NEUTRALISING

Shannon News, 20 December 1927, Page 2

NEUTRALISING

If ,a set with neutralising means is to be operated to the best it is necessary that such neutralising deviee should be flsoperly adjusted. The method of neutralising a set with one stage of high-frequency amplification and some method of regeneration 9 et out below, and will be found simple and effective. The object of the .method is to find an adjustment of the neutralising condenser which will allow the greatest setting of the reaction control to be used without producing oscillation. To begin with, the neutralising condenser and the reaction control are both set at a minimum and then the tuning condensers are both set so that the two tuned' circuits aro in step with each other. At these settings .it will probably be found that the set is in a state of oscillation, and this state may be observed by touching the grid terminal or those plates of the tuning condenser connected to it, and listening for the dick both on placing the finger in position and on withdrawing it. As it will possibly be found that the set will only oscillate when the two circuits are in tune, the presence of oscillation will indicate that they are in this state.

The capacity of the neutralising *i condenser, should now be gradually y increased, testing for oscillation, as this is being done, and soon it will b> v, observed that oscillation has ceased/ \ and will not begin again, even when | the tuning dials are readjusted. The \ next step is to increase the reaction

until the set commences to oscillate ivogain, and then increase the neutralisi. ing condenser setting until the oseilla- > tions cease. Beadjust the tuning coni:,'denser slightly, so as to make sure

that the set is stable under these cou

ditions. .. Cany on these operations till it is found that the setting of the ncutrar Using condenser has been increased too much. When this state is reached it will, be found that further increases in the capacity of the neutralising condenser will not cause oscillations to cease, hut will have the effect of making them stronger. .Bring the setting of the neutralising condenser back tc the point which is thought to be the correct adjustment, and it will b< found if this is the correct point that when the two tuned circuits are in step and the set is brought to the verge of oscillation by the reaction control, a slight movement of the neutralising condenser in cither drection will im mediately cause oscillations to be set up.

It is desirable to carry out the above operations at some point noaT the middle of the tuning range of the set and, of course it is not necessary to state that the whole set should be in the final state, with all the etc., in the position which they will occupy ultimately.

In eases where two-volt or smilaT valves are used with standard screened coils or other types of intervalye coupling, it may be found that poor signals and other difficulties arise. These may be due to the fact that such eouplngs were designed primarily for six-volt valves. An increase -at the plate voltage of the high frequency stage may help, and it is often quite worth while increasing the grid-plate capacity, of the high frequency valves by connecting an ordinary neutralising .condenser between the grid and plate, setting it about half in. The neutralising condensers are then adjusted in the usual manner.