INTRODUCTION TO SONAR 



WER CORD 

 ND PLUG 

 TOWED) 



CARRYfNS 

 HANDLE 



Figure 



20.345 

 10-15. — Capacitance-inductance-resist- 

 ance bridge, type ZM-ll/U. 



tance value in micromicrofarads (niJif). The right 

 dot in the lower row indicates the decimal multi- 

 plier that determines the number of zeros to be 

 added to the right of the two significant figures. 

 The center dot (lower row) specifies the tolerance, 

 which is the possible deviation of the actual capaci- 

 tor value from that given by its dot markings. The 

 left dot on the lower row deals with temperature 

 coefficients and applications. 



By way of explanation, a capacitor with upper- 

 row dots colored black, red, and green (reading 

 from left to right, according to the directional 

 indicator) would mean a mica capacitor with the 

 significant figures 2 and 5. The lower row of dots 

 (reading from right to left) are brown, red, and 

 red. The brown dot requires the addition of one 



zero to the value 25, giving the result as 250 /ufxf. 

 The red center dot indicates that the actual capaci- 

 tor value may be greater or smaller than the 250 

 /^/jf by plus or minus 2 percent. The left red dot 

 means it is a bypass or silver mica capacitor. 

 Some micacapacitors have only three dots, indica- 

 ting the first and second significant figures and the 

 multiplier. Their tolerance is 20 percent, and they 

 have a 500-volt rating. Try reading some of the 

 values of the mica capacitors you see in electronic 

 equipment when they are exposed to view. Learning 

 to read capacitor values is a matter of practice, 

 much like reading resistor values. 



TUBES 



Electron tubes are essential components of 

 electronic equipment. If the average tube is not 

 overdriven, nor operated continuously at maxi- 

 mum rating, it can be expected to have a life of 

 at least 2000 hours before the filament opens. 

 Tubes do fail, however, before achieving their full 

 life expectancy. Some of the more important condi- 

 tions affecting the life expectancy of an electron 

 tube are (1) the circuit function for which the tube 

 is used; (2) deterioration of the cathode (emitter) 

 coating; (3) decrease, with age, in emission of 

 impregnated emitters in filament-type tubes; (4) 

 defective seals, which permit air to leak into the 

 envelope and oxidize the emitting surface; and (5) 

 internal short circuits and open circuits caused by 

 vibration or excessive voltage. Because of the 

 attendant expansion and contraction of the tube 

 elements during the process of heating and cool- 

 ing, the electrodes may lean or sag, causing exces- 

 sive noises (microphonics) to develop. Other 

 electron tube defects are cathode-to-heater leak- 

 age and nonuniform electron emission of the 

 cathode. The defects, of which only the most 

 common are listed here, contribute to about 50 

 percent of all equipment failures. For this reason 

 it is good practice, when you are troubleshooting 

 the equipment, to eliminate immediately any tube 

 that may have contributed to the equipment's 

 failure, but avoid blind replacement of good tubes 

 by fresh spares. In a glass envelope tube, visible 

 evidence of a tube defect may be present when the 

 filament is open, when the plate current is exces- 

 sive, when the tube becomes gassy, or when arcing 

 occurs between electrodes. When metal-encased 

 tubes are warm to the touch, it is an indication that 

 the heater is operating. A tube may be tapped 

 lightly with the finger while operating in a particu- 

 lar circuit to provide an aural indication of loose 

 elements or microphonics. 



166 



