Electronics > Repair
Brother industrial sewing machine repair / reverse engineering (repair success)
max.wwwang:
[Edit]
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...
[/Edit]
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.
Psi:
Without the solenoid connected there may not be anything to switch on/off and so you might be measuring something that is now floating. Are you sure it's a push/pull driver that will function without the solenoid in circuit?
Also keep in mine its not just earth connections you have to be aware of when deciding where it's safe to clip the scope ground.
Neutral is connected to Earth at the house fuse box, so electrically they are the same wire.
And of course phase has voltage with respect to both those as well.
If the device has a transformer and you can see it's totally isolated from mains and the case is not earthed then it's probably fine to clip anywhere with a single scope probe.
max.wwwang:
--- Quote from: Psi on February 08, 2023, 02:48:40 am ---Without the solenoid connected there may not be anything to switch on/off and so you might be measuring something that is now floating. Are you sure it's a push/pull driver that will function without the solenoid in circuit?
--- End quote ---
It's not a push/pull circuit or something like that. It's a digital circuit beautifully made without an MCU chip but with only logic gates (flip-flops). The existence of a waveform, with only the power on (but without any action on the pedal lever), is not expected and is probably a problem on its own. But now what baffles me is the different behaviour of the oscilloscope with different ways of probing the two pins.
Yes, they are probably floating. But, if I understand correctly, notwithstanding the floating pins, the relative voltage difference should be correctly reflected between the two poles of the probe. And clipping the probe to one of the two terminals will set it to the ground potential anyway. Am I right here?
And true, the behaviour is different when the load is connected (again no voltage differences or waveforms). I'm not sure if that is due to the existence of the flyback diode?
max.wwwang:
--- Quote from: Psi on February 08, 2023, 02:48:40 am ---Neutral is connected to Earth at the house fuse box, so electrically they are the same wire.
--- End quote ---
Are you sure about this? If this is the case, I will detect a positive connectivity with a multimeter between ground and neutral of a wall socket. Really?
Edit: just checked this out - indeed neutral and ground of the wall socket are electrically connected. I still have not truly understood how ground protection of the grid works. |O
Any recommendation of good resources about this? I watched videos of TPAI on YouTube, thought I understood, but obviously I have not.
The good side of this is, this makes me understand (really?) why an isolation transformer is useful/important in repair jobs (I made one at one point by the way, though have not really used it).
srb1954:
--- Quote from: max.wwwang on February 08, 2023, 02:32:04 am ---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.
--- End quote ---
The 2SD720 transistor cases, which are also the collector connections, don't appear to be insulated from their respective heat sinks so you should assume that these heat sinks are not grounded and you can't clip your scope earth lead onto them. Remember, for a standard scope, the earth clip is connected through to the mains earth connection. Clipping a scope lead to such a heat sink will likely short out an active signal.
Assuming that your device has a mains power transformer it is best to connect your earth clip to the circuit common, which will be found on pin 7 of a 14-pin logic chip or pin 8 of a 16-pin chip. The heat sink of the regulator (the vertical one on the RHS) is also likely connected to circuit common (check with an ohmmeter) and could be safely clipped onto.
If your device doesn't have a mains transformer then you will need to power it through an isolation transformer before you can safely connect your scope earth lead to the circuit common.
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