Radio and television today (Jan-Nov 1941)

SERVICING WITH A 'SCOPE Finding intermittents and weak performance on actual set with signal generator, tube and condenser tester, and cathode-ray equipment. Visual signal tracing and the interpretation of oscillographs are important servicing aids that should be understood by all servicemen. Beside the obvious technical advantages of signal tracing, the visual system has a large amount of customer appeal. For the benches that are located in the window of the shop, or in any other display position (and all of them should be), the "wiggly green line" of the cathode ray tube holds plenty of fascination for passers-by and for those customers who like to watch the checking of their defunct receiver. This prestige building type of service method can be a real business asset. Briefly, the visual method consists of analyzing the wave shape and response of the signal passed by the set by means of a signal generator and cathode ray oscilloscope. Misalignment conditions, low or no stage gain, distortion, noise and intermittent conditions show up on the screen. Once the patterns of characteristic troubles become familiar, it will be fairly simple to run down faults in short order. TYPICAL CASE An example of how one enterprising serviceman, James A. McRoberts of Long Island City, N. Y., does the job is as follows: A customer brought in a near-new Packard deluxe auto set made by Stewart-Warner. The complaint was intermittent noise, and weak reception when driving in the country. Plenty of things might be the cause for this type of performance. Tubes, alignment, by-pass capacitors, and other parts could cause these troubles. The problem, of course, is to hunt out the trouble in the least possible time. The equipment used for the following tests were a Supreme 582A signal generator, and 546 oscilloscope. Bypass capacitors were tested on a Solar analyzer and the tubes were checked on a Hickok tester. A check-up of the tubes of the set showed four of them to be under the performance of new tubes. The tubes were operative, but since the maximum performance was desired, they were replaced. EQUIPMENT CONNECTIONS After the tube replacements, the set itself was analyzed with the oscilloscope and associated equipment. Connections of the equipment can be understood by referring to the block schematic diagram. The points in the circuit to which the equipment was attached are numbered in the order in which the tests were made. Photos of the oscilloscope screen for the various adjustments and corrections made in the circuit. These are all RF and IF response curves. See text for reference to individual curves. The "hi" terminal of the vertical input for the oscilloscope was connected to the first audio grid at point 4 rather than the customary diode load position. Either connection will prove satisfactory. The signal generator is equipped for frequency modulation of the carrier and a total deviation of 30 kc, 15 kc. each side of the carrier, was used for all except the audio tests. The horizontal gain control of the oscilloscope was adjusted to extend the base line to the marks ruled on the celluloid reference screen corresponding to a 30 kc. sweep. The set volume control and 'scope vertical gain control were set at maximum for the RF-IP tests. INITIAL TESTS For the initial check of the performance of the circuit, the weak tubes having been replaced, the signal generator was connected to the antenna terminal through a 250 mmfd. capacitor. A very strong signal was supplied to the set and oscillograph 1 is the picture of the output. The broadness of the curve, and the projection below the base are due to static and the internal intermittents. The time exposure of the photo graph caused these irregular streaks to blend into a solid line. The peaks on the ends of the oscillograph base line are one-half of the center peak and are due to the sweep circuit of the oscilloscope being adjusted to a multiple of the frequency of modulation of the RF carrier. They are unimportant in the analysis of the wave. FINDING THE INTERMITTENT The roughness and irregularity of the wave of graph 1 led to a check of the voltages on the tubes as well as the plate and screen:. circuit by-passes and the cathode by-pass capacitors. Replacement of several by-pass capacitors brought the voltages back to normal and caused the response curve to become steady. Normal input in microvolts for the set gave the curve in graph 2. The response shown in 2 is weak and the base is very broad, thus indicating misalignment. To go back a bit, the testing of the set thus far has included the replacing of four tubes, the removal of the intermittent trouble causing the irregular and jumpy test pattern through the replacement of faulty by-pass capacitors. From the examination of graph two, it is obvious that alignment is necessary. To prove this contention, the output of the signal generator was increased about four times and graph 3 shows the image obtained. The separate peaks readily indicate that all the tuned circuits are not aligned to the same frequency. Incidentally, the 34 RADIO TOD A Y