Improving Radio Reception

Radio Record, Volume II, Issue 31, 15 February 1929, Page 28

Improving Radio Reception

ee Points that are Usually Overlooked.

LOUDSPEAKERS of the horn and cone types suffer a gradual loss of efficiency where the output leads of the speaker are wrongly connected to a set with no output filter. Under these conditions the steady qnode current of the last valve tends to demagnetise the magnets of the loudspeaker unit. This results in weaker and weaker signals as time goes on, and it may be some timé before you realise what is happening. It igs then generally too late to reverse the connections and the only thing to do is to return the unit to the agents who, in most cases, will repair the damage for a small charge. The same thing, of course, happens to the ’phones if wrongly connected to the receiver, and, if possible, they sbould be compared with another pair khown to be in good condition. Saving the Speaker. AN improvement which is well worth while making in all sets where the last valve is a power valve is to substitute choke bypass output for direct output to the loudspeaker. This modification consists of a special output. choke, wired in the anode circuit of the last valve, one side of the loudspeaker being connected to one end of this choke through a 2-mfd. fixed condenser, and the other side to L.T. negative, Full description of this was given last week. This obviates all risk of leakage when the loudspeaker leads are extended to other rooms and, there being no direct anode eurrent through the loudspeaker windings, the magnets cannot be demagnetised. The efficiency of the output system is also considerably improved. o Important Accessories, [HE importayce of such accessories as H.T. and L.T. batteries is apt to be overlooked. Too often the efficiency of these accessories is thken for granted and the receiver blamed for poor results and defective quality. The B battery is more often than not the source of such troublés, due chiefly to its high internal resistance producing low-frequéncy instability and even oscillation at audible frequeticies. The internal resistance of dry batteries increases considerably as they become run down, and is sometimes quite high even in wet cell and accumulator batteries due to one or more defective cells. It will, theréfore, repay you to overbaul véry thoroughly all your batteries nd sources of power supply, replacing dry B batteries by fresh ones, preferably of large capacity (heavy duty). Search carefully for and replace cracked or leaky containers in wet and ac cumulater batteriés, and clean up any busbars or terminals which havé suffered corrosion through creeping of the electrolyte. When testing your B batteries with a Yoltmeter it is as well to remember that a ched4p. instrument may give very misleading teadings. Sueh a volt-

meter might take quite a considerable current, more than the total plate cur‘vent of your .receiver; and certainly more than is good for the battery. Under such excessive load the voltage of the B battery, as measured by the voltmeter, is much less than its actyal value when working under normal conditions. Whether you decide to instal a new B battery or not, it is essential to go over every B wandey-plug, cleaning them up, and, where necessary, gently prising apart the split pins so that they fit firmly and seundly in the battery sockets. I have known of cases where insecurely fitting wander-plugs have been responsible for some queer and erratic faylts. : The same attention should be given to all battery connections with the view to obtaining clean and firm contacts at all points of attachment to the battery, whether A, B. or C. In the majority of cases the filament current supply will be derived from aecumulators and the principal attention required here will be ‘to the terminals and connecting lugs. These should be removed, any corroded parts seraped clean, and when replaced all terminal parts should be well smeared with vaseline. As in the case of B batteries, it is just 48 important to ehsure a sound contact at the points of attach; ment. Your Battery Leads. For preference, leads to the accumulator should be fitted with substantial spade terminals, thus making quite sure of a large area of firm contact. Before the winter season commences it is well worth while having your accumulators overhauled at a recharging station, where expert attention can be given to any faults which may be present. When you are feeling more satisfied about the general health of your patteries, it is as well to examine all the battery leads as 4 final precaution. Thesé are quite important cannections and should not be made with haphazatd lengths of any wire which comes first to hand. The best wire for such leads is high quality flex of gené6rous gauge, soundly insulated and covered. In any case you should éxamine each battery lead to see t@it the insulating covering has not detetiorated. Neglécted Terminals. HYD attachment points at the receiver end should also receive the attention of a piece of fine emery paper. Very often the leads to the set are made to terminals right at the back of the cabinet where thy remain neglected and forgotten. Under thesé conditions the amount of tarhish and corrosion which can take place is surprising. All this dirt must, of course, be removed, and terminals and thé ends of battery leids brightenéd up with emery paper, following up With a ligh smear of vaséline, which preverts fut ther cortrosioi, afid actually impfoves contact, despite the fact that it is itself an instilating substancé; exactly how it does it is still something of a mystery. byt the fact is a weéll-tstablished one.