lid (Adrian Tuddenham) writes:

wrote:

Anyone know how these old singer foot pedals work? There's not much
to this thing, but I can't for the life of me figure out how it's
supposed to work!

The one I have contains a long ceramic tube filled with a stack or
maybe 100 graphite disks. One end connects to the sewing machine
motor (I assume), and at the other end there's a contact that's
brought closer to and eventually touching the contact on the end of
the tube as the foot pedal is depressed.

The foot pedal was dropped and I'm trying to repair it, but so far I
can't get any variable speed out of it - I get either off (when the
contact isn't touching the end of the graphite-disc-tube) or ON-HIGH
when the contact touches the end of the graphite-disc-tube.

The contact resistance between the carbon discs is varied by the
pressure on the stack (in a similar way to the action of a carbon
microphone). There will be some sort of compression spring between the
operating lever and the top of the stack (it might be a leaf spring).
When the spring is almost fully compressed, a contact by-passes the
resistance stack and gives full power.

It sounds as though the compression system isn't functioning or the
by-pass contacts are closing too quickly. You need to identify an
adjustment screw somewhere in the linkage to the compression spring and
set up the pressure on the stack so that the compression is minimal when
contact is first made with the stack and increases until the motor is
nearly at full power before the by-pass contacts close.

I can supply a photograph of the inside of the controller for a 1950s
Model 201K (for 150 to 250 volt operation) if that would help you.

It may also be possible to simply buy a replacement foot-pedal rheostat
but it would have to be matched up in terms of resistance range.

Of course, if you want authenticity, you'll have to repair it!

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