To Cure "Overlap"

Radio Record, Volume IV, Issue 14, 17 October 1930, Page 26

To Cure "Overlap"

A Common Fault

(QQNE little trouble which is not very clearly undérstood by many people is the reaction irregularity known as "overlap," and it may be useful to some readers to try to explain just what it is. In a normally working set you should find that if you gradually increase the reaction condenser setting: the set will presently break into dscillation at a certain reading, and will ‘stop again at the same reading if you reduce reaction once more. / For example, if you find that the set goes into oscillation as the reaction condenser passes the 50 degree mark, it will stop again when the condenser passes 50 degrees once .more as you bring it back. That is what happens in a set behaving properly, In one afflicted with overlap you would find that although it might go inté"oscillation as the reaction condenser reaches, say, 50 degrees, it will not stop again uutil the reaction is slacked off "to perhaps 45 degrees. The two points overlap, hence the name, Where the overlap is.at all serious it can be an extremely annoying fault, and in a set depending-to any great extent on properly applied reaction for its sensitivity, can make long-distance reception almost impossible, The trouble

ee = ’ a eed ry wot ery ry c is that if you adjust the set to its most sensitive ‘state a little below the oscil» lation pdint it is working in a thorough-, ly: unstable condition. If a strong mospheric comes along the‘ set will probably break into oscillation, and. will not stop again until you slack backs the reaction considerably. : Each time you have to do this "OF course, you probably lose the station altogether, because by the time the Set stops oscillating the reaction has beén reduced so much that it is a long way off its most sensitive adjustment... You then have to bring it up once -more to the sensitive setting, just short of oscillation, presently it goes over:' the edge once more, and you begin again. Altogether a most annoying aes both for yourself and for listening to the same station in your neighbourhood, It is not, you will see, the same thing as "ploppy" reaction, although the causes are much the same. Probably it should be regarded’ as a more acute form of the same complaint, ploppiness run wild, so to speak. ... Here are. some of the iikely from which you can pick out the ones most likely to be responsible in any particular case; wrong size of reaction coil, wrong B voltage, unsuitable detector valve, detector valve losing emisSion, grid leak of wrong resistance for valve, faulty grid leak, B battery run down, coupling troubles in B eliminator, radio-frequency currents leaking into audio-frequency stages, faulty’ Kighfrequency : choke, and a run- down (A battery. . by LSS22NERS. must _ attach ‘this ,/9 coupon to all queries sent: to": the. Technical Editor (Box. 1082, Wellington), Questions: arriving without it are likely to go astray or be delayed. . Name of set occ scecacccccccccesicoss Number of Valves secececesesscescs Name Oo esccaencceseeeseceseeores AddreSS oe ecsecccccecccccccesigeces Nom de plume ......ecccsveccecccces To be kept in subsequent inquiries. * Date Shree eee setae es eeeeeeeeerennes Please Note:(1) Be specific and brief, tabulating, if possible. (2) Write and on one side of the paper, "p (83) We do not design circuits, but accept suggestions for feature articles, . 42 Solving trouble, as different "from advice, is difficult by correspondence and while letters are ‘given every consideration, answers are not necessarily correct-they are only our opinion. based on the -:matter supplied, which may be quite inadequate. Intricate and involved specifications cannot be — supplied without a specialist’s fee.