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It started out as the motor just sounding funny. Gradually,the bottom half of the range on the speed knob started disappearing. I'd have to turn the knob up 1/2 way just to get the spindle started,and at top end the speed was about 1/2 what it should be.
I tried adjusting the belt tension,no help. I disengaged the belt,and turned the motor on and it sounded worse. I looked at the brushes,and they seemed fine. Once I put them back in,the motor wouldn't do anything.
It has to be the motor,or something in the speed control.
This particular motor is a 110v DC,250 watt,virtually identical to this,other than the color. Tommy D
These are notorious for problems with the circuit board. I'm no electronic technician but there are supposedly power mosffets that go bad on the circuit board and are replaceable. Little machine shops site has a trouble shooting chart that narrows down all electrical problems and lets you know what is wrong.
https://www.littlemachineshop.com/Reference/DriveTroubleshooting.pdf
There is also a fellow who will rebuild the circuit board.
Dick