I have just checked a friends grinderette after he had fitted new
brushes and found the sparking was much worse than ever....

I found the fine copper tail had been wound up to the point where it
was compressing the spring and the brushes were just barely making
contact with the commutator. When fitting them make sure the fine
copper tail is not wound up and that the spring applies adequate
pressure on the brush.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk