fan mail

[julian macassey]

    Here  in  Los Angeles we are  currently going through  a heat 
wave.  It is in the hundreds during  the day and plummets  to the 
low  seventies  at night.  My  3b1 lives  with me  in  an ancient 
apartment with no air conditioning so we both swelter together. I 
do  not have enough power  available (25 Amps 110V total) to slap 
in a small air conditioner.

    The other evening I cam home and sat at the console, suddenly 
the  cooling  fan revs suddenly  dropped. I thought  the  fan was 
about  to stop and my heart damn nearly  did. That is how I found 
out that the Unix-pc has a 2 speed fan. Regular noisy for regular 
cooling and super noisy for super cooling. 

    This  was  a revelation.  I  mailed Jim Gottlieb  (jim at denwa) 
about this and he told me that yes  indeed, there was a two speed 
fan and it was controlled by a thermistor, possibly on the  Power 
Supply board.

    Recently  on  this  group there  has  been  discussion  about 
ripping out the installed DC fan and inserting an AC fan from Rat 
Shack. Obviously unless modifications were made, any change to an 
AC fan would loose the 2 speed feature. It could be done, I could 
do  it  with a bit of fiddling  -  schematics would help  though. 
Obviously  for a margin of safety, as a single speed fan,  the AC 
fan at single speed would have to shift as much air as the DC fan 
at   high  speed.  Unless   your  computer  lives   in  a  luxury 
airconditioned room. Beware of airconditioned offices, they often 
kill the air over those hot August weekends to save energy.

    So  it would appear that  some care should be  taken over the 
cooling  fan thing. This brings  me to a concern that  I have had 
since  I  first  turned on  my  3b1.  I think it  runs  too warm. 
Whenever I put my hand behind the machine I feel warm air  coming 
out  of the fan vent, no  matter what speed the fan is running at 
or  whatever the room temperature. Yes my un-airconditioned  dump 
also has no heat and it  gets to the mid forties in the winter. I 
believe as far as electronic equipment is concerned,  exhaust air 
should  not  be warmer to the  touch  than ambient and  heatsinks 
should never get uncomfortably warm. The reason for my heresy  is 
that  heat kills electronic  components - sooner  or later. So if 
you hate fixing stuff, or just having it die, keep it cool.

    I  am  seriously  considering  throwing  another fan  (AC) in 
series with my installed DC fan, just to increase the airflow. If 
I can get an AC fan that really pushes wind, I will remove the DC 
fan  and replace it with the super AC fan.  This will also remove 
some  of the load from the  Power Supply which will  further help 
with cooling - assuming the fan is powered by the Power Supply.

    As far as I know, there  is no current source for replacement 
power supplies. This makes keeping the thing cool and alive for a 
long  time all the more important. Also when power  supplies die, 
they  have  a nasty habit of  doing  it in a vindictive  way  and 
taking expensive boards with them.

    Any thoughts and suggestions  re cooling and power greatfully 
received. If you have messed with the fan, I certainly would like 
to hear of successes and failures, tips and traps.

Yours


-- 
Julian Macassey, n6are  julian at bongo    ucla-an!denwa!bongo!julian
n6are at wb6ymh (Packet Radio) n6are.ampr.org [44.16.0.81] voice (213) 653-4495