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[PacketCluster Home Page ] | Last Update: 23 May 1999 |
by Peter Simpson, KA1AXY
The Raw Materials
We started with a freezer that was generously donated by Pete, W1VAB. It was a bit rusty (OK, more than a bit), having been stored in a basement with an earthen floor. One corner of the base was eaten almost entirely away.
Preparing the Box
My first task was to get rid of the freon, since the refrigerant coils were part of the shelves, which needed to be moved. After a morning of calling appliance stores, I found a guy who would remove and recycle the Freon for $40. The process was very anti-climactic. A large plastic bag and 30 minutes later, I was four ounces of Freon (and $40) lighter, with an environmentally clear conscience. I unscrewed the cooling fins from the back of the unit, and removed the compressor and its mountings from underneath. Then, I cut and removed the tubing from the inside of the unit, leaving the shelves intact, but cutting the connecting tubes. I also removed the old thermostat.
I ground down the rust with a small abrasive wheel mounted in an electric drill, then primed and painted with 2 coats of Rustoleum. This spruced up the outside of the cabinet so it wouldn't shock the generous provider of the site for the node.
Ventilation
Next, came drilling the four ventilation holes. This required a heavy-duty drill and a 4-1/4 inch hole saw. If you work slowly, and drill a pilot hole first, it's not too hard. I used light oil and a low speed on the drill to avoid burning up the ($50) hole saw. The freezer is actually two metal boxes, separated by four inches of fiberglass blanket. This means two holes, one in the outer box, and one in the inner box, for each of the four vents.
Since I intended to move air through these vents, I needed to form airtight ducts from the inside to the outside of the freezer. I used expanding insulation foam and four inch PVC pipe wrapped in plastic film. The PVC pipe is wrapped with plastic film to prevent the foam from sticking to it, then inserted through both holes. The foam is injected around the pipe, which acts as a male mold, so it expands into the space between the inner and outer boxes. When the PVC pipe is removed, the foam can easily be trimmed with a keyhole saw, forming an airtight tunnel.
I used 12 volt muffin fans on the top holes. These were salvaged from outdated equipment we were disposing of at work. The fans suck air through the bottom vents and blow it out the rear of the unit. All the vent openings, inside and out, are protected with grilles and wire screen, sandwiched between the grille and the freezer box. This is to prevent fingers, mice, or insects from finding their way into the box or the fan blades. The muffin fans are plugged into a controller, made from a regular household thermostat and a 12 volt relay. They are powered by the same supply that runs the radios and TNCs. The relay activates when the thermostat reaches its set point, and continues to run the fans until the temperature inside the enclosure drops, and the thermostat deactivates.
Power
The power cord goes through a 1/2" piece of copper pipe, glued in place with expanding foam. It connects to a surface mount outlet box which holds a single duplex outlet, and a relay. The relay switches half of the outlet off when it is energized. This will act as a power off reset to the PC and packet TNCs when we install our remote reset logic. The other half of the outlet is powered continuously. A power strip is plugged into the switched outlet, leaving the unswitched outlet available for the control logic.
RF Penetrations
RF enters and leaves the enclosure through three 8 inch feedthroughs. I couldn't locate these, so I made them up by taking two, four inch barrel connectors, and one double-male adapter and threading them all together (using Loctite on the threads). Then, I drilled oversize clearance holes through the freezer boxes, and made two reinforcing plates from aluminum sheet. I drilled mounting holes in both reinforcing plates, then mounted the feedthroughs to the first plate, which I attached to the outside of the freezer. With the feedthroughs now extending into the inside of the freezer, I mounted the second reinforcing plate and completed the mounting of the feedthroughs on the inside.
Holding It All Together
I attached everything to the metal freezer with #6 Phillips self-drilling sheet metal screws, which install easily with a power drill. I needed to reinforce the base of the unit, where it had been damaged by rust. I removed the adjusting feet and used the three remaining threaded mounting holes to bolt a piece of 3/4 inch plywood to the bottom of the freezer. To this, I attached 1x6" pressure-treated decking board. Four heavy-duty casters bolt to the decking board, allowing the enclosure to be repositioned by one person if necessary.
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[PacketCluster Home Page ] | Last Update: 23 May 1999 |