[CC] How to Use An Analog Multimeter for Quick Health Check of Diode & Capacitor; & DC NULL 

The center null function was very helpful back in the old days, where almost all transistor radios were using at their audio output two small transformers (driver and output), where the output transformer was usually working in push-pull mode on its primary winding. The center null function was ideal in measuring the voltage balance between the end terminals of these primary windings, without inputting any signal, i.e. at idling mode of operation. This differential measurement could very easily indicate shorted turns in one of the two subwindings of the primary, causing the needle pointer to deflect towards one side (left or right of null point). The same effect existed if one of the two output transistors was leaky, whereas a shorted transistor was causing full deflection of the needle pointer (the voltage indication being depended upon the working voltage of the stage of course). The same technique was applicable on vacuum tube push-pull end stages of audio amplifiers...This measurement, in other words, is very useful when we want to see if there is a voltage deviation at some point of a circuit from a voltage reference point, especially when such a deviation is not expected to be there. The probes of the instrument are conneted accordingly, one to the reference point and the other to the suspected point. The intrument's needle pointer, under normal condition, should stay at the null center point. This measurement is very similar to the "delta" voltage measurements most high end digital multimeters have available...

La escala DC NULL es muy aplicable en fuentes de audio devido a que hay DC POSITIVO Y DC NEGATIVO FUENTES ASIMETRICAS.