>   I've two AlphaStation 200 4/166 machines here which are missing
> their front fans.  The power supply seems to have some sort of
> "sense" leads (in addition to the DC power leads) running into the
> area where the fan would be...it seems the power supply won't start
> unless it "sees" the fan in some way (tach, I'd imagine).
> 
>   Anybody know where I might find a replacement?  Any help or advice
> here would be greatly appreciated.

The DEC partnumber for the fan 
Fan, 3.15in. Tube Axial DC                 12-24701-35
    
Systems equipped with a 210W powersupply will sense fan failures. There
are also 180W PSUs that don't sense fan status. Apparantly you have a 210W
one, which will not start supply power unless the fan is running.

Wilko