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Although special receiving equipment is undoubtedly essential for television reception, good results can be obtained from other ultra-short-wave transmissions, including the sound accompaniment to television, by using the ordinary broadcast receiver in conjunction with a suitable converter. The apparatus described in this article is of simple nature and enables good reception to be secured over the 6-9 metre waveband.
The commencement of broadcasting from the Alexandra Palace will undoubtedly greatly intensify the present interest in ultra-short-wave reception even among those who are not yet interested in television itself. The sound transmissions are to be on a frequency of 41.5 MHz (7.23 metres) and the vision signals on 45 MHz (6.66 metres), the power used being 3 kW for sound and 17 kW for vision. For television reception of the quality necessary for a good picture a special receiver will be needed, but this does not apply in the case of sound reception. It is possible to secure very good results from the sound transmissions by using any ordinary medium waveband set in conjunction with a converter.
In essentials, an ultra-short-wave converter differs in no way from a short-wave converter. In practice, however, much greater care in design is required if efficiency is to be secured together with freedom from interaction between the signal and oscillator frequency circuits. Experience has shown that the triode-hexode type of frequency-changer gives the most satisfactory results provided that the correct oscillator circuit is used. This design uses the M-OV X41. No difficulty is found in obtaining oscillation at wavelengths well below 5 metres, but with most oscillator circuits parasitic oscillation causes much trouble. An efficient and smoothly operating frequency-changer is out of the question so long as this trouble persists, and it is very important indeed to avoid it. The writer's experience has been that the commonly used Hartley oscillator is rather prone to generate parasitic oscillation, but that the Colpitt's oscillator is usually quite free from the trouble.
Fig. 1. - The complete circuit diagram of the converter shows that a triode-hexode frequency-changer is used and that there is one signal-frequency tuned circuit.
It was accordingly decided to adopt this oscillator in the converter, and the results have amply justified the choice, no sign of parasitic oscillation being evident at anytime during its development. Since the tuning capacity adopted is quite small, it is actually unnecessary to employ a split-capacitor in the manner of the conventional Colpitt's oscillator, and a single capacitor can be used, the effect of a split capacitor being given by the inter-electrode capacities of the valve. This leads to a very simple circuit, will be seen from Fig. 1, for the oscillator tuned circuit L2 C7 is connected between the anode and grid of the triode,a 0.0001 μF. capacitor C6 being interposed to isolate the grid supply. The grid is returned to the cathode through a 50,000 Ω resistance R41, while the anode derives its HT supply through the 75,000 Ω resistance R5.
One signal frequency tuned circuit L1 C1 is used, and the control grid of the hexode is connected directly to a tapping. This is done partly to reduce the stray capacities thrown on the circuit and partly to reduce the valve damping. At the very high frequencies at which we are working the input impedance of a valve can no longer be ignored for it may be only a few thousand Ohms. With the coils used in this equipment it is found that with the grid connected to a tapping near the centre signal strength is no less than with it joined to the top of the tuned circuit and tuning is noticeably sharper In addition, the reduction in stray circuit capacity leads to a reasonably wide wavelength range being secured.
The screen grid derives its supply of voltage from the potentiometer, consisting of R1 of 30,000 Ω and R2 of 20,000 Ω, and a 0.0005 μF by-pass capacitor C2 is shunted directly to cathode. Capacitors C3 and C4 of 0.0005 μF capacity are also joined between heater and earth in order to prevent modulation hum. Grid bias is derived from the 200 Ω resistance R3 shunted by the 0.01 μF capacitor C5 in the cathode circuit. The output is taken from the hexode anode by means of a screened primary transformer T1, decoupling being obtained by the 1,000 Ω resistance R6 and the 0.01 μF capacitor C8.
The method and shape of the internal screening as well as the arrangement of the tuning capacitors and coils are clearly shown in this photograph.
The photographs and drawings accompanying this article show the details of construction, and little need be said about the mechanical work involved beyond stressing the necessity for very short leads and for rigidity in the wiring. The capacitor C2 is mounted directly on the valve-holder, being slipped between the pins so that the heater pins lie on one side of it and the cathode and screen grid pins on the other. Its tags are bent round and soldered directly to the sides of the cathode, and screen-grid pins. The grid leak R4 is similarly joined directly between the cathode and oscillator grid pins, the wire leads to it being not more than one eighth of an inch in length.
The by-pass capacitors C3, C4, C5 are all connected directly between the appropriate points on the valve-holder and one anchorage point on the chassis, while R5 is joined between the oscillator anode and the supporting pillar for the HT connection. This resistance must be mounted directly on the valve-holder with a ⅛ in lead; the length of the lead for its other connection to HT, however, is not important. C6 is joined directly between the oscillator grid and one terminal of the variable capacitor C7.
The coils are wound to the dimensions given here with No. 14 enamelled wire.
Details of the coils are shown in the drawing above, and are most readily made by winding about 20 turns of No. 14. enamelled wire on a ⅝ in diameter tube. Let the wire slacken and it will be found that it springs out slightly so that the coil is a loose fit on the tube. Then pull the turns apart slightly so that an evenly spaced winding results and cut the coil in halves. Two coils each of ten turns will thus be obtained. At one end unwind half a turn and straighten the wire out for connection to the fixed vanes of the capacitor, at the other unwind one and a half turns, straighten the wire and bend it appropriately for the connection to the moving vanes.
With such heavy wire the enamel is easily removed by scraping it with a knife. Some little care is necessary in removing the enamel for the tapping on L1, however, for it must not be removed also from the adjacent turns. This tapping is not at the exact centre of the coil, but at 31 turns from the high potential end, or 4 from the earth end.
A view of the converter with the end of the chassis removed, showing the components beneath the output transformer.
The aerial coupling coil La is wound on a length of ⅜ in. diameter ebonite rod screwed to the front of the chassis. It consists of eight turns of No. 24 DSC close wound and is suitable for the average aerial. Provision is made for the use of a dipole aerial by fitting two aerial terminals and then about 3 turns only for La will be needed. Under normal conditions, however, the aerial is joined to one of the aerial terminals and the other is strapped to the earth terminal, to which the earth also should be joined.
No provision is made for ganging adjustments, The intermediate frequency available being necessarily within the medium waveband, it is so low in relation to the signal frequency that it is not practicable to attempt to differentiate between the signal and oscillator circuits. The theoretical difference of tuning between them is considerably less than the probable ganging errors. Both circuits are, in consequence, nominally tuned to the same frequency, and this results in there being two tuning points for every station and no protection against second-channel interference. This is unavoidable unless an intermediate frequency of at least 5 MHz is used, and this is impossible with a converter.
Correct ganging is consequently secured by making the inductance and capacity of the two circuits equal. If the coils are carefully constructed they will be sufficiently alike, and if the wiring is carefully carried out the stray capacities will be equal on the two circuits. No trimming capacitor is consequently fitted. The ganging is readily checked by slacking off the coupling between the two capacitors and adjusting the circuits independently. It all is in order it should be found that for one of the two oscillator settings the vanes of both capacitors are enmeshed to the same degree.
Fig. 2. - A valve-holder should be mounted on the receiver and wired to it in the manner shown here, to take the plug from the converter.
The receiver should be set to operate on the medium waveband and the two output terminals joined to the aerial and earth terminals on the set by a foot or so of twisted flex. Power for operating the converter can be obtained from the receiver by fitting it with a five-pin valve-holder to take the plug connector. The connections in the receiver should be made as shown in Fig. 2.
So far nothing has been said about C9, which is shown in Fig. 1 connected across the primary of T1. This is because it is not always necessary. Whether or not it, is required depends on the design of the input circuit of the receiver with which it is used. When the capacitor is needed, it should be a 0.0005 μF compression type, and should be adjusted for maximum signal strength, its setting depending upon the tuning of the receiver. It is not required with sets such as the QA Super and Variable-Selectivity IV, and with these sets a capacitor of 0.0001 μF should be joined in series with the inter-unit lead to the aerial terminal of the receiver.
Full details of the construction and wiring of the converter are given in this drawing.
In general, the receiver should be tuned to its most sensitive point in the medium waveband, but not, of course, to such a wavelength that interference is experienced from a medium wave station. Tuning on the converter is sharp, but depends greatly on the selectivity of the receiver. With selective sets it will probably be necessary to stand the converter on sponge rubber to avoid howling due to acoustic feed-back from the loud speaker to the vanes of the variable capacitor.
Too much should not be expected in the way of signals at first, for most present transmissions are of an experimental nature and irregular in their operating times. In most urban districts, however, it_ should be bpossible to gauge the efficiency from the amount of ignition interference experienced when a car passes the house. This should be fairly loud with a sensitive receiver and at its greatest around 7 to 8 metres.
The tuning range of the converter is from ea little over 5 metres to about 10 metres, 7 metres being almost exactly half way round the dial. The range may, of course, be extended lower if it is required to take in the 5 metre amateur band by using slightly smaller coils. This will naturally involve a curtailment of the upper wavelengths, the ratio of maximum to minimum wavelengths being constant whatever coils be used.
Tested in London with the QA Super, the converter brought in several experimental transmissions at full loud speaker strength, and proved sensitive throughout its tuning range. Tuning was critical but actually rather less so than many ordinary short-wave sets.
An upper view of the oscillator section of the apparatus.
Parts List
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