Tonight I was working on a Singer 758 sewing machine. This machine looks to have been used very heavily. I adjusted and oiled it, then noticed the foot control was one of those that is either on or off, no variable speed in between. The machine has two speeds but having variable speed control would still be nice to have, so I ventured into the depths of the foot control.
The ubiquitous Singer clamshell controller...
To open one of these, you need to skewer the rubber backing, then unscrew the screw that is behind the rubber backing. Feel around on the bottom of the controller until you find an indentation. That is where the screw is. I just poked the screwdriver through the rubber to get to the screw.
Unscrew the screw, then open up the clamshell, like so.
Next, vacuum out the decades worth of lint and spider remnants. You can see a carbon pile resistor (long white thing). The way this works is when the long copper strip is pressed in the center by a protrusion on the top cover (shiny spot on the copper strip), the strip bends or bows down. The first thing this does is the top strip contacts another copper strip just under it. The underside strip supplies power to the top strip. Bow the top strip down further and it tilts the left-hand side of the copper strip inward. You'll see a carbon button attached to the copper piece on the left. This button presses against the carbon pile resistor.
The harder the contact point pushes against the stack of carbon, the less resistance that the carbon pile has, which is what provides speed control of the motor. At the extreme end of travel, when the pedal is pressed all the way, the bowed copper strip contacts another copper strip that bypasses the carbon pile resistor and gives full power to the sewing machine motor.
On my controller, there was a gap between the carbon contact and the carbon pile. When the pedal was pressed all the way, the contact point had not even started contacting the resistor, which is why my controller provided no variability. What I did was remove the contact point on the right side of the carbon pile. It is an L-shaped brass piece. I then elongated the screw hole so I could have some adjustment left to right. This next photo shows the elongated screw slot in the brass bracket.
I reinstalled the bracket and slid it to the left as far as it would go, then tightened the screw. This took out all the slack between the left-side carbon contact, the carbon pile, and the brass bracket on the right. One other thing I did was to take a pair of needle-nose pliers and carefully rotate the carbon button 180 degrees. This presented a new surface of the button to press on the carbon resistor (the old location was looking a bit worn and rounded).
Now you just need to close up the controller and screw the cover screw back in. I don't know how there got to be such a gap between the various parts, if it was just heavy usage over the years or if some part is missing. In any case, I now have a functioning motor controller. And it didn't cost me a cent to fix.