I have taken the motor apart and cleaned the commutator with very fine sandpaper. It was sparking a lot on the brushes. After reassambling the machine works fine since a couple of months.
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=== Update (07/02/2020) ===
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There is a very nice youtube video about disambling a Kennwod machine https://www.youtube.com/watch?v=AdiNEwVQWCA
@loubylou23
I have taken the motor apart and cleaned the commutator with very fine sandpaper. It was sparking a lot on the brushes. After reassambling the machine works fine since a couple of months.