Chapter 10 — SAFETY; TEST EQUIPMENT; TEST METHODS 



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NEON 

 LAMP 



(O) NOISE TEST m) 



A-C 

 LINE 



1.75 



Figure 10-19. — Basic circuit used for short circuit and noise tests. 



as a result of the heating process. The short 

 circuit test is similar to the test for detecting 

 noisy (microphonic) or loose elements. Because 

 the only difference between the two tests is in the 

 sensitivity of the device used as an indicator, the 

 noise test is discussed as a part of the short cir- 

 cuit test. 



Figure 10-19 shows a basic circuit used for 

 detecting shorted elements within a tube. With the 

 plate switch set to position 2, the plate of the tube 

 under test is connected to the leg of the trans- 

 former secondary containing the neon lamp. All 

 the other elemente are connected through switches 

 to the other leg of the secondary. If the plate of 

 the tube is touching any other element within the 

 tube, the a-c circuit of the secondary is completed 

 and both plates of the neon lamp glow. If no short 

 exists, no indication will be present, or only one 

 plate of the neon lamp will glow. Each of the other 

 elements is tested by means of the switching 

 arrangement shown. Resistor R2 limits the cur- 

 rent through the neon lamp to a safe value. 

 Resistor Rl bypasses any small alternating cur- 

 rents In the circuit that might be caused by stray 

 capacitance, thus preventing the neon lamp from 

 indicating erroneously. Tapping the tube lightly is 

 recommended to detect loose elements that might 

 touch when the tube is vibrated. 



The noise test is, in effect, no more than a 

 very sensitive short circuit test. In figure 10-19 

 two leads are taken from either side of the neon 

 lamp and brought to external receptacles labeled 

 "noise test". A liigh-gain amplifier (with speaker) 

 is connected to the receptacles. Perhaps the hand- 

 iest amplifier for tills test is an ordinary radio 

 receiver. The antenna and ground terminals of the 

 receiver are connected to the noise test jacks, and 

 a normal short circuit test is made while tapping 

 the tube. If tube elements are loose — but perhaps 



not loose enough to indicate on the neon lamp — 

 loud crashes of noise (or static), over and above 

 the normal amount of noise that is present, are 

 heard from the receiver. The noise test also may 

 be made without the use of the high-gain amplifier 

 merely by Inserting the leads from apair of head- 

 phones into the noise test receptacles. The latter 

 check, of course, is not as sensitive as the test 

 made with the amplifier, but ordinarily is more 

 sensitive than the short circuit test made with the 

 neon lamo as an indicator. 



SEMICONDUCTORS 



Semiconductors have been used in electric 

 circuits for several years. Copper-oxide and 

 selenium dry-disc rectifiers are two of the more 

 f am? liar types. The outstanding characteristic of 

 a dry-disc rectifier is that it will conduct current 

 easily in one direction, but very little or no cur- 

 rent in the opposite direction, thus acting like a 

 vacuum tube diode. Dry-disc rectifiers are too 

 bulky and heavy, however, for many modern elec- 

 tronic requirements. Experiments in nonconduct- 

 ing materials led to the development of the tran- 

 sistor, maldng possible the microminiaturization 

 found in modern electronic equipment such as 

 computers and missiles. 



Transistors are constructed of semiconductor 

 materials. The most widely used semiconducting 

 elements are germanium and silicon. In their 

 regular form, these substances ai'e nonconductive. 

 When impurities are added to them, however, they 

 behave as conductors. Adding arsenic to germa- 

 nium, for instance, results in the presence of a 

 large amount of free electrons (negative charge). 

 The material is then a semiconductor of the N-type 

 with a negative current. When indium is added to 



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