On 09/11/2014 03:52, Trevor Wilson wrote:
On 25/10/2014 8:31 AM, just fixed it wrote:
On Wednesday, April 22, 2009 3:17:37 PM UTC-5,
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.

Sorry for the miserable description - it's been about a month since
I had the thing apart - just figured I'd post here to see if anyone
had any suggestions.

thanks!

-Pete

Just fixed one of these and found this ancient thread. Thought I
might put this up for people finding this thread. The carbon discs
get burned, so go through the stack and discard any that have broken,
then take a very fine sand paper (600 grit)and lightly sand both
faces of each disc. Make up the space for any discards or
accidentally broken while sanding discs with some small washers. It
is important that the gap be filled with something strudy, not foil
or steel wool. We don't want springinees to the stack. The discs are
poor conductors and need to pushed very hard together to decrease the
resistance, therefore increasing the motor speed. You don't want the
little metal nub at the end of the tube that engages the arm to be
mobile. Do not bend the leaf spring unless you think it has been
changed, it's tension is set to not break the discs. Good luck.


**Yep. My SWMBO thinks I am a hero. I fixed her foot control more than a
decade ago, using your method. It still works fine. More recently, part
of the system which feeds the cloth was dodgy. I stipped it down and
fixed that too. Bugger me, those damned things are complicated.



Is the resistance variation due to surface effects rather than bulk effect.
I dug out a couple of ancient boxes , 1930s?,of some locally made
"Health Ray Infra-Red Carbons" from when you could say on the box that
such rays cured everything except cancer and no namby-pamby H&S concerns
about covering the heated rods to stop toasting forks or whatever.
Anyway (before mm days but) about 8mm diameter and 150 mm long.
The minimum resistance over that length about 0.7 ohms but is highly
critical of how pointy and pressured the DVM probe tips are.