Need to know how to replace a carbon brush in Bernina motor?

Hi -
I collect & enjoy older SM's. Are there any schematics or photos showing how to replace a carbon motor brush in a Bernina motor - specifically a vintage 840 (1983)? I unscrewed the brush cap and cannot see anything... I am not sure of what the exact dimensions of these brushes are, have not bought a replacement yet. The groups mention that there should be a spring as well.

I need to learn to do this because my other machines can probably use the same help..

thanks