The A.C. Browning-Drake

Radio Record, Volume III, Issue 10, 20 September 1929, Page 36

The A.C. Browning-Drake

RE. and Detector Unit

(Bp

the Technical Editor)

Accompanying i is the R.F. and detector stages of the A.C. Br owning-Drake. The: audio stages have been omitiéd, beeause the writer considers. that the constructor will be well advised te build up the ‘A.C: amplifier and get it working before attempting the detector and radio stages, A.C. constriction is simple, and providing instructions are followed carefully, no difficulties will be encountered, but most constructors do otherwise, and then the trouble Starts, —

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HB name "Browning-Drake" needs little introduction, Ii is a class name given to ‘those receivers which embody the invention of Glen H. ‘Browning: and his associate, Drake, two Americans -who. first developed mathe-

Iatically, and :.then practically, .the tonception of a primary slot wound with fine wire, the impedance of ‘which is balanced against. the ‘impedance of. the preceding © radio frequency valve. . The original circuit was a 4-valve D.C, A radio Btage was added, and latterly the coils have been adapted so that the screen grid valve can. be incorporated, but in this it losés its individuality. Further improvements -haye been made. in the ‘method of neutralising, split. primary, ‘an addition tothe secondary winding and parallel feed to the R.F. stages ‘have proved themselves great improve‘ments, but all ‘these have taken some ‘time to perfect, and have very rarely been combined in one set.. The A.C, "‘Browning-Drake, the last of its kind, Gncorporates all‘the advances made by the models which have preceded it. Its sensitivity equals that of the 4-R.F. ‘'B.D., while tone by the addition -of ithe ’ push-pull amplifier, is improved considerably. The receiver, unless ‘used with a wavetrap, is notoriously ‘anselective. Single dial control has en added, not because ‘of its greater efficiency, but because of its marked simplicity. Shielding is another adaptation which, although not essential, ‘simplifies wiring to a remarkable ‘degree, as a glance: at the lay‘out of the completed receiver will ‘show. The writer has selected the moving‘eoil tickler, not because he considers it an improvement on the condenser ‘control (for in many ways the latter

4s to be preferred) but because. the greater number of owners of Brown-ing-Drakes, who wish to convert their sets will have on hand these coils. Moreover, it fits better. However, full explanation is given whereby capacity reaction may be employed, The 227 type of valve is nocessaty in any detector stage, in. all A.C. sets, and it has been selected ‘for the radio stage because there is no. possibility of A.C.:hum,. By .a slight adaptation, the’ 226 may be used as a radio. valve, "Making. the Coils. BTAIN about 10in. of 23-inch tubing, either cardboard’ or bakelite, ‘Wind on 84 turns of 24 gauge D.C.C: wire or 68 of D.S.C., in each case without spacing. Other gauges of wire may be‘used if the constructor has them on hand. . The numbers of turns for these can be. obtained from ‘the table. At the 18th turn pierce the former and, making a- loop in the wire, pass. it through and secure another piece of wire on to this,. preferably of heavier gauge. This will be the aerial. tapping. Having completed the aerial coil, leave a ,small-space, and. proceed with the wiring of the regenaformer. This will comprise 65 turns .of 24 gauge D.G.G. wire, or 57 turns ‘of D.S.C. If eapacity .reaction is to be employed

tap the coil at the 15th turn from the secondary .proper. Wind a_ further 26 turns on the end. of thié coil, tapping it as before at the end of the secondary winding. proper. Separate both coils with a sharp knife. The primary for this regenaformer is jumble wound and placed under the

first turn of the secondary coil. It. coniprises 14 turns of 30 guage .D.S.C. . wire. There are three typical ways of making this coil, the diameter in every case being such that the coil can be slipped inside the end of the secondary without leaving an appreciable space between the two coils, though there must be proper insulation between them, otherwise a "short" of the B potential may result. ‘The: three methods are shown in a diagram, One is a bunch of turns -of the requisite diameter, bound. together in "places with twine. .The second method is to puild up a slotted former of cardboard, the slot round the periphery measuring barely 3-16in. each way. The third method is to turn a similar former out of wood. ‘Rotating ticklers are closewound upon a short former of small dimensions; the turns should not be spaced on ticklers, There is no need to:endeavour to make the tickler as large as can be rotated in the end of the secondary.- A small tickler with a few more turns to compensate will give even better results, Bring’ out gil. the connections and ‘:gsecure them by small- brass. bolts at one end and label them: Aerial Coil‘Top G; bottom, BE; tapping A. . Re--genaformers.Secondary. tap G;._bottom B-; tapping N. Primary-tIn-put P; output B-,.and join to both os, * . . Any convenient method of -mounting y may be employed, the diagram suggests , y , d d d y

method is to secure to the end opposite to that carrying, the terminals, two or three brackets from a meccano set. To one ‘ofthese brackets. should pe secured the "W" end of the aerial coil, while on .the. regenaformer both It’s go to a bracket ‘and. so. to the shield. This connection is’ shown. quite clearly in the theoretical. diagram. . .. For those who do not féel. confident or who have not the ‘time and patience to construct their’ own tuning’ coilsy there are very many commerciallymade Browning-Drake outfits... These have been specially designed so as to suit the 201A type. of valve, and .as the’ 227 is the A.C. version of this valve, the primaries of these are of the right impedance, . Some difficulty may be encountered in neutralizing the type of commercial

coil employing the. additional to secondary winding method. . In_ these "N" must be disregarded and a fresh tap made as described. Again, some commercial coils require the 0005 condenser first according to the size of the coil, That with the. larger secondary winding requires the smalier condenser. It is immaterial which condenser comes first, but the ¢oil must suit. — See table. Mounting. the. Components, . OR the panel a piece of ebonite of _formica undoubtedly makes the finest job. The constructor who does feel .disposed to ‘buying a. suitable panel may make quite an excellent one out of a piece of three-ply which has been immersed in. a*solution made by dissolving old gramophone records in methylated spirits. Another way is to spray with duco. — " The lay-out of the panel is very simple, but owing to the -yariance in. the sizes of drum dials and. components, it is almost impossible to gire definite measurements. In the cartoon , ‘of each dial, there is found ‘a’ template’ which indicates: quite .cledrly where "each cut is to be made. This should be pasted on to the centre of the dial, so that when. .fixed:.into -position- the -moving vanes of the. condensers will not touch the ‘baseboard. .This means that ‘the spindle on. which the con-densers-are mounted should: be at-least two inches from the baseboard. e Three other mountings remain for

— the panel, the balancing condenser, the tickler, and the volume control. This latter (not shown.in diagram) may be placed anywhere on panel to give balance, and be connected to cathode and grid of ‘detector or B- and grid of RF. If a condenser is used in control reaction, some little trouble will be experienced in fitting if so as: to clear the tuning condenser. Reference to the diagram shows the position of the various.knobs. | __ ’ The Baseboard Mounting. BTAIN a piece of 15in. x 10in. x din. timber. or three-ply, .shellac it well; and secure to the under surface "two small strips sufficient to raise it from ‘the ground. This is to permit sub-baseboard wiring. Mount the drum dial ‘and shield boxes. and find out where to ‘drill spindle for condensers, "tickler, volume control and balancing \conderiser. Dismantle and drill. Replace "shields and mount condensers. In’ all probability it will be necessary ‘on one: of these condensers to'shift the projecting sHaft through’ to'the other side. This‘ condition will: be determined if’ the’ constructor ‘finds’ that ‘he cannot put on’ one’ condenser because it appears back to front. Arrange them eso that. the plates will move outwards from the panel, and downwards to the baseboard. "° To shift the shaft of the condenser loosen the two screws that appear on the side of the moving plates opposite the half on which they are secured. Now determine the lay-out of the rest of the shield boxes. Mount coils and valve sockets.so that G of each will be as near as possible to. the valve sockets. The.sockets are nearer the

at front to allow the condensers to clear coils of the variable condensers. Mount the neutralising: condenser on the floor or side, as the type may require.. If to be mounted on the side drill a very large hole and place mica washers over thread to prevent any portion earthing.. In this manner the

contents of each‘ box may be fastened into position. Keep coils as near the centre as possible. Screw each firmly to the bottom of ‘the-shield. It may. be necessary to drill the baseboard -a-little in order to allow the heads of the nuts to settle down without putting the

shield box off the level. Mount the R.F.C. and’ the 1 mfd. by-pass condenser at the back of the dial between the’ two shields. The Wiring. ROM the top of the aerial coil G run wire to the fixed plates of each of

the .0005 condenser, the neutralising condenser, and to "G" of the valve socket. By securing the tuning. and balancing condensers to the shield the moving plates are automatically connected with the--shield which should be connected to the earth terminal. A sol-

-- wes dered joint cannot be made if aluminium shields are used, so'slip a washer underneath one of the corners of the box shields: and connect this to the terminal. The aerial is brought ‘in to the tapping and the moving plates of both balancing and tuning condensers connected. ' Drill a hole through shield and base board -near the legs of. the socket marked "F," "Ca," and "P." From the "p" terminal take a connection’ ‘under neath the baseboard to the radio fre~ quency choke and to one sidé-of the the by-pass condenser ,(C5),.. The’ ‘other side of the by-pass condenser goes" to the free terminal of the primary.(P), The other side, it will be remembered, is connected: to the | secondary and shield (B- to B- to BX). The shields are automatically connected: by the spindle of the drum dial. Jdin fixed plates of neutralising condenser N to N of regenaformer by an under baseboard connection. Take the "G"’ terminal of regenaformer and connect it to the fixéd plates of .the C2 condenser and: to one, side of. the grid. leak (R2) and condenser (C8). ‘;The other side of this is connected with the valve socket "q." Keep this grid. leak and zone denser clear from the shields by mounte ing it on the valve socket or ona sHeet of mica. The plate of the valve. ig connected with one side: of ‘the tickler (P1), the other side (T) to the O04 fixed condenser (7) and to a piece of flexible wire or terminal (T). Cone nect the detector cathode and the other side C7 each to the shield. Take a piece of. twin flexible wire and connect the: two filaments underneath the

= baseboard, and from one of the sockets connect to a long.lead of twin flex; The battery cable is connected thus: one. wire to the L5, another to the cathode of the R.F, valve, two to the dial light and one to the shield (B--), Connect to the eliminator or power pack as follows:-Lead from Ld to B -+-.130 or variable, cathode to the terminal that was placed on the panel of the amplifier Jabelled "cathode." In other words, it is connected with the cathode of the amplifying valve which is connected to a series resister and a by-pass condenser to earth,See theoretical diagram for alternate eonnection. The dial light. is connected to appropriate voltages, usually 5, and the valves. to either 4 or 2. 5 If they are 227 they will require 2.5, Other. indirectly heated valves volts.’ Neutralising. VERYTHING is now ready for a try-out, Connect one lug of a pair of phones to free terminal of tickler end the other lug to B+ detector, turn the current on and moye the reaction condenser over until a plop is heard; This will indicate that the set fs oscillating. If it does not oscillate, look for faulty wiring, bad connections, or wires that have dropped off, If the A.O., amplifier has been constructed and is working examine the inside con-

--a nections to the termisjals to see if all is intact. 5 , : To neytralise, tune in a station about the frequency of 1YA and advance reaction until the set oscillates. Move neutralising condenser until it stops, ‘Readjust until a point. is found when further advancing the neutralising con-

denser will-not prevent oscillation. The set is now neutralised.. The Diagram, "THESE are mainly self explanatory. It will be noticed double — pro-

vision hag been made in diagram 1 for bias to edthede,. Hither can be used, not both. Hither take GA to cathode of amplifier or insert a resistance of 8,000 ohms (R1) and a by-pass con: denser 1 m.£.d. (G6) as shown. Tf there is 2 tendency to hum, connect a, centre-tapped resistance of about 40 ohms (R3). across the filament of the detector, . earthing the centre tap. This connection must be from dead centré. C8 and R2 ¢an be arranged so that it can be cleared by the moving vanes of C2. The letter X denotes a connection to the shield by a washer under the corner support. A small cirele indicates a hole in the floor of the shield. | It is hoped to publish photographs next week,