Troubleshoot an Audio Amplifier with an Oscilloscope

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WisdomAugust

I don't know exactly what you are looking for but a couple things come to mind. Assuming the scope has at least two channels to display. If you feed the test signal to one channel, and the output to the other channel, then adjust the two scope channels for the same size traces. Now invert one channel, and set the scope to ADD. If the two signals are exactly the same, no distortion, then the inverted should exactly cancel the non-inverted, leaving a trace at zero. Any difference between the input and output would result in the two not exactly cancelling, and the result would be a display on screen of the remaining distortion itself.

Another technique would be to put the scope into X-Y mode. Put the test signal on the vertical, and the amp output on horizontal. If the signals are exactly the same, you get a slanted straight line on the screen. Adjust the amplitude or the scope sensitivity to make the line a 45 degree slant. If the output is not identical, any distortion or phase difference will result in the slanting line spreading into various shapes.

We used to use that method to look at stereo signals. If you applied left and right to X and Y axes, and fed it mono, you got a straight slanted line. But when the channel signal differed, you got a more chaotic picture. Also used that method in tape head alignment sometimes.