A Simple Short-Wave Superhet.

Radio Record, Volume V, Issue 13, 9 October 1931, Page 14

A Simple Short-Wave Superhet.

Constructional Details and Operating Notes —

By

Cathode

} HE final circuit of the shortwave super complete, with such frills as are considered necessary, is given in Vig. 1. It will be seen that the design follows very closely that developed in the explanatory discussion given last week, the principal changes being the replacement of two of the tuned plate intermediates by a transformer coupling for the input and a tuned grid eoupling next following. A very crude volume control in the form of a variable resistance across the phones is also shown. This muy be replaced by some less primitive control (e.g., a potentiometer across the audio transformer secondary) if desired, The arrangement shown was chosen by the writer because of its simplicity. The receiver was intended for use in an amateur transmitting station, and the small output valve employed overloaded on any signal above a certain strength. ‘The resulting distortion on broadcast reception was anything, but pleasant, but the rig had ‘the considerable advantage (for reeeption of Morse) of reducing even a bad burst of static to something the same, level as the signal being received Using an Ordinary Set. PHE. oscillator-detector part of the circuit is so arranged as to enable anyone possessing a short-wave receiver of normal type to use it for this part of the super. The shortwave set is, of course, set to oscil: late steadily all the time. The autodyne first detector shown fs tuned by a .0001 mfd. variable condenser used in conjunction with valvebase coils. For the S0-metre coil it will be necessary to fit a celluloid or thin bakelite extension over the yalye base. The tickler coils are wound between the last two turns at the filament end’ of the secondary coil. Such close coupling is normally to be avoided, ow ing to the tuning effect of the reaction eontrol. Here, however, where the adjustment for oscillation is set once and then left alone, the tuning effect of the oscillation control is immaterial, while a more compact coil is achieved. Oscillation of the first detector is controlled by a 100,000-ohm variable resistance. This is shunted by a 1 microfarad condenser to absorb the clicks and noises which would otherwise result from its adjustment. The Intermediate Stage. HOR the input transformer { the intermediate amplifier any commercial 80 k.c. transformer (e.g., the R.C.A. UV 1716) may be used, the secondary being tuned by a semi-variable condenser of .0003 mfd. maximum capacity. If the transformer has a small laminated iron core this should be removed. A suitable transformer may also be constructed in the followjug manner: A turned former of bake:

lite, ebonite, or thoroughly paraffned wood, should be finished to a diameter of about 24 inches. .A winding groove should then: ‘be turned to a depth of g-inch, its width’ being ?-inch. Alternatively, a former of the same dimen. sions may be built up of discs of ebonite or bakelite bolted together with a thin brass screw. The secondary is wound on first, and consists of 1000 turns of No. 80 d.ec. Two or three layers of Empire cloth, silk, or paper, are then wound on, followed by a prim iry comprising 250 turns of the same wire, This transformer will tune to about 380 kilocycles, this frequency having been chosen as about the lowest which cin be used without introducing noise and difficulty: in keeping if. out of the audio amplifier. If a higher intermediate frequency is used, signal strength is reduced as a result of the greater detuning of the oscillating detector necessary in order that the oscillation frequency and the signal frequency have the requisite greate) frequency difference. The .01 mfd. mica condenser connect"dt between the condenser tuning the input if. transformer and the low

potential end of the secondary is to allow one side of the condenser to be grounded and the low side of the secondary to be brought to the battery side of the fixed filament resistance. thus securing the necessary grid bias for the first screen-grid valve, A somewhat similar expedient is adopted in the tuned circuit following this valve in order to ground one side of the semi-variable tuning condenser; the erid bias in this case is applied through a grid leak. The coils for the second and third intermediate couplings are ordinary 1000-turn honeycomb coils obtainable at very low prices from firms specialising in clearance stock. A 250-turn honeycomb coil is employed as a tickler for the last intermediate, the con. trol being by thé old-fashioned swing-ing-coil method; a two-coil holder is very handy here, It is not a bad idea to replace the semi-variable tuning condenser in this stage by a fully-vari-able type; by this means the beatnote heard in Morse reception may be uwdjusted to any desired pitch; more. over, the tuning effect of the swinging coil when putting the second detector in and out of oscillation may be « 1pensated for by this control.

The Intermediate Amplifier. HE most important part of the super-the intermediate amplifier -is assembled inside a three-compart-ment metal screening box, to which the A-+ and B- connections, as well as a large number of internal connections, are also made. In the experimental model a box was roughly assembled out of thin sheet copper, the joints being goldered. A much better-looking job could be made with aluminium sections bent and bolted together. If a copper job is decided upon, it will be found that a fairly heavy soldering bit is necessary, a small bit being too readily cooled. The dimensions of the box are: Length 16 inches, width 8 inches, and depth 7 inches. It is divided lengthwise into three compartments, the first two taking up a space of four inches each, and the third eight inches. The lid may be either hinged or removable. This box is mounted at the rear of a baseboard 12 inches in width, thus leaving a space four inches wide at the back of the front. panel. This space is occupied by. the main tuning condenser, the reaction and volume controls, the plug-in coil unit, the first detector’ valve, and such condensers, terminals, etc, as seem best disposed of here. A rough idea of the layout of the original model is given by fig. 2, and while it is quite unnecessary for the constructor to adhere rigidly to the layout suggested, the diagram may be found helpful in rapidly finding a suitable location for the various pieces of apparatus,

The Copper Compartments. THE first compartment of the copper box houses the intermediate transformer and its associated semi-variable condenser; the first intermediate valve and its rheostat are also located here, as well as a couple of by-pass condensers. The second compartment of the box contains similar apparatus, except that the transformer is replaced by the 1000-turn coil L5. The larger compartment of the box, in addition to the coils L6, L7, and the swinging holder, contains the audio transformer and the audio valve. The handle of the swinging coil-holder serves, of course, as the reaction control for the second detector, enabling a beat note to be obtained for Morse reception, This control is therefore arranged to project through the box, where it falls conveniently to the right 4nd of the operator. Alongside it is he knob of the intermediate tuning condenser C6, which the operator may wish to adjust occasionally. The Wiring. 1B wiring it will probably be found most convenient to take the bulk of the low-potential wiring under the baseboard, which can be raised slightly for this purpose by a couple of strips of ebonite or three-ply screwed to its underside. Wiring is not a critical matter, and it will be found that, apart from that portion of the circuit preceding the first detector, much greater liberties can be taken than is usually the case with even a much less sensitive set. Wiring can be most rapidly done with American "push-back" wire, or with No. 22 tinned copper run in insulating sleeving.

Operating Hints, To put the receiver in operation, after inserting the valves and connecting up the batteries, the first adjustment is to set the first detector so that it oscillates over the entire tuning range. The second detector is adjusted to a point just below oscillation. The main tuning dial (first detector) is then turned until a good strong signal with plenty of. modulation is tuned in. Then the three condensers tuning the intermediate stages are adjusted. A convenient method is to set the second and third tuning condensers at about half their capacity, and then adjust the input transformer condenser for maximum signal. When the optimum setting has been found, transfer attention to the last tuning condenser C6 and adjust this for best results. Some slight readjustment of the other condensers may then be called for. It is worth while trying the effect of reversing the connections to the input transformer, as signals are at a maximum when this is properly poled. After. these adjustments have once

been made, they are left alone, and the set is operated by the single tuning control] on the front panel. For Morse reception the second detector is set oscillating by adjustments of the swinging tickler. For ’phone reception, this control should be set to give maximum regeneration without actual oscillation. This condition is quite easy to maintain. The value given for the by-pass condenser C17 (.001 mfd.) may sometimes be found too small, this condition being indicated by a thin, highpitched squeal: heard as a signal is tuned in. The obvious remedy is to increase the size of this condenser to .002 mfd., or, if necessary, .003 mfd. It should be emphasised that this super is intended for short-wave reception. It will work on the broadcast band, but owing to the considerable detuning of the first detector necessary, results are almost certain to be a disappointment to the constructor. In its own sphere, however, it is a first-class little set, while its construction would be a really excellent exercise preparatory to the construction of the ambitious mains-operated superhet shortly to be described in these columns,

Dual Wavetrap : rey our issue of September 4 was described.a wavetrap designed to separate 2YA and 2ZW. Unfortunately, in

one of the diagrams a wire was omitted, and of course a trap made from this diagram would not work. Accompanying is a corrected sketch. Had those who made the trap failed to get it to work followed the theoretical sketch they would have seen wherein the trouble lay. The wire connecting Li and L2 was omitted from the sub-baseboard wiring.

Pw SUESEOURCECUCSCEEERALSRRT EIST SEED HGUANDUOSSSUUCRSSEURREQEAUREN ROARED ORECAST SECONDARY COIL. Band. Turns 10 metre 2 20 metre 5 40) metre 15 SO metre 32 Spacing. One diameter One diameter Close wound Close wound Size. 20 d.c.e. 20 d.e.e. 26 d.ec.e. 26 d.c.e. TICKLER. Turns. "CUO eb Size. 34 d.s.c. 34 d.s.c. 84 d.s.c. 84 d.s.e.