DEC BA11K Fan repair

Jon Elson elson at
Sat Sep 26 14:45:08 CDT 2015

On 09/26/2015 11:25 AM, tony duell wrote:
> I'll tell you one that you can't get -- the fan in the PDP11/44.
> That one is a standard outer dimension alright, but electrically
> it's plain odd. 35V 70Hz AC or something. It runs from a full-H
> circuit from the (possibly battery packed) 36V supply. There have
> been suggestions to kludge in 12V DC fans, but I would not do
> that, the fan circuit _is_ part of the design of the PDP11/44 and
> should be preserved.
Oh, wow!  We had a 44 a LONG time ago, I never knew about that!
> There is also the issue of keeping the machine as original as possible.
> And the fact that a lot of the cheaper (i.e. $15) modern fans are very
> poorly made. I think a rebuilt oriignal DEC fan would easily outlast one
> of those.
Well, fans come in many varieties.  There are Chinese 
knockoffs, and name-brand fans.  You can get plain sleeve 
bearings, "hydro-wave" bearings and ball bearings.  Good 
ball-bearing fans have a very long life, but may get a bit 
noisy near the end of their life.  These can also be 
re-bearing-ed quite easily, and it essentially makes them 
brand-new.  I've done this on my Dell desktop machines a few 
times. Yes, maybe the best fans do run over $15.


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