The "Combination" Receiver

Radio Record, Volume II, Issue 39, 12 April 1929, Page 28

The "Combination" Receiver

A Small Set with Wide Application

By

PENTODE

‘HE small crystal receiver and a single stage amplifier will : always be popular among those desirous of listening to the local station only, but after a while a little discon- : tent is sure to arise when it is desired to reach a little further on the local’s silent day, even though it be only on the _ headphones. Then, again, there may be a time when all but one member of the family do not wish to listen to the loudspeaker, and the crystal and amplifier has to be partly unassembled in order to use the ’phones only. The little set to be described has several uses and can be changed from one to the other merely by changing the *phone or speaker plug or throwing over a miniature switch. : Using but a crystal and valve the following combinations are available :- 1. Crystal receiver alone. 2. Crystal set and amplifier. 8. Single valve detector employing regeneration. To build, it is quite inexpensive; in fact, only a few shillings more than if designed for crystal and amplifier alone, and the construction calls for no great skill or previous experience. When completed, the front panel contains a tuning dial and reaction or volume control. Mounted in the centre is the erystal detector and just below is a double pole double throw switch. At each corner at the bottom is a jack and these are arrange so that to use the valve in either capacity one has merely to insert the plug to switch on the filament. When inserted in the crystal set plug the filaments are not connected and the batteries not being run down. Controls have been reduced to a minimum, and if built into a eabinet can be totally operated from the front.

Considerable latitude can be given in the choice of components and the only parts that require care when buying are the two jacks. Smull diagrams are given and the constructor will be able to tell by the appearance whether he is getting the correct article. The Coil Construction. ETAILS for the construction of the coil are given, which has to be made before any assembly can be attempted. If of cardboard, the coil former will have to be thoroughly dried and given one or two coats of shellac varnish or celluloid solution in amyl acetate and acetone. This drives out all the moisture which is often present in heavy proportions in cardboard. The secondary and reaction coil are wound as one coil, and a tapping taken for the connection to the filament of the valve. Starting half an inch from one end, wind on 55 turns and make a loop in the wire. Continue to wind on a further 25 turns in the same direction, and about 1-8in. from the first coil before the loop. Finish off the ends, either by a soldering lug or passing the wire in and out of two small holes drilled conveniently. The aerial coil has to be wound over the lower end of the secondary coil, and consists of twenty turns separated from the inner coil by several layers of empire cloth or paper. Reference to the diagram will make the position of this coil quite clear. Arrange the leads to this coil so that the three from the reaction and grid coil run out from one side, and the aerial and earth leads run out from the opposite side. This facilitates wiring.

To avoid any absorption of moisture the whole can be given a thin coat of celluloid solution and allowed to dry -in a warm place. The easiest way in which to mount this component is by using two strips of 1-8in. ebonite or fibre 4%in. long, and drilling holes jin. from each end. One side of the coil can now be clamped when two screws are passed through these holes and screwed to the baseboard, a block being used to keep the coil away from the wooden base. Unless the builder wishes to make his own R.F. choke, the coil is the only eomponent that requires actual construction. As the choke is used only to throw back sufficient radio frequency energy to get reaction it need not be very efficient and may consist of two or three slots wound full of No. 36 s.c.c, wire. In fact, the writer used an ord-

inary cotton. reel filled with this guage wire, with a screw passing through the centre hole to mount to the baseboard. The Assembling. SSUMING that all the individual cou sonents are on hand, the next procedure is to mount them in their respective places on the baseboard and front panel. It is always best to start with the panel first as the amount of available room at the back can better be judged with the variable condensers, etc., in position. Screw the front panel to the front edge of the baseboard, and see if, when the condensers are mounted with the centre spindle 3in. from the top of the panel, the fixed or moving plates foul the baseboard. If so, then the variable condensers will have to be mounted a little above the centre of the panel. A suggested place for these two components so that the whole will give a symmetric apearance when finished can be obtained by drilling the spindle hole 3in. from the top and 3in. from each end. This gives sufficient room between the dials for the crystal and switch. At each of the lower corners of the panel..are. mounted the two jacks, while: between -the ‘dials are--helow, the six connections to the DP. I).T. switch and-aboyve, the crystal detector. : The diagram given represents a plan of the receiver as it would appear if

the front panel was hinged and tet down. This view constitutes the rear view of the panel. A word or two on the jacks required will not be amiss. Jack No. 1, or that on the left-hand side of the panel, is one that makes the substitution of the telephones for the transformer primary a matter of merely plugging in the *phone plug, and is represented diagramatically. The right-hand jack is known as the single circuit filament jack, and is shown also. Two small terminal strips, one holding two and the other four terminajs, are screwed at the back of the bade board. The available space can no be utilised by arranging the remaining components to best advantage. On the left is the main tuning coil, with the reaction end towards the left, and the three connections coming out towards the front panel. The rheostat has been fixed to the baseboard,, and a suitable one will have to be chosen when buying the parts. It could, of course, have been fixed to the front panel, and used as a

switch, but the aim was to cut down all unnecessary controls. In front of this rheostat lies the inter-valve transformer. One should be chosen having a high ratio, as distortion is unlikely to be noticeable in a single stage of amplification. The R.F. choke and valve socket complete the fixed components, as the grid leak and condenser can be made self-supporting by eal ing bus wire. The Wiring Connections. { OR the wiring, No. 18 or 20 tinned ’ eopper wire, with lengths of insulating sleeving, is recommended. It is quicker and easier to use, and ehances of short circuits are less possible. In wiring any set it is a good plan to complete the filament and battery systems first and then go on to the wiring of the coils, etc. The point-to-point method of describing has helped many to better understand little points not made clear: by the diagrams. Join left-hand terminal, on the strip of four, marked A- to one end of reheostat and to C- on transformer. (This C- terminal may be marked F-, C-, or G.B.) Join the free end of the rheostat -to.one of the filament terminals on the valve socket. Joi the two middle terminals on the strip together, and carry the wire to one of

the top lugs of the jack No. 2. Join the lug on the same jack with which the previous makes contact when the plug is inserted to the following points. Remaining terminal on valve holder; the loop made in the tuning coil; the moving vanes of the first, or left-hand, variable condenser; one of the outer

lugs of jack No. 1, and also to one side of a .001 mf.d. fixed condenser. Join the remaining two lugs on the jack No. 2, one to one end of the R.F. choke, and the other to right-hand terminal on the strip of four, marked B-+. Join the remaining outer lug on jack No. 1-to the free end of the .001 fixed . eondenser, and also to one end of the

erystal detector. Join the two centre lugs of jack No. 1 to the P and B+ terminals of the transformer. Connect the fixed plates of the lefthand variable condenser,to the free end of grid coil and to one of the centre lever arms on theD.P.D.T. switch. The opposite switch arm goes to one end of the grid leak and condenser and also to the grid terminal of the valve. The various other switch terminals

run, as shown, to various pointsnamely, (1) terminal marked G on transformer, (2) free end of crystal detector, (3) free end of grid condenser and leak. The fixed plates of reaction condenser are formed to plate terminal of the valve and also to the free end

of R.F. choke and the moving vanes of this condenser to the free end of the reaction coil. The aerial and earth wires, joined to their respective terminals, complete the wiring of the set. Connecting Up. HE most interesting part lies ahead, and this comes after the batteries have been connected up and all is ready to try out. Commence first of all with the switch arm thrown over to the left and plug the ’phones into jack No. 1. Adjust the crystal and tune in the local station. with the reaction condenser at zero. Now plug the ’phones or speaker into the right-hand jack. If everything is in order the station will roar in at speaker strength. With the *phones still in jack No. 2 throw the switch to the right and we have a single-valve detector, the crystal and transformer being thrown out of commission. Gradually increase the reaction condenser and with the receiver on the verge of oscillation search for distant stations on the left-hand dial. Adjust the-rheostat-at the back until the valve’ operates ‘successfully in the capacities: of both detector and amplifier with the minimum of A voltage. This is done with a purpose, as no © battery is provided and a cer.

tain amount Of bias is available across the rheostat resistance:

AQUOS When operating this set care must be taken not to allow it to oscillate, as it is capable of radiating and -dis-

turbing neighbours’ reception for miles around unless carefully handled. Finally, there is no necessity to disconnect the batteries or cutting off the rheostat when closing down. Merely take out the speaker or ’phone plug from jack No. 2 as this automatically cuts off the filament supply.

Components Ne ecessary

1 variable condenser .0005 mfd. 1 variable condenser .0003 mfd. 1 fixed condenser .001. 1 fixed condenser .00025, with grid leak and clip. 1 erystal detector. 1 double pole double throw switch. 1 valve socket. 1 R.F. choke. 2 jacks as specified. 1 intervalve transformer. 1 rheostat 20-30. ohms. Panel 12in. x 16in. x 3-16in. Ebonite or cardboard tubing, 22in. diameter, 33in. long. 2lb. 24 S.C.C. wire. doz. terminals. Connecting wire, baseboard, ete.