Radio and Television Today (Jan-Nov 1941)

Block schematic of the auto set tested with the various test points numbered in order. 'Scope remained connected to 4 for first six oscillographs and was then reconnected to points 5 for audio tests and graph 7. Refer to text for sequence of tests and adjustments. "center" frequency at this point was 1600 kc. The signal generator frequency was ( hanged to the IF and applied through a 0.1 to the converter grid, point two on the block diagram. A 500M isolating resistor was placed between the converter grid and the grid clip. IMPROVING ALIGNMENT After a thorough alignment of the IF transformers, the response curve for this end of the set looked like graph 4. The procedure in aligning the transformers is one of adjusting the trimmers or permeability cores until the response curve is symmetrical, steep sided, and in the case of high fidelity BC sets, has as broad and flat a peak as possible. In most cases it will be OK to go back over the adjustments several times in order to get the best curve. Oscillation in the IF channel will show up as small peaked waves on one side or the other of the highest point of the response curve. LOW FREQUENCY ADJUSTMENT Further improvement of the performance of the set was obtained as the RF end was brought into alignment. Graph 5 shows the finished job on alignment at 1550 kc. The signal was moved back to the antenna, point 1, for these adjustments and, of course, the input level was reduced to the normal value for the set. A low frequency adjustment was made at 590 kc. The plates of the oscillator section were bent slightly to give the maximum height of the curve on the screen. This curve is shown in graph 6. The double image is the result of the stronger signal used and is due to the detuning effect of the AVC voltage. The height of the image is the most important thing at this point in the alignment. AUDIO TESTS The audio testing of the set was conducted by reconnecting the oscilloscope to the voice coil, marked 5 on the block diagram. The input signal was RF modulated at 440 cycles and supplied to the antenna terminal of the set. The 'scope picture is shown in number 7. At this point, watch for any distortion of the wave. The shape of the curve with distortion present depends upon the phase angle between the fundamental and the harmonic. As a rule, even harmonics give symmetrical positive and negative waves, while odd harmonics give unsymmetrical waves. The oscilloscope test routine is thus fast, convenient, and informative. Remember that the 'scope remained connected to the first audio grid for all the RF-IF tests, and to the voice coil for all the audio tests. The signal generator lead was moved from point to point as the different parts of the circuit were investigated. Audio response curve taken at 440 cycles. Modulated RF signal was fed into the antenna post. Powder Eliminates Tire Static A powder which eliminates static interference and static shock caused by the friction of rotating parts including wheels, fan belts and tires, has been developed by United States Rubber Company. One tablespoonful of powder blown into each tube (where it remains in suspension like dust in the air) is declared to be sufficient to neutralize the static in any make of radio, with any kind of antenna, on any make of passenger car, with any make of tire or tube, on any type of road surface. Application of this powder is made with the tires on the car, but the tubes must be deflated, valve cores removed, the powder blown in with an applica tor, unit limes Inflated. In case of a puncture loss of powder is so small that full effectiveness remains. Static electricity is developed by friction between moving parts, the tires and road, the car body and surrounding air, etc. Radio Interference occurs when there is an irregular discharge of electricity between various parts of the automobile, or between car and ground. FACTORS OF STATIC Of the various factors influencing static electricity in automobiles — such as atmospheric conditions, clothing and car upholstery, road surfaces, and moving parts of the car — it seems that tires are the only one which contributes doubly. Tires not only build up electricity through friction against the road, but they also serve to insulate the car from the ground, thus preventing any and all charges from flowing to the road surface. The new powder, which has been tested for many months, will be available within 30 days. Cost of treating five tires will be about one dollar. Television Sound Goes FM With the July 1 commercialization, television stations changed over from AM operation on its sound channel to FM and from 441-line picture definition to 525 lines. The servicing problem of converting the several thousand sets will be a complicated job. However, experience in operating the old sets on the new standards seems to indicate that quite satisfactory performance can be obtained with a few adjustments. The shift from 441 to 525 lines is easily made by adjusting the horizontal "hold" or "speed" control slightly. The 441-line image required a horizontal frequency of 13,320 cps., while 525 lines call for 15,750 cps., a value well within the range of the "speed" control. FM PERFORMANCE Most of the sets give fair performance on FM sound with no change whatsoever. This is due to the high intermediate frequency channel, of 8.25 mc. These IF transformers are more than broad enough to pass the 150 kc. swing used with the FM signal, and there is enough of a slope somewhere on the characteristic to convert the frequency variations into amplitude variations. Improved performance may be obtained by adjusting the tuning control, or by slightly de-tuning the sound IF transformers. This tuning shift puts the carrier down on one side of the peak where the slope is more pronounced and conversion from FM to AM is more complete. For true FM performance, however, a standard limiter and FM second detector will be required. Manufacturers are planning to announce details for the complete conversion of their sets including the new No. 2 channel, 60-66 mc, not previously covered. J U LY , 7 94 1 35