RE-NEUTRALISING SETS

Poverty Bay Herald, Volume LII, Issue 16200, 25 November 1926, Page 3

RE-NEUTRALISING SETS

ELIMINATING INTERFERENCE. Many of (lie so-called non-rogeneva-tive or non-osciallating receivers, although perfectly balanced at the factory with certain types of valves and ■with certain aerials, become the worst possible offenders in service as fat as radiation is concerned. One of tin 1 principal causes is tiie variation it valves. For instance, tiie neutrodyne made about iwo year*? ago, oiiginally balanced on the TJV2OIA valves with the brass base has been found quite unsuitable when using the new TJX 201 A valves. Also in nine eases out of ten a set neutralised with Radioiron valves in the sockets will be out when other makes of 20.1 A valves are used. This, of course, only applies to the radio .frequency portion of tiie set, and is brought about by the difference in the internal capacity of the valves.

In the ease of nun-regenerative sets, a simple remedy ds to decrease the plate voltage on the radio frequency amplifiers. With most sets of this typo both the radio and audio frequency amplifiers- take tho same amount of ‘ls” battery voltage. To reduce plate voltage on 1 ho radio tubes alone it is therefore necessary to remove the set from the cabinet, trace out the wiring and -separate the ”B” positive wires from the radio frequeue, v valves. These are then connected to a. separate terminal ol' their own or to a flexible insulated wire vlnch can be carried direct to the ” B ” battery, a voltage being selected so that the sot does not go into oscillation. A common by-pass condenser being usually employed for both audio and radio frequency amplification, a second 1 m.f.d. by-pass condenser should be added, connected from the radio frequency coil ”13” positive ;terminals to the “A” negative terminal, preferably within the set itself, in order to shorten I lie leads to the condenser.

The straight-out neutrodyne may bo rebalanced, the process being a simple one. If care and patience are used, no special apparatus is necessary. A strong signal is tuned in, and the first E.F. valve is removed from its socket and replaced with a piece of paper over one filament pin or leg so that the valve will not light. The neutrodon sleeve is then moved along by means of an insulated'rod until it is at a point where signals are weak

or disappear altogether. If the micrometer type of neutrodon is employed, a stick of wood, one end sharpened to a screwdriver point, may bo used to turn the screw until a point is reached where the signals are weakest or disappear. Usually only a very small movement is required and once the correct point has been arrived at the neutrodon is sealed and the first R-.F. valve is neutralised. The process is completed in the second R.F. valve in exactly the same way. completing the neutralising operation. The aerial or earth wires running parallel with a neutrodyne set will very often make it burst into oscillation on certain wave-lengths. The cure is obvious, keep both wires away from the sot.