From the foto, it looks like the speed control board is similar to the American KB units…..
I'd be checking the thyristors to see if one or both are shorted, that would cause the full speed symptom….
why the the little trim is burnt? Hmmm strange but possible….if it's the min or max speed adjustment then they are both in series with the normal speed pot, stepped across a fixed supply rail…..if the voltage is higher(much higher than normal) then the current flowing maybe the cause of the burn up…..this is all supposition of course…..and I could be completely wrong……
A better foto of the board would help…..
How are your electronic repair skills?