TO BROWNING-DRAKE CONSTRUCTORS

Radio Record, Volume I, Issue 20, 2 December 1927, Page 12

TO BROWNING-DRAKE CONSTRUCTORS

A FEW HINTS BY THE WAY

Many constructors have. built the four-valye Browning-Drake described in these columns, and those who read this page regularly will have noticed that some constructors have had immediaie success, whilst others have been confronted with trouble. THose who have kept near to the specifications have not, as a rule, experienced much trouble, whilst those who have altered some details and left out others entirely, particularlv fixed condensers, have found results’ somewliat disappointing. | However, the remedy is simple-merely tc go through the description and see that cyery detail is properly carried out. Whether American or: English valves are used is not the question, so long as the equivalent type is selected. It is in the matter of primary turns, tickler turns, and higti-tension voltages that there is room for much experiment, and no constructor should give up tinkering and altering until he is getting reproduction equal to the best he has heard, at Icast, for the four-valver is capable of it. And whatever volume the owner demands should be accompanied bv mellow, pleasant tone, with an entire absence of jarring. distortion or harshness when main New Zealand or Australian stations are tuned in under fav- ourable conditions, of course excluding interference from outside sources.

Soldering Sometimes tfie Trouble. One point ‘"Megohm’ is not too sure about is the soldering. He has just heard of a set in which scarcely one of the joints was soldered in such a way as to be efficient. Many of the joints were practically only held toher by resin, which is a fine insulator, No wonder results were disappointing. Tt is a good plan for those who are not too sure of their soldering to cut out as manv joints as possible by running from rheostats a scparate wire to each valve, and aseparate return from each valve to the battery terminal. In place of running one wire and branching off with soldered joinis, this cuts out a good number of possible weak spots. All such A battery wires, which must. have a good itisulating covering, can be run alongside as inuch as possible and bound with twine to form one cable with wires branching out to their respective valves. ‘This method helps to earth stray high-frequency currents which are detrimental to good tone when present on the audio side. A well soldered joint will not show any crevices round the edges of the solder, and the solder will appear to form a well-curved and thicker portion of the wire or surface to whieh it is attached. Having to place the iron on a gas stove to heat it causes many amateurs to manage "just ome more joint’ before re-heating, and that joint will probablv be a "dud," because, although the iron melts the solder, it is not lot enough to cause proper incorporation with the metal the solder is intended to join. ‘Che electric soldering iron is conducive to good work, because there is no anxiety about the heating part of the business.

Clean the Vaive Legs. The occasional cleaning of valve legs with glasspaper is a wise precaution, and songctimes their fit and consequently reception can be improved by opening the legs if of the split variety. Noisy or erratic reception can easily be caused by a badly-fitting vaive-leg, giving poor contact : The Neutralising Condenser. It was pointed out in the original article that the neutraliser was to have vety small capacity, and this was obtained in many cases by purchasing,a midget condenser and removing all plates but two, leaving one fixed and one moving plate Anybody who replaces old solid-woutd coils with the space-wound ones as specified will have little luck with the neutralising condenser formerly used, as the neutraliser must be much reducea in capacity to match the greatly reduced seli-capacity of the coils. "Megohm’s’" own neutralising condenser is a home-made one with two diminutive aluminium plates each about three-quarters of a square inch area, and at correct neutralisation these are only overlapping at 10 degrees on the scale, 50 degrees being full in. It was not considered necessary t6 describe this, as the midgets can be utilised and save the making. Most of the bought neutralising condensers appear to have too great a capacity for this circuit. General Remarks, Tt must be remembered that the aim of a receiving set is to reproduce as exactly as possible the broadcast sent out from the transmitting station, and no constructor should let things rest until the very best tone, coupled with good volume, is obtained from his set. "Megohm" wishes every constructor of the Browning-Drake to achieve this end, and has on that account replied by post, to a good many queries in order to give the: individual particulars required, although personal correspondence is not intended to be a feature of this column. A nuinber of constructors lave reported getting very fine tone and volume, but there must be many who have not yet done so, but careful observation and trial of different condenser values and other details as pointed out will bring due reward, : There is probably some difficulty experienced in the matter of testing various condenser values on the audio side, owing to the expense of providing the necessary set of fixed condensers of different capacities, and also the difficulty of obtaining’ the grading required. To get over this dificulty "Megohm" will next week describe his system of making fixed condensers, of which a full set can be made for testing purposes at small cost. Any constructor providing himself with such a sect will soon learn all about the influence of condensers upon tone, and will have a ready means of providing and testing for correct values for any purpose.

Mainty About Construction

(Continued from page 12.)

B ELIMINATOR (Continued). | New Zealand would cost much more! than the price of the ready-made article, Smalt Output Affects Tone, An eliminator should be well up to its work, or tone will suffer, and that is why. double-wave rectification is necessary to get best results from any but small sets. Testing Fuses. YTinfoil fuses as specified should he included at all points, for the small amount of extra labour ensures freedom from a burnt-out valve or a serious breakdown later on. Sometimes in a set of such new fuses there is a weak one that blows unnoticed. The fuses should all be tested across with a pair of *phones and dry cell, and a defective one replaced. If soon after the elimjnator 1s put into use it refuses to work, test fuses again, and if a weak ene has blown and is replaced there will be no further trouble except when one acts as a safety device. A blown fuse can scarcely be detected by the eye, so the quickest way to locate one is with phones as above. A side elevation of transformer was marked ‘"‘half-scale,’ but is really quarter scale, as the drawing was re- duced to half-size. Care must be taken to pack tight the laminations inside transformer windings. | In operating, filaments must always be turned on before the eliminator, and ewhen closing down the eliminator is switched off first If the eliminator is working on no load, a high peak voltage is thrown upon the condensers,

which may not be able to stand the abnormal strain, This remark applies to all B eliminators. Extra output voltages can be provided by adding a variable resistance with a 2 mfd. condenser across in the same way as those already shown. Work by the diagram marked ‘‘Cortect Eliminator Circuit.’’ When adjusting filament current for PM4 valves, care must be taken to commence with the minimum, that is, with plenty of resistance in circuit, Tor these valves an ordinary 30-ohm_ rheostat would be a convenience rather than none at all. |