micromotor repair anyone?

[parb]

hi all,

besides the hobby on this site i also do some silversmithing. i purchased a micromotor (similar to what dentists use) and i use this for both polishing and stone setting.
its this machine: link

however i think i broke it yesterday as i was polishing up a piece i just finished. the speed control simply stopped working and the device operates at full speed only no matter if i use the foot pedal or the potentiometer on the front to adjust the speed.

i'm going to make a go at repairing this, but just in case that the device can't be repaired, it seems relatively easy to build a mosfet pwm motor control. this motor is 32volt and max 80w which makes me believe that its not too coomplicated to replace the controller with a generic design?

anypne built a circuit like this before? anything i should watch out for?

the one thing im not sure how to design is a current limiter to stop the current if the motor gets overloaded. any ideas?

thanks!

par

[Bone]

Be careful as some motors especially coreless ones don't like pulsed supplies. I expect the controller is a linear one and the series transistor has gone short. All you probably need to do is to replace it and possibly the driver transistor. The circuit is normally quite simple.

[parb]

the more i've been thinking about this the more i'm sure its just the power transistor that has gone short. not sure if its a mosfet or not, but this seems likely.