I have a fan in my MicroVAX II which sometimes does not start without a
push, which means that I can't really put the machine back in its external
enclosure. I have been unable to find an illustrated parts breakdown to give
me a part number for the fan, so I was wondering if anyone here knows the
part number, or has a suggestion for how to get the fan to work properly
Is there something special about this fan? Or will any fan of the right
dimensions and operating voltage work? DEC rarely used strange fans (the
11/44 being the obvious exception!), so you might get a generic
replacement that will drop straight in.
As regards repairing the fan, I've done this many times (time I have
plenty of, money to buy replacement fans I don't :-)). I can give you
some pointers..
Firstly, with the machine off (and the fan power wires disconnected if
possible), try rotating the fan by hand. Check there's no 'catch' at any
point of the revolution, which may point to bad bearings.
Most of the time a fan that will only start when flicked is a DC fan.
The ones I am used to have a 2-phase stator, a permanent magnet rotor,
and a simple circuit using a hall effect device (to sense the rotor
position) and a few transistors (to switch the windings). A fan that will
only start when flicked normally means that one widing isn't being
driven, this is either because the power transsitor driving it is dead or
the widing is open (most often the latter.
The bigest problem is getting to it. Oftne you have to peel back or cut
away the label on the fan housing to reveal either secrews or the end of
the spidnle. If the latter, theyre's a circlp to remove, then assorted
washers, and the rotor slides out the other side. If it's bad bearings
you can now get to them (either to replace the ball races, or make new
sleeve bearings as applicable). If it's an electronic fauly, yo may have
to remove the stator lamination stack from the bearing tube to get to the
control PCB -- it's often stuck in place. Gentle heating with a soldering
iron may get it free. Then you can test the windings and transistors. It's
not hard to rewind one of these stators if you have to, BTW.
-tony