Radio Broadcast (May 1928-Apr 1929)

264 RADIO BROADCAST SEPTEMBER, 1928 A MANUFACTURED 5-METER RECEIVER This receiver is of the detector-audio type, and is made by Parmeter Products, of Lansing, Michigan one has a little practice in 5-meter reception and has gotten the sending set steadied up such a set is of little practical value because of the difficulty of following the transmitter wave — or finding it initially. If radiophone is to be received the best device is certainly the double-detection receiver, and as second choice the regenerative detector with audio amplification, to which the stage of 5-meter r.f. irUiy be added later. The perfectly commonplace circuit of such a 5-meter set was shown in the August Radio Broadcast article. For c.w. reception we have a much more powerful tool in the form of the double-detection receiver which is shown in diagrammatic form in Figure 3. This consists simply of an oscillating detector ("autodyne") followed by an i.f. amplifier and a second detector with a single stage of audio. To secure a beat note from the c.w. signal there is provided a long-wave (intermediate frequency) heterodyne which beats upon the intermediate frequency. This heterodyne may be removed and a 5-meter heterodyne put in whereupon the set becomes suitable for 5-meter phone work. The first detector in such a case is left regenerative but not oscillating. However as it stands the set can receive phone in a somewhat "chewed" manner by careful adjustment of the first detector and removal of the tube from the heterodyne oscillator socket. A further improvement is to replace the i.f. amplifier shown in Fig. 3 by a single ux-222 tube with a band-pass filter either before or after the ux-222. MAKING THE RECEIVER WORK '"THE troubles of a 5-meter receiver are those * of any regenerative receiver or doubledetection receiver plus the additional job of persuading the tube to oscillate without howling at 5 meters. The solution of this begins in making the receiver oscillate at all, and ends in eliminating the yelling. Usually it is easiest to start at about 7 meters, guessing at the proper coils, and then work down gradually to such dimensions as mentioned in connection with the receiver. Do not take the dimensions too seriously since such a trifling thing as a change in the make of socket may demand an entire turn to be taken off the coil. It is therefore a matter of "cut and try," with constant attempts to find a click or thump on the General Radio wavemeter by turning its dial slowly while it is held near (1" to 1') the tuned system and the phones are worn around the neck or otherhow far enough removed from the ears to prevent damage from a sudden savage scream. Changes in the tickler diameter, number of turns and location, together with alterations in the grid leak and filament current will finally produce a combination which can be taken in and out of oscillation quietly and at the same time has the proper range of tuning. It is well to spend an evening or two in getting this strictly right, after which the regeneration control condenser, Co, should be left severely alone to prevent changes in calibration. Use the rheostat, R, to control the regeneration thereafter. In general it will be found best to use a tickler (L2) which is rather close to Lj and has turns between one half and three quarters of the diameter of Li made of a wire fine enough to handle easily when making changes but not "floppy." Both Li and L2 should be substantially secured into position to avoid vibration noises. On the other hand the ux-i 12A tube must be mounted in a springless socket set on sponge rubber and fed by wires of the greatest softness, such as small bare or silk covered single strands of No. 30. These wires must be kept mutually apart since changes in their separation will cause noise and loss of signals. The filament wires need not be handled so carefully, but the grid and plate wires should depart at something like right angles from each other so that motion will not change their separation greatly. The grid leak and condenser may be mounted on the tuning condenser with a flexible lead to the socket, or on the socket with flexible leads to the tuning condensers. Leaks of a value above 8 megohms or below 1 megohm will work about equally well. H THE RECEIVING ANTENNA [AVING caused the receiver to oscillate decently and smoothly we may now connect it to an antenna and see if it will continue to perform decently. Here we find from experi RFC-2 Rl.Supply Jil. Supply 0 I. TYPICAL TRANSMITTING CIRCUITS All these circuits are of the ultraudion type. They operate smoothly after the chokes have been made correctly. As a starter these may be wound with No. j6 d.s.c. or s.c.c. wire spaced slightly on a \" form about 2" long. Sometimes a foot of small wire stretched out straight is almost as good. The center taps consist of double 100-ohm resistors with each half shunted by a capacity of about 0.001 mfd. The filament leads may be choked to advantage outside the diagram. For the tuning circuit, U, L% one may start with a total of 3 turns of _j" diameter in circuit A and a condenser, C\, of 100 mfd. maximum. For circuits B and C the capacity, C\, need not be above 25 mmfd. and the entire inductance from grid to plate need not be over a foot long. Co in B and C can be anything around 200 mmfds., capable of withstanding the plate voltage. Make good connections. Ordinary grid leaks make trouble and a grid leak and condenser consisting of two aluminum wires immersed 1" in water is a good thing to use. Changes may be made; the inductance of A may be a single turn 10" across. The chokes in these diagrams are not coupled to the tuning coils.