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I have an old Pfaff 97 sewing machine that suddenly started to run at full speed regardless of the pedal input.

I disassembled and checked the foot pedal and it is working perfectly.

I took out the motor and found a strange capacitor/filter. Can anyone help me decipher the schematic so I can replace it using individual capacitors?

The black and white wires got shorted and this is the problem.

(from my understanding of German language: "grau" = grey; "gemeins. Pol weiss" = common pole white; "sw" = black; I have no idea what b in a circle with the grounding symbol means as the motor has no grounding)

enter image description here

enter image description here

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    \$\begingroup\$ looks like a dual start/run cap. in metal film that are shorted \$\endgroup\$
    – Tony Stewart EE75
    Dec 2, 2021 at 16:58
  • \$\begingroup\$ Got a dremel??? Open it up and see how it works. \$\endgroup\$
    – Kyle B
    Dec 2, 2021 at 18:14
  • \$\begingroup\$ grau is grey, wiess is white, and sw is black, if that helps. \$\endgroup\$
    – dandavis
    Dec 2, 2021 at 20:14
  • \$\begingroup\$ @KyleB Good point, I will open it at some point. But for now, after writing the question, I realised that if I disconnect the white wire it may work. And it works indeed. I am not sure if this setup will do any damage to the motor on the long run. \$\endgroup\$
    – paul.v
    Dec 2, 2021 at 22:58
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    \$\begingroup\$ I forgot to mention it is an universal motor so I doubt it needs any start caps. \$\endgroup\$
    – paul.v
    Dec 2, 2021 at 23:09

2 Answers 2

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Here is my guess:

enter image description here

I believe that you are correct. It is a universal motor and the capacitors are used to filter the noise of the commutator and the rheostat sliding contact.

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  • \$\begingroup\$ from the label I assume there are 4 capacitors inside: 0.06uF, 2 x 2000pf and 0.02uf but I may be wrong \$\endgroup\$
    – paul.v
    Dec 2, 2021 at 23:03
  • \$\begingroup\$ I think the 2 x 2000 pf caps are connected from gray and black to the capacitor enclosure case which is assumed to be ground and probably would be ground if the metal parts of the machine were grounded. \$\endgroup\$
    – user80875
    Dec 3, 2021 at 1:42
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About:

Can anyone help me decipher the schematic so I can replace it using individual capacitors?

I found here this quad-capacitor showing the diagram more clearly - just in case, I marked the capacitance values if you decide to follow them in commercial values:
2x 2200pF + 1x 22nF + 56nF.
Minimum voltage should be 400V, better if 600~630V.

enter image description here

Other site shows different versions of Foot control pedal and this one discusses exactly function of the capacitor (green highlighter), the problem you described (red highlighter) and their proposed solution (in yellow); but instead of simply removing it.

Instead of completely removing the capacitor, I would prefer to use a single capacitor as a 1nF x 600V ~ 1KV (ceramic or film), or even better if can find: a Safety Class X2 of 1nF x 400V, if available to you.

enter image description here

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