On 06/03/2020 07:54, The Natural Philosopher wrote:
On 05/03/2020 23:10, Roger Mills wrote:
Anyone know how these work?

The foot control of my wife's 1980's Singer sewing machine started
emitting smoke a couple of days ago and, at one stage, the motor ran
without having her foot on the pedal.

Mr Google suggests that it contains a capacitor which, if it develops
a short circuit, can exhibit these symptoms. I opened it up to have a
look and there was was a nasty smell from the vicinity of the
capacitor, but nothing obviously burnt out. I've ordered a new
capacitor, anyway, and will fit it when it arrives. Meanwhile, I've
blown all the dust out of the foot control with an airline, and tested
it - and it seems to be working ok.

But speed control isn't very progressive - it tends to be virtually
all or nothing, like a car accelerator which only has two positions.
[This isn't new - it was like this before it started smoking.]

So I'm trying to understand how the speed control is supposed to work.

https://app.box.com/s/inzt3elo35uq13srz5fqi7qg15ei7z8x shows a photo
of the foot control with the hinged top open. The brown and blue wires
are connected together via a ceramic cylinder which has a piece of
carbon rod protruding from each end. The one at the left hand end is
spring-loaded.

When the lid is closed and pressed down, a post on its underside
presses down on the centre of the long copper? strip so that it makes
contact with another strip below (hidden in the photo) which is
connected to the brown wire - swiching on the motor. As the lid is
pressed down more, the upturned part at the left hand end of the
copper strip moves to the right and presses on the spring-loaded
carbon rod. I assume that it is somehow meant to increase the motor
speed, but I don't understand what's going on inside the ceramic
cylinder.

Can anyone throw any light on this?

May be a primitive carbon variable resistor. More pressure = less
resistance. Or the ceramic cylinder hasÂ* a sliding contact on a carbon
rod to be a variable resistor.


You can get repair kits for these (
https://www.ebay.co.uk/itm/VINTAGE-S...S/262084594501
)Â* - that silvery cap has GONE - but I'd definitely be looking to
replace with something electronic.


Thanks, but that 'kit' consists only of a capacitor - and I have already
ordered a similar one from a different source. Look like I could so with
replacing the carbon pile rheostat - but no-one seems to sell those.
--
Cheers,
Roger