A few months ago my best buddies Wife asked me if I could have a look at her Singer 201K as when being used it would randomly speed up/slow down and just generally be a pain. So I agreed to have a play when time allowed, and took the machine home with me. Fast forward to last night when I finally unboxed and had a look. It’s not in too bad a condition with the rods/gears/linkages appearing reasonably well oiled, the handwheel turns freely, and there’s no weird clunks or noises either.
Given that the speed of the machine is controlled through the footpedal I decided to chuck the pedal (and the lead attached to it) into my bag and bring it to work with me today. I popped the footpedal open on the workbench and proceeded with the inspection/repair.
Here’s what I found. The plunger on the right hand side (by the screwdriver) pushes down on a linkage, which in turn, pulls a rod with a plunger arrangement through a porcelain block, and when fully depressed, shorts the internals of the porcelain on a second set of contacts.
This shot shows the aforementioned shorting contacts, with what I am assuming is a supression capacitor between the line and load connections. As the knob is depressed, the plunger is pulled towards the bottom of this photo, first making contact with a resistive element inside the porcelain block thus giving a low-speed run. When fully down the silver threaded plate meets the external contacts, reducing the resitance in the circuit, allowing for a high-speed run. Such a simple and elegant solution to get two running speeds. I really like it!
This is the plunger I was mentioning previously. The plunger/contact arrangement is held in place via a screw that runs through a spring and out the porcelain insulator on the other side. You can see the black build up on the contact pad at each end, which partly explains the dodgy speed control that was evident. I cleaned these off by gently sanding them with 200 grit wet and dry sandpaper. You don’t need a lot of pressure to ge the stuff off either.
Here’s what’s inside the porcelain insulator. There’s what appears to be a carbon rod on each side which is connected to the top terminals forming a resistive loop when the plunger moves inwards and contacts them. The faces of these were also quite blackened, and you can see that I’ve partly cleaned the one on the left hand side by gently scraping it with a screwdriver. I’ve also hosed the inside of the block out with a non-conductive electronics cleaner, which is why I’m wearing gloves.
The unit reassembled after testing. The resistance when the internal plunger is contacting the carbon rods is several thousand ohms, and it drops to 0.4 homs when the two external contacts are bridged. I’ve also pulled apart the machine plug ont he lead and cleaned the contacts as they were quite filthy. No photo’s of that part I’m afraid. The only thing that remains to do is replace the wall plug and then I can test run the machine. I will be sure to update with what happens, even if I blow something up.
It works very nicely now. It turns out that the footpedal is a proper variable speed control, not the two-speed that I had assumed previously. I suspect that the compression applied to the carbon rods alters there resistance, which in turn alters the speed of the machine. Regardless, the speed control is easy now, and has a very smooth realtionship to how hard you press on the plunger.