Chapter 10 — SAFETY; TEST EQUIPMENT; TEST METHODS 



The easiest way to check a suspected mal- 

 functioning transformer is to measure its voltage 

 output. Tiiis test can be made by using a volt- 

 meter to measure across the proper terminals, 

 and comparing the reading obtained with the proper 

 voltage, as indicated in the technical manual. If 

 only one reading is in error, it indicates that only 

 that winding is at fault. If all windings read in 

 error, the trouble could possibly be the fault of 

 either liigh- or low-input voltage. Consequently, 

 it always isnecessary to measure the input voltage 

 as well as the output voltage. 



If improper voltage is measured at any of the 

 output windings, the next step is to measure the 

 resistance of each winding. For this measurement, 

 the connections to the terminals must be dis- 

 connected first. Power to the equipment must be 

 secured in order to disconnect a transformer. 

 Once the leads are free measure the resistance 

 across each winding with an ohmmeter set to the 

 proper scale. Check each winding carefully, includ- 

 ing any center taps. Next, set the ohmmeter to a 

 high scale and test for shorts between windings 

 and between windings and ground. 



If no faults are discovered in the voltage or 

 resistance readings, or in the tests for shorts, 

 the transformer is assumed to be in proper operat- 

 ing condition. It then can be connected back in the 

 circuit. 



If a replacement is required, use as the 

 replacement only the one indicated in the manu- 

 facturer's technical manual. It is always good 

 policy to test for a short and make a resistance 

 check on a new transformer before installing it in 

 the circuit. 



CAPACITORS 



Capacitance, inductance, and resistance are 

 measured for precise accuracy by alternating 

 current bridges. These bridges are composed of 

 capacitors, inductors, and resistors in a wide 

 variety of combinations. The bridges operate on 

 the principle of the Wheatstone bridge, in winch 

 an unlmown resistance is balanced against known 

 resistances. The unlmown resistance is calculated 

 in terms of the laiown resistance after the bridge 

 is balanced. One type ofcapacitance bridge circuit 

 appears in simplified schematic form in figure 

 10-14. When the bridge is balanced by adjusting 

 the two variable resistors, no a-c voltage is de- 

 veloped across the input of indicator tube VI, and 

 the shadow angle is maximum. {VI is a "magic 

 eye" tube, commonly used as a radio receiver 

 tuning indicator.) Any slight imbalance produces 

 an a-c voltage, which, in turn, develops a grid- 



20.344 



Figure 10-14. — Simplified schematic of capacity 

 checker. 



leak bias and lowers the plate current of VI, 

 reducing the shadow angle. 



In the basic Wheatstone bridge circuit using 

 d-c voltages and simple resistances, the balance 

 is obtained when the voltage drops are equal 

 across the ratio arms. In the a-c capacity bridge, 

 it is insufficient to have equality of voltage drops 

 in the ratio arms. The phase angle between cui'rent 

 and voltage in the two arms containing the capaci- 

 tors also must be equal in order to obtain a 

 balance. When a balance is obtained, the current 

 is equal on both sides of the bridge circuit. 



The capacitance-inductance-resistance bridge, 

 type ZM-ll/U, shown in figure 10-15, is widely 

 used to measure capacitance, resistance, and 

 inductance values in addition to special tests, 

 such as the turns ratio of transformers and capa- 

 citance quality tests. It is a self-contained instru- 

 ment, except for a source of line power. It has 

 its own source of 1000-Hz bridge current with a 

 sensitive bridge balance indicator, an adjustable 

 source of direct current for electrolytic capacitor 

 and insulation resistance testing, amd a meter with 

 suitable ranges for leakage current tests on 

 electrolytic capacitors. 



Many capacitors have their value printed on 

 them, but some types use a color code system to 

 indicate their value. The color code used is the 

 same as that used for resistors, but the methods 

 of marking the capacitors vary greatly. The 

 method used for fixed mica capacitors is shown 

 in figure 10-16. 



A black dot in the upper left corner signifies 

 that the capacitor has a mica dielectric. The 

 center dot in the upper row indicates the first 

 significant figure, and the upper right dot indi- 

 cates the second significant figure of the capaci- 



165 



