Upon checking found that control of DC motor speed control circuit board faulty. Not sold on the market, for ease of maintenance, based on real draw the circuit diagram, as shown in the figure (diagram does not indicate the parameter elements of SMD type).

220V mains by transformer t buck for Exchange 24V, again by D1~D4 bridge type rectifier, C1 filtering, R1, and VD regulator Hou supply IC power, IC and R2, and RP1, and R4, and C2 constitute common of low frequency oscillation device; IC3 feet output control pulse entered VT1 base very, regulation RP1 on change has output pulse frequency and pulse width, thus by VT1, and VT2, and VT3 regulation DC motor m of speed (no level adjustable speed) ; RP2 is for DC motor 24V operating voltage calibration at maximum speed.
After looking up for high-power transistor VT2, VT3 two damage, welding test for b-e junction breakdown, with model with the new canal replaced circuitry is working. VT2, VT3 substitution models 2SC2750, 2SD1706, 2SD1840.