A Corner for Beginners

Radio Record, Volume II, Issue 36, 22 March 1929, Page 11

A Corner for Beginners

, _The Neutralisation Process. Vw the grid and tue piate of the valve a capacity elect known as the anode grid capacity is set up, and unless there is some method of preventing the energy so generated from feeding back into the aerial, the set is going to act as a transmitter, and all the neighbourhood wili be iterlered with. ‘'o combat this, in the radio frequency stage, a method known as neutralisation is employed. ‘This is quite Lrequently referred to as balancing. Vhe object of balancing is to provide a second feedback between the various other external circuits to connections outside the valve. The second feed-back is arranged so that energy passing through it is equal in amount to the valve feed-back but is opposite in phase or polarity. he effect of the valve feed-back is then exactly balanced by the external feed-back. The result of combining these two feed-backs is to destroy the effect of both, so that regeneration and oscillation are prevented, A simple analogy may make this clearer. If an acid is spilt or taken internally, the simplest method of _ preventing its action is to neutralise it _by an alkali whieh, though in itself as dangerous as the acid, when applied to the acid neutralises its effect or kills it. Exactly the same thing happens when the set is balanced. Principally, there are three methods of balancing. Of these the most popular balanced cirenit was the neutrodyne. rom the condenser that tunes the grid coil of the aerial circuit a lead was taken to a tap near the filament end of the secondary winding of the following transformer. This connection was broken by a neutralising condenser. Thus the feed-back from the grid was intermingled with the feed-back from the transformer which was opposite in polarity, and so one destroyed the other. The Roberts method is very similar except that the balancing energy is secured from a special winding in the radio frequency transformer, and the balaneing condenser is adjusted so that it allows enough feed-back to just compensate for the internal feed-back of the valve. The Rice method of balancing requires a centre tap on the grid coil of the aerial cireuit which is connected to the filament negative. The plate of the radio frequency valve is then connected to the lower end of the grid coil through a balaneing condenser, The Neutrodyne method is that generally used and is the one employed with the Browning-Drake. Adjusting the Balance. i ean be veadily seen from the foregoing that the amount of feed-back trom the secondary of the second R.F. coil must be equal to the feed-back from the valve, and this must be regulated by the balancing condenser. Thus the capacity of a balancing condenser is proportional to the internal enpacity of the valve. It is rare that the internal eapacity of one valve is similar to that of another valve. so that it is only to be expected that the eapacity of

7 ee sere noe the balancing condenser has to be altered every time a new valve is inserted in the radio frequency socket. Very few radio enthusiasts realise this, and very unsatisfactory reception following the change of radio frequency valve is quite common. Receivers that howl or whistle very frequently require reneutralisation. With all the circuit connections pro-. perly made, and with a neutralising condenser, set at about one-half of its total capacity, signals from a station on about 200 metres are tuned in to a maximum. All the signals must come from the antenna, so remove the antenna and decide if this is the case. If not, tune to a weaker station. . The filament of the valve to be balanced is turned out by turning off its rheostat, by removing the filament control resistance, or by disconnecting a wire from one of the filament terminals. No other changes are made, and no other valves turned out, and the tuning controls are left unchanged. The signals from the station previously tuned in will still be heard with fair volume, because of the energy that passes through the internal capacity of the valve. The balancing condenser is then carefully adjusted until the signals are not heard or until they are at a minimum. This means that the internal capacity of the valve has been neutralised by: the feed-back from the second coil. The set is now neutralised, and when the valve is turned on ugain, the signals will come in at far greater strength, and there will be an absence of noise caused by feed-back. The procedure should now be gone over again with a different station, preferably one with a higher wavelength, then on one with a lower wavelength. If it requires a great deal of alteration to the balancing condenser an adjustment should be made so as to strike a happy medium. When two stages of radio frequency are to be neutralised the stage nearest the detector should be neutralised first and quite separate from the first stage. Batteries! Batteries! Batteries! UITE frequently from this corner the need of watching the batteries is stressed. but there are still very large numbers whose trouble: can be traced down to this very simple cause. A radio service man tells the story of a happening just within the last few days. A set of his installation commenced to cause trouble some while after it had been installed. Unfortunately.

the owners were that evening entertailing some friends, and when the set retused to function, the radio man was called. His first suggestion was: "HOw are your batteries?" The batteries were all right! ‘They had only -been in use two or three months, When it was suggested that they should be tested, it was explained that they could not possibly be at fault; there must surely be something else. To satisfy the family, the radio trader travelled many miles with a considerable amount of radio gear, including a voltmeter. His first action was to test the "B" battery. One registered 0 volts, the other 18 volts. Again the radio trader asked the man how long he had had his batteries, and then, and not until then, did he realise that the batteries had done eight months’ service. The Grid Bias Cells. HE "C" or grid bias cell is. one which usually receives little or any attention at all, although it certainly requires the least of the three. It must be remembered that a dry battery does not last for ever, even if no current be taken out of it. in fact, a life of twelve months under these conditions is all that can reasonaubly be expected. The electrolyte in the battery gradually dries up completely in the course of time, rendering the cell useless, even if it is not used at all during the time, so that the grid bias battery should be renewed as a matter of course every nine months. ‘The vyaltage, us shown on a meter, will decrease gradually owing to the increase of resistance of the battery, due to drying up of the electrolyte. If the grid bias to the audio amplifier value is too small, or becomes too smull, serious distortion will be noticed, and a serious drain on the "B" batteries will take place.. Also, the life of the valves will be materially shortened. It would not be out of place to point out the great importance of carrying out the valves manufacturers’ advice as to providing the correct grid bias for each stage of amplification. The usual all-purpose valves require in the vicinity of 9 volts grid. bias, with 135 volts on the plate. But if a power valve is to be used fn the last stage it will be found that the manufacturer recommends 15 tw o°4 yolts bias. terme --