Brother industrial sewing machine repair / reverse engineering (repair success)

[max.wwwang]

Is there a connection at the household end between N and PE as indicated with a green dash line in the figure below?

There may or may not be, it depends on the local electrical code. But in most localities the protective earth and neutral conductors are indeed electrically bonded on entrance to the property. They may, or may not, also be connected to a ground rod.

Sorry,  though it should have been clear enough (with "TT" in the figure), I could have included "in the TT system" to make it clearer (have done now). Do you mean, specifically for the TT system, that is still the case? If so, why are other people saying there is no such connection in TT, and there is indeed no such connection in many references that I mentioned above?

[max.wwwang]

Putting an isolation transformer on a scope or other test equipment is can be somewhat acceptable if you understand what side effects it creates and why it's so dangerous, and you take adequate precautions , like using test equipment with no exposed metal and only using 1 probe.


The main issue is that if you isolate the scope and then clip the scope ground lead onto 110/220 phase (or any HV source.)
Then the scope case and all metal terminals are now LIVE !!!

And of course if you put CH1 gnd clip onto HV source then all the other scope probe ground clips are also now LIVE!!

You can see why it's so dangerous and it requires such extreme care, and why its killed many people.

Thanks. Of course whoever using an isolation transformer should know what they are doing. Agree with all you said. That's exactly my counterargument in Reply #24. By using an isolation transformer with the DUT, we must know what we are doing as well --- because by isolating, we have taken away the safety protection provided by PE deliberately.

[max.wwwang]

You can find a description of all the different arrangements here:

https://en.wikipedia.org/wiki/Earthing_system

Thanks. That's been covered now along with a handful of other resources.

[tautech]

Aha! Are you saying in the good old days experienced people used to use this trick?! It sounds like this is not entirely a dumb idea then!

Don't get me wrong, I now know the existence of potential probes and isolated scopes. I am also not desperately trying to put my life in danger. I am just fiercely attacking the problem (and in the meantime grilling you guys) and trying to distil the essence out of it for the sake of knowledge and deeper understanding.

I'm not going to walk away satisfactorily with an advice "you should do this not that" but without knowing/understanding WHY.

This is where we walk the tightrope of informing how it was once done when EE's didn't commonly have the tools to do it more safely or expanding on what was and still is, a mortally dangerous practice. 

None of want it on our conscience after explaining in detail such practices so we universally advise strongly against it. 

Now that's not to say many of us haven't done it but in doing so we realise how dangerous it actually is.
As I now have stocks of differential probes you won't catch me doing floating measurements again. 

[gf]

The main issue is that if you isolate the scope and then clip the scope ground lead onto 110/220 phase (or any HV source.)
Then the scope case and all metal terminals are now LIVE !!!

The same also applies to battery powered scopes, though, except for isolated ones. In fact they are equally unsafe per se as a floating bench scope, but nobody cares.

[alm]

The same also applies to battery powered scopes, though, except for isolated ones. In fact they are equally unsafe per se as a floating bench scope, but nobody cares.

That's why reputable brands like Fluke and Tektronix have fully isolated inputs, isolated BNC jacks and probes with insulated reference leads on their battery powered scopes.

[2N3055]

The same also applies to battery powered scopes, though, except for isolated ones. In fact they are equally unsafe per se as a floating bench scope, but nobody cares.

That's why reputable brands like Fluke and Tektronix have fully isolated inputs, isolated BNC jacks and probes with insulated reference leads on their battery powered scopes.

As do R&S, Keysight, and Siglent isolated scopes..

[IanB]

Sorry,  though it should have been clear enough (with "TT" in the figure), I could have included "in the TT system" to make it clearer (have done now). Do you mean, specifically for the TT system, that is still the case? If so, why are other people saying there is no such connection in TT, and there is indeed no such connection in many references that I mentioned above?

In a TT system, there is no connection between protective earth and neutral at the consumer premises. If there is such a connection it is not a TT system, it is something else such as a TN-C-S system.

When I say there may be such a connection, it means that such places are not using the TT system. For example, there is usually (always?) such a connection in North America, but this also means that North America is not using TT.

[james_s]

This is where we walk the tightrope of informing how it was once done when EE's didn't commonly have the tools to do it more safely or expanding on what was and still is, a mortally dangerous practice. 

None of want it on our conscience after explaining in detail such practices so we universally advise strongly against it. 

Now that's not to say many of us haven't done it but in doing so we realise how dangerous it actually is.
As I now have stocks of differential probes you won't catch me doing floating measurements again. 

It's a lot like driving drunk. Lots of people have done it and lived to tell the tale, technically it's still possible to do, but nobody is going to suggest you do it. It's needlessly dangerous both to yourself and anyone that happens to be nearby.

[tautech]

This is where we walk the tightrope of informing how it was once done when EE's didn't commonly have the tools to do it more safely or expanding on what was and still is, a mortally dangerous practice. 

None of want it on our conscience after explaining in detail such practices so we universally advise strongly against it. 

Now that's not to say many of us haven't done it but in doing so we realise how dangerous it actually is.
As I now have stocks of differential probes you won't catch me doing floating measurements again. 

It's a lot like driving drunk. Lots of people have done it and lived to tell the tale, technically it's still possible to do, but nobody is going to suggest you do it. It's needlessly dangerous both to yourself and anyone that happens to be nearby.

Excellent point James, the risk to others that might attempt to rescue an electrocuted person from a hazardous situation.....wouldn't be the first time innocent parties have paid the ultimate price when helping others. 

[james_s]

Excellent point James, the risk to others that might attempt to rescue an electrocuted person from a hazardous situation.....wouldn't be the first time innocent parties have paid the ultimate price when helping others. 

Or somebody who happens to walk over and twiddle with the scope not realizing it's floating, or move it out of the way to reach something, or brush against it while walking by, you might say you'll always turn off the power when walking away but it only takes one mistake. More than once I've managed to leave a burner on the stove on when I'm done making dinner. Humans make mistakes.

[max.wwwang]

Excellent point James, the risk to others that might attempt to rescue an electrocuted person from a hazardous situation.....wouldn't be the first time innocent parties have paid the ultimate price when helping others. 

Or somebody who happens to walk over and twiddle with the scope not realizing it's floating, or move it out of the way to reach something, or brush against it while walking by, you might say you'll always turn off the power when walking away but it only takes one mistake. More than once I've managed to leave a burner on the stove on when I'm done making dinner. Humans make mistakes.

Makes good sense. I need to do a bit more homework on the subject of isolation transformers. Vaguely understand it but probably not fully.

[max.wwwang]

Sorry,  though it should have been clear enough (with "TT" in the figure), I could have included "in the TT system" to make it clearer (have done now). Do you mean, specifically for the TT system, that is still the case? If so, why are other people saying there is no such connection in TT, and there is indeed no such connection in many references that I mentioned above?

In a TT system, there is no connection between protective earth and neutral at the consumer premises. If there is such a connection it is not a TT system, it is something else such as a TN-C-S system.

When I say there may be such a connection, it means that such places are not using the TT system. For example, there is usually (always?) such a connection in North America, but this also means that North America is not using TT.

Really appreciate your patience here. It's now crystal clear now. So it is --

For TT systems, there is no (never) direct connection in the household switch box between N and PE.

I believe this is correct because this aligns with two (possibly more) references mentioned above.

So if I understand correctly (as always, please correct me straight if I'm wrong), in the five earthing systems described in the IEC standard, which are TN-C, TN-S, TN-C-S, TT, and IT, only TN-C or TN-C-S systems have a direct connection between N and PE in the household switch box.

I believe this is the most accurate statement so far in this discussion with regard to this subject. Edit has been made in Reply #41 for easy reference of people who might be reading and are interested. Any of you who have contributed if don't mind, can please move to that post and make sure I'm not making any mistake, misunderstanding, or misinterpretation, there.

[tggzzz]

Excellent point James, the risk to others that might attempt to rescue an electrocuted person from a hazardous situation.....wouldn't be the first time innocent parties have paid the ultimate price when helping others. 

Or somebody who happens to walk over and twiddle with the scope not realizing it's floating, or move it out of the way to reach something, or brush against it while walking by, you might say you'll always turn off the power when walking away but it only takes one mistake. More than once I've managed to leave a burner on the stove on when I'm done making dinner. Humans make mistakes.

Both of those have happened to me. In the latter cases (plural ) I kick myself. In the former I gave a verbal kicking to the Dunning-Kruger-candidate perpetrator who ignored safety instructions, making sure that everybody knew why/how it was potentially lethal.

No harm has occurred, so far.

[max.wwwang]

Excellent point James, the risk to others that might attempt to rescue an electrocuted person from a hazardous situation.....wouldn't be the first time innocent parties have paid the ultimate price when helping others. 

Or somebody who happens to walk over and twiddle with the scope not realizing it's floating, or move it out of the way to reach something, or brush against it while walking by, you might say you'll always turn off the power when walking away but it only takes one mistake. More than once I've managed to leave a burner on the stove on when I'm done making dinner. Humans make mistakes.

Both of those have happened to me. In the latter cases (plural ) I kick myself. In the former I gave a verbal kicking to the Dunning-Kruger-candidate perpetrator who ignored safety instructions, making sure that everybody knew why/how it was potentially lethal.

No harm has occurred, so far.

Thanks for sharing your experience. That's the reason why I try hard to understand that it is a bad idea, should be avoided, and (more importantly) why --- and sharing this with whoever comes by.

[max.wwwang]

Carefully probing and testing. It's slow and no progress here.

Another strange thing I don't understand --- when probing a checkpoint on the board (with GND lead clipped on the negative of the 37V DC rail, which is believed to be, or may be considered as, the common GND of the circuit), I got two straight lines on the scope: one 0V another 12V DC.

Do you have an explanation for this? Only one channel is being used and on display. Thanks for any help.

[tggzzz]

Thanks for sharing your experience. That's the reason why I try hard to understand that it is a bad idea, should be avoided, and (more importantly) why --- and sharing this with whoever comes by.

You are welcome.

It is always a pleasure to see someone that thinks, asks important/good simple questions, and listens to the responses. That isn't guaranteed

[james_s]

The scope is not triggering properly, either the sweep is the wrong speed or the trigger setting is wrong for the signal that is there.

[max.wwwang]

The scope is not triggering properly, either the sweep is the wrong speed or the trigger setting is wrong for the signal that is there.

Thanks, James.

I traversed all the stops of the speed (time/div) dial (along with the different settings of other related trigger switches/dials), but failed to get a meaningful display of signal. In the meantime, using a fluke DMM, I got 1.17V AC and 3.90V DC readings across the same two spots.

[james_s]

What happens if you turn down the sweep speed very slow so you can watch the dot moving across the screen? It's an analog scope so the vertical position of the dot is directly controlled by the voltage at that instant in time. The only way you can get two parallel lines is if the beam is sweeping across at both voltage levels in succession.

[srb1954]

The scope is not triggering properly, either the sweep is the wrong speed or the trigger setting is wrong for the signal that is there.

Thanks, James.

I traversed all the stops of the speed (time/div) dial (along with the different settings of other related trigger switches/dials), but failed to get a meaningful display of signal. In the meantime, using a fluke DMM, I got 1.17V AC and 3.90V DC readings across the same two spots.

It is probably a pulsed waveform so the DVM is measuring the DC average of the pulsed waveform.

It looks like you have your scope configured for auto trigger, which is not suitable for triggering off slowly varying waveforms.

Slow the sweep speed of your scope down to 10ms/div or slower. Select NORM trigger and INT source. Adjust the LEVEL control and polarity until you get a stable waveform display.

[max.wwwang]

What happens if you turn down the sweep speed very slow so you can watch the dot moving across the screen? It's an analog scope so the vertical position of the dot is directly controlled by the voltage at that instant in time. The only way you can get two parallel lines is if the beam is sweeping across at both voltage levels in succession.

When I turn the dial CCW (towards greater time per div), the two lines first start to flash (when the dial is at 5 ms/div), then look like shooting from LHS to RHS (on the same voltage levels), then become two dots moving at the same direction in an increasingly slower speed. Nothing special is observed. Thanks.

It is probably a pulsed waveform so the DVM is measuring the DC average of the pulsed waveform.

It looks like you have your scope configured for auto trigger, which is not suitable for triggering off slowly varying waveforms.

Slow the sweep speed of your scope down to 10ms/div or slower. Select NORM trigger and INT source. Adjust the LEVEL control and polarity until you get a stable waveform display.

Thanks. Indeed I was using auto trigger (though the source was internal). Tried what you suggested but the same remains.

But anyway, with more patience, more combinations of the "Level" knob and the "SWP VAR" knob, I finally got a meaningful sweep (with time/div set at 100us/div or less, and still with auto trigger mode -- the same for normal). It's a square waveform between 0V and 12V (or so) with a period of 44us or so, which is about 23kHz.

Thanks for the help! This 12V or so DC voltage is the same, or similar, voltage for the Vdd pins of all the logic chips. Does this ring a bell with you?

[max.wwwang]

Some tiny progress. I think the signal detected is ok.

It's a "checkpoint" on the board (cp2, a metal stud on the LHS of the top logic IC in bay #6. Refer to the photo). It goes to pin #5 of a TC4011BP chip - a Quad 2 Input NAND Gate. Pin #5 is one of the two input pins of one of the four(4) NAND gates. According to the datasheet, the voltage range for inputs is from 0 to V_DD. V_DD is about 11V (pin #14). That is about right.

Also, (a little bit) excitingly, this (input) signal --- its frequency --- responds to the pedal movement. The frequency increases when the pedal is pressed forward. This makes sense. This is the first meaningful response to the pedal action I detected from the circuit so far. This means at least the pedal sensing module is working (though not necessarily in full order). The opposite is unlikely but this has to be demonstrated by readings or test runs.

Attached is the component side of the board I'm talking about. There is another board of similar size, and a third small board for pedal movement sensing, all enclosed in one single beautifully made metal control box.

[max.wwwang]

Trying to trace down where the issue is. I start from the output side, that is the connector going to the clutch solenoid. It's supposed to output 37V or so voltage, either stable or switching. Refer to the upper left corner of the further updated photo.

The base of the darlington for this, RHS of the two big ass power transistors, is fed by an upstream transistor (the RHS one of the two, RHS of the letter 'A' on the PCB), whose C&E lie between the 11V rail and GND, with transistors on both sides. Its base (B) goes to one end (upper end) of a resistor ladder (highlighted with a blue rounded rectangle). I suppose the analog signal comes from this ladder which in turn comes from the logic chips, with the ladder working as a DAC. However, the bottom end of the ladder goes to a TC4011BP chip, pins 1&2, which are the two inputs of one NAND gate, not an output pin (as I hoped).

I would appreciate any help here working out this part of the circuit, particularly the ladder and the logic chips around it.

[james_s]

It would be much easier to follow if you draw a schematic of the area in question.

If you can identify which output(s) of the logic chip control the device you're trying to test then it should be straightforward to determine if those outputs are doing what they're supposed to do. Then test the transistors using the diode check feature on your multimeter.