The "Night-Owl Three"

Radio Record, Volume V, Issue 18, 13 November 1931, Page 16

The "Night-Owl Three"

For World-wide Short-wave . Reception

' (By the

Asst. Tech. Ed.

ISTENING-IN on shortwave is undoubtedly becoming very popular in New Zealand-a state of affairs for which‘the New Zealand Shortwave Club and the amateur transmitters’ are largely responsible. Constructors who previously

coniined their attentions to the broadeast band are now becoming "shortwave minded," and are realising that listening on the higher frequencies provides thrills unobtainable on the longer waves. And this fascination is easily understood. The thought that with a few pounds’ worth of the simplest apparatus one has the world literally at his finger tips is only a part of the lure which keeps the shortwave listener at his set controls for hours at a stretch. Thus we have dedicated the shortwave receiver to be described below ‘to this

enthusiastic band of listeners whose keenness, which often keeps them out of bed until the wee sma’ hours, has earned for them the soubriquet of "the Night-owl Club." . Now for a few words about the circuit of the "Night-owl Three." It will be noticed from the diagram that the set employs three valves-a detector employing Schnell control of reaction, followed by two stages of transformerceupled audio. Many constructors whose experience of radio has been gained in the building and operating of sets working on the broadcast band will no doubt be a little surprised that we should design a three-valve receiver which does not use a screen-grid valve. However, it has been definitely established that on shortwave, radio frequency screen-grid valves, while often materially reducing hand-capacity trouble, do not give the enormous lift

that they do on the broadcast band. Thus, from the home-constructors’ point of view, the increased-expense involved in using this type of valve is not justified by results. After all, a well-design-ed three-valvye receiver such as the "Night-owl Three" is capable of picking up strong signals from stations in all parts of the world, so what more is wanted? The Cabinet, Panel, and Baseboard. THE completed set may be enclosed in a suitable cabinet such as that shown in the photograph on this page. In regard to this the constructor can use his own ingenuity, and we will give no further details beyond saying that the lid is hinged at the back, and that the front panel is recessed into the eabinet so that it fits flush with the sides. Two one-inch holes are drilled in suitable positions in the back to

allow, the battery cable and aerial and ‘earth to pass through. The front: panel, which is of ebonite, measures 15 inches by 7 inches, and is 8-16in. thick, while-the wooden baseboard is 15’ inches long, 84 inches deep, and is of 4-inch timber. Both the back of the panel and the top of the base‘board are covered with thin aluminium sheets to overcome undesirable coupling ettects and hand capacity. Drilling the Panel, rp disposition of the tuning dials, filament switch, loudspeaker’ or phones’ jack, and the rheostat may be clearly seen from the photograph, and all details of measurements to be fol-. lowed: when drilling the panel ap shown in the layout sketch. The pa‘nel components should be mounted first of all, not forgetting the aluminium backing sheet. The Baseboard Components. "THE layout of the baseboard components is given in the diagram, and the.mounting of them should present no difficulty whatever. ‘This layout should be followed as closely as possible for, in a shortwave receiver, instability is much more easilv introduced by wrongful placing of components than in a broadcast set. Now for a word or two about the components themselves. It will be noticed that a midget variable condenser of capacity .00005 has been included in series.in the aerial lead. This has been introduced for several reasons -firstly, the uSual broadcast aerial is

r rather too long for shortwave work and & series aerial condenser is the easiest way of reducing this; secondly, "dead spots," which are due to too tight a coupling between the aerial and secondary coil, may often be overcome by varying this midget condenser, The next refinement is a 400 ohms baseboard . mounting potentiometer, which is placed across the filament of the detector valve, thus enabling the grid to be made more or less positive with respect

to the filament as desired. By correctly adjusting this, very smooth control of reaction is obtained-a very important factor in shortwave listening. Two further refinements are the .001. mfd. de-coupling fixed condenser placed between the plate terminal of the first audio transformer and earth, and the .25 megs. resistance across the secondary of the same transformer. By their inclusion. the objectionable "fringe" or "threshold" howl often en- _ countered in tuning is overcome. The 4 resistance should not be lower than the value stated, or otherwise the volume will. be appreciably reduced. Incidentally, it may be found that this resistance is not required. . If you have a couple of spare leads in your battery cable, do not be tempted . to place the grid-bias battery with the others, outside the set. Instability will almost undoubtedly result. Always keep the grid-leads short by placing the battery within the set. Another point which may appear inexplicable is the inclusion of a .002 fixed condenser in series with the reaction condenser. This is purely a safety measure, for otherwise, if the vanes of the reaction condenser are accidentally shorted, a new set of wires will be needed. While on the subject of reaction condensers, we might menTge ren Olas a

tion that if the constructor so desires, a differential reaction condenser may be incorporated in this receiver. The two sets of fixed vanes are connected one to each side of the reaction coil and the moving vanes are earthed. A point to notice in the layout sketch is that while the moving vanes of the reaction condenser are earthed, those of the tuning condenser apparently are not. This is not so, however, for these vanes are earthed via the aluminium sheet backing the front panel, which is itself earthed through the moving vanes of the reaction condenser. Wiring the Receiver. HEN all the baseboard components are mounted, the wiring may be a

commenced, No difficulty whatsoever should be experienced with this, as both layout and theoretical diagrams are given. Constructors are advised to wire always from the theoretical diagram rather than from the layout, for the latter is given more to show the disposition of components than the actual wiring, and mistakes are liable to creep in. At first the theoretical diagram may seem hard to follow, but

dentally, if you ever come across a discrepancy between the theoretical and layout diagram, always follow the theoretical. The Coils. FoR the convenience of those who constructed our "Differential" series of receivers and who might desire to build the "Night-owl Three," we have provided for the use in the latter receiver of the "Differential" shortwave type coils, These are wound

if you stick to it and puzzle things out for yourself you will be surprised to see how easy it really is. Ineion a five-pin American type 224 valve base. The following is the procedure in making up one of these coils: First of all work the socket loose from the glass bulb of the valve by twisting it backward and forward until it comes ‘out. Then apply a soldering iron to each of the five pins to open the ends. Next identify the five pins by comparison with the valve holder or with the diagram reproduced on this page, and with a sharp steel point scratch on the valve base opposite each pin F-+-, F-, P, G, and C, as the case may be. Now commence with the reaction winding at the bottom of the base. The wire should be threaded through the correct pin and taken to a hole drilled in a suitable place in the side of the valve base. The correct nunrber of turns is then wound on and the (Concluded on page 30.) Reeee EE 4

The "Night-owl Three" (Continued from page 17.) wire taken back inside the valve base aund-down through the correct pin. It is then soldered into position by touching the end of the pin with the soldering iron. The secondary is now wound on in a similar manner, and finally the primary. The coils should now be placed in its socket and a continuity test applied to see if the coils are connected to their correct pins. The procedure for all the coils is the same.

though for the 40-metre band coil and above, valve base extension former will be required. This may be fastened to the buse by means of two boot brads, which. are driven through the bottom of. the former and into the valve base after the latter has been drilled, A table giving full particulars of these coils, with correct gauges of wire, etc., is reproduced on this page ‘and should be adhered to strictly. Though designed primarily for shortwave work, this receiver may also be used on the broadcast band, and particulars of a suitable valve base coil, together with useful hints on operating the receiver, will be given next week.

List of Parts for the "Night-Owl Three" a () a) eS

on . Wooden Baseboard, 8% x 124 x 3. Ebonite Panel, 7 x 124 x 3-16th. One .00015 Tuning Condenser, with double spaced vanes. One 60005 Variable Midget Condenser, One 5-pin Base for coil, with 5-Pin Valve Holder. . One .0002 Grid Condenser with 6meg. Grid Leak and Mount. One Baseboard-mounting 4000hm. Potentiometer. Three UX Valve Sockets. , One R.F. Choke. One .001 Fixed Condenser. One .00025 Reaction Condenser.

One .002 Fixed Condenser, One .25meg. Grid Leak, with Holder. Two Audio ‘Eransformers (one 3-1 and one 5-1). One 300hm Rheostat. | One On/Off Filament Switch. One Loudspeaker Plug and Jack. ° Two Vernier Dials. One BRheostat Dial. Clips for mounting Bias Battery. Aerial an‘ Earth Terminals on mount. Set of Shortwave Coils. Une Battery Cable. Three Wanderplugs for Bias Battery. Giazite, screws, etc,