I had this problem and the 0.5 Ohm resistor in series with the fan that gives the low speed was burnt out.
If you disconnect the resistor and check it with a meter you should be able to see if it is still 0.5 Ohm. If not, you can replace with any resistor of the correct value providing it is a large wattage. It gets very hot and will need a nice cooling jacket on it.
Good luck!
Euan