Questions and Answers

Radio Record, Volume II, Issue 27, 18 January 1929, Page 11

Questions and Answers

Push-pull Resistance Coupled. wal should be grateful if you will supply me with the following information :-- (1) Are the results satisfactory when A.C. is used: on the filaments.of a_re-sistance-coupled amplifier? I am assuming that potentiometers are used across the filament leads for balancing out the hum. (2) Would the output of a trans-former-coupled push-pull stage be much greater than that of a resistance-coupl-ed push-pull stage, assuming that the imput be. the same in both cases and the same tubes be used in each case?G.T.R., Gisborne. ANSWERS :-(1) No, unless special A.C. valves are to be used; raw A.C. nnot be used on any other than the valve. (2) Yes, the output from a trans-former-coupled amplifier would be greater because of the stepping up propensities of the transformers themselves, Seratehing Noises. AM wondering if you will kindly give me a little instruction concerning my factory-built wireless set of six valves-the faults are a crackling on Christchurch and Auckland, like static noises. Wireless expert says everything in set in good order, also loud speaker. When earth wire and aerial wire are off the noise does not come in so am completely at a loss to know what to do-F.M. (Island Bay), ANSWER :-The correspondent has gone about finding his trouble in the right manner when he disconnects the aerial and the earth to see if the trouble is in the set or in the aerial and the (earth systems. The results go to show that the noises are originating, not in the set itself but in the aerial or the earth’ connections or are caused by a power leakage. The first. duty of the trouble-seeker in this case would be to compare notes with another radio enthusiast in the same neighbourhood to ascertain whether or not the trouble is being caused by a power leakage. This being the’ case, a complaint should be addressed to the District Telegraph Engineer. It is more likely, however, that in this case the cause of the disturbance lies In the aerial and the earth connections. These should be carefully examined. ; First examine the. connections to the set itself and make sure that these are clean and making good contact with the terminals. Proceed from these points to the ground and to @: aerial, examining carefully for ulty joins, wires broken within their insulation, and any such defects. It may be that there is in the immediate neighbourhood an electric instrument that is leaking and causing the trouble, A faulty electric motor of any description would cause this trouble, and, in these days of vacuum cleaners, electric sewing machine motors and violet ray apparatus, trouble might be unwittingly caused. The "Rotorua" Portable, I have lately completed the portable set described in your pages, and it is a great success, I have tried it 20 & and 30 miles from 8YA and speaker trength was good although at 30 miles the volume was not great. Hveryone comments on the tone, and it is ideal for local station work. Wellington is also heard distinctly on the speaker, using the frame aerial, With the main

aerial attached, 1YA and 4YA are good, and 2BL and 2FC distinct but not loud. Iam using my old speaker still as the dealers in town have been unable, so far, to get any information re the P.C.J.J. unit. However, the set is naturally rather unselective, and I have been wonder-

ing of there is some way in which a "shielded grid booster" could be added when the main aerial is in use. The frame forming the grid coil makes this rather difficult, but I would be glad if you could give me any information re this point-D. A. LANE (Christchurch),

ANSWER:-A shield grid booster such as that described by Megohm some time ago could easily be added to the portable set described by "Pentode" in our special issue. The method of connection is this:-Connect the main aerial to the loop. From the loop take leads to the booster input, from the output leads go to the set in the usual manner. This is the only position in which a booster can be employed with this set. Charging Rate for Accumulators. HAVE purchased a new six-volt accumulator, and the following instructions are given on the outside:First charge: Fill with acid, 1.300 sp. gr., and allow to stand for 12 hours before charging. Charge at 6 amps. for 24 hours, and add acid during charge, 1.300 sp. gr...." Does the "charge at 6 amps." refer to the first charge only, or is it the charging rate for all charges given the battery? Would it be harmful to charge a battery with a charging rate of 6 amps. with a charger giving + amp?-B. §. D’ATH (Wanganui). ANSWER. — These instructions should be strictly adhered to when making the first charge, as the life of the battery largely depends on this early treatment. There is generally a reason for everything. The specified sp. gr. is correct for that type of battery. Allow to stand to let the acid thoroughly impregnate the more or less porous plates, Air bubbles rise

as the acid soaks into the lead compound and the acid level must be kept above the plates. The first charge must be at a fairly high rate, so that the chemical change that takes place will be completed within a certain time. Until this change has taken place the paste is

ee eee comparatively soft, and if the battery is jarred is liable to fall out. The process of charging binds, as it were, this compound together. It is always recommended that the first charge be given by a firm of repute, after which a trickle charger can be relied upon to keep it in order. Battery Voltage.

‘"Wuyr do I have to discard my batteries when the voltage drops but still shows that a certain amount of

current is left? Why won’t my valved use up all the electricity in the cells?" ‘THE voltage as measured by the volt» meter shows pressure. Take & simple analogy. Imagine a tank of water to be connected to a tap which is slightly higher than the middle of the tank. When the tap is turned on when the tank is full the water flows easily and gives a plentiful supply of water, but when the water is low the pressure falls off and there is a difficulty in obtaining water; at last the flow ceases though the tank is yet half full. The batteries of the wireless set work in a somewhat similar fashion. The pressure is the voltage; the amount of water supplied the amperage. Owing to the resistance offered by the set all the electricity cannot be drained off and unless the batteries are not kept well up a copious supply cannot be afforded the valves. Lightning Danger. "TS there any likelihood of an aerial being struck with lightning, and what steps could be taken to prevent this happening?" AERIALS are only rarely struck with lightning, so that there is ne eause for anyone refraining from installing a set because of the lightning risk. However, one precaution is essential. The regulations of the Fire Underwriters of New Zealand stipulate that a lightning arrester be used in every wireless set. All that is necessary to instal this inexpensive piece of apparatus is to connect one end to the aerial and the other to the ground or to the earth terminal of the set. This is a sure precaution against lightning entering the set, for should it strike the aerial and enter the arrester it merely runs to earth. After an aerjal is struck in this manner the set would refuse to function until the old arrester was removed, as the aerial and the earth were joined by the lightning.

Shh LN ACO Important Notice Regarding Enquiries S our service is now being fully availed of and as almost every mail brings in a large number of enquiries on various topics, it has become necessary to adopt some system of uniformity. Correspondents are therefore requested to observe carefully the following points. Failure to observe these may mean delay in answering and even the loss of the letter. 1, Addresses.-All technical correspondence, whether inviting reply or not, must, in future, be addressed: "Phe Technical Editor," "N.Z. Radio Record," Box 1032, Wellington. This includes letters to "Pentode," "Galena," and "Observer," as well as those that have in the past been erroneously addressed the Editor. 2. Each letter to bear (inside) the department to which it refers:Construction, Crystal Corner, Questions and Answers, or General. 3. Letters inviting reply-must be accompanied by a stamped and addressed envelope, but the right to answer any letter through the columns is reserved. Correspondents should watch the columns carefully as one letter may be answered in more than one section. 4. Advice requiring discrimination between factory-made sets or between makes of components cannot be given. 5. All letters to be signed, but a nom-de-plume may be added.