Radio today (Apr-Dec 1939)

should be adjusted so that the transmitted picture just fills the screen on the tube. Adjustment of the vertical and horizontal hold (synchronizing controls) should produce a steady picture. With the receiver picking up a signal from the transmitter, the antenna should be rotated for the best possible picture — which is maximum signal when no ghosts are present. If ghosts or multiple images are present, rotation of the di-pole may eliminate the difficulty. More generally, it will be necessary to move the antenna about on the roof, in an effort to find a location where reflected signals are non-existent. ELIMINATING GHOSTS Since it is essential to watch the picture on the receiver while moving the aerial, two servicemen using a telephone may be necessary. Where the distances are short, it will probably be possible to shout back and forth. When it is impossible to eliminate the ghost by positioning the aerial, directive arrays may be necessary. These types of aerials are extremely directional and will cut down reflections arriving at large angles to the line of sight and at the rear. If a single image is obtained that seems weak, it may be necessary to find another location on the house where a stronger signal is obtained. Because of these "ifs", it is best to make the installation temporary, until it has been determined that a satisfactory (and best possible) image is obtained. Haywire installations will not be satisfactory for television. The lead-in cable or transmission line should be securely fastened for swaying may have an adverse effect on the picture. In order to keep the transmission line out of the field of the doublet, the lead-in should run down at right angles to the doublet for at least % wave The position of yoke in a set having magnetic deflection determines the size of the picture and its position with relation to mask. Diathermy interference produces the pattern shown above. The wavy lines drift across the picture. length. For a 6 meter signal, this is about five feet. CONFORM WITH LOCAL LAWS In many cities there are local ordinances that require that all wires be 7 or 8 feet off the roof. Care should be taken that the antenna and transmission line be kept high enough off the roof so as to conform with the law and prevent damage. At the window where the transmission line enters the room, a double type lightning arrestor makes an excellent terminal block as well as conforming with the Underwriters' regulations. Inside the home, light-colored wire may be used to harmonize with the surroundings. Because of the extremely high frequencies of 44 to 108 megacycles, care should be taken to use nothing but the highest grade of transmission line, in order to keep line losses at a minimum. When the manufacturer supplies a ground terminal on the set, the serviceman should be sure to provide a ground for the receiver. Neat tiiple sockets and plugs are available that accomodate the two aerial wires and the ground connection thus making it simple to disconnect the house receiver when it is necessary to move the set for house cleaning, etc. TYPES OF INTERFERENCE After the set has been installed, it is possible, but not likely, that various forms of interference may be present at certain times. The serviceman should be able to recognize the common forms of external interference. Diathermy is the one form of interference for which there is no cure, unless the offending machine can be located. It makes its appearance in the form of wavy streaks of varying widths that may move up and down on the image. Illustrated herewith are two conditions of diathermy interference. One is when the diathermy signal is comparatively strong, and the other when it is extremely bad. The interfering patterns move about on the screen. A second form of interference is auto ignition. It creates globs of white on the image. Placement of the antenna as far away from the travelled roads may reduce the interference. The only cure is to equip all cars with suppressors, an impossible procedure unless laws are passed to this effect. Tube hiss on a weak signal gives a snowstorm effect when viewing the image. The cure is to increase the signal strength. Beats from an amateur transmitting station cause the formation of fine bars, a sort of cross-hatch pattern — that drifts across the picture. When the receiver is improperly tuned, the video tuned between the sound and picture carriers — a fine cross-hatch pattern is the result. Cure is to properly tune the receiver. Cross talk between the sound and picture signals causes relatively coarse black and white horizontal bars. When tuning the receiver the sound channel of the station is tuned in. The sound channel is fairly broad in tuning, so it is possible to adjust the receiver slightly for the best possible picture. DANGER— HIGH VOLTAGE Because of the high voltage in these receivers — it is urged that servicemen do not make adjustments inside the receiver while the power is on. Protective interlocks that remove the primary power when back is removed will be used on most sets. And on the service bench, the servicemen should be extremely careful not to contact the high-voltage circuits. If voltage measurements are necessary, be sure the insulation in the test leads and instruments is adequate. A good procedure to follow would be to check circuit continuity and parts values with no power on when loss of supply voltages to the picture tube is suspected. Di-pole antenna with reflector suggested by DuMont for reduction of ghosts and increasing the received signal strength. APRIL. 7939 79