Mainly About Construction

Radio Record, Volume II, Issue 21, 7 December 1928, Page 26

Mainly About Construction

Trouble Tracking on the F.F.

(JOMPARED with almost any other ' wireless component the high-fre-yuency transformer is a’ trouble-free ind blameless piece of apparatus. Being in esséntials merely several coils of wire properly spaced and. brought out to pins for connection to the rest of the set, it. can hardly fail to work provided it is of suitable type. Yet for all its simplicity the work it carries out is of a highly complicated character, ‘so that it is quite possible for a small fault in an H.F, transformer to make a big difference to the working pf a set. ; H.¥F. transformer troubles are best flealt with according to the way in which they affect reception. It is not very often that a fault in a transformer completely. stops reception, but it will very often make.a stable set unstable, give rise to flat tuning, or in other similar ways make the set misbehave. A good instance of the type of fault which a faulty HF. transformer will cause is the one in which the symptoms are that the set will not neutralise. Generally, in such a case, the set does not fail to work altogether, but it works in an abnormal way, with peculiar symptoms, Why Won’t It Neutralise? 1B the ordinary course of events, a . receiver which contains an H.F. stage will give pretty good _ results, even if the neutralising is not properly earried out, Failure to neutralise. probably means that when the circuits are exactly in tune with one another the set is. prone to oscillat at that point; anid when the condenser nettralising condenser is readjusted to its correct position this instability disappears and one set becomes: perfectly easy to handle. =. ‘Not infrequently, however, one comes across a set which simply will not neutralise properly. Adjustment of the neutralising condenser results in the

most violent yells and whistles imagin‘able. Usually there is one pretty well defined point where this tendency to instability is ata minimum, but if the neutralising condenser is moved away from that point violent. oscillation sets

in and frequently spoils reception. In other words, the neutralising condenser appears to be acting rather like a reaction condenser, except for the. fact that one cannot reduce reaction suffi: ciently by it,.and the reaction effects are altogether too ‘lively. Such symptoms .are very often due to the fact that the connections to tho neutralising windings have been re

‘versed. If the coil is 4 home-made-one, matters ‘can soon be remédied by undoing it and changing over the two: ends of the neutralising winding. Another puzzling fault is the appearance of unusually. flat tuning. Generally, { the aerial cofidenser appears to ei rs PD eet re te ated

tune quite well, but the tuning on the high-frequency condenser is inéxplicably broad and instéad of stations .being confined to one or two degrees on the dial they dre spread out over quite a wide area. | When the N.C. Goes oni Strike. ERY often in such cases the set appears to neutralise fairly well at one point, but not over the whole range. (Wat tutting of this typé very often occurs where 4 h mé-made H.F, transformer is employed, or it thay be due to the tise of a very high imped: ance R.C. valve.) ) Such symptoms generally arise. because the primary winding has too many turns for its particudlr valve, so in all such eases 4 reduction in the humber of tirns of the primary winding should be tried. All that is necessary is to take oif a few turns at a time, say, half a dogen, until the desired degree of selectivity has been obtained, ink h inant

Another rather puzzling fault that high-frequencey ‘ amplifying séts are: prone to dévélop is a"general falling- . -off in: the sharpness and degreé of high-. frequency amplification. When the set is neutralised in an attempt to restore good amplification, it is discovered that adjustment of the néutralising condenser makey no difference whatever. . In such cages, whatever the position’ of the vaties--whether all in or all out -no -difference’to results is apparent. Very often the set is unduly lively ‘When the H.F. circuits are in tune, and, in fact, it may oscillate badly at this point. If you are experiéncing this type of tréuble you will probably: find it is due to a break in the neutralising winding. Hither the wite itself has broken, o1' is making unsatisfactory connection to the rim of the tratisfortner, or else the pin of the transformet Which carries the neutralising winding is failing to make proper contact with the rest of the set. Consequently, the sét is béhavitig as though. it wére not hetitralised at all, and when. ‘the connéctions are restored its beha-_ viout shotild become:the normal one of a properly netitralised receiver, ‘Weak Signals and Ohoking. NOTHER. peculiar fault worth men‘tioning .has ‘a Similar cause, but quite different, effect. In this case the main trouble is weak signals. Probably the set has no tendency to oscillate at all, and it may, éven Show. signs: of choking. In this latter condition, reception, which is not too good to start, seems to fade away periodi--cally, the stations getting . ‘weaker dnd weaker until they disappear altogether. Generally, one finds that if one of the wires joining the grid-leak or condenser is touched, the set ‘shows signs of ‘life- again, only to "pop off" again at the shortest notice. The word "choking’ seems exactly to describe this condition, for the éffect is as though the set were trying to‘ wotk all right, but the signals are edntinually choked back and suffocated. Very often this state of affairs. ean be traced to the fact that there is a ‘preak in thé secondary winding of the H. ¥, transformer. ‘Possibly the wire ‘is continuous throughout, but is not making propér contact to one of the pins, or one of the pins is not making proper contact to its corresponding -socket to which © the rest, of the circuit is wired. a an ee ae aer | ae