43 

 FAULTS 



The number of types of fault which commonly overcome individual compo- 

 nents is not great, but the list of symptoms which these faults can produce in 

 complex electronic equipment is endless. The most that can be undertaken 

 in this chapter is to list the more important component failures, and to 

 indicate the kinds of diagnostic procedure to adopt when complicated units 

 break down. 



COMPONENT FAULTS 



These fall into two categories; failures which occur suddenly, and those 

 which are progressive. The fact that some components are more subject to 

 one type than to the other is of considerable value in the diagnosis of equip- 

 ment faults, since a gradually developing component failure is usually 

 mirrored by steadily deteriorating equipment performance. Common com- 

 ponent faults may be tabulated as shown on page 666. 



EQUIPMENT FAULTS 



We consider now faults within a complete unit of apparatus. We may 

 conveniently divide these into: 



(1) complete failure, in which no part of the apparatus works 



(2) partial failure, in which part of the apparatus works 



(3) intermittent failure, in which all the apparatus works some of the time 



(4) illusory failure, in which it is in fact the operating procedure which is 

 at fault. 



(/) Complete failures 



Complete failures are those in which, on switching on, either nothing 

 happens or else the fuses blow instantly, and do so again when replaced. 

 In-so-far as in most electronic circuits the only thing which is common to the 

 whole equipment is the power supply, attention should first be directed 

 towards the power pack and HT divider circuits. If nothing happens on 

 switching on, see if the valve heaters are lighting up: if they are not, check 

 the mains transformer primary circuit; if they are, the trouble is probably 

 absence of HT voltage — find out if this is so. If experiment confirms this, 

 check the alternating voltage across the transformer secondary winding, 

 then find out if there is an appropriate direct voltage across the rectifier input 

 capacitor. Follow the d.v. through the smoothing choke. Advance by steps 

 in like manner in logical sequence until the cause of the trouble is found, 

 perhaps an open-circuited choke. Unless the apparatus has been built 

 recently by oneself and one's memory is good, it is almost essential to have 

 a circuit diagram at hand ; given this, the procedure is perfectly simple and 

 straightforward. 



The apparatus which blows its fuses is rather less easy because one has no 



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