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ASSEMBLING A MOTOR COACH WIRELESS SET.

26th February 1924
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Page 13, 26th February 1924 — ASSEMBLING A MOTOR COACH WIRELESS SET.
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The Final Instructions for Buildin a Receiver Suitable for the Motor Coach are Given in this Article.

IN THE previous articles, constituting the series dealing with the construction of a wireless ireceiving set Specially designed for the peculiar condiirons appertaining to its use on a motor coach, we have covered the question of the type of components and their positioning in order to secure the best results, and last week we gave instructions'for preparing the ebonite panels and the wooden base of the receiver, It now remains to detail the wiring operations and the testing and handling of the set.

The wiring is carried out in No. 16 S.W.G. soft copper wire and is bare—that is, it is not insulated in any way. The only insulated wire that should be used is that for carryirlg the filament heating current. For this part of the work ordinary electric bell wire should be employed.

The 16-gauge wire will be found sufficiently stiff to maintain its position and the shape given to the leads and, thus, if the wiring is carefully carried out, there will be no danger of wires coming in contact with one another. The theoretical wiring_ diagram has already been given in an earlier article, and it is given here in perspective and should be followed exactly, the theoretical diagram being watched and used as a check. Owing to the careful disposition of the parts in this receiver, it will be found that all wiring is very short, and since, at the same time, there is plenty of room for everything, no excuse is possible for cramming up the wiring to the detriment of the efficiency of the set. The, only wires that should he run closetogether are the low-tension battery ones,. that is why we specify that they should be insulated. The wires should stand away from each other and particularly should he shaped so as to give ready access to the valves for changing and replacing.

So far as possible, and not only for the sake of neatness, but also because it makes a more rigid job, the necessary bends tin the wiring should be made with a pair of pliers and should be sharp. It is not necessary that all wires should run at right angles to one another—in fact, it is much better to avoid parallelism so far as possible and make the crossings at more acute or obtuse angles. It does not, how eVer, add to the efficiency of the wiring if the wires straggle about all over the place, so they should be kept short and direct, and the necessary bends in them should be as sharp as, the pliers will make them.

Square braas bits bar wire one-sixteenth of an inch thick has lately come into vogue for wireless sets became it is firmer than the soft round copper, wire and keeps its position better -should the fingers touch it.

Four valves will be required, and these may be of the ordinary R type, or dull emitters may be employed. If the former, each valve will take between .8 and 1 ampere forAlcatingcurrent, so that up to 4 amps. may be drawn-from the battery at one time. This means that a battery of at least 60-amp. hours should be employed, and this would run the set for about ten hours continuously. For loudspeaker work, the last valve shouldivreferably be a power amplifier specially made for this class of work. These valves have a greater output than an R valve and give the necessary volume without .distortion.

The Advantage of Low-current Valves.

Dull-emitter valves may now be obtained which have nearly as great an output as the R valve and yet consume only about one-third of the current for heating. These, therefore, make a much smaller demand on the battery and, again, they are particularly robust and do not grow greedier with age. For the second note magnifier, however, a power valve should be employed, as the average dull emitter will not function properly in this position. Power valves with dull filaments may now be obtained. They are expensive, but they are worth the money. Reference to the layout will show that there are two small batteries connected in the first and second note magnifier grid circuits respectively. These are for the purpose of keeping the grids of these two valves at a definite negative potential and so avoiding distortion due to partial rectification of the already rectified signals. The value of these bias batteries varies somewhat with the actual make of valve employed, but, as a rule, two to three volts are required on the first grid and five to six on the second one. As the current required from these batteries is infinitesimal, small pocket flash-lamp batteries answer the purpose excellently.

The high-tension battery is in three sections : one of sixty volts and two of thirty volts each. These are connected up in series to give a total voltage of 120, and wires are run to the appropriate terminals so as to give the voltages marked. The negative end of the 60 volt battery is connected to the terminal marked —, the + end of this battery to the terminal above it and to the — end of one 30-volt unit, the + end of this latter te the next terminal, and so on.

When the set is finished it should be tried out first, preferably, on a good P.M.G. aerial, such as one finds on a house. The valves sheuld first be inserted and only the accumulator connected. The main mwitch should then be turned on and the master filament resistance knob turned, if all is well, the four valves should light up. Next connect the hightension batteries as just detailed and also either a low-resistance loud-speaker or a pair of low-resistance phones. The reaction knob should be turned to its minimum position, so that the reaction coil is coupled loosely to the anode coil, and the knobs controlling the variometer and the anode 'tuning condenser should then be turned slowly in opposite directions until signals are picked up. So soon as this happens, stop turning the condenser knob and adjust the variometer until the loudest signals are received. Then readjust the anode condenser until a further increase in strength is obtained. The reaction coil should now be brought closer to the anode coil, so that the coupling is increased until the greatest strength of signal is received. The reaction must not, however, be so tight that, if the aerial terminal is touched with a damp finger, it will produce a click—which means that your receiver is radiating energy and interfering with other listeners. After this, slight readjustment of both condenser and variometer and, perhaps, also of the brightness of the valves is all that is necessary, The four-valve set built by the writer to this design will fill a large room with dance music from 2L0, 150 miles away, on a P.M.G. aerial. A set constructed in a motor coach garage -should certainly do as well if the directions have been carefully followed.

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