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This is a thread on my repair and reverse engineering project with a Brother industrial sewing machine.
It started off with a question "Why does the polarity of oscilloscope probe matter?" then touches on mains earthing protection (or grounding, depending on where you live) in some good depth, then aspects of the project. And it goes on...
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I'm repairing another vintage industrial sewing machine (the other one is
here). I found something that I can't find an explanation for so would appreciate any help from you.
I'm aware of the risk of blowing a piece of equipment by using an oscilloscope in the wrong way (particularly when using two probes), after having watched the vids from David. So I would not put the Earth clip of the probe to anywhere that is not connected to Earth in a circuit lightly. In this case however, it appears Earth is not connected to any point of the circuit, so it's safe to clamp the clip to anywhere I want as long as I'm using only one probe.
I'm testing the output terminals intended to drive a solenoid. The circuit is a typical design, consisting of darlingtons, flyback diodes and associated power resistors.
Without the load (solenoid) connected, when connecting the probe in one way, I can see a switching between ~40V DC and 0V, as a waveform on the oscilloscope, across the two pins. When I change the polarity of the probe to another way, the wave form becomes a flat line.
What's going on here? Thanks for any insight.