DEC BA11K Fan repair

tony duell ard at
Sat Sep 26 06:36:52 CDT 2015

> I have a BA11K power supply out of a pdp 11/34 that has frozen up fans.
> They are frozen up pretty bad. I heard from another list member here they
> are repairable, any advice on how to do so?

I have repaired many fans over the years, I am not sure if I have done those
ones specifically. 

First remove them, take off any grilles, finger guards,etc so you are left with
the bare fan. Then, assuming an AC fan (I am pretty sure the ones in a BA11K
are 115V AC), measure the resistance between the terminals. If it is totally
open-circuit, then the winding has failed. Rewinding is often possible, but
not trivial, particularly not for something that has to stand mains voltage,
and you may decide it is better to replace the fan.

If the winding seems OK (or if the fan was running, albeit with a lot of 
noise, before you started this), then it's time to take it apart and deal with
the bearings. Theres is little consistency in how fans are assembled, alas..

Very occasionally, the outer housing is in 2 parts, held together by screws on
one side. If so, separate that first.

You now need to find out how the spindle and rotor is fixed : 

Look for a cap in the centre of the blades held down by a circlip. Remove the 
circlip, the cap, oil felts and washers (keep them in order) and you will see a tiny
circlip on the end of the spindle. Take that off, then any washers (which may be 
keyed to the spindle) and slide out the blades/rotor


If there are screws on the hub of the blades (uncommon, but I have seen it), undo
those and remove the blades. This reveals the motor, there is a little circlip on
the end of the spindle as above, remove it and slide out the rotor


There may be screws hidden under the label on the other side of the fan that retain
the motor stator. Take those out and carefully separate the motor from the housing. 
Carefully, so as not to damage the wiring connections. Then again there is a circlip, 
remove this and separate the rotor from the stator.


The worst on is to have a cap hidden under the label. Once you have found this, 
remove it, then the circlip (getting familiar now), washers, and pull out the 
blades/rotor from the other side.

Now for the bearings. These will either be sleeve bearings or ball races. If the latter
(more common in DC fans) you can extract them from the housing and either replace 
(they will be a standard size) or take the covers off, flush out the old grease with a 
suitable solvent (petrol (gasoline) is good, but of course highly fammable), and repack
with high melting point grease. If sleeve bearings, try just cleaning everything and relubricating
with machine oil, If that doesn't help, you can extract them, but will probably have to make
(rather than buy) replacements from phosphor bronze or similar, which needs a good 
workshop. This sort of thing is worth doing on the PDP11/44 with its odd fans, possibly not
on more common ones.


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