I have a wormdrive that is 23 years old (what a workhorse) but lately it has started to rattle a little like maybe a bearing or the brush set is worn out. I can see a bit more arcing at the end of the saw than I have in the past and if it gets the least bit hot the speed really slows down. Any ideas??
Slowing could be either the comutator or field coil brushes or all three.
Rattle may be bearing breaking down.
Doubt you'd get any spares that old so retire it and get another but try first for spares you never know.
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Try fixfault.com to fix circular saws.