The Two R. F. Shielded Browning-Drake

Radio Record, Volume I, Issue 44, 18 May 1928, Page 12

The Two R. F. Shielded Browning-Drake

Distance-Getting and Quality Reception Combined

By

Megohm

= ERE is the reliable ° Browning-Drake circuit with the addition of an extra radio-frequency valve, for which so many Seosinies readers have been asking. The whole receiver has been re-designed for accommodation within the necessary shielding, and a fine quality receiver is the result. Not only is there a wonderful increase in distance-pulling, but alteration of details, including the cabling of battery leads, has produced a sharpness and clearness of tone that leaves little to be desired. As an instance of added range, it may be stated that with the new five-valver in Wellington, both 1YA and 4YA can be received at full loud-speaker volume in the daytime, and this could never be done with the four-valver. The main Australian stations come in with tremendous volume, with reaction at zero, and the lesser stations in due proportion. For the R.F. stages, ordinary valves such as have previously been used in the R-F. stage, are used, but a method of utilising a shieldedgrid and other low-capacity valves will be described at a later date. The three stages are fully shielded with copper. The amplifier is not to be deseribed, and it is recommended that constructors use a separate ampl-fier unit, which, in the case of those already running a Browning-Drake or similar circuit, will be made up from the components already in use. The separate amplifier is a good proposition, as it may quickly be hitched up to any circuit, permament or ex. perimental, or short-wave adapte;. -THE BASE AND SHIELDING. Tr basebcard is made of 3-Sin. rimu, 19 by 114 inches, with three battens underneath of the same thickness, one at each end and one in the centre, each measuring lin. by 113in. Shellac dissolved in methylated spirits makes a neat finish for the woodwork. The shielding is constructed of sheet copper, 28’s s.w.g. gauge, tinned one side or plain both sides. This is usually obtainable in sheets 4 feet by 2 feet. If tinned, let the tinned side be the inside, and finish the outside with black cycle enamel. The panel may conveniently be made of zinc or aluminium of 14’s or 16’s gauge, measuring 19 by 88 inches. The side of the shielding boxes that comes against the back of the panel is omitted, and the end of the box butts close against the panel. Each copper box stands on its own shielding tray, which is screwed to the baseboard, a quarter of an inch teing allowed between each tray. Half an

inch is turned up on three sides of the trays, and where a wire passes from one shield to the next, a V notch is cut in both the sides of the tray and the lower edge of the shielding box, and a short. piece of spaghetti covers the wire to ‘Pro--tect it from damage.

FoR the shields, a whole sheet of copper will be required, and in addition a piece 21 by 19% inches. The full sheet ‘must be cut up us shown in the diagram, otherwise more copper will be required. By

cutting, as shown, the three trays’ come out of the full sheet with two boxes, and the third box is made from the extra piece. All edges of the boxes are turned in half an inch to give strength and stability, and this éxtra must be allowed for in eutting as shown. Tach box is cut

in one piece, the back having a halfinch, turned in at each side, which is turned in and soldered to the sides. This soldering cannot easily be done with an ordinary iron, as the copper conducts the heat away very rapidly. | A blowpipe, even a jeweller’s mouth | blowpipe and methylated spirit flame, . will answer quite well. | LL bending should be done over , the edge of a suitable piece of wood or metal, and care must be taken to have the boxes a shade under rather than over the dimensions. A coat of black cycle enamel completes the copper boxes. All bends are shown by dotted lines in the diagram. THE panel is drilled along the bottom edge to screw to the front edge of baseboard. At the top corners a 1-8in. hole is drilled, through which a brass bolt is passed to secure the 3in. brass strut at each side, 18’s gauge brass being used, the total length being 1lins. The panel comes below the baseboard and hides the space left by the battens. It will be noticed that the centres of the condenser dials are not central to the boxes, but this arrangement is necessary. Care must be taken to drill the central holes in a: position to suit the particular make of condenser purchased, but the positions as shown will fit most makes.

It is to be particularly noted that the right-hand condenser controlling the detector stage must not have either fixed or moving plates earthed. To accomplish this, a square of ebonite is to be bolted to the back of ‘panel, and to this the condenser is to be fixed, all holes in the panel

being drilled well oversize to clear the spindle, etc. A piece of ebonite about 4 by 2 inches may be. sufficiently large. No metal part of the condenser is to contact the panel, THE AERIAL TUNING COIL. | HIS is construction on-the | "low loss plan, of 20’s s.w.g tinned copper wire, spaced with 24’s, Sixtyfour turns are to be put on. The ‘coil is made in the usual way for space-wound coils, on a 38-inch cardboard or other former which is afterwards removed leaving the turns supported by three 8-8in. strips of transparent celluloid inside the coil, and three other strips outside. ._ The celluloid is cemented to the turns with liquid acetone in which chips of celluloid have been dissolved. A triangular piece of 8-8in. rimu Jis cut as shown in the plan of set, the corners being sawn off to give a small flat, to which the lower end of the celluloid strips is serewed. The lower end of all coils is to be a full inch clear of the copper tray. ’At the 18th turn, counting upwards from the bottom, a tap is to be provided for connection to the small series condenser in the, aerial circuit. This condenser may be placed in the most convenient way. It is shown mounted in clips on a strip of ebonite 14 by 3-in., and the bolt at one end of this attaches to a small brass bracket which is in turn bolted to the' back of the aerial terminal. This terminal is mounted upon a piece of ebonite 1% by i18ins. screwed to the edge of baseboard as shown. A small piece is cut from the side of the copper screen to clear this. The earth terminal is bolted direct into the edge of the tray. The panel and all trays must be electrically connected. This is done by bending together the front end (at the panel) of adjacent turned-up sides and soldering them together. To connect the panel to the trays, solder the strut to the tray near aerial terminal. It is handy to have the fixed aerial condenser in clips so that it can be changed if necessary. A variable condenser in this position takes up too much room, and is practically no advantage.. THE NEUTRALISING CONDENSERS. WO are required, maximum capacity .000025 mfd. The make used by the writer is the Igranic (Continued on Page 13.)

The Two R. F. Shielded Browning Drake

Continued

micro-condenser, which is particularly suitable. In this condenser the plates dre easily removed and replaced. The correct value is obby removing two fixed and one moving plate from each. The condensers are fixed by drilling a hole in the centre of the bakelite cover, Which can then be secured to the baseboard by a screw and the condenser placed in position. Across the knob of each a slot is to be cut with a hack-saw so that the condenser may be turned with a thin stick cut at the end like a screwdriver. A hole is to be drilled in each R.F, shield directly above the knob of the condenser so that it may be turned’ by inserting the neutralising stick through the hole whilst the shield is in position. ‘VARIABLE CONDENSERS. HE values are .0005- for the first stage and .00025 for the second and detector stages. The three vernier dials should have a.ratio of not less than 10 to 1. The two rheostats should, be wire-wound, of 30 ohms each, | RF. TRANSFORMERS. | HERE are two R.F. transformers, ‘ only the one in the detector stage requiring a tickler. The coils nearly alike as possible, 82 turns of 22’s s.w.g. tinned wire on each, for these should both be made as spaced with 24’s wire, which is wound off before cementing to the celluloid. When these coils have been fixed in position, examine them very carefully to make sure that no turns have become misplaced so that they are in contact with an adjacent one, as.such a happening may cause quite an amount of mysterious trouble. Three strips of celluloid are put on both inside and outside as for ‘the aerial coil. The coil that is to bear the tickler must have a wider piece of celluloid, half inch, put on at the back, and at the top, above the turns, a piece of celluloid should be put in between the two, and cemented in place. This is to form a thickness through which a iin. hole is drilled to act as a bearing for the back end of tickler spindle, which is Zin. ebon ite rod. The lower end of each coil is secured to a wooden triangle with serews, the lowest turn of the coil to be a clear inch above the copper tray, so that the celluloid strips must be left projecting from under the turns when winding on the former. "THE primary coil for each transformer is made by winding irregularly the prescribed number of turns for the valve in use. These turns are wound irregularly round a formera battle will do-slightly smaller than the inside diameter of the coils. The wire to be used is 30’s d.c.c. The turns are then bound together in three places with strong thread. In order to thoroughly insulate the primaries from the secondary turns, a in. strip of celluloid is fastened round the inside of the lower end of each secondary coil, projecting below about a sixteenth. This is secured by cementing to the upright strips, the joint being made on one of these. The primary coil is then placed inside the lowest turn of each.

secondary and secured there by tying to the celluloid strip with fine thread. Without this precaution of insulation there is the possibility of a: "short’ between the two coils, which are connected to B positive and B negative respectively. The upright position of the coils and limited space within the shields necessitates a special arrangement for the tickler control. The back end of the tickler spindle works in a bearing in the celluloid strip. The lower side of the spindle is to be 4-inch above the top turn of the coil. The front end of the spindle is supported by a bearing consisting of a strip of wood screwed to a cross-

piece fastened to the base. A wooden pulley of 1 to 12 inch diameter, the latter best, is placed upon the spindle, and another pulle;; of the same diameter works upon a 4in. spindle near the baseboard as shown, th front end of spindle working in the panel and back end in a small wooden bearing. A strong thread is passed twice round each pulley and fastened to each at its central point on the pulley, which is conveniently done by drilling a hole through diagonally and pulling a loop through, fastening on face of pulley. There can be no slip with this arrangement. The only way to dispense with it is to place the condenser

lower, put the rheostat at top of | panel above condenser. O doubt some readers will be writing to ask if the shielding can be dispensed with. It cannot. The whole success of this receiver depends upon the shielding, and the extra R.F. cannot give amplification without it. Moreover, the construction of the shielded receiver is paving the way for the addition of the screen-grid valve at an early date. The writer is expecting the arrival in New Zealand of stocks of a special low-capacity R.F. valve, which may be very suitable, and there is shortly to arrive the English type of screen-grid valve. This receiver will need little alteration to accommodate the new valves. ' There will probably be queries as to the advisability of reducing the size of the copper boxes. Well, constructors may alter sizes if they wish to do so at their own risk. The nearer the shield goes to the coils, the more damping will there be. After all, we are not building these sets for export, and thinking as much | about packing space as efficiency, so why need the size be pinched? Next week further particulars and helpful diagrams will be given, probably concluding the article. MATERIALS REQUIRED.

QUERIES BY CORRESPONDENCE. 1. Every communication enclosing queries is to be addressed to ‘‘Meg: ohm,’ Box 1032, Wellington, and must be accompanied by a stamped address: ed envelope for reply by post. 2. Questions must be written so that a space is left in which the reply may be added. 3. No charge is made for replies. CEE EEE EEE EEE EEE EE

Variable condenser, .0005 2- Variable condensers, 00025 ..° .: 1 8 Vernier dials, 10-1 .. 2 Rheostats, 30 ohms 2 UX valve sockets 1 Benjamin socket, sprung Fixed condensers, .00025, 001, .0001 2 Neutralising condensers, max. .000025 + Grid leak, 4 megs, and clips 4-Ib. 22’s s.w.g. tinned wire 3-Ib. 20’s s.w.g. tinned wire Knob for tickler .. ° Panel, zine or 19 by 82 ee Copper for shielding Connecting wire, screws, etc., ete. oe ee ut ror) 12 me we GO Ct bet ~] Tp oo OH pep ww Slo SOSH COnuHWPO Seo Aanoaoeo oe