Hi,i have a xbox360 falcon motherboard,my rear fans are running at about 3 volts?

i need to replace a transistor or 2 maybe?.i can direct run 12v or 5 volts,but i dont really want to do that.my main question is,were exactly is it located(transistors?) were can i can i find BCP51 or ZT751 transstor on my falcon motherboard...i have another old motherboard,its a xenon i think,so im thinking of taking that parts off and replacing the parts onto my falcon board...need guidance thnx.the transistors control my fan speed.