Generally, there are several ways to find faults:
Voltage measurement.The first thing to be confirmed is whether the voltage of each chip power supply pin is normal, and then check whether various reference voltage is normal, in addition, whether the operating voltage of each point is normal.For example, the BE junction voltage is 0 when the silicon triode is conducting.About 7V, and the CE junction voltage is at 0.3V or less.If the BE junction voltage of a triode is greater than 0.The 7V (except for special audion, such as darlington tube, etc.) may BE open with the BE junction.
Phi method of signal injection.Add the signal source to the input end, and then measure the waveform at each point in turn to see if it is normal to find the fault point.Sometimes we will use a simpler method, such as holding a pair of tweezers to touch the input at all levels to see if the output side is responsive, which is commonly used in amplifying circuits such as audio, video, etc. (however, it should be noted that the circuit of hot baseplate or circuit with high voltage cannot be used, otherwise it may cause electric shock).If there is no reaction at the first level, and there is reaction at the second level, then the problem is at the first level, which should be checked.
There are many other ways to look, listen, smell, touch and so on."To see" is to see if the components have obvious mechanical damage, such as rupture, blackening, deformation, etc."Listening" refers to listening to whether the work sound is normal, such as something that shouldn't be loud, where it should be quiet or the sound is abnormal.Smelling means checking for odors, such as burnt or capacitive electrolyte, that are sensitive to an experienced electronics repairer.A "touch" is a test to see if the device's temperature is normal, such as too hot or too cold.Some power devices generate heat when they work, and if they are cold to the touch, they are not working.But it doesn't work if it's hot where it shouldn't be or if it's too hot.General power triode, voltage control chip, work below 70 degrees is completely no problem.What's the idea of 70 degrees?If you press your hand up, hold on for more than three seconds, which means the temperature is about 70 degrees.