Radio-gram Instability.

Radio Record, Volume VII, Issue 5, 11 August 1933, Page 25

Radio-gram Instability.

jz is occasionally found with eombination sets that when they are switched over from radio to gramophone howling results. It may be found that the pick-up itself cannot be touched without getting a squeak from the loudspeaker, while in other cases, when volume is increased, instability results. ‘There are two mdin causes of .this trouble-firstly, the amplifier itseli, and secondly the position of the pickup. In the first place it may be found jthat the howl takes place principally iwhen using the pick-up and the ampli‘fier is being given a far greater input ‘yoltage than that used with radio. This increased voltage when amplified is sufficient to cause audio oscillation. To re-design the amplifier completely is ‘perhaps the best cure in this case. } Far more common is the trouble due ,to long pick-up leads. Long loud- | speaker leads, which run near the aerial or the aerial circuit of the set, tend to cause instability, when the set is working on radio. In a similar way the pick-up leads, when trailed near the audio end of the set, or when running near the loudspeaker may cause trouble when. the pick-up is put into operation. If long pick-up leads are necessary it is quite a good plan to use transformer input at the set end. An ordinary audio transformer will be quite satisfactory in most cases, though there are | few pick-ups which do not operate well when used with input transformers. Winally, never run pick-up leads near electric’ light power wires or an. induction hum will almost certainly be. picked up.