Crystal Set Construction

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No. 96

CRYSTAL SETCONSTRUCTION

SIMPLE· INSTRUCTIONS FOR MAKING

NINE CRYSTAL SETS

"" B. B. BABANI

There is to-day Q marked revival of interest ill cry,,.receivers. and this book has been specially prepared to introduDt

1M hom e con stru ctor to the design and building of tM vanow typa.

TM text Is simple an d free from techn icalities, so tM t '1'm tIM .

young readel' m ay con fidendy embark on tM con stru ction 0/ any o f, .

t~ receive r« shown ,·

CONTENT !>

I. AERIALS.

2. BAaTHS..

• , CR.YSTAL DBTECI'ORS.

.. COILS.I. THE BEOINNERS' CRYSTAL

RECEIVER. •

" Fl.AME ABlUAL RECElVElL.

1. LONO DlSTANCB RECEIVER..

I. JUGB GAIN 1lBCIIIVEIl.

t. JUOB SELECTIVITY IlEcmVEIL

10. BATI'ERY AIDED Cll....STA.LltECEIVER •

n, MEDIUM AND LONG BANDRECEIVER.

l2. ULTRA SBNSII1VB ALL WA.ftllECElYBR.

11 AN ADVANCED CltYSTAL DlODIlLECElVEll.

"THER .E ARE '' 2 DIAGRAMS

L '0 N D.O N BERNARDS (PUBLISHERS) LI~ITED

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PI8.1 SYM IOLS USED IN TH E CIRCUITS D ESCRIB ED ..



_ _ _

I. Aeria ls..

One of the essentials for satisfactory recept ion with crysta l sets is an efficientaerial. Four different types are suggested and are shown in Fig. 2.

There are several important points to be noted when erecting an aerial fora crystal set.

( l) See that it is suspended as high as possible and is not screened by taU

buildings, obstructions, etc. .(2) The aerial itself and the lead-in from the aerial must be adequatelyinsulated. Be sure that the porcelain or plastic insulators do not permit theaerial or lead-in to touch the building, tree. or post from which they aresuspended. The lead ..in should be brought into the house through an insulatedrod.

(3) It is exceedingly important to use insulated copper wire. either singleor multi-stranded. for both the aerial and the lead-in.

2. Ear ths. .An adequate and efficient earth contributes to the good performance of any

well-designed crystal set and Fig. 3 shows the bes t method of obtaining 8R

efficient earth where there is a garden or patch of around available at a shortdistance from the receiver.

Obtain a 3..ft. length of i-in. ~vani.sed iron pipe and at one end drill abole right through. Insert a bolt WIth spring washer and locking nut in tho hole,Connect the earth lead-in wire to the bolt; the other end of the lead ..in to theearth connection on the receiver. Leave botb ends of the pipe open. Insertthe end opposite the bolt into the ground vert ical ly until only JMin. are showinaabove the surface of the surrounding earth.

It is important to see that the surrounding ground is kept damp, if necessaryby occasionally pouring a pint or two o r water down the open projecting endof the pipe.

Ready-made copper earthing rods with a pointed end for easy hammeringinto the earth arc sometimes obtainable from radio dealers.

Should a garden not be available, an alternative earth is a cold water f i p ecoming from a rising main. Simply file very lightly the pipe at the back 0 thetap and wind the copper wire earth lead-In round the filed part and cover thejoint with insulating tape.

In no circumstances should a gal pipe be used for aD earth.

3.. C rys ta l DetectoR ..Most radio dealers can supply very efficient permanent or semi-permanent

crystal detectors of proprietary makes. However. there arc alw,\)"S alf8l.!e........quite a number/of' ex-Government surplus units which are called crystal val\faand, though these were originally designed for a very different use in serviceequipment, they are easily adaptable and will give extremely efficient results.when used in circuits designed for them. To enable the constructor to purchase

suitable types for this purpose-and they cost very little-the following listJives the Government reference numbers.CV 10], CV 102, CV 103, CV nt, CV J 12 , CV 113, CV 226, CV 241, CV 246,CV 247. CV 253t CV 291. CV 351, CV 361, CV 364, CV 367, CV 727, CV 749~and CV ]785.

One of the satisfactory proprietary makes of this class of crystal detectorit the 8TH CS7A, which, incidentally, is the commercial equivalent of the CV 253.

For all these Government types, one connection should be made to eachend by arranging a holder to fix them into or by carefully soldering the appropriateconnecting wire s to each end.

1



~ . ~ - - - - - - - - ~ 6 0 0 ' - - - - - - - - - - - - -~NSULATOR

( AS HIGH ....avE GA.OUNO

AS POS$IILE.

UA,D-IlriPO'-I .,,/OR TREE.

~INVERTED- L' AER(AL

PLATFORM AERIAL

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LtAD-IN

AERIAL SYSTEMS



4 . Coils .In Fig. 3 two methods are shown offixing the coil to the chassis, However,

the ingenious constructor may evoJve another arrangement. .The important thing to remember is that the coil should be fixed so that h

is completely insulated from the metal chassis that may be used.It is also important to remember that if a metal chassis is used, one must

see that all components and wiring, with the exception of earth connections,

are adequately insulated from this chassis so as to make sure that no shortcircuits take place. It is recommended that all crystal sets should be builton a panel, and that both panel and chassis should be of bakelite, paxolin orperspex, as these three materials are efficient insulators in themselves.

5. The B eginne rs ' C rys ta l R e ce ive r .The popularity of the crystal receiver among beginners is undoubtedly due

to the very modest outlay necessary, and the ease with which such receiversmay be built. It is not always appreciated. that, to obtain worth while results.

TOAleINE •

EARTHSYSTEMS

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FIGURE3

WI" 'D~NG

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YI.RTICAL. MOUNTING

COIL MOUNTING METHODS

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rar more care in avoiding losses is necessary than with a three or four valvereceiver. ODe reason for this is the fact that the crystal receiver can offer nohelp by way of amplification and it is therefore dependent on an efficient aerial}earth system, low loss design within the receiver itself, and the use of sensitivebeadphones. The experience gained is well worth the time expended since,when more ambitious receivers are attempted, the constructor will rememberthe benefits gained by efficient construction and installation with the result that

each receiver attempted will be an instrument capable of giving a firs t classperformance.

Losses within the receiver itself may be considerably reduced by efficientcoil design. Unfortunately, selectivity is a factor which must riot be overlooked;it is this factor which governs the sharpness of the tuning and permits the separa-tion of adjacent powerful stations. The greater the selectivity, the greater theease with which signals may be separated; but at maximum selectivity thereceived signals are at minimum strength. InFig. 4, the coil is tapped at fourpoints, which enables the best point between selectivity and sensitivity to bechosen. The nearer the aerial tap to the: earth end of the coil, the greater theselectivity. The pre-set condenser Cl also aids selectivity ~ at minimum capacitythe selectivity is high, and vice versa.

U s ed in conjunction with the coil taps, the constructor should have no

-tifficulty il l selecting a condenser setting and tapping point which permit the~eption of strong signals and at the same time effect the required separationof powerful sta lions.

The coil is constructed on a former 3-in. diameter x 2-in,! and consistsuf S O turns 36 S.W.G. enamelled wire close wound and tapped at every tenthturn. These taps are brought out in the form of loops twisted firmly so thatthere is no actual break in the winding continuity. After completion theiO ·~ ndn g m a y be pain ted with Durafix to prevent any movement, and the tapsand two free ends cleaned with emery cloth to remove the enamel. The layoutand wiring is shown in Fig. 4. The headphones should have a resistance oftome 4,000 ohms or more and should be of reputable manufacture. Any of.... terials previously described are suitable for use with this receiver.C2 .0005 variable condenser. 2 oz. 30 S.W.G. enamelled copper

Cl .0002 pre-set condenser. "ire.C3 .001 mica condenser. . 1 C k n . t termer 3-in. dia. x 2-in.t Crystal detector, semi-permanent I Ebonite or Bakelite panel 6-in. x

pattern. S - m . x i - in.4 terminals, connecting wire, etc.

~ Frame Aerial Receiver for Local Station useon Medium wave bands.

This receiver was specially designed for constructors who are within5 to 10 miles of a powerful station and who have no facilities for constructinga good outdoor aerial.

The frame itself consists of two i-in. x t-in. square section wooden rods

each 36·in. in length. These are placed together at their centre fine to form across: they are fastened by a nut and bolt and these in turn pass through anupright piece of wood i-in. x l-in. section 2·ft, in length. This upright supportis then screwed into a baseboard 8~in. x 8-in.x J -in. thick. At each of thearms of the 36-in. cross a 4-in. strip of i-in. square section wood is placed.Reference to Fig. 5B will give the details of the construction of this aerialQuite clearly.

It will be noted that approximately 85-ft. of 20 g,W.G. double cotton-~ .vice arc required for the induetance to cover the medium wave broadcast'band, The wire should be wound around the four crossbars, thereby creating

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Ll CI CI

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'IG ..... THE BEGtNNER~CRYSTAL RECElVER .

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a square wire frame, and each tum of the wire should be spaced approxunatelyi-in. The best way to effect this spacing accurately is to file on the 4·in. crossbarsa number of small nicks at !-in. intervals. The two ends of the wire shouldthen be brought down on the two terminals which may be screwed into the",-.are baseboard and two leads taken from there to the crystal set itself.

The actual set may be constructed on a small ebonite, paxolin or perspex

panel S·in. x ' - . i n . which may be screwed to one side of this base supportingpiece of wood, thereby making the set virtually self-contained except for theearphones. Fig. SA shows the circuit of the receiver.

To obtain the best results with this receiver, the aerial should be rotated byhand until the signals become loudest.I Semi-permanent crystal detector.Cl 1 .0005 mfd. variable condenser.C2 1 .0005 mfd. f ixed condenser.1 condenser dial marked 0 to 100 or

o to 180.

I pair of high sensitivity earphones.8S-ft. to 90-ft. of 20 S.W.G. double

cotton covered wire . (80L)Sufficient connecting wire for wiring

purposes.

7. Long D i s ta nce R e ce iv er .

This receiver has been specially designed to achieve high sensitivity, Witha sufficiently good outdoor aerial effective reception has been obtained up to150 miles .

The circuit is quite straightforward. as can be seen from Fig. 6 and theonly special point is in winding the coil Ll.

This coil consists of a 2i~in.diameter former on which are wound a totalof 5] turns using 24 S.W.G. double cotton wire . Starting at one end, tap everytwo turns until there are eight taps~ wind a further 15 turns and tap; thena further 10 turns and tap; finally, add 10 turns to end the winding. The wire·should be close wound, t.e., with turns touching.

To obtain the best resul ts , the two crocodile clips A and B should be triceon the various C"il taps unti I the loudest reception is obtained. It may b e '

found in use that the loudest reception of any particular station causes anotherstation to be heard at th e same time; care should, therefore, be taken to select

the appropriate tapping point to separate any two powerfu! stations that mayinterfere with one another.4 oz. 24 S.W.G. DeC copper wire .Coil former 2l-in. dia. x 3..in.r:1-2 single variable condensers

.00035 mfd.2 Condenser dials marked from

o to 100 or 0 to 180.

1 Permanent crystal detector.C3 .001 f ix e d c on de ns e r.1 pair of high sensitivity earphones

(preferably between 4.()(M)ohmsand 8 tOOOohms im pe da nc e) .

2 Crocodile clips.

8_ H igh Gain Rece iver.

This r ec eiv er has b e en d e si gn ed to give good reception up to 50 m ile s dis tancefrom any normal power broadcasting station in the medium band. It is a very

lClective circuit and will therefore enable the user to separate closely situatedstations.

Itwill be noticed inthe circui t (Fig. 7) that the variable condenser CI; hasan on/off switch SI to throw it out of circuit if necessary. It is used to increasethe selectivity of the receiver and, in operating, it should be tried with thiscondenser either in or out of circuit on the various tappings on the cou Lt.Ll consists of 90 turns of 22 S.W.O. double cotton covered wire wound

on a 2-in. diameter former and tapped every ten turns. L2 separately woundon a 2-in. former, consists of 90 turns of 22 S.W.G. double catv. Cf....-ad. . . . r n . also taoped as U. The componenta required are given on 1JaII ;

......... .... - .. .._ .._ - _ ..------------------



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CI

FIG. SA FRAME AERIAL CRYSTAL AECE1VER

IoftAVY e . t . s £

FIG. 56. DETAILS OF FRAME AERtAL

FIG.6 LONG-DISTANCE CRYSTAL RECIE IVCA

C3

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CI-2 Single variable condensers.00035 mfd.

51 Single pole single throw switch.Qi .0001 pract condenser.1 Permanent crystal detector.C3-4.001 mfd. fixed condenser.

1 Pair of high sensitivity earphoneL8 oz. 22 S.W.G. DeC •2 Dials for variable condenser

marked 0 to 100 or 0 to 180.

9. H igh Se lectivity Rece iver .This circuit (see Fig. 8) is for use in high saturation strength areas where it is

deI i red to separate powerful stations. The circuit is not complicated and isextremei)' effective.

It wiU be noted that Cl and C2. each .00035 mfd. capacity, are 2 gang unit.The only special point in this set is the design of coils LI~ 2 and 3. which

are constructed as follows: Ll, 60 turns of 22 S.W.G. double cotton coveredwire on a3 x 4 inches former tapped every fiveturns; L2~ 15 turns of 22 S.W.O.double cotton covered wire on a 3 x 6 former; and L3. 85 turns of 22 S.W.G.double cotton covered wire wound on the same former as L2, the two windingsbeing separated by approximately i·in. (see Fig. 8).

Note that the aerial lead is . brought down to one of the tappings on wire LI.

Thia should be tried on each of the tappings and the one giving the required.lectivity should be used.Cl ..2 2-gang variable condenser

.00035 mfd.2 3-in. fo rm er s fo r coils Lt-2.-:J.I oz. 22 S.W.G. DCC.

C3 .OOO~fixed condenser.I Sem i . .permanent crystal detector .

10 . B atte ry-a ided Crysta l Rece ive r . .Fig. 9 shows a design which utilises a 9-volt grid bias battery to improve

pcrf'ormance.The charge built up in the .02 mfd. fixed condenser is changed to a positive

current when it passes through the coil Ll. This, in turn. is superimposed on

the positive cbarge that is commencing to be rectified by the crys.tal detectorand thereby increases the signal strength.Coil Ll consists ora 4-in. diameter former wound with 85 turns of24 S.W.G.

double cotton covered wire tapped at . s , 25,45 and 65 turns from the ear th end.The earth end and these four taps are brought out to five small sockets and theconnection from the earth is taken to a small socket. The plug should be triedin each of the five positions to find which one give!!:the greatest signal strength,

The 9-volt battery is connected across the .02 mfd. condenser and specialcare should be taken to see that the battery is connected correctly, t.e., negativepole to earth.1 Permanent crystal detector.1 4-in. dia. x s-in, former.8 oz. 20 S.W.G. DCC copper wire.Ct Variable condenser .0005 mfd,C2 Fixed condenser .005 mfd.

C3 Fixed condenser .02 mfd.1 pair of high sensitivity headphones.1 elix plug.S Clix s oc k e ta .

1 9-volt grid bias battery.

II. Medium and Long wave band Receiver.This set has been designed for reception on two wave bands.Care must he taken in winding the coils LIt 2,3 and 4. These are wound

on a single 3-in. diameter former, Ll consists of 30 turns of 28 S.W.G. Dectapped at 8, 16 and 24 turns. L2, 60 turns of 32 S.W.O. Dec. L3, 40 tUlDJ c J I

28 S.W.G. Dec, and U, 80 turns of 32 S,W.G. OCC. All a re close woundand are separated from each other as followa~ ~tweeo Ll aad L2. 3/16-m.1

9



II

C!

LI

CI

FJ~ 7 HIGH-GAIN CRYSTALRECEIVER

FIG. 8 HIGH SEL ECT1V1TV CRYSTAL RECENER

c. _:r...-J.1

tv. G . ' . "'fTl.R't'

. .FIG.9 BATTERY-AIDED CRYSTAL R.ECEIVER

10



be tween L2 and LJ, t-ln.; and between L3 and L4, 3jl()..in. See Fig. 10.CI-2 single variable condensers C3 .001 mfd. fixed condenser .

.0005 mfd. 1 Pair of high sensitivity earphones.2 Variable condenser dills marked I 3-in. dia. x g·in. former,

o to 100 or 0 to 180. 202 of both 28 and 32 S.W.G.1 Clix plug. double .cotton co....red wire to3 Clix sockets. make these 4 coils.

81 Single pole double throw switch. I Permanent crystal detector.In use. the plug should be tried in each of the three sockets to find the pohn

which gives the best results. The switch is used as a wave band change switch.

12. Ultra Sensitive All Wave Receiver.

The receiver shown in Fig. 11 is for long distance reception. It is a novelcircuit and, though apparently complicated in construction, it is really verysimple to assemble wi th the minimum of tools and labour.

Itwill be seen from the circuit diagram that there are two banks of sockets,five black and s ix red. When u s m g this set the appropriate black and red plugsshould be inserted in whichever of the similar coloured sockets give the best

results. The coil is quite easy to construct and consists of a primary windingwhich is inserted at the earth end of the secondary winding. For the mediumwave band, the secondary winding L2consists of 54turns of20 S.W.G. enamelledor cotton covered wire. This coil has four taps inaddition to the starting pointand the end connection, and these taps are taken at six turns, 14 turns, 27 turnsand 40 turns from the earth end. The taps are taken out and a connection itmade to an appropriate socket. as. shown in the diagram.

The coil is wound on a 3-in. diameter former which may be a paper tube,bakelite, paxolin or perspex, and spacing is approximately 17 turns to the Inch,

For short wave work down to about 30 metres. L2, the secondary coil, willconsist of the same diameter former wound with I S turns of the same gauge wire ,the four taps being taken at 3,6,9 and 12 turns from the earth end.

LI~the primary coil, is used for both wave bands and consists of 11 turnaof similar gauge wire to that used for L2, wound on a 2-in. x I-in. former, thewinding to be spaced the same as L2. The coil is then inserted at the earth endof whichever secondary coil L2 is being used.

The crystal detector may be one of the semi-permanent types freely availableto-day; the old-fashioned eat's whisker pattern; or one of the silicon permanentdetectors which are sold by many surplus dealers.

The Iphones should be of a standard type 4,000 ohms impedance although'phones of other values will probably operate quite satisfactorily with thispowerful receiver. It should, however, be stressed that the better the bead-hones used, the more satisfactory the reception.

Coils for other wave bands may be experimentally designed by the con-structor, remembering that four tappings are advisable for the secondary coil L2..

A long aerial is very desirable with this receiver, placed as high as possible.It is recommended that a minimum length of 70·ft. be allowed. to include the

aerial and lead-in.The values of the components are shown in the following components list.The constructor should experience no difficulty in obtaining excellent resultlfrom this cleverly designed receiver.CI .0005 variable condenser.C2 .002 fixed condenser.I semi-permanent crystal detector.I coil former 3-in. dia. x 4i-in.Icoil former 2-in. dia, x I-in.

Headphones,

ilb. 20 S.W.G. enamelled or DCCcopper wire.

6 red wander plug sockets.6 black wander plug sockets.~ red and I black wander pl111 •

•fl'lDm.... -"mnecting wireII



CI

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FIG. 10 MEDIUM AND LONG-WAVE CRYSTAL RECEIVER

ei

j!'IG. II ULTRA-SENSITIVE AlL-WAVE CRYSTAL RECEIVER

" i " "

FIG.12 ADVANCED CRYSTAt. DIODE RECEIVER



13. An Advanced Crys ta l D iode Rece ive r.

The modern crya taJ diode of the radar type does not require any adjustmentand 'thus has a distinct advantage over the ordinary crystal detector which needsdelicate adjustment-even the sem i . .permanent type.

Such crys ta ls a rc now obtainable on both the regular and the surplusmark~ and a popular type is the B.T.H. CS7A silicon crystal (also codedas CV 253).

A rather different circuit from the usual crystal set arrangement is neededto sui t the dtaractcristica of a cryatal diode. The diode must be tapped on tothe tuning coil, and it is found that the tuning circuit itself lives best results it

a series-tuned acceptor circuit ilused, The circuit of a radar crystal receiver i,shown in Fig. 11. Tho typo of aerial UICd with this circuit has a very great.bear:ing on the behaviour of the receiver for~ in effect. the tuned circuit, with.aeries resistance equivalent to tho rcftectod crystal load resistance, is in serieswith tho capacitance of the aerial to earth and the aerial's effective serieeresistance.

At resonance-when the combination is tuned to any particular signal-the inductance resonates with the ca~tancea of the tunina condenser and theaerial in ae r i e s , and the fi na l e ffe ct q that the roftcc:tcd load of the crystal. inIer iea with the coil", R.F. rea i J taDoe , II paralleled aero .. tbe aerial's seriesresistance.. For maximum power transfer, the cryJtalload rea1atance must be made- -qual to tho sumaC tho acrial lCrics resistance and the coil R.F. resistancc. and10 the method in which the crystal is tapped. into tho tuninJ coil,-A D d the e xa ctcapacitance required to tunc the receiver to any required aiJnalt must dependto a very areat extent on the amialltlClf.

At the sa me tim e the crystal r ea it t ln ce v ar ie l with the aianal atrcnath. th eresistance bcina hiah for weak aignalJ and droppinl by .. much u S O per cent.lnd more for strona : sipata. 10 that this eBoc\ alJ o has a b c a r i n a : on the correct:oil tap. The output impedanca of tho crystal aIIo varies similarly, affectmathe matching of the headphonea into the crystal diode, and 10 for any set of

conditions. the receiver requires to be matched up to both the aerial and theligna) being received for bes t results, nul would mean a ser ies of coil tapaand (thcorcticallYt not practall)') a matching transformer be tween the diodeand the headphones; but in pracnee itwill be found that a receiver using standardparts can be built up to give very good rcsulllunder various conditions.

It has been shown aJready that the tuninl of the receiver depends to a greatextent on the characteristics of the aerial, and while a .000' mfd, variablecapacitor is shown in Fig. 12 as the tuner, the constructor must be preparedto experiment with different capacitance velues until th e required station istuned in. The range of reception given by the receiver is quite good. if a reallylong and high aerial, and a good earth connection, are used; but no more thanthe local station signal can be expected. and the tuning therefore must be adjustedl(l suit the station frequency.

It must be mentioned that the headphones are shown parallel connected.High resistance headphones of the 4,000 ohm type must be used , and iftwo ·ofthese are connected in parallel rather than in the more usuallCries me thod , theywill provide a roughly accurate match to the crystal. If more than one pair ofheadphones are to be connected in, then the pain of headphones may be leftseries connected in the usual manner; the sets of headphones being connectedin parallel.

The capacitor across the headphone terminals completes the crystal R.F.circuit. and any value be tw ee n a bou t 0 .0 01 and 0.005 mfd. will serve. The highercapacitance will. of COUTSe. by..pass some of the higher audio frequencies. 10

13



that if good steady reception IS obtained some experiment with this condensrris also worth while.Cl, .()(x)Smfd. variable condenser.C2•• 005 mfd. fixed condenser.4oz. 26 S.W.G. enamelled copper wire.

Coil details are as follows: ISO turns of 26 S.W.G. tapped at 20, 25, 30 ,3S , 40 and 45 turns from the earth end. At each tapping-point the wire should

be twisted up into a loop and the windinS then continued without breaking thewire; .when the coil is completed and the ends anchored the tappin, loop' . . . . a a

be bared.

M.R.l CV 253 Crystal diode.Coil former II-in. dia. x 4i~in.



BERNARDS RADIO BOOKSNo.56. Radio Atrid BuAGo!!;57. UltnI-IIIorttt'wN a-Aook51. RMJo. RIO No... ' ...

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