Radio Broadcast (May 1929-Apr 1930)

RADIO BROADCAST A new view of the Clarke Laboratories, Danville, l a. (Continued from page 226) to neutralize one of these sets, and, although he was able to balance out the oscillations in each stage, still the set would go into oscillation on certain parts of the band. "Looking at the set from the bottom, a brass plate will be seen under three of the tuning condensers. This plate is insulated from the frame of the condenser by a very thin square of bakelite. On the side of each tuning condenser is a small leaf similar to a book-type condenser, which is adjustable. At first glance the serviceman will put these down as trimmer condensers, and here is where the big surprise comes in. Of the four tuning condensers equipped in this way, only one of them is connected as a trimmer condenser, and that one is in the detector input. On the other three, the brass plate under the tuning condenser is connected to the movable leaf, and this constitutes a variable radio-frequency bypass condenser to the filament. "By choking off the r.f. currents from the C-bias lead, and providing a variable bypass to the filament, the engineers of this clever system have incorporated a losser adjustment, which is necessary in conjunction with neutralizing, to prevent oscillations. This is probably necessary due to the fact that the set is unshielded, and although neutralizing will prevent coupling due to tube capacities, this still leaves the coupling due to coil feedback and circuit capacities. " However, the main point of interest is how to adjust this system for maximum sensitivity, selectivity, and freedom from oscillations. Fortunately, the largest part of the job is the understanding of what goes' on in the circuit. My method is to neutralize the set first. If it oscillates after this is done, I then loosen all the condenser plates very slightly, or until oscillation stops. Unfortunately these little by-pass condensers do act as trimmers at the same time, and this fact must be kept in mind. So if it were necessary to stop oscillation by loosening these condensers, and at the same time the circuits went out of resonance, I would re-resonate the circuits by continuing to loosen one or two of the condensers, as needed, until selectivity is regained. Then the circuit is reneutralized with the oscillator, and the job is done. If at first, after neutralizing the set, I lind, instead of a tendency to oscillate, that the set lacks pickup and selectivity, I introduce more radio-frequency gain by tightening all the condensers a little. Care must be taken to keep the circuits in resonance at the same time. The adjustment is right when the set hisses slightly as it is tuned into a carrier wave, and tunes sharply. I finish by neutralizing again, and make a linal check on the resonance, by pushing slightly with a screw driver on the tuning condenser end plates. "I have found that most of these sets can be tremendously improved by the intelligent use of these adjustments, and the customers are delighted in every case." A. H. Goud, So. Portland, Me. An Automatic Switch Try suggesting to some of your customers the utility and convenience of having their sets turned on or off automatically at a predetermined time. Your service editor finds it quite in accord with his general indolence to roll into bed at ten p. m., listen to the slumber music waft in from the living room, with the serene knowledge that it will turn itself off at midnight. More ambitious fans may put the device to the perverted use of an alarm clock, substituting setting up exercises for the bell. Vernon W. Palen, an engineer with the Telephone Company shows how easy it is to do. "The clock described in the following paragraphs was constructed because a certain member of my family had the bad habit of falling asleep at night with the radio going. As a result, I found my batteries prematurely exhausted on several occasions. "The time clock shown in the illustration effectively ended the above evil since it is always set to turn the radio off at some hour after midnight at which time the household is usually fast asleep and most radio programs are ' Off the air.' "A dollar alarm clock furnishes the backbone of the mechanism. The clock is mounted in a small wooden box which is constructed to fit. A circular piece is cut from the front of the box through which the face of the clock protrudes. The rear of the box is then fitted with the switch mechanism. "The face of the clock extending through the circular hole of the box helps materially to hold it securely in position. A strip of wood placed across the back of the clock and fastened to the box with screws holds the clock fast. It is convenient to mount the brass contact springs on the strip of wood above mentioned as will be seen in the illustration, Fig. 1. "To the alarm winding key, a piece of sheet brass (bent 'U' shaped to give it rigidity) is either soldered or bolted. A wire is connected to each of the two stationary brass contacts and brought out to the radio set and these are used to connect the contacts in series with the A of the set supply or a.c. line. The brass contacts are adjusted so as to touch the brass arm on the alarm key when the key is rotated to wind the clock. The alarm is set in the usual manner and when the alarm sounds, the key rotates the contact arm, breaking the connection with the two stationary brass contacts. It will be noted that the brass arm on the alarm key will strike the side of the box after approximately a half revolution. This stops any further unwinding or ringing. A half turn of the winding key is all that is necessary to reset the switch since it never completely runs down. "By a slight variation in the contacts, the device can be used to turn the receiver on at a given time. " I finished the box in black lacquer and by means of the two screw eyes hung it under the edge of my radio table. There it serves conveniently as a safety device for my radio set and as a timepiece for the living room." In the Service Laboratory Determining Transformer Ratios: "As a rule there are several power transformers in operating condition laying around the service shop that have been taken from power packs, sets, etc. At a time when one of these transformers would come in handy for replacement on a rush job, the serviceman is often ignorant of its high-voltage characteristics. And as a rule the a.c. voltmeter at hand reads only to 150 volts. "A quick and easy way to obtain the high-voltage reading is as follows. First measure the line voltage. Connect the secondary leads to the line. (In the case of a full-wave transformer, connect one outside terminal and the center tap.) Take a voltmeter reading across the usual 110-volt primary. The line voltage divided by the voltage across the primary will then give ARM WILL • STOP WHEN IT STRIKES BLOCK OF WOOD HERE TO A BATTERY OR 110-VOLT LINE Fig. 1 the ratio of transformation. In other words, if the line voltage is 108 and the voltage across the primary is 36, the ratio is 3 to 1 and the transformer, when connected properly, will give 324 volts on the high side (108 volts times 3). 228 • • FEBRUARY 1930 •