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Old 27th Oct 2021, 12:20 pm   #1
Uncle Bulgaria
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Default Bernina 707: four suppression capacitors?

I'm repairing this 707 for a friend. It needs a good clean, oil and the replacement of a cracked cam-stack nylon gear which had jammed the mechanism. From the sticky base I thought it had been excessively oiled in an attempt to free the movement, but it turned out to be this leaking capacitor in the motor housing.

From the diagram on it, there appear to be four capacitors inside (there are none in the pedal). I've drawn it out in the attached photograph, and in writing it's: 2 x 2nF (Live - Earth & Neutral - Earth); 0.06µF Live - Neutral; 0.02µF Live - Motor.

The neutral goes to a fast/slow range switch and some diodes, out of which assembly comes the other wire to the motor.

There's a 2M2 resistor between Live - Neutral.

With the recent success of an X and dual Y three-terminal capacitor in a Jones machine, I've ordered a similar 0.47µF X2 and 2 x 2.2nF Y2, but am wondering about the extra 0.02µF to the motor.

Why so many capacitors, and why the resistor across the mains?
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Old 27th Oct 2021, 2:22 pm   #2
kellymarie
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Default Re: Bernina 707: four suppression capacitors?

I think that is just a decent quality suppressor you say there are rectifiers they need suppression as well as the motor. Also the caps will help reduce the level of transient over voltages getting to the rectifiers. The resistor is just there to discharge all the caps otherwise its possible you'd get a nasty bite from the mains plug
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Old 31st Oct 2021, 9:46 pm   #3
Uncle Bulgaria
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Default Re: Bernina 707: four suppression capacitors?

Thanks kellymarie. I suppose the diode (it turned out there was just one) has something to do with a particular position of the min/max speed switch.

The new three terminal capacitor fitted in fine. I soldered its black legs to the coloured wires I'd snipped off the old capacitor - useful, as I haven't got any 2.8mm Lucar connectors to slide onto the contact otherwise!

I added the fourth with a spare I had which fitted in the motor housing too. It all runs and nothing in the house is complaining of interference. I count that as a win.
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Old 2nd Nov 2021, 1:05 pm   #4
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Default Re: Bernina 707: four suppression capacitors?

Yeah definitely a win it's all good.
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