A Superheterodyne Short-Wave Adapter

Radio Record, Volume IV, Issue 16, 31 October 1930, Page 16

A Superheterodyne Short-Wave Adapter

A Set for the "Advanced" Constructor

By

CATHODE

HERE should be no need to enumerate the failings of the usual type of short-wave adapter by way of justification for the piece of apparatus about to be described. The inconvenience of continually plugging it into the detector socket and subsequently disconnecting it, and its failure to make proper use of the broadcast receiver to which it is attached, are in themselves sufficient to render almost certain its eventual discontinuance. The adapter now described may, by the inclusion of a switch on its panel, be connected permanently to the aerial terminal of the receiver; the receiver requires no alteration whatever, functioning as the intermediate and audio ainplifiers of a superheterodyne, of* which the first detector and oscillator is provided by the adapter. The intermediate frequency is the minimum frequency to which the broadcast set will tune-usually between 550 and 500 kilocycles. It is not intended to enter into any discussion of the theory of reception on the superheterodyne principle. The mode of Operation of the adapter, however, is as follows :-First, the receiying Set is turned on and the dial (if single-control) is set at the minimum frequency, which we will assume is in this instance 500 kilocycles; if the receiver has two or more tuning controls, these must be brought into resonance so that all the tuned circuits are adjusted to the same frequency. Then the first detector in the adapter is tuned to the desired frequency in the short-wave

band (somewhere between 1500 kilocycles and 20,000 kiloeycles), and the oscillator is tuned to a frequency 500 kilocycles above or below the detector. A "beat-note" of 500 kilocycles is generated between these two circuits and ‘arries the original modulation; this new frequency is then applied to the aerial terminal’ of the receiver end, since it corresponds to the. Frequency. umplifier and detector cirenits haye previously been tuned, it is amplified nnd then rectified by the-detector (now the "second detector" of the super). Finally, signals are stepped up still more by the audio amplifier of the receiver. If the usual ‘type of seven or eight valve a.c. receiver is used,. and the apparatus correctly adjusted, it Will be possible for the constructor to put his highly developed imagination away on the shelf along with his oldtype adapter. Neither will be further required.

"THE most difficult task which the constructor will have to face is the winding of the detector and oscillator coils. The job can be simplified very much by employing midget formers of the Pilot or Silver-Marshall type, but the most economical proposition is. to thake the coil formers by winding a scrap of thin celluloid into a: cylinder of such a diameter as just to slide tightly’ over an old valve base of the UY or. American five-pring a.c. heater type. The usual celluloid cement made by dissolving scrap celluloid in amyl acetate or acetone or both, can be used both-for cementing the cylinder up and for sticking it to the valve base. The plug-in mounting bases are, of course, ‘simply five-prong UY valve sockets.

The antenne coils L should be separated from the winding L1 only by a sixteenth of an inch or so. The plate coils L4 are wound close to the filament end of L3, and the oscillator coupling coils L2 at the grip end of L3 for 7500 and 15,000 kilocycles; for the 1800 and 3750 kilocycles bands, coils L2 should be wound over the grid end of L3. Details of the connections are given in Fig. 1, while the numbers of turns for the four pairs of plug-i -in coils are as follow :-

All windings may be made with either silk-covered or enamelled wire. It is sometimes necessary to increase the number of turns in L2 slightly to obtain optimum results, as some valves do not give the same output'as oscillators as others. -Generally speaking,

though, the number specified will. be about right. As will be seen from the circuit diagram given in Fig. 2, the adapter is arranged more particularly for broadcast receivers of the all-electric type, certain modifications being necessary if it is desired to use battery-type valves in the adapter itself. It should be-made clear, though, that it is. quite permissible to employ the adapter as it stands with a battery-operated receiver, so long as lighting mains are available for heating the heater elements of the 227 type. valves employed. A separate transform:,) er giving 2.5 volts is used for this pay. pose in any case; a 60 ohm resister is connected across the heater terminals and the center tap grounded as a. precaution against hum. The plate voltage is secured from the power supply unit or the batteries of the main receiver, and although 45 volts is specified, this may be increased if so low a voltage is not available. Grid bias is provided by resistors in the cathode leads, by-pass condensers of 0.1 mfd. shunting these. The radio frequency choke in the plate circuit of the oscillator is a critical cor_ponent, and the constructor should be at some pains to obtain a good one. The effect of a poor choke

here will be a cessation of oscillations from the oscillator valve at certain points in the tuning range, and consequent ‘dead spots’ at these points, "THE purpose of the tuned circuit C4 L5 in the plate circuit of the detector is to render the adapter suite able for receivers in which the inpv stage is untuned. This circuit is peal{ed at the same frequency as that t which the broadcast receiver is tuned, and, once adjusted, does not have to

be touched again. For this reason @ condenser of the semi-variable compression type is used for C4, while the coil Ld may consist of a straightforward single layer winding of 85 turns of No. 30 wire on a 13 or 1#-inch former, C4is also necessary to the proper operation of the oscillator, so do not

try to dispense with this.extra tuned circuit, which is quite worth inclusion for.the extra selectivity it :gives. Where the first stage of the broadeast receiver is of the untuned type employing a resistor or r.f. choke to couple. the aerial to the -grid circuit, it will usually be found of advantage to connect the lead from the adapter to the grid of the second r.f, valve; where this is done, a reduction of 05 to .00005 mfd. or less ‘will improve selectivity. . (CRAMPING should be avoided in constructing the adapter. A metal panel is of advantage in reducing

any hand-capacity effects, while if the . sub-panel is raised an inch or two above the floor of the cabinet, the by- — pass condensers and the heater and low potential wiring can be located out of ' ‘sight beneath it. The 2.5 volt heater —

circuit should be wired with No. 14 or 16 S.W.G., the ‘two wires being twisted together and kept well away from other wiring and components to prevent hum being picked up. The adjustment of the adapter is very simple, particularly where the input stage of the receiver proper is of the tuned type, which is usual, The aerial is first disconnected from the broadcast set and connected to the adapter, the earth wire being treated likewise, or an additional earth connection used for the adapter. Next the B + 45 and B leads must be brought out from: the broadcast receiver power-pack and connected where shown to the adapter. The lead from the adapter may then be connected to the aerial terminal of the broadcast set; where two terminals marked "long" and "short" are provided, make the connection to that marked "short." The power may now be switched on to the filament transformer. With the broadcast dial set at maximum, tune in a weak shortwave station, remember-

ing that the two tuning controls on the adapter run more or less in line. Lastly, adjust the condenser C4 and the detector bias resistor: for best -Tesults. Once the initial connections and. adjustments have been made, it is only

a-‘matter of changing over the aerial connections and switching on the filamen‘: transformer to put the adapter into operation. It is only a broadcast receiver haying considerable radio-frequency amplification which is suitable for use with an adapter of this type.

requency an (kilocycles), + 7,500 .... 3,750 eeee ~ 1875 ... 1 to ATA bo 99 : ah . 22 . 22 . 22 Li 6 No, 18 No. 25 No. 45 No. 22 22 28 L2 3 No. 5 No. 10 No. 18 No. 28 28 28 L3 7 No. 15 No. 31 No. 60 No. 22 22 28 380 L4 5 No. 28 7 No. 28 10 No. 28 15 No. 28