Slider pot for Janome controller?

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I have offered to examine/repair, for a friend, a Janome sewing machine
control pedal. It is a fairly simple circuit to decipher - foot controlled
resistive divider circuit feeding a diac/pulse transformer connected to the
gate of a triac. The potentiometer is a slider (rather than rotary) and has
a hinged and spring loaded mechanical bracket fixed to the slider arm and
then screwed at one end of the potentiometer body. The longitudinal travel
of the slider covers only one half the length of a normal slider range from
the centre position to the end away from the anchor. The hinged (top most)
point of the bracket pivots on a pin that also holds a small nylon wheel in
contact with the underside of the pedal.
Here is a crude representation (in plain text)
..........................................................
...............................o.........................
.............................../.\........................
............................./....\m......................
...........................V---------V..............

The problem facing me is a broken track in the potentiometer. It is
nominally a 1Mohm part. The potentiometer is not of normal construction. The
resistive "track" is low impedance from one end to the half way point (this
feature is not utilised in this controller) other than to provide a
"tougher" end zone for the wiper contact.
There are several options for repair, but one or more may not be
achievable....
i) find a replacment part (Janome don't supply component parts, according to
recent advice). The part has a marking TO42Z1Mn where the O has a thick
diagonal slash within it and the n is a narrower/taller script font.
ii) find a 2Mohm slider pot that is mechanically compatible and change the
bracket
iii) use a conductive epoxy (or similar) material to try and repair the
broken track segments. I don't hold much hope for this option since I reckon
it is subject to reasonably severe mechanical stress,
iv) build a new circuit, try and fit a different pot, with perhaps a opto
triac driver and monostable circuit to replace the pot/diac trigger
adjustment.
v) find a different foot controller and modify the connections to suit this
machine
vi) buy the recommended replacmenet controller (~$80)

Q1. Has anyone had any success in track repairs in slider pots? Any tips?
Q2. Are there any other sources of these sorts of parts (besides WES)?
Q3. Any other commenst/suggestions?

Many thanks,

Don




Re: Slider pot for Janome controller?


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Sounds like a normal (i.e. crappy) approximation to a logarithmic pot,
such as is used in audio gear. They have exactly this feature.
It's most unlikely that Janome got custom pots made, so you should
just try a normal log pot.

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