—more expensive, but far more reliable. Screw contacts are also much more reliable than plugs and sockets. As for the environment, the best solution is to keep it out, which leads us directly to the concept of the sealed module. Previous experience with sealed units has not been uniformly happy, and there are three basic rules which must be respected if the technique is to be successful. First, the modules must not rely on sealing for their proper functioning; secondly, it must be possible to test them fully without unsealing them; thirdly, they must be easy to unseal and reseal if there is a fault. The sealed modules used in the Sixty Series are designed to meet all these requirements. The leads are brought out on screw contacts instead of a plug and socket, and each contact is a test point, so that modules can be checked without even taking them off the chassis. The sealing area is small—only two inches square—and the seal is made with a moulded neoprene gasket; only four screws are used for fixing. The modules are filled with nitrogen, to give a dry inert atmosphere, though this is not essential. Nitrogen filling is, however, very simple, requiring no special equipment, and gives an added measure of protection. Compact, but accessible, the Marconi 6401 VHF receiver incorporates circuit boards which fold out from the main chassis Such a sealed module has virtually all the advantages of encap-sulated circuits, but with the big difference that it does not have to be thrown away if a fault develops. Furthermore, circuit modi-fications can be incorporated. However reliable components are, their values will change duringthe course of their life; this is in addition to the normal manu- facturing tolerances. If a circuit is to be reliable it must be designedto accept these changes. Part of the Sixty Series reliability pro- gramme was to test circuits under all environmental conditionsat the extremes of component tolerances, applying additional factors for ageing. This technique is equally applicable to mechanical design, andthe mechanisms used in the Sixty Series were checked at their extreme tolerances with artificially induced wear. The design Moving parts tend, by their very nature, to be less reliable thanstatic ones, and much has been done in the Sixty Series to replace mechanisms by electronic circuits. For example, there are no movingparts, not even relays, in the marker receiver; and in the AD 360 ADF receiver the original crystal switching unit, consisting ofthree switch banks, three electro-magnetic clutches, a gearbox and a motor, has been replaced by a diode matrix. Nevertheless, there are some cases where mechanisms are thebest way of doing a job. For example, a moving Doppler aerial gives much greater accuracy than a fixed one. In these cases, theuse of new techniques and materials can greatly improve reliability. In the mechanical tuning system of the AD 360 ADF receiver thebearings are made out of sintered PTFE/bronze, requiring no lubrication and no attention during their life. In miniaturized equipment, particularly when it is transis-torized, the problems of maintenance need very special consider- ation. Electronic equipment generally works best if it is left aloneand there is a laudable trend in airlines towards "on condition" maintenance, rather than overhaul at regular intervals. Thisbecomes feasible with the reduction of mechanical parts and the construction of those remaining in such a way as to need littleroutine maintenance. When a set does fail it is essential that the fault should be easy to locate and repairable with the minimumdisturbance to the rest of the equipment. In the Sixty Series great attention has been paid to accessibility of components and theprovision of adequate test points. Sealed modular construction is a great advantage in mainten-ance, because the faulty module can be identified without any danger of damaging the circuitry, and then unsealed for repair. Sets canalso be serviced rapidly by replacement of modules, the faulty module being repaired later. It is enclosed in a rigid case and willnot be damaged in transit. A further advantage is that sealed modules have a virtually indefinite storage life. Reliability of manufacture Elimination of the major causes of failure throws into reliefthe minor ones, so that even greater attention must be paid to the quality of manufacture. Methods of component selection andtesting and quality control procedures have been thoroughly reviewed to ensure the maximum consistency of the finished product. It is also vital that there should be a rapid feed-back of infor-mation from the user to the manufacturer so that improvements, based on field experience, can be incorporated quickly. An efficientservice organization, in constant touch with the user, is an essential part of any reliability programme. Theoretical assessments of the probable reliability of equipmentcan be made, but the reasons why sets fail are complex and the estimates are often wide of the mark. Ultimately, the proof ofthe pudding can only be in the eating. All that can be said is that, in the design of the Sixty Series, a really thorough attempt has beenmade to analyse all the causes of failure and to do something constructive about every one of them. The introduction of trans-istors has provided the opportunity for a completely new approach to the problems of reliability. It is certain that much will have beengained. Exactly how much, only time will show. The author wishes to thank the Engineer-in-Chief, Marconi's Wireless Telegraph Co Ltd, for permission to publish this article.
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