Shortwave Club Meeting

Radio Record, Volume IV, Issue 51, 3 July 1931, Page 11

Shortwave Club Meeting

Some Interesting Questions

HE monthly meeting of the Short- | wave Club held on Thursday, June 25, was only moderately attended. | However, the weather was anything | but favourable, and this probably ac- — eounted for the absence of many. The reception of letters and routine business was proceeded with. The most interesting letter was one from a member in Hawke’s Bay, who had cleared up the French station which many

members had reported but could not identify. The chairman, Mr. R. Spence, remarked that a member of the society, Mr. G. R. McCarthy, was at the present time in hospital in a serious condition. A vote of sympathy was passed by the meeting with best wishes for a speedy recovery. The evening was one set aside for "Questions Night," to be conducted by Mr. McCarthy and Mr. Roy Clark. In the absence of the former, Mr. Clark answered the questions himself. For a few minutes, Mr. Smith, of the "Radio Record," spoke on the calcutation of time. Mr. Clark’s conduct of the queries aren to be interesting and enlightenWithout notes or preparation, Mr. Clark took his place in front of the meeting, who asked him all manner of questions. To these he gave a very full explanation, at times with blackboard illustrations. Probably the -most interesting query was one on hand capacity. A member stated that he had been troubled with this and had tried all the usual methods to overcome it, The set was completciy shielded, and the returns from all condensers were. earthed. A screen-grid valve was used, and it was.completely shielded. Different combinations of grid-leak and condensers had been tried, likewise the valves had been varied. The leads to the batteries were short and direct. Mr. Clark, in reply, stated that with hand -:apacity it was rarely as difficult to eliminate as this. Usually when the moving plates of the condensers were earthed and the battery leads kept down to a minimum,

hand eapacity disappeared. In this case it would seem that the only thiug that could be done was the erection of a false panel. In front of the ordinary panel another metal shiele should be erected and the condenser shafts extended until they could be fitted on to this. The two panels should be separated by two inches. If this panel were earthed, all hand capacity must disappear. . Speaking of battery leads raised another question. "Would it be of advantage to shorten my battery leads? They are now three feet long." Mr. Clark, in answering this. question, pointed out that the "CO" battery leads were a continuation of the grid return, and if these were long not only would selectivity be impaired, but the set was prone to oscillation and _ similar troubles. "It is advisable," said the lecturer, "to keep the ‘C’ batteries within the set, for by this means the ‘O’ battery leads can be kept down to a minimum, A very great amount of trouble arises through long, straggling leads, even although these be enclosed in a battery cable.’ If an eliminator was used, it must be shielded, and un- der these circumstances can be placed within a foot or so of the set. In reply to a question on coils, Mr. Clark remarked that it was a bad practice to coat short-wave coils with "dope" or celluloid cement. "It quite spoils selectivity and will broaden the bands out too much," he said. Another rather difficult problem was raised by a member who had a superheterodyne broadcast receiver. He had been sold a super-heterodyne converter on the understanding that this would fit in front of the ordinary super-

heterodyne, but coupled up in this way it would not work, When it was connected to the grid of the first. intermediate frequency, it worked well: The lecturer said it was rather hard to a@count for. Of course, a double oscillating process was taking place in the converter and in'the set, and under the circumstances he thought it. advisable to connect the converter where. it was now being connected. No harm cowd be done, and if the set was giving satisfaction the matter could be left at that. Another complained that his set was. not giving enough lift, although everything seemed to be in order, Mr. Clark remarked that a pentode would probably have the desired effect. Very many people had put in pentodes and not had satisfaction, although he had found them particularly good. A péentode must not follow a stage of audio, but if coming immediately after the ¢etector would have the effect of two stages of audio. It goes best if eoupled in a specially matched output unit. It is a valve with which very good results ean be had on short-wave. At the end of his lecture, Mr. Clark was cordially thanked, and a ‘request was made that at some time in the future he bring along a typical shortwave set and explain to members the points to look for in constructing @ -short-wave receiver. Mr. Clark said he: would do his best to comply with this request. om