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I have a Black and Decker LST 136 grass trimmer. Its a 36 volt model and is apx. 4 years old. While using to trim,i noticed that the power was not as it usually is. them the unit would would start to die and i would have to shake to keep it going. then it stopped and a little white smoke came from the motor , located on the unit assembly on top of the cutting head next to the ground. Replaced the battery with a new one , but no luck getting it going. I wondered if it is possible it could be the PCB board and not the motor or is it for sure the motor ?

Dave
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When the magic smoke escaped from an electrical device, it's dead...

Seriously, smoke means something is dangerously overheating. To avoid doing further damage you should remove power from it and not attempt to use it until the problem has been found and fixed.

keshlam
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    Usually the smoke is insulation burning off one of the coils. If you haven't let it smoke too long and short out, you can sometimes bring the motor back by taking it apart, cleaning and lubricating. – Wayfaring Stranger Jul 11 '16 at 11:56
  • @wayfaringstranger: Right. And if so, (a) that can become a cascading failure, where losing insulation and partly shorting the coils causes more heat and so on, and (b) it is probably not going to be a simple just-reconnect-or-move-something fix. – keshlam Jul 11 '16 at 12:03
  • ... but yes, if you catch it immediately and the problem is just that something seized mechanically, you may be able to free that so the motor is no longer straining and overheating. (Higher-quality tools often have a thermal cutout to keep the motor from overheating enough to damage itself. Cheap consumer-grade tools, alas, may not.) – keshlam Jul 11 '16 at 13:18
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    +1 for "magic smoke" , although keeping that contained is much more important when maintaining pinball machines :-) – Carl Witthoft Jul 11 '16 at 13:24
  • Most of the time, caused by long term clogging of any air cooling passages with grass flakes and dust. Gotta keep the air passageways clean or the heat eventually burns off insulation and electronic controls. A toothpick (soft probe to move matted vegetative materials) and compressed air can prolong its life. – Fiasco Labs Jul 11 '16 at 17:02