The 3 speed reversible Omnivent. (Probably other models apply)
The #1 common fault.....stuck brushes in the motor.
Indoors, remove the 4 screws holding the fly screen and remove screen.
remove motor wires from switch (theres only two and may be a resistor between them, do not seperate) taking note which goes where.
the connectors are hard to get out of the small hole in the plastic motor mounting frame so i cut them and fitted new connectors on reassembly
Hold the fan blades and remove the spindle nut, its threadlocked and tight, undo anticlockwise.
remove the fan but MAKE A NOTE OF SPACER WASHERS !!! Inc the very thin nylon/fibre one.
In my haste I think i got it wrong as on assembly the fan wouldnt turn.
on the roof remove the 2 motor retaining screws and lift out the motor.
using a thin driver seperate the top cover from the motor body, the driver may need a tap with a small hammer etc to start it moving.
its hard to seperate as the permenant magnet tries to hold the cover on.
in the cover are the brushes....one will be stuck in its housing.
dont attempt to remove either as the wires need desoldering first.
i simply worked the stuck brush in and out in its housing until it moved freely.
i also used some fine wet n dry paper to clean the commutator segments which, because of the stuck brush, has burnt slightly.
reassembly is the reverse ensuring the brushes dont get trapped as the lid goes back on.
i used a new nyloc nut on the spindle instead of the plain nut and threadlock.
check the fan spins freely, if not you need to slacken the nut slightly...but only enough to allow the fan to spin....this may be where i got the spacers wrong.
Start the fan on 1, then increase to 2 then 3.