The "Record" Short-Wave Receiving Set

Radio Record, Volume I, Issue 23, 23 December 1927, Page 12

The "Record" Short-Wave Receiving Set

HOW TO CONSTRUCT A "CONVERTER" USEFUL HINTS FOR OPERATING THE SET -_-_---

CONSTRUCTING THE SHIELD. In order to make the construction of the copper shielding as clear as possible, a diagram is given of the respective parts in relative _ positions. As already mentioned, the back of cover has a half-inch flange on three sides and these flanges go inside the main portion of cover and are soldered to it to hold the back in place. The ordinary radio soldering-iron will not be equal to this job, which was easily carried out with a small blowpipe blown with the mouth, heat being supplied by a tuft of cotton-wool soaked with methylated spirits and placed in the lid of a metal shaving-soap container. Ordinary ‘"‘killed acid" flux is most suitable for the work. All: bending of the copper * sheet should be done over the edge of a piece of wood to ensure a straight,neat bend. The portion to be bent ever should be pushed down with a piece of wood, a small amount all along, then go over again, pushing further, and so on, until the required bend is arrived at. BUILDING THE CONVERTER. The converter, or adapter, is a means whereby the one-valve portion of the short-wave set alone may be built, and by means of three wires connected to an old valve base, the necessary batterv connections to the converter are made by plugging in the valve-base to the detector valve socket of any ordinary broadcast receiver, and at the same time this connection puts the broadcast amplifier into use to amplify the short wave signals. ‘‘he main components constituting tlie two-valve amplifier are two audio transformers, and two valve sockets. Jn constructing the converter, these are omitted, thus allowing the total width of baseboard to be reduced _ to 144 inches instead of 20 inches. The rheostat must then be placed on the front panel between the two condensers, anid the position of the radio choke will be slightly altered to place it about centrally in the space provided. All other spacing of components ind above-board wiring will be just "he same as for the full set, except that one end of the choke, instead of connecting to a transformer, will go under the board to cénnect to the wire ronning to the plate pin in valve base plug. Tt is unnecessary to give fresh diagranis of shielding, as if this is adopted, all dimensions will be the same, with the exception of those for widths, which will all be reduced bv 54 inches, giving 144 inches in place of 20. The old valve base is hest obtained from an old yalve, British or Ameri- | —

can, to suit the detector socket on the broadcast set. The connecting wires should first be unsoldered from the pins, and then the cement fixing the glass to the base is dissolved with methylated spirits, and the bulb removed. Three lengths of single lighting fley are now taken, each sufficiently long to reach fram the converter to the brdadcast detector socket. To one end of each of these a wire is soldered, which must be thin enough to go through the holes in base. Each thin wire is then threaded through its respective hole and soldered to the valve pin. The three flexes can then be taped together with adhesive tape) just above the valve base, and may be plaited together and then connected to their respective points on the converter. The diagram shows an American socket, but if a British socket is used the same connections are made; that is, to the two filament pins and the plate

or anode pin. The grid pin is not used in either case. Every broadcast amplifier is not the same, and there may be cases in which the converter does not work at its best } unless some small temporary change is made, perhaps in condenser value across the first audio transformer. CONNECTING TO REFLEX SETS. Reflex sets employing a crystal detector have no detector valve socket, so in order to make use of the aimplifier for the converter it will be necessary to wire in a socket that will not affect the ordinary working of the set, and yet is always ready for the application of the converter. Taking the All-Amax three-tube as a typical reflex of this class, a Klosner or similar UX socket and a doublepole double-throw switch would be re-

OPERATING FULL RECEIVER OR quired. ‘The two ends of the primary winding of the first audio transformer are then both conuected to the two arms of the swicch, and the two connections that went to the transformer are now wired to a pair of the switch contacts. ‘The two other switch’ contacts are wired respectively to the plate socket of extra valve holder, and to a special B positive voltage tap for high tension on detector. The two filament sockets *of holder are wired to respective A battery leads. Tor normal use the switch is put over to connect as usual, When the converter is ty be used it is plugged into the extra socket and the switch thrown over, when all is ready for short-wave. A diagram shows this arrangement. ‘fhis has been worked out in response to a number of requests, and appears to be the simplest way of utilising the double amplification of the type of receiver in question. CONVERTER. The novice at short-wave receptiom ulust remember that until a fair amount of practice has been put in, tuning is inore difficult than on the broadcast wave-lengths, ‘The reason for this is that upon the lower wave-lengths the frequencies are so great, and a small movement of the dial that in the broadcast would cover less than one kilocycle, would on short-wave cover severa! Ikilocycles. At 40 metres the frequency is 7,500,000 a second, and at 15 metres 20,000,000. This means that unless the dials are turned very’ slowly indeed stations will be passed by unheard and unnoticed, | To start with, detector high tension voltage must be cut down to 15 or 20. volts, and the filament voltage kept at a minimum. The reaction condenser is now set to zero, and if the set has not_ a "liye" sound when off oscillation, increase filament voltage slightly. When the "‘live’’ condition has been reached; turn the reaction condenser about half in, when the set should begin to oscillate quietly and smoothly, with a louder characteristic hiss or liveliness off oscillation. To search, carefully turn the secondary condenser, all the time keeping the set just oscillating by following with the reaction condenser. If reaction comes on with a click, then filament curreut must be increased; if it starts with a bang and a squeal, then try_ decreasing filament and plate voltages With different coils a slight adjustment of a filament may be required in order to maintain oscillation at all parts of the secondary dial. Aerial coupling should be quite loose, reducing static and enabling the set to oscillate more easily, Remember other listeners, and keep the oscillation as gentle and quiet as possible, which will also make your own 1eception easier.

Usual suitable amplifier voltages are employed. SUITABLE VALVES. Quite a number of valves have been tested as detectors, including a couple of anti-capacity types, and the two sixvolt that came out best are the UX201A and the PM5X. ‘The PM83 makes a very good four-volt detector, and the PMILF a good two-volt. American valves were not tested in the lower. voltages, but no doubt the equivalents will be found equally satisfactory. When using a four-volt detector it will be necessary, in order to get the low filament voltage required, to include resistance in addition to the 80-ohm theostat, and this may be done by means of an extra rheostat in series be- | hind the panel, or by a length of re. sistance wire wound on fibte. The advantage of an extra rheostat is that it can be cut out at any time by @ turn of the kno,

When using the converter, the broad-_ cast set must be disconnected from earth. There is plenty of room on the audio side of the receiver for the later installation of a push-pull stage in place of the present second stage. Shielding of the battery cable of wires is secured by passing it through a spiral metallic gas tubing of half-inch outside diameter, ‘This tubing is admirable for the purpose, as it is constructed almost entirely of metal. ‘The batteries are shiclded by constructing a wooden box or frame to hold them, and then covering it with tin. A_falldown front will in many cases be found convenient. No trouble will be experienced with hand-capacity with the shielded panel, and the operator can remove both hands from the dials without affecting the tuning. lor good results. keep as near as possible to specifications, and especially to spacing of components and wiring. Plenty of space is good for the R.V. side of a short-wave set, and many fac-tory-built sets are too crowded in this respect to give full efficiency.