Singer Serger Repair

My wife's Singer Sewing Serger stopped starting.  Weird way to put it, I know, but when you have three s's in a row, how can you stop?

A serger is like a sewing machine, except that instead of sewing it, well, serges, I guess.  Whatever it does, it stopped doing it just as my wife was finishing off a Christmas gift she was making.  And mailing, after it was finished.

The trouble, it turns out, wasn't the machine itself, but the foot pedal that turns it on and adjusts the speed.  And the serger hadn't completely stopped working, it was just coming on and shutting off randomly after she depressed the pedal.

Taking Apart the Pedal
The pedal is easy enough to take apart.  The top slides over the bottom.  On on end there are two bars that hook together and keep the pieces from springing apart.  On the other end, it's held together by two protrusions on the bottom that snap into holes on the top.  I just pried next to the protrusions to pop them out of the holes.

Yeah, it did seem like it could break, but it didn't.

Inside there's a printed circuit board with a lever switch, a potentiometer and a few other parts.  It's a pretty simple mechanism.

How it Works
As you push down the pedal, a wedge (see last picture) on the top part pushes down on a roller attached to the slider of the potentiometer, which moves the slider up.

The pressure from your foot, via the wedge, is acting against resistance from a spring pushing the opposite direction. As the potentiometer moves, the resistance goes down causing the machine to go faster.

I suspected the potentiometer because it was all gunked up.  It's heavily greased to keep it moving smoothly and, since the pedal sits on the floor, the grease had picked up a lot of carpet fibers and other stuff.

The two contacts for the potentiometer are at the top end. An ohm meter showed it was OK; it's an open circuit until the slider hits about the halfway point, then the resistance starts high (I didn't write down how high) and drops down to a short as the slider reaches the far end.

The spring arm pushes on the
slider and holds down the lever switch.

The pedal doesn't rely on the potentiometer to turn the machine on and off.  Right next to the potentiometer there's a lever switch with an arch at the end of the lever.

The end of the spring that pushes against the potentiometer slider crosses over the switch lever.  When the pedal is in the "off" position, the spring wire pushes down on the lever (at the bump) and keeps the switch off.

As you push on the pedal, the wire from the spring moves off the bump, the lever pops up and the switch turns the serger on.

It doesn't immediately start serging because the potentiometer is still too far down to let any current through.

The Problem
Turns out that switch was broken.  The same ohm meter proved that.
It wasn't the contacts, they were soldered on securely.  The switch was just bad.

The desoldering tool sucks
away the molten solder.

I melted the solder with an iron, then sucked it off with a handy desoldering tool.

You cock it by pushing down a plunger through a cylinder.  When you press a button to release the plunger, it springs up, creating a suction in the cylinder and sucking the molten solder in through a nozzle.

The Fix
I found a reasonable replacement for the switch at Radio Shack.  It's a SPDT (single pole, double throw) Submini Lever Switch.  "Reasonable replacement" was the best I could do on a Sunday.

The replacement switch
has a shorter lever.

The replacement switch had two salient differences from the original: the contacts were perpendicular to the switch and the lever was both shorter and lacked the bump.

With a pair of longnose pliers I twisted the contacts so they would fit through the circuit board.  They bent easily enough and didn't break.  There was just enough contact after the twist to fit through the board.

Old lever glued to new lever.

It soldered in just fine.

I couldn't figure out a decent way to get the new lever to work, so I cut the old lever off the broken switch and glued it on top of the new lever.  It worked fine.

The Putting-It-Back-Together Glitch
Yeah, there's one of those, but it's not bad.

The ramp on the top of the pedal that moves the slider is like a hill; it goes up, peaks, then goes down.  To work right, the roller on the slider has to be on the back side of the ramp.  If you just lay the top of the pedal over the bottom, the roller lines up on the wrong side of the ramp.

All you have to do is push the slider forward with a screwdriver while you align the top of the pedal over the bottom.  Once they're kind of together, you might peek inside with a flashlight to make sure.  Then you just pry the protrusions back into the holes and pray that it all holds together long enough for you to take credit for fixing it.