[PLUG] Shuttle ST62K fan failure

Keith Lofstrom keithl at kl-ic.com
Sun Dec 3 20:38:10 UTC 2006


A year ago, I bought a Shuttle ST62K small format PC for my wife's
office, and it has been running SUSE 10.0 and automatically updating
and doing nightly backups back home to the server.  All well and good.

However, the fan has a problem, and from a google search I've learned
that many people have the same problem.  Either the fan device itself,
or the power controller on the CPU board, have problems, and I suspect
both.  About 6 months ago, the fan stopped running, so that the CPU
heated up to >100C and then shut down the system.  Moving the fan plug
from "FAN1" to "FAN3" cured that.  A week ago, the overheating started
occuring again.  The fan will come on under some circumstances, but
mostly not, and the machine never stays running for long.

The fan will run directly off the 12V supply, but it is very loud
and cools way more than necessary.  For normal operation on the 
ST62K, running the fan between 6 to 8 volts keeps everything properly
cooled, and the fan is much quieter.

The expedient hack suggested by some folks on the web is to connect
the fan from the 12V to the 5V connection on one of the 4 pin disk
power supplies, hard wired, bypassing the speed control and powering
the fan with 7V = 12V-5V.  The problem with this hack is that electric
motors without speed controllers look like dead shorts until they are
turning, so that at system startup the motor could drag the 5V supply
rail up to the 12V supply.  This would also occur if for some reason
the fan jammed or was shorted.  Bye-bye peripherals running on 5V. 

The hack I came up with was a little more work - I powered the fan off
the 12V supply to ground, but dropped the voltage with 8 power diodes,
which lowered the voltage to around 7V.  One advantage of the diodes
is they have a "negative temperature coefficient", which means the 
voltage across them drops a bit when they get hotter, and that means
a little more voltage for the fan when the system heats up.  I have
the diode strings running in front of the airflow to the fan (surrounded
by heat shrink tubing).  A major hack, but it keeps the system cool.
Perhaps too well, the CPU temperature is at 38C right now and it could
stand to be higher, while I imagine the system sounds pretty loud in
my wife's office on a quiet weekend.  I may add some more diodes.

BTW, I could not quickly find a gnome panel applet that works with
SUSE 10 that displays cpu temperature.  That would help my wife keep
tabs on the machine.  What I do instead is run a little script:

 #!/bin/bash
 while true
 do
 date | perl -pe 's/\n/ /'
 acpi -t
 sleep 60
 done

That prints the temperature every minute, into a two line text window.
I run the same script from my home office over the vpn, so I can watch
this kludge for a few days.

Keith

-- 
Keith Lofstrom          keithl at keithl.com         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs



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