

Cooling fan installation for the Atari STe.

By: Niles Russell

May 1992


Have you ever noticed how much heat is generated by the power supply
in your STe?  I did. So I decided to mount a cooling fan to improve
the cooling efficiency of the power supply.  It is a well know fact
that component life is greatly extended when kept cool.  In fact, I
noticed that when I opened up my STe (this happened many times) and
inspected the power supply, the area on the printed circuit board
that has the transformer soldered to it is starting to discolor,
indicating the build up of heat.

The Ole' Disclaimer.

This whole project is put together as a suggestion, offering tips and
hints to add a cooling fan to your computer.  So, I offer no
guarantees whatsoever, nor will I be held responsible in anyway if
the information provided is incorrect or modified.  In other words, it 
is strictly up to you to do the job correctly and insure its safe
operation, as this is an electrical modification to your computer.
If your computer is still under warranty, this will most likely void
and null any further warranty, as this is a modification.  So check
before you open up your case and start soldering!  


                      *******************
                      *  W A R N I N G  *
                      *******************


Since this project involves working with electricity, disconnect the
computer from the power source.  In other words, don't work on your
computer with it plugged into the wall!  Nothing is worse than
getting shocked, or possibly electrocuted.



Okay, now on to the fun stuff!

What kind of fan should I use?

The fan I selected is sold by Radio Shack, part number 273-244.  It
is a 1 9/16" DC brushless micro fan.  It is rated at 12VDC (volts
direct current) and displaces 3.5cfm (cubic feet per minute).  

I was concerned about how much current the fan would draw, as I
decided I would hook it into the power supply.  Radio Shack states on
the back of the package that it will draw 100mA (milli-amps) max.  I
hooked the fan up to my ohm meter to find at full load it draws right
at 72mA.  I could live with this!  The project was on.




Why wasn't a 120 volt fan used instead?

Well... space was a consideration, as you will find there is not a
lot of it.  Most of the fans offered were too big to fit in the space
where I wanted to install the fan.  And the biggest reason of all,
I didn't want to hack my case up, and this installation will not do
that.

Where to mount the fan.

Viewing the picture, FIG_1.NEO, will probably help you get an idea of
where I located my fan.  These pictures were created with the help of
VIDI-ST, and although they are not as clear as I wanted them, I hope
it gives you an idea of how the project was accomplished.

Now it's time to remove the case.  There are some screws on the
bottom of the case that need to be removed.  Remove only the ones
that are in a square type hole.  Once you complete that, remove
the screws that holds the sheild covering the power supply.
  
The fan will be mounted on the underside or bottom of the shield 
covering the power supply.
The measurements are approximate, so check and double check your
measurements before you start drilling and cutting.  The fan was
strategically located on this sheild because of the heat sinks on the
power supplies printed circuit board, FIG_2.NEO.  Take note, however,
that your power supply may be different than the one I have.  Mine is
made by MITSUMI ELECTRIC, TC1-A75.  If yours is different, then you
may have to adjust the location of your fan.  

You will notice a red box I have drawn in FIG_2.NEO.  This is the
space that the fan will take up, so you will need to position the fan
in such a way that it will not come in contact with heat sinks, power
resistors or other components.  I found that R108 (R is for Resistor)
sits up fairly high, and contacted my fan, making cool grinding
noises.  After carefully bending the resistor down slightly, the
problem was taken care of.  

Once I had located where I wanted to place the fan, I carefully
layed it out and marked it with a pencil.  In fact, I wire tied the
fan to the sheild making sure it cleared everything before I started
cutting.  I then drilled a hole in the sheild large enoungh to
accomidate a nibbler.  A nibbler is a device that will cut metal or
plastic by cutting small pieces away on a marked pattern.  Radio
Shack sells one of these too, catalog number 64-823.  Other methods
of cutting a hole in the sheild might include: 

*  A hole punch, proportionate to the fan size.
*  A small coping saw with a metal cutting blade.
*  A jigsaw with a metal cutting blade.

Once I cut the hole in the shield, I then drilled 1/8" holes for the
screws to mount the fan to the shield.  I used 4-40 machine screws
to mount the fan.  To make sure they don't rattle loose, I put some
non-corrosive silicon rubber on the nuts.  

One note:  I mounted my fan so that when turned on, the air will be
drawn over the components of the power supply and forced up and out
of the computer.  So far, this has worked exceptionally well, as I
can feel the warm air being forced out of the power supply.

Power Supply Hook-Up.

Again, please take caution when working with electricity.

The next step is to remove the four screws holding the power supplies
PC (printed circuit) board in.  On my power supply, the manufacturers
were kind enough to drill two extra holes.  These holes allowed me to
solder the fan wires directly to the PC board eliminating the need to
"tap" into the wires, as this is not really a good idea, and takes a
lot of the professional look away. The blue wire is + or positive
12VDC.   Just to the left of it is a blank hole.  This extra
hole is where I soldered in my positive fan wire.  The extra hole for
the negative wire is located just to the right of C205 (C is for
Capacitor) as shown in FIG_3.NEO.  I found these power points by
disconnecting the plug from the mother board, plugging the power
supply in, and using a volt meter to test voltage output.  I suggest
that you cautiously do the same, to double check that you are
receiving the correct 12 volts at these points.  If you are uneasy
about doing this, I would suggest you have a friend help you who is
comfortable working with electricity.  While you are testing for 12
volts, and once you find it, should you find the need to slightly
increase the voltage, you may do so by adjusting trim pot VR201.
Mine was off slightly, so I adjusted it up about .5 volts.

Once you locate these points, carefully solder the wires from the fan
onto the PC board.  While you have the solder iron hot, you may want
to check the connections on the board for cracked solder joints.  I
found one and resoldered it.

I found that soldering the fan wires to the PC board, then bolting 
it to the shield was the way to go.  The wires on the fan are long 
enough to permit this.  In case you were wondering, I did route the
fan wires through the coil with the other wires.

Installing the Shield.

Almost there.  Carefully install the power supply and secure it with
the four screws.  Now, being careful not to entangle the fan wires
with everything else, position and mount the sheild, attaching the
screws to hold it in place.

When I got to this point, I wanted to make sure that everything
worked like I wanted it to.  So I plugged my machine into the wall,
popped a disk in drive A: and turned it on.  It booted just fine, and
the fan came to life. 

Once I installed the cover, I found the air from the fan was impeded
by the venting slots in the cover.  Still not wanting to hack and
cut on my case, I investigated further possibilities of trimming
some plastic, without changing the outside appearance of my computer.
This will be hard to explain, but I'll try.  Look at the vent slots.
Notice they are cut at an angle.  There is a certain amount of
plastic that can be trimmed away without causing structural or
cosmetic damage.  This best way to illustrate this is by the cross
section I have symbolized below.

                        



                     Top of case                        
      
        
Cross section        |\_|\_|\_|\_|\
                --->   ^  ^  ^  ^                                             
                                                      
                     Bottom of case

The symbol, ^ indicates the area of plastic to be removed to increase
air flow.  I used a sharp Xacto knife to accomplish this task.

Once all that is complete, you are basically finished, except
for mounting the top case to your computer and installing the screws.

How well does it work?

I have been using my STe with the fan mounted for about 3 weeks now.
It is used everyday, and is usually left on for quite awhile.  I
have had no problems at all.  I have 4 megs installed and run
VIDI-ST, VIDI RBG, and assorted MIDI devices.  By running the
cooling fan off of the power supply, it has never failed to give the
best in performance running all my software and hardware.

This project may be of interest to Sysop's that leave their ST's on
24 hours a day.

If you would like to give me feedback, or leave suggestions, my GEnie
mail address is:  N.Russell.  Or I can be found roaming around ST
Base BBS at (602) 285-9246.

Please feel free to distribute this.  Do not erase the warnings or
disclamer, and include all files in this archive.

STE_FAN.TXT
FIG_1.NEO
FIG_2.NEO
FIG_3.NEO

Have fun and be cool!



ST and STe are trademarks of the Atari Corporation
VIDI-ST and VIDI RGB are trademarks of ROMBO LTD.