THE LOFTIN THREE

Radio Record, Volume IV, Issue 19, 21 November 1930, Page 14

THE LOFTIN THREE

Powerful, Simple, Economical ---~ the Set You Have been Awaiting

}ROBABLY the intending con- \ structor who has read our schedule in last week’s issue will, at the present moment, be debating as to which receiver to construct. Much, of course, depends’ upon. his own particular = circumstances, but for those who have the inclination to rebuild we would suggest their using the circuit that will be published next week. The difference between this and the one published this week, while only small, generally makes the set more satisfactory, though there is no reason why those who use all their parts in the present layout should not get the best. of results. For those who are commencing to build the outfit for the first time we recommend next week’s adaptations. The groundwork for both circuits will, however, be laid this week, for. after all, the differences in layout are only slight and after the directions this week have been read one should be ready to start upon either model without further instructions. The difference lies in the values of the main chain of resistances only. Where the original amplifier is to be unchanged,

a different style of connecting the radio amplifier and the pick-up must be resorted to. This will in turn be described, In the first place we must , nent’ on the utter simplicity and cheapness of the receiver. Tf carefully made and the parts first class there can be no questions as to the results. In fact there is nowhere in the whole receiver where distortion can enter and the limitations to almost perfect quality will be only those of the reproducing devices such as the speaker, pick-up, ete. The Parts List. We shall bandy words no further, but commence upon the construction of the Loftin Three. First consider the list of parts published here. These do not take into account the parts required for the amplifier, for it is assumed that the constructor already has this, though for those who are starting from "scratch" the additional parts for that section of the receiver ‘are also published. Again, we must stress the point that they must be of the highest grade, The sub-panel measures 16% x 10 x

23, and should be of heavy aluminium, say. 18 gauge. It is not any too big, and although the parts need not be ° crowded there is no waste space and there is plenty of room underneath for large condensers. The tuning condensers should be of the low-loss t-pe and well made, for as they have to be ganged any discrepency in their values: will most certainly affect the tuning. The same applies to the coils, the construction of which appeared last week. If this wire is not spaced evenly the inductances will be unequal and the two condensers will not be able to track properly. Those- who do not have the necessary winding apparatus should have the coils built for them. The valve sockets might be of the sub-panel type which can be affixed under the panels so that only the top is visible from above, but their use entails cutting a fairly large hole in the aluminium and unless the requisite tools are to hand this is a difficult task. Oz course, a dealer would b~* able to do that part of the business fur those who aesire the extra neatnei: involyed by their use. The ordinary sockets, however, are the easiest to fit. Four additional .5 condensers are required and these should be tested to withstand 500 volts, or in other words that will have to work safely at a voitage of 250. Preparing the Base. YOW that everything is to hand, study the picture diagrams accompanying, and lay your parts out naecording to the top view. This is the more important for the resistances and condensers underneath will = fall into position when the components on the top are right. It will be seen that ~

the major component on the top is the power pack. This has been described previously or it may be one purchased ready made. It will be noted that the condensers in the power supply are placed in the container and those whose apparatus is more\than usually bulky should make allowpaces in the size of the panel. At least 18 inches length may be necessary, so determine this factor beforehand. Where there is any doubt, lay the parts out on paper and then decide the size that will be necessary to accommodate them all. When they are roughly placed, indicate the positions by lightly marking the panel, then remove them and square up the marks so that the layout will be symetrical. The idea of the squareness can be seen from the accompanying diagram. Now everything is replaced ready for drilling and with a sharp instrument indicate where each hole is to be drilled and then with a suitable driil make all the holes necessary. Now do the same to the under base panel components, making quite certain that none will foul the components on the top. The positioning of these under- . base parts is not critical, but if alterations must be made, see that filament and plate wiring is kept well apart. Having now drilled all the . holes, mount the above board components with the exception of the coils and the condensers, as these are easily damaged when the set is reversed to attend to the under base wiring. The under base parts should ocecasion no difficulty, though the greatest of care must be taken that the lugs of the resistances, except were indi eated, should not come into contact ~

with the panel which is at earth potential. As all grounding is made. to the panel and this is of aluminium it, will be necessary to attach soldering tags to certain of the screws that are attaching the components to the base. /Before proceeding with the "wiring it. ‘is best to tin all lugs‘and resistance ends that will come into the business of wiring. It will be found that this simple expedient will save a great deal of time and will also guard against drz joints. ‘ Wiring the Set. WIRING is not a job that can be accomplished -in a few minutes. It necessarily requires a good deal of eare, and a mistake may do a great amount of damage to resistances, condensers or. valves. All wiring below the base should be carried out with glazite or other similarly covered wire, but constructors are advised against heavy wire of the busbar. type, as this is not easy to work with and does not well. The theoretical -diagram. should be followed and the layout used as a .check,. as it is much easier to go astray when following the latter than when following the former. When wire has to pass through the shiefd it should be well insulated by a piece of rubber, shellacked cardboard or other insulator as well as the covering on the wire. See that the filament wires, are kept clear of one another and that a separate winding is provided for the power filament. This is essential, as the potential with regard to the ground is not the same in each. Remember that the current comes through the 245 valve, through the balancer resistance and then through the main chain of resistances. A little flows up through the high value resistances to provide the plate voltage of the amplifier 224 and the bias of the 245. The resistances in this main chain must be _ to pass the current supplied by the TY adapting to actual practice many of the angles of. wire shown in the diagram can be omitted. Generally -avoid wires crossing. Everyone who has done any construction will have. his own method of actually doing the wiring, but we have found the easiest method is to measure off the distance between the points to be wired on the glazite and then cut the wire. The ends are bared about a fin. from each end by running a sharp knife round the insulation, which then comes off cleanly. Where crossing

wires have to be joined. it is simplest to place the wire, then make the cuts round the wiré on either side of the joint. No. more than a fin. should be bared, for with a hot. iron there will be no trouble in making a join in this space and the likelihood of anything shorting on to the, bared’ wire is effectively minimised. It will be noticed that thé humbucking potentiometer is placed at the rear. Once the silent point is found there will be no need to alter the potentiometer. The plate resistance is likewise at the back, for once ‘the set is properly adjusted there will be ‘no need to alter the potential of the plate. R12 controlling the voltage on the screen acts as a very effective volume control and is consequently on the front. Above the Panel. LITLE wiring has been left for the top of the panel. Wires to connect with the coils should be left long so that when the coils themselves are brought into place there will be no shortage of wire. Before completing the sub-panel wiring a word must be said about the jacks. The output can be of the simple two spring type and is connected with the output transformer attached to the speaker. The other is a four springed type and is arranged as is shown so that when the jack is pulled out the two wires leading to it are connected. ‘This condition is shown clearly in the diagram.

Of the above base wiring there is only one point of note, and that is the grid wire. This. is taken through the top of the can to the grid of the valve and to the fixed plates of the condensers. The fixed plates are connected with solder lugs attached to the base. The two taps on the aerial are brought out to a suitable mounting and the ground wire is attached to the frame anywhere. The plate wire, if neatness is essential, can be taken under the base, but it gets'a much better run if allowed to pass through the bottom of the can and run above the panel to the plate of. the first valve. Where the English type valve is used some adaptation in the wiring -will be necessary, as the plate is brought to the top terminal.

Trying Out. LTHOUGH this is the most inter_esting part’ of the whole job, it should not be rushed. Check and double check all the wiring and look particularly for ‘resistances shorting to the" ‘panel when. they are not supposed to. be at earth potential.: When satisfied in "this direction, .see ‘that the valves are in their right places and switch "on the current,’. Watch the rectifying valve and if’ it turns. a bluish colour and begins to spark, ‘éut off the electricity ‘quickly ‘and j.leok: for a short. circuit. If a -milliaimeter is available insert this in the leadto the B+. It_should: read: about. 30: piiland if too many. are : "shown there is.‘a-.short between.one: of the resistances and: the. earths. ‘Tf these are "in order, look. for. an open circuit in the: "bias" chain: or’ a broken-down condenser. .. a Presuming that the milliameter after reading, say, 50 fora few -second settles down to 30 and the ‘rectifier does not show signs of distress,’ £ ney the next stage can be proceeded: Attach the gramophone where indicat: ed and flick the needle. This: shoult produce a grating sound in the speaker, If it does not a defective connection in the chain of pias resistances or some wires not connected is indicated and examination should reveal the trouble. The signals coming from the loudspeaker should be loud and clear; weakness and distortion indicates faulty resistances, particularly. those in the plate 224 bias 245 chain. Other troubles may be that the jack or the volume control is not insulated from the panel. . Do not worry about the initial hum as this quietens down when the filament of the 224 has had time to warm.up. There will still be (Concluded on page 28.)

Loftin-Three

. (Continued from page 15.) a residual hum and this should be balanced out by adjusting the humbucking potentiometer indicated in the diagram, The Radio Side. Now that the gramophone pick-up is working properly, try the tuner by rotating the condenser until the local station is brought in. Adjust the moving plates of the condensers until the signal is loudest and then, if there is not a great deal of difference between the settings, tune to another station and bring it to its loudest point by adjusting the balancing condenser. Then loosen the coupler and see if.any alteration in the setting will make the signals louder. If it will the coils are not properly balanced and it may be necessary to remove a turn from the coil, the condenser of which requires to be moved outward to get the loudest signals. If the set is prone to oscillation, remove some of the turns on the primary of the r.f. transformer for the greater the number of turns the greater the fransfer of energy and the greater the liability to oscillation. If signals cannot be received, bring the aerial to the grid of the amplifier 224 when the local will be heard. This indicates a mistake in the wiring of the r.f. stage. Make sure the coils are connected the right way round. 4 \