Chapter 10 — SAFETY; TEST EQUIPMENT; TEST METHODS 



COLOR 



BLACK 



BROWN 



RED 



ORANGE 



YELLOW 



GREEN 



BLUE 



VIOLET 



GRAY 



WHITE 



GOLD 



SILVER 



CAPACITANCE 



SIGNIFICANT 

 FIGURE 



DECIMAL 

 MULTIPLIER 



1 



10 



100 



1,000 



0.1 

 0.01 



TOLERANCE 



20 PERCENT 

 2 PERCENT 



5 PERCENT 

 10 PERCENT 



CHARACTER 

 ISTIC 



BLACK DOT IDENTIFIES 

 MICA CAPACITOR 



DIRECTIONAL 

 INDICATORS 



SIGNIFICANT 

 FIGURES OF 

 CAPACITANCE 



CHARACTERISTIC ' \ \ ■ IN //^f 



CAPACITANCE TOLERANCE-^ ^-DECIMAL MULTIPLIER 

 CHARACTERISTICS 

 A - ORDINARY MICA BYPASS 

 B - SAME AS A-LOW LOSS CASE 



C - BYPASS OR SILVER MICA (±200 PARTS/MILLION/C) 

 D - SILVER MtCA (1 100 PARTS MILLION/C) 

 E - SILVER MICA (0 TO t 100 PARTS/MILLION/C) 

 F - SILVER MICA (0 TO +50 PARTS/MILLION/C) 

 G - SILVER MICA (0 TO - 50 PARTS/MILLION/C) 



20.376(71) 

 Figure 10-16. — Color code for fixed mica capacitors. 



Several types of tests may be conducted to 

 determine the condition of a tube. The most 

 common test is substituting a good tube for the 

 suspect one. Other tests, requiring the use of 

 a tube tester, are emission, transconductance, 

 gas, short circuit, and noise tests. 



Substitution Test 



Substituting a tube known to be in good con- 

 dition is the most reliable method of deciding 

 the quality of a questionable tube. In high fre- 

 quency circuits, tube substitution should be car- 

 ried out carefully one at a time, noting the effect 

 of differences in interelectrode capacitances of 

 the substituted tubes on tuned (aligned) circuits. 

 The substitution method of testing cannot be 

 used to advantage to locate more than one faulty 

 tube in a single circuit. If both an a-f amplifier 

 tube and an i-f amplifier tube are defective in a 

 receiver, replacing either one does not correct 

 the trouble. If all the tubes are replaced, there 

 is no way of knowing which tubes were defective. 

 Under these or similar conditions, the use of 

 test equipment designed for testing the quality 

 of a tube saves valuble time. 



Emission Test 



An important indication of the condition of 

 a tube is obtained by a comparative check of the 

 cathode or filament emission. In most instances 

 a pronounced lower-than-normal emission, or a 

 complete loss of emission, indicates that the tube 

 has reached the end of its useful life. Both 



multigrid and diode tubes are tested for cathode 

 emission. 



• Multigrid tubes: For a test of the emission of 

 a multigrid tube, the tube is connected as a 

 half-wave rectifier. (See fig. 10-17.) The plate 

 and all the grids of the tube are connected 

 together. A milliammeter and a variable resistor 

 are placed in series with the tube, and the entire 

 circuit is connected across a transformer second- 

 ary. Because of their common connection, the 

 plate and all of the grids are at the same poten- 

 tial with respect to the cathode. As a result, the 

 tube functions as a diode rectifier, conducting 



PUSHBUTTON 

 SWITCH 



Figure 10-17.- 



1.73 

 •Basic circuit used for emission 

 test. 



167 



