On Wed, 21 Jun 2000, Gregory McGarry wrote:

> Chris wrote:
> 
> >     While I've never opened a switching reg. before, I imagine that the fans
> > have their own voltage regulator, they may also me variable speed depending
> > on CPU temperature. (My 5000/240 has a t/3/misc/overtemp in the test list)
> > It may just be that a 12V regulator or a triac went bad. Assuming one of the
> > fans isn't shorted out and/or wired in series of course. Barring those
> > symptoms, the fans probably don't need full wave rectified DC, so I bet they
> > tap off the 12v AC line before it gets bridged down and filtered. Perhaps a
> > rectifier/diode bit the dust; not likely, but possible. Then there's the
> > most often not-checked cold solder joints. Just some thoughts.
> 
> It is not possible to get a 12v DC output from a 12v AC input.
> Rectification occurs before the switching.  Other more crucial components
> are likely to be clagged before the fan if the bridge is stuffed.
> Could be dry solder joints, but not after all these years.
> 
> I would recommend cleaning the dust out of the machine and manually
> giving the fans a spin.
> 
> 	-- Gregory McGarry <g.mcgarry@ieee.org>
> 

	I think the fans on the DS5000/1xx are speed controlled so a
likely culprit is the speed control regulator or temperature sensor. Might
also just be the fan connector to the Power supply card.
	Be _VERY_ careful if you poke around in the switching power
supply, If you dont know how to discharge the input filter capacitors, its
best to leave the power unplugged for a least a full day before poking
around. Most switching supplies use a half-bridge input circuit which
means you have large electrolytic capacitors charged with 300V just
waiting to bite...


Peter Wallace