FAULTS 



with this information, transfer the oscillator output to the output stage grid 

 (or one of the grids, if a differential amplifier), still using the 4 /j.¥ capacitor, 

 and check the output stage gain, distortion, signal handling ability, etc. 

 Continue in like manner towards the amplifier input until the defective stage 

 is found. Once again, the work will be easier if the stage gains to be expected 

 are kept in a log book. Remember that if the amplifier gain control works 

 by variable negative feedback, the gain figures at low-gain settings will be 

 rather independent of the condition of the valves. Only the high-gain values 

 are of much significance. 



Having established the defective stage, use the voltmeter to check the 

 valve potentials as before. 



(3) Intermittent faults 



The remedying of intermittent faults is among the most exasperating of 

 the tasks which befall anyone concerned with the maintenance of electronic 

 equipment. With the fault 'on', one begins one's logical chain of experiments 

 and deductions, eventually making some alteration which seems to put 

 matters right, only to find later that the same trouble reappears, and to be 

 forced to the regretful conclusion that one has been on quite the wrong track. 



Intermittent faults are usually caused by components on the verge of 

 breakdown, which alternate between normal and abnormal working. If the 

 apparatus can be left on for long enough the breakdown will probably 

 become final and permanent in the end, when the problem can be attacked 

 in the normal manner. Unfortunately it is not always convenient merely to 

 sit down and wait. 



It may be possible to accelerate the final demise, or at least to cause the 

 fault condition to appear at will, by a little gentle prodding and tapping at 

 likely components. Alternatively running the apparatus on 10 per cent over- 

 voltage (in gear with unstabilized power packs, for example, by using the 

 200 V mains transformer tapping on 220 V mains) may be successful, though 

 it is scarcely a practice to be recommended. 



There is one type of intermittent fault to which direct-coupled amplifiers 

 are particularly prone, which appears on the cathode ray tube face as a 

 random flickering of the base line between two definite levels. The trouble 

 may be caused by a senescent HT battery or faulty power pack, but if this 

 should prove not to be the case it is probably attributable to a defective 

 high-value (e.g. 100 k) wirewound resistor, of which such amplifiers contain 

 a number. The reason seems to be that in course of time the weld between 

 one of the lead-in wires and the resistance element fractures, causing unsatis- 

 factory contact. The equipment needed to find the faulty resistor is quite 

 simple. If the amplifier is single-sided, take a good quality 4 fi¥ paper 

 capacitor and earth one terminal. Connect the other to a yard or two of 

 flex, ending in a crocodile clip. If the amplifier is double-sided, two such 

 capacitors are needed. Any part of the amplifier circuit to which a clip is 

 attached now becomes 'frozen' in potential, so far as rapid changes are 

 concerned. 



The procedure is, with the amplifier input short circuited, to v/ork 

 steadily through from the output end towards the input, 'freezing' the 

 signal path, until the stage containing the faulty resistor is found. Begin at 



668 



