Radio today (Jan-Mar 1939)

PRACTICAL DYNATESTING OF SETS Case histories of how service problems were rapidly solved using dynamic testing procedure By Vinton K. Ulrich, Service Editor This twelfth installment of the dynamic servicing procedure is to show the radio man how the method may be used for every-day radio repair work. Typical examples illustrate that dynatesting is a simple and most logical method of analysis. In the articles appearing in Radio Today for the past year, much emphasis has been placed on dynamic rather than static tests. The latter type of analysis should not be overlooked by the practical radio man, for in many instances it is an essential test. However, it is the writer's viewpoint that dynamic tests should always be made first, for if the dynamic conditions are proper, the static conditions are satisfactory — and the reverse is not true. Thus, with a single dynamic test it is possible to check both the static and dynamic conditions— if either one or the other condition is not proper, it will be shown up by a dynamic analysis of the stage or stages under consideration. Stage-gain requirements The readers who have been following Radio Today have by this time discovered that Dynamic Testing is not just a measurement or indication of stage gain, but a method for testing many of the individual circuit components and circuit conditions within a stage. In order to avoid the necessity of taking actual stage gain measurements the test procedure starts at the loudspeaker and works toward the antenna. The only gain or amplification requirement for a stage is, that the stage will drive the following one to its proper level or output. This idea is in keeping with the design practice of many engineers. For instance, the radio set engineer decides a set must deliver 5 watts output. Requirement is that the output tubes de liver the power and the speaker handle the output. Suitable output tubes are then chosen. Next step is to drive the output tubes to full capacity. This means that the driver tube must be capable of supplying the necessary signal voltage without distortion. And likewise, the previous stage must be capable of supplying the driver tube with sufficient signal voltage or excitation. If the driver stage can deliver the required sock and each preceding stage enough signal voltage to properly drive the next one, then the stages have sufficient gain or amplification. In dynamic testing as developed by the writer, this principle is basic. As long as sufficient gain is present, it does not matter what the actual value of the gain is — and because of this relationship it is possible to neglect actual values of gain with dynamic testing. As described in previous articles, it is necessary to move the signal generator from stage to stage in order to accomplish the desired results. Movement of the signal generator from stage to stage has the additional advantage that signal voltage measurements in critical circuits are unnecessary— thus simplifying the tests. Receiver dead The first receiver difficulty to be discussed is that of a set that was dead. Set was inoperative — all tubes tested okay. Signal generator (audio) was connected to grid of output tube — no signal. Next connection made to voice coil, which showed speaker was operative. Finally the audio oscillator was connected to the primary of the output transformer as shown in Fig. 1-A — signal was still present. Obvious conclusion was that the output stage was dead. Voltages of output stage were checked — none was present. Since the field was used for a filter choke and it was properly excited, it seemed Fig. 1 — In circuit A the audio oscillator was connected to the primary of output transformer and resultant signal showed speaker to be okay, yet output stage was dead — fault was shorted filter. In B an intermittent coupling condenser was discovered by moving the oscillator from one terminal of condenser to other. that the B plus might have been shorted. Actual inspection showed the second filter condenser to be shorted. Time consumed for the analysis was but two minutes. Static tests would have provided the same conclusion, but it is not likely that the serviceman would have started at the defective stage. With six tubes in the set, the chances are only one in six that the voltages or resistances of output stage would have been tested first. Intermittent receiver Case two is that of a set which operated intermittently. The volume would suddenly decrease after several minutes operation — turning lights on and off in house would bring it back to normal sometimes. Set had stumped several servicemen and manufacturer's jobber. First step was to connect audio oscillator to grid of output tube as shown in Fig. 1-B — no intermittents were noticed after quite a few minutes' operations. Next the oscillator was connected to the plate of the first audio tube. After a few minutes the decrease in volume occurred — symptoms same as those reported by owner. Coupling condenser was suspected as the cause — unit replaced and set performed okay. Condenser later checked on a standard condenser tester which showed unit to have no defects. However, replacement of condenser, additional tests showed, was the proper cure, which indicated that it was one of those rare cases where a defective condenser tests okay. The third example was also an intermittent— but different from the usual type. Set would sometimes operate properly for a week and then go mushy. Because of the rare occurrence of distortion, it was decided to try using an autotransformer with an over-voltage to hasten the phenomenon. This time luck was present. After about 3 minutes' operation with a line voltage of 122 the set went mushy. With the set in a continuous state of distorted reception (Fig. 2 shows wave form), the tests were handled in the usual manner. The output was undistorted when the oscillator was directly connected from grid of output stage to ground. Circuit of set shown in Fig. 3-A. Output became distorted when oscillator was connected to plate of first audio tube. The ear was used to determine the presence of distortion and the scope used for a visual record. Next the coupling condenser was checked for leakage — none was present. Grid resistor was of proper value. Lowering value of grid resistor reduced distortion but also decreased gain. Even though the tube tested okay, it was replaced with a new one. New tube gave perfect results under all line voltage conditions. Later tests with the old tube showed grid was as 38 Radio Today