Sound to me that you has a (N) Diode, a transistor or a capacitor that is open or shorted out. Check the board near the Negative (N) side of the fan where the N wire are connected and check the Diodes, capacitior or transistors in that ares. To check the Diode just unsolder (Float) one end of the Diode get a DMM set it on Diode. First look at the Doide and you will see a sort of a white stripe on one end of the Diode, that end is the Cathode the oposite end is the anode, now get the DMM with it sat on Diode place the red or + probe on the anode then place the black probe on the cathode and you should get a reading depending on the size diode but the reading should be somwhere at 8.79 good reading, now change the prober on the other sides with the red probe on the cathode and the black probe on the anode, you should get no readind at all, if you get any reading the Diode is shorted and should be replaced. Now look on the Diode, get the letter/s and numbers and try to cross it with a NTE, ECG there should be an electronic supplier in you area. Now you can't stop here, you still have to look for the signs of a capacitor with a slight bulge on top, if you see either one with a bulge unsolder one pin of the capacitor look at the capacitor side for the voltage and uf (farad) set the DMM set it on uf or this sign ( -I(- ) check the uf value, it more then 4uf below the rated value change it out, if it read above 10uf change it out. Now we come to the Transistor, there should be two transistors near this area, first look on the bottom on the board whare these transistors are soldered, look for a black or brown spot near the solder pins, look with a magnyfing glass and check to see if the pins don't have cold solder joints (CSJ's) or loose, you has to look at these solder CSJ's real close, sometime these loose solder joints are hard to see, if you come across any CSJ"s clean and resolder the pin, if there are no un-soldered pins unsolder the two outer pins, but before we start the transistor check you have to now there are several types of transistors but the most common is the NPN or PNP, the PNP with the transistor facing you the left pin is (B) or base, center (C) or collector, right pin is (E) emitter, the NPN transistor facing you the right pin will be (E) emitter, center pin (B) base, right pin (C) collector, to realy know which transistor it is you will have to get the number of the transistor then look it up in an NTE cross reference book, just let us say this is a PNP transistor, place the red probe on the center pin and the black probe on the pin on the right side, there should be no reading at all, good, now place the black probe on the center pin and the red probe on the left pin and again there should be no reading, good, now take the red probe and set it on the left pin and the black probe on the right pin there should be no reading, good, now place the red probe on the right pin and the black probe on the left pin and there should be a deading close to 2.725 if you get a reading on more then two legs the transistor is shorted, replace it. Now this sounds like a lot of work, believe me, it's not, I almost sure that when you open the unit, look at all the components on the inside, check to see that there are no burnt or smell burnt components, turn it over you will probably come across a CSJ and if you do, clean and re-solder it then ckeck the unit, sound like an easy fix. Good luck.