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My sewing machine foot pedal appears to short circuit intermittently, causing the sewing machine to be at full power randomly. Given the limited number of components on the PCB, I suspect the component labeled VT1.

Sewing machine foot pedal PCB

On component VT1 it says "50k".

There are three pins for this component. Top and bottom-right are connected to neutral, bottom-left is connected to the wire marked 'L'.

What part is VT1 and what is its purpose in this foot pedal?

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    \$\begingroup\$ VR1 seems more likely. \$\endgroup\$ – Robert Endl Jun 6 '18 at 19:59
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    \$\begingroup\$ 'live' and 'neutral'? No, not on that board with those parts. Ignore the 'L' and (presumably) 'N' labels on the PCB. \$\endgroup\$ – brhans Jun 6 '18 at 20:27
  • \$\begingroup\$ What brand sewing machine is this? \$\endgroup\$ – Olin Lathrop Jun 6 '18 at 20:53
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    \$\begingroup\$ @brhans - I absolutely would not. It was common in the past for sewing machine pedals to literally just be a wire-wound rheostat in series with the mains AC going to the motor. This may be just that, only with more modern processes. \$\endgroup\$ – Connor Wolf Jun 6 '18 at 22:19
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    \$\begingroup\$ @ConnorWolf - the components we can see on that PCB are clearly not capable of surviving anything near mains voltages, and are also clearly not wire-wound rheostats. Those are low-voltage, low-power parts. \$\endgroup\$ – brhans Jun 7 '18 at 0:26
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Something related to the VR1 slide-pot is the most likely culprit.

It may not be the slide pot directly, although those are notorious for getting "scratchy". However, that wouldn't likely cause sudden full speed operation.

I see that this is a single layer board. Most likely the solder joints of the slide pot pins on the back of the board have partially failed. This is common with single-sided boards and parts that take mechanical stress.

Single-sided boards don't have plated-thru holes, so the solder is only stuck to the board on the back side only in a ring around the pin. The pin sticks thru the blob of solder, which is also what holds the pin in place. These arrangements can develop hairline cracks in the solder such that the pin and a little solder around it becomes a free-moving "plug" inside the rest of the solder blob. It will make contact much of the time, but then sometimes intermittently not. That seems to be exactly what's happening with the pin for the low speed end of the pot travel. Without that pin connected, the voltage out of the pot is that for high speed, largely regardless of the pot setting.

The slide pot is probably mechanically linked to the foot pedal, and therefore gets regular stress. A single-sided board for mounting something like that is really the wrong tradeoff for everything except price. This thing was cheap in more ways than one. Single sided boards are cheap in high volume because they can be stamped instead of routed, for one thing. You may want to note the manufacturer and think carefully before buying anything from them again.

Reflow all the solder joints on the back of the board, and add a little fresh solder while you're at it. That should make it last another few years until you need to do that again.

Added

For completeness, VT1 is a 50 kΩ trimpot used as a rheostat (variable resistor). It looks like its purpose is to set minimum speed for when the slider is all the way at the slow end.

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  • \$\begingroup\$ There also appears to be a pair of unpopulated screw holes in the linear pot that possibly could be used to add further structural support. Of course, making sure not to short out anything in the process. \$\endgroup\$ – AkselA Jun 6 '18 at 21:40
  • \$\begingroup\$ Nice post, but it wouldn't hurt to also identify the trimpot (i.e. answer the question) while you're at it. \$\endgroup\$ – user234461 Jun 7 '18 at 9:46
  • \$\begingroup\$ @user: Others have already explained the trimpot. I answered the larger question of "What is going on, and what do I do about it?". \$\endgroup\$ – Olin Lathrop Jun 7 '18 at 10:33
  • \$\begingroup\$ @OlinLathrop thanks for the extensive response. Your answer indeed addresses the larger question, but should I mark it as my accepted answer if it does not actually say what component VT1 is? I hope this SE site has no problems with me doing so. \$\endgroup\$ – Daan Jun 7 '18 at 18:41
  • \$\begingroup\$ @daan The accepted answer is defined as "the one which helped you the most", and it is totally up to you to choose it. Whether it answer the question as asked, or the question you should have asked shouldn't matter for the other people of the site. \$\endgroup\$ – dim Jun 7 '18 at 20:31
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The 50K trimpot looks fine to me. It is a calibration adjustment, to adjust either the maximum or minimum speed (minimum, I would think).

I would think it's more likely VR1 (especially if something got into it) or something on the other PCB with the actual speed controller.

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It looks like a trimpot, or miniature potentiometer.

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I can't say what its purpose is in the foot pedal without a schematic, but the part is definitely a single-turn 50,000-ohm potentiometer.

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VR1 seems more likely. It's a linear potentiometer and is probably going intermitently open.

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