Wow, just bought 5 pairs of carbon brushes and just after replacing the very worn out set in the tool, the speed controller died on me.
As cera@1967 said in the past, this was to be expected. 
I am right now wondering if the above mentioned Hilda 180W rotary tool features a God's way speed controller. And not just :???: wondering. If any of you has something worthwhile being said, please do so.
