Juki TL2010 sewing machine speed control

Thread Starter

VernonLS

Joined Oct 9, 2019
42
I have been trying to replace a Juki TL2010 sewing machine speed control (foot pedal) with a hand operated variable resistor to no avail. I set up some measurements as shown by the diagram thinking that I could just replace the foot pedal with a hand operated variable resistor (rheostat) but have been unable to control the speed that way. The internal parts of the foot pedal seem to be nothing more than a metal tip tracing over a coated surface. There are no detectable diodes or any active element involved, just resistance. While I can get the machine needle to move when I hit the sweet spot on the rheostat, it only goes a stroke or two and stops. This happens at about 4.05 VDC measured voltage and about 0.4ma and about 60 Kohm. In order to get further motion, I have to increase the resistance to above 100K and then slowly reduce it again, but again I get only a stroke or two. I have tried and tried to find an internal circuit diagram for the sewing machine, but the best I can do is a layout of the circuit board which tells next to nothing about the circuit.

Has anyone dealt with one of these sewing machines/foot control before? It seems like it should be so straightforward, but it is not. See the diagrams of the measurements taken with the sewing machine/foot pedal and the attempted rheostat replacement. Does anyone have an insight into what kind of a circuit this foot pedal could be controlling? Rheostat.jpgJUKI TL2010 SEWING MACHINE.jpg
 

AlbertHall

Joined Jun 4, 2014
8,756
Could you repeat the speed and voltage measurements (current is not very useful as it can be inferred from the other two) using a standard linear pot of 50k or 100k and report the results here?
 

Thread Starter

VernonLS

Joined Oct 9, 2019
42
Could you repeat the speed and voltage measurements (current is not very useful as it can be inferred from the other two) using a standard linear pot of 50k or 100k and report the results here?
OK - I set up the circuit illustrated using two pots wired as shown. From the results it seems like I have to start with a total circuit resistance of about 100K or more and then I have to suddenly switch from the large resistance down to about 13K to get the sewing machine to go up/down. Once started up/down I can smoothly control it by varying the resistance, but once motion ceases (resistance gets to high) I then have to return to my starting point of 100K or more. But starting with an open circuit and suddenly switching in the 13K fails to start the machine. So I am thinking that I need a fixed resistor of about 100K shunted by a switch in series with about a 20K pot to make it all work from a hand control. That is, I could use the switch to start/stop the machine and the pot to control it once it is started.

I still have no idea what kind of a circuit inside the machine that I am controlling. My thanks to Delta Prime who posted the link to a description of various kind of speed/foot pedals and to Albert Hall who asked for exactly the circuit that I post here. I would still welcome additional comments/suggestions/better ways to control the Juki. verntwo-pots.jpg
 

AlbertHall

Joined Jun 4, 2014
8,756
You can get pots with a built in switch. The switch is open at the end of travel of the pot and then closes as you rotate the pot. This would do just the function you need. 100k in parallel with the switch and that cmbination in series with the pot.
 

Thread Starter

VernonLS

Joined Oct 9, 2019
42
You can get pots with a built in switch. The switch is open at the end of travel of the pot and then closes as you rotate the pot. This would do just the function you need. 100k in parallel with the switch and that cmbination in series with the pot.
Thanks, I will look for a pot with a built in switch.
 

Thread Starter

VernonLS

Joined Oct 9, 2019
42
What does it do with the original foot pedal?
The original foot pedal behaves the same way. That is, if I shut off the machine, press the foot pedal part way down and then turn the machine on, it will fail to operate. Releasing the foot pedal then depressing it again restores normal operation.
 

AlbertHall

Joined Jun 4, 2014
8,756
The original foot pedal behaves the same way. That is, if I shut off the machine, press the foot pedal part way down and then turn the machine on, it will fail to operate. Releasing the foot pedal then depressing it again restores normal operation.
That might be an intentional safety feature so that it won't start unexpectedly when you switch it on
 
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