Tips and Jottings

Radio Record, Volume II, Issue 5, 17 August 1928, Page 30

Tips and Jottings

The R.R. Selective Crystal Set. A PETO2r:E reader states that he has made up ten different types ot -crystal set and finds the R.R. the best. With » 100-foot aerial 30 feet high and a water-pipe earth, the set has frequently to be toned down when using ‘phones. A Brandes table speaker can sometimes be heard in every room. This correspondent writes with the object of getting a crystal circuit that will beat the above. We have not come across one yet, but when we do it will be worth publishing. A Dunedin reader can eut out 4YA any night and receiye 2YA, using the R.R. selective.

What Causes Oscillation Interference? HAT actually causes the noise in neighbouring receiving sets when "reaction is applied to the aerial in such i way as to make it ‘oscillate? The receiving set then acts as a transmitter, and sends out a continuous wave. As the set is tuned to a broadcasting wave-length, this wave will be very near to the frequency of the carrier wave used for broadcasting, Either of these waves alone would be inaudible, but when they are present in a receiver simultaneously beats are formed between them which occur at 2 .dible frequency, and which give rise to a note in. the ’phones. If the tuning condenser is turned so as to bring the wave-lengths nearer to -. further from one another the beats will alter in frequency and the received note will rise or fall accordingly, giving rise to the all-too-familiar "chirps" associated with oscillation. Exponential Horns. CONSTRUCTOR recommends that corners be joined with the light aluminium angle strips that are used on the running-boards of motor-cars, This is soft and can be easily bent to shape and fixed with jin. screws, staggered, making a strong job. The idea seems quite good. It must be remem-

bered that the inside of joints must have a fillet of glue or secottine run in to make absolutely airtight. Reaction "Plops." ! THE cause of reaction coming in and out with a "plop" (backlash) is geenerally owing to incorrect potentials ‘on grid and plate. If the rheostat is in the positive lead and the grid return is connected to A positive, then there is ; an excessive positive potential on the, grid of the detector. In such a case the erid return should be connected to the negative side of the valve filament. -Rheostats are best placed in the nega-

tive lead. Detector plate supply should be easily variable, and the -eorrect value found by experiment, to give smooth reacti¢1. Another cause may be that there are too many turns on f, re tickler. If the tickler is adjusted f certain: voltage and a dry B gradually falls off in voltage. a time. will probably come when a further ad-; justment must be made to compensate* for the reduced voltage. Stray capacities can ‘also help to introduce backlash, and its presence in a receiver makes it more liable to cause interfer: ence,