Correct Neutralising.

Radio Record, Volume II, Issue 11, 28 September 1928, Page 31

Correct Neutralising.

Causes of Failure. T is common experience among home builders that preliminary experiments with various forms of neutrodyne circuits, or other receivers, such as the Browning Drake, embodying the

i capacity neutralising principle, are disappointing. Neutralised radio frequency receivers as a type are probably more widely used than any other type of receiver, except, perhaps, the crystal receiver, and their efficiency when properly constructed and adjust. ed is never doubted. Quite the most common cause of failure in home-made neutrodyne sets is incorrect. adjustment of the neutralising condenser. The purpose of this condenser is to. compensate for the unavoidable capaeity which occurs in the receiving valves, and to a lesser extent in other parts of the circuit, but to be effective its adjustment must be made with great exactness. The effect of the accidental and unavoidable capacity in the radio frequency amplifying yalves and associated components is virtually to form a partial short circuit across the radio frequency amplifier, with the result that energy picked up by the aerial is passed more or less completely to the detector valve without being properly amplified by the amplifier. Great loss of sensitivity results. The method of adjusting the neutralising condensers to avoid this effect ig simple. The receiver is tuned in to a fairly strong local station, and the filaments of the radio frequency amplifying valves are then turned off. It will be found that signals from the local station will still be heard, though. generally at considerably reduced volume. The neutralising condenser is then adjusted until the signals disappear or become so faint that they are practically inaudible. If more than one valve is used in the radio frequency amplifier, first one valve should be adjusted. When signals are cut off by the adjustment of the neutralising condenser on the first valve, that valve should then be lighted, the second still being turned off. It will be found that signals will again become audible, and the second valve should then be adjusted until they disappear again. The receiver will then be completely neutralised. It should be. borne in mind that it is generally necessary to make a slight readjustment of the neutralising condenser if the radio frequeney amplifying valves are changed. This will certainly be necessary if valves of one type or make are replaced with a different type.