Hi all,

I'd be very grateful for any help. I have a Dual Action Polisher from a company called Maguiars and it's a very common fault that the speed controller fails. The symptoms are the motor speed increased and decreases, and seems to work most consistently at a higher speed. From my research, the speed is controlled by a PWM AC Speed Control circuit board (picture included). Would anyone know the most likely failed component to cause this sytmptom? Would it be the MOSFET?

Thanks in advance!

 

Here is te underside of the PCB.

 

That looks like a simple mains dimmer circuit, better pictures of both sides of the circuit board would be better.

 

Hi Dave, thanks for your reply. Plese find better pictures. The issue is intermittent - sometimes it functions as expected, other times the motor speed is erratic, sometimes running way too fast.

 

If it's erratic, then I suspect the variable rotary resistor, try cleaning it with contact cleaner, or WD40, check the solder joints too.

 

Thanks Dave, really appreciate your help. Will give it a go and report back.

 

I would also suspect the variable resistor being at fault; either due to wiper contamination or a solder joint becoming intermittent.
If the main Mosfet were at fault, it would be shorted and the device would run at full speed always.

 

I would also suspect the variable resistor being at fault; either due to wiper contamination or a solder joint becoming intermittent.
If the main Mosfet were at fault, it would be shorted and the device would run at full speed always.

It's a Triac, BTA12-600

 

Most component faults are not intermittent. So the variable control is certainly the first suspect. Other possible causes would be a failing solder connection or contamination causing electrical leakage. leakage The solder joint near the "R4" label looks a bit suspect, but that might be just a lighting issue.