Variable-speed Unix PC fan

[~XT6561340~Tom J. Toeller~C29~M19~6092~]

In article <1990Jun25.205650.7200 at craycos.com>, jrbd at craycos.com (James Davies) writes:
> I have a Unix PC (2 MB, 40 MB disk) which I bought new in
> November, 1987.  Just during the past few months I have
> noticed that occasionally the cooling fan sounds like it is
> slowing down or speeding up - i.e. as if it has two speeds, and
> periodically switches between them.  Other than this, the system
> has been rock-solid reliable (hardware, anyway) for as long as I've
> had it.
> 
> My best theory for this behaviour (assuming that this isn't a
> problem) is that the fan is adapting to cooling requirements -
> when the system starts getting hot, the fan speeds up, and
> then slows down again when things have cooled off sufficiently.
> Does anyone out there know if this is the case?  Or do I have a
> hardware problem?
> 				Jim Davies
> 				jrbd at craycos.com


Yes, the power supply is setup to drive the fan(s) in either low, or
high speed. The speed directly related to the power supply temperature.
As the machine heats up, the fan will go into high. If the temp keeps
rising, the power supply will shutdown. If you ever notice your machine
rebooting sporatically, it could be as a result of over heating. A 
common problem with these machines is that the fans, especially the
one behind the power supply, can be slowed or stopped by pressure from
the wiring coming out of the rear of the supply. You must be careful of
this routing when you remove/replace the top of the unit.

In summary, this is a feature. You may want to remove the top and make sure
that there is nothing pressing against the center of the fan. It doesn't
take much pressure to slow down the fan. You should also check for dust
build-up inside the vent slots, as this will impede the airflow.