The "KESTREL THREE"

Radio Record, Volume V, Issue 3, 31 July 1931, Page 16

The "KESTREL THREE"

Continued from Last Week

By

the Technical Editor

| HEN all the components are in position and firmly. screwed down to the baseboard, and the panel mounted in position, we are ready for the wiring. Now, just before commencing the wirinz, ~ check over the position of all the components to see that they are right. Make quite certain that all the holes indicated on the layout have been drilled. Probably by now those who have had a little experience in construction will prefer to go on without further comment, but just in case someone who .does not know too ‘much about construction is building the set, we will detail the wiring from point to point. uo ; From the aerial terminal take a short wire to the moving plate of the midget condenser. The fixed plates are connected with "G" of the valve socket, with "G" of the coil socket, and witb one end of the .0001 fixed condenser. The other end of this condenser is connected with the fixed plate of the rf. tuning condenser nearby. Two wires go from the moving plates of this condenser, one to "P" of the coil socket and the other to the .5 mfd. fixed condenser. A hole must be drilled in the Shield just above this connection and a bolt passed through it with a’ solder lug on either side. This terminal of the’ condenser must connect to earth Now join the solder lugs to the condenser. From this same point a wire goes to "EF" of the valve socket. The free side of the .5 mfd. condenser connects with "C" of the valve socket, but there is also another connection to go underneath the baseboard, which we shall make a little later. Connect ‘I’ of the valve socket (that of course is a free guess) to the right-hand side (that is, when you are looking at it from the pack) of the switch. The wire from here will also go under the baseboard, but we will talk of that later. The left-hand side of the switch is left: free for the time being. 'The earth terminal is connected to the screw that is holding the .5 mfd. condenser to the shield. We have now finished the wiring of the radio side. From "P" of the detector socket, a wire runs back to "RH" of the coil socket. Another wire goes from this point to one terminal of the radio frequency choke and from this same terminal a wire goes to one of the set of fixed plates of the differential condenser. The grid terminal of the valve socket is connected with one end of the .0002 condenser nearby and with one end of the grid leak. The other end of the grid leak is connected with the centre point of the 400 ohms potentiometer. The free terminal of the .0002 fixed condenser is connected with "G" of the coil-holder and through the screen to the plate of the screengrid valve. As this is not in position, leave a fairly long length of flex connected with this point on the .0002 condenser. We will have more to say about this connection later on. Another wire is taken from "G" of the coil-holder, and this time it goes to ‘the near end of a .0001 condenser, the other of which connects with the fixed plate of the detector tuning condenser. The moving plates of the detector con-

denser connect with the lower tag of the phone jack. There is also another connection from this point, but we will come back to this, The side of the 400 ohms potentiometer nearest the panel is connected to one of the terminals of the rheostat and the remaining terminal of this potentiometer is connected to three places, first to the moving plates of the differential condenser, secondly to the nearer terminal of the .05 by-pass condenser and thirdly to "I" of the detector valve © socket which has also a connection going underneath the baseboard. It will be noticed that.near the connection we made to the .05 condenser there is another bolt with a solder lug, so we con-

nect this terminal of the condenser to this solder lug. If we now turn our ‘attention to the radio frequency choke, we will see that there is one terminal not yet used, so we connect this with "p" of the transformer, although we connect "G" of the transformer to "G" of the audio valve socket. "P" of this valve ig connected to the top terminal on the phone jack. We are now ready to complete the set with the under baseboard wiring. We shall start at the switch. Pass a wire through the hole on the right hand side of the switch (looking from hehind), and take this along to the hoie underneath the free terminal of the rheostat, and eonnect it to this. Also

to this terminal join another wire, pass it through the same ‘hole underneath and run it along to the "I" terminal of the audio valve. There are two "FH" ters minals, and if they are marked positive or negative, connect it to the positive terminal, that is, the one on the lefthand side at the back: The other "F" terminal of this valve socket is connected through a hole to "HK" on the detector valve. One "F’"’ is free and the other has a wire going to the potentiometer, so we join this wire to the one that has already a wire joined to it. This, by the way,.is "F-." The only free terminal on the detector valve™ socket is "F+-," and this goes the ‘hole to the terminal of the 400 ohms potentiometer nearest the panel. This, you will notice, has already been joined with the rheostat. Another connection goes from the lower tag of the phone jack to the plate terminal of the letector coil holder, and another wire goes from this point underneath the baseboard to the free terminal of the .05 mfds, condenser. This completes the under-baseboard. wiring, with the exception of the battery cable. To make a neat job of this, cut out-a piece of the flap supporting the baseboard, just sufficient to allow::the cable to fit in without lifting the.set:off its base. Then screw a piece of metal across to hold it in position. Slip eight or nine inches through, cut away the braid for about nine inches, and take one strand through the hole.underneath the left hand: terminal of the switch. This is "A+." and’ should be the red wire. The black wire is "A-" and must connect with one of the bolts that hold (he screen to the baseboard. If you have followed the diagram there will be one just handy. ‘The brown wire in the enable goes through the hole to the plate of the r.f..valve and connects with that terminal. The blue or green wire goes through the baseboard and connects with "}"’ of the audio transformer. This will be the grid-bia. connection. The brown wire is taken to the other hole near the audio transformer, and is connected with "B-+." The remaining strand in the cable is taken through the hole near the .05 mfd. condenser and connected with the terminal on which there is already a wire going underneath the baseboard. . This is the rear terminal, and connects with .the lower tag 9, the phone jack and with the coil ; We have now almost finished. but shall make a few comments about the connection to the top of:.the screen grid valve. This must go. through the screen, Although, if you drill an 1-Sin. hole and pass through a fairly heavilycovered flex you will get quite satisfactory results, yet we have found the best plan is to use specially armoured cable to make this connection. Short lengths of about four or five inches with a terminal at each end are available, and if a threaded bush is used a very eat job can be made of fitting this through to the hole in the screen. A connecting wire can then be fitted between the panel side terminal of the grid condenser and the terminal on the connector. The other side will have a spade terminal fixed tu it. and will he used for connection with the plate of the screen-grid valve.

4 The Coils, BH are now ready to make the coils. For the broadcast band we are going to use 2in, former and valve bases for the short-wave. Mounting a two-inch former on valve bases will present somewhat of a problem, but there are several ways of tackling it. One way, and probably the best, is to have or cut a dise of. three-ply ‘which will just fit into a: 2in. former. Place this over a valve socket and mark the position of the four terminals. Now drill through with.'a .1-8in. drill and fix into those holes’ four valve pins.’ These can be prised out until they fit the holes tightly. ~'

If you are using American bases you will find it a little difficult to make the thin pins fit the larger large hole in the base comfortably, but if you use English bases you will have no difficulty. Furthermore, the use of Dnglish bases for the coils precludes any possibility of your putting the valve in the place that is intended for the coil, To do this would mean the tuination of your valves, and if you put the coil in the valve socket, the ruination of your battery. The moral is, be careful. It is better to paint the two coil bases a conspicuous colour so that you cannot make any mistake. Another method, instead of fitting valve pins, is to cut an ordinary valve base crosswise, so that only a quarter of an inch or'so is left above the four pins, and then screw this to the dise of three ply in such a manner that the hole in the three ply corresponds with the pins. The disc can be held into the former either by screws or by thin brads. Fix in position temporarily. It will have to come out again, so do not make a "&ood job of it. Identify each pin of the valve base by pushing it into a holder and marking the terminals in pencil on the former. By doing this you will bring the end out to the right terminal. We will explain the making of the coils using the measurements system, as it is much ‘to be preferred. Threeeighths of an inch above the plate terminal drill a small hole, and lin. from the bottom and immediately over the grid terminal drill another hole: 2iin. over "I+", another hole, and the last, 3in. above "F-" prong. These instructions, of course, are for the regeneration coil. Take out the. valve base and thread the 26 d.s.c. wire through the lowest hole and slip the former on to a winder... If a solid

wooden core is used as a former, tintacks can be pushed through the hole and the wire can be twisted round them. Commence winding with the 26 d.s.c. and continue until the next hole is encountered. Make the wire fast to this, or to a tack, and commence winding with 30 gauge wire immediately above it. Finish up to the top hole. If you wish to wind the coil by hand and count the turns you will find the numbers in the accompanying panel. You proceed with the r.f, coil in the same manner as before, but it is necessary to drill two holes only in the threeply disc. That is the hole over the plate

at the grid. Take your 2in. former and 38-16in. above the grid terminal drill. a hole. The next hole is 3}in. above the plate prong. Wind this space with 24 d.s.c. wire. Note particularly that the grid terminal connects with the bottom of the coil; it is unusual, but it is absolutely necessary in this circuit. Your coils are now made and you must assemble your batteries. Of course most constructors will use what they have on hand, but we recommend two 60-volt batteries for the "B," and either two, four or six-volt "A" supply, and nine-volt grid bias. For this circuit you can use the new 230 type valve and an air-cell or the 230 type with an ordinary two-volt accumulator or dry cell. If you use dry cells you had better get a voltmeter from somewhere and put in a resistance until the voltage is down to two. The rheostat will not control voltage on all the sereen grid valves, so if a battery in any way higher than the valves used is employed. the voltage must be broken down with a suitable resistance. A ten ohms rheostat would probably be sufficient for most purposes. "A-’, "B-", and "O-+" are connected together. The connections to the battery are made in the usual manner. You will find the detector will work best with from 223 volts to 45 volts on the plate. The detector connection goes to "B" of the audio transformer and in our description, we called it the brown wire. ‘The highest voltage, 120, can go to the plate of the rf. valve and to the plate of the power valve. In our description it was the white wire and went to the top of the .05 condenser. The screen voltage must be half that of the plate, so if you use 120 on the plate you must use the 60 terminal tap

for the screen voltage, that is, the one going to the plate of the valve-holder,: and which, in our description, was brown. If you have an old valve handy, one in which the filament lights up but the emission of which is gone, put it in each of the three sockets in turn and turn on the switch. ‘The filaments should light up-in each. If it does not you will know that- you have made a mistake in the filament wiring somewhere. If it flashes out, then you will know that you have done the same thing, but be very careful. before you put in the actual valve.

A fuse can be fitted to this circuit between "A-" and "B-." If this is done, separate connections must be made to " -_?? and "R.??, «Be is not connected to "A-" as was recommended for the usual set, but to one side of the fuse. The other side of the fuse is connected to "A-". Thus the only way the "B" current has of getting back to negative is through the fuse. If you instal a. fuse read over. what we have said about "A-" and "B-" before you put it in. You can quite easily put.it in wrongly and pass all your "A" current through it. Assuming that the valves light up, lyou can rest assured that your voltage is correct as far as the filament is concerned, and can put your valves and get work. -If it does not go we cannot tell you what is wrong. Just look round and check up everythine nntil vou find the mistake.

By turning over the differential condenser you will get the set to oscillate. Adjust the potentiometer until reaction is smooth. By turning it toward the positive side the set will become very active and probably ploppy. Turning toward the negative deadens it, so that an intermediate point will ‘give you the best reaction. The set, if constructed to design, will not be unstable. If it is, make quite certain that the high potential end of the r.f. coil is nearest the base-that is the one that is connecting with "Gq" of the valve socket. This connects with "G" of the valve and the aerial. It must be the lower end. The other side connects to earth and is uppermost. This reverses the field

and prevents interaction-otherwise you will never get the set stable: You will find that you will have to alter the midget condenser in the aerial to get best results. Do‘not leave it out, thinking you are going to get still stronger results than we have ourselves obtained. You may get yourself into trouble if you do so because the set will probably radiate. Furthermore, it will be unselective. " Now, while we are on the subject of selectivity, we might mention that this set is not selective. We do not know of any single stage of screen grid that

is, unless it uses.a pre-selective circuit such as a band-pass filter, and the cost, of this does not justify inclusion. If. you wish to separate the local station, use a wavetrap. It will be quite ef-. fective and will do the job well. On shortwave the set is as selective as is desired, the midget condenser in the aerial having the required effect.