Radio-Gram. Instability

Radio Record, Volume V, Issue 27, 15 January 1932, Page 23

Radio-Gram. Instability

Several Causes qf is occassionally found with combination sets that when they are switched over from ‘radio to gramo-' phone 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 yolume is increased, instability results. There are two main causes of this trouble-firstly the amplifying itself, and secondly the position of the pick-up. In the first place it may be found that the howl takes place principally when, using the pick-up and the. amplifier 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 completely re-design the amplifier is perhaps the best cure. in this case. Far more common is the trouble due. to Jong pick-up leads. Long louilspeaker leads, which run néar_ 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 radio transformer will be quite satisfactory in most cases, though there are few pick-ups which do-not operate well when used with: input transformers. Finally, never run pick-up leads near electri¢ light power wires or an induction hum will almost certainly be picked up.