Jul 171999
 

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I’ve wanted a metal detector since I was a kid, and I’ve considered getting a surplus mine detector for many years. I finally broke down and ordered an AN/PSS-11 mine detector from Fair Radio Sales. It’s a solid-state unit with a 9″x11″ search head and a handle which adjusts from 29″ to 57″ long. It can be folded and disassembled in order to fit in an aluminum transit case about the size of a small suitcase. It uses a custom battery which is not commonly available, so I wrote this brief article to describe how I built a substitute. Luckily, my mine detector came with a dead battery for me to measure and dissect.

 

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There was some more information about the PSS-11 at the Lone Mountain Electronics web page (link no longer works). They recommended using four CR-123A lithium batteries, without an adapter. I haven’t tried that yet.

Two items not shown in the above pictures are the canvas bag which lets you attach the electronics package to a standard military pistol belt (I got mine after I wrote this article), and a protective cover for the detector head (which I haven’t found yet).

 


Battery Fabrication

The AN/PSS-11 originally used a 10.4V mercury oxide battery called the BA-1389/U (NSN 6135-00-691-3603). This battery was composed of 8 “camera-style” button cells connected in series to form a battery pack about 0.65″ in diameter and about 5.2″ long. Each cell is connected to the adjacent cells by drawn metal cups which are spot-welded to the cell terminals, and the whole assembly is covered in a shrink-wrapped plastic tube. Note that in mercury oxide cells, the case is positive and the “bump” is negative. The original battery has to be over 70°F to operate properly, so a battery adapter is supplied which allows the operator to tuck the battery inside his clothes when the temperature is too low. According to the tech manual, the mine detector is designed to operate from 7.25 to 10.8 volts DC. The BA-1389/U has a rated life of 35 hours of continuous use.

Due to environmental concerns, the Army began developing replacements for mercury-containing batteries such as the BA-1389/U around 1995, and by 1997 the BA-3389/U (NSN 6135-01-442-4580) was deployed as a replacement. The new battery uses the more common, and supposedly more environmentally-friendly alkaline battery chemistry.

For collectors, the bad news is that you can’t buy a BA-1389/U or BA-3389/U battery at the local grocery store. The good news is that it’s not too hard to come up with an acceptable substitute. The battery is a simple cylinder with a contact at each end; unlike a lot of other military batteries, you don’t need to scrounge or build any funky connectors. Based on Mike Murphy’s suggestion, I used a pair of Eveready 523 cells. I ordered mine from Mike; I don’t know how common they are. They are not listed on Eveready’s web page, so I’m wondering if they were discontinued. The packaging says that they replace batteries PX21, RPX21 and V21PX.

The 523 cells are alkaline cells meant for photographic applications, and generate about 4.5 volts. They are about the right diameter, and two of them will provide enough voltage to run the metal detector. They are about 1.96″ long, so a pair of them isn’t as long as the original battery. Thus, it’s necessary to make a 1.3″-long “dummy battery” to fill in the extra space in the battery compartment. It’s pretty easy to make one from common junk you may find in your garage (or at the local hardware or hobby store). It’s not necessary to assemble the cells into a battery pack; you can just insert them in the mine detector’s battery compartment with the dummy battery. Your milage may vary if you assemble a battery pack using a different cell.

 

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To build my dummy battery, I took a piece of wooden dowel with an appropriate diameter (actually, I cut the end off the wooden handle of a disposable foam paint brush), and I cut it so that it’s length would be about 1.3″, minus twice the thickness of some copper strap I had lying around. I flattened one side, then cut a strip of copper to use as a conductor. I attached the copper strip to the flat side with some 5-minute epoxy, and carefully trimmed and bent the ends. Then, I wrapped a couple layers of electrical tape around it, and covered the whole thing with a piece of 1″ heat-shrink tubing. In the pictures below, the bump around the middle of the finished dummy battery is where two strips of electrical tape overlapped. Make sure that your finished dummy battery has a large enough diameter to stay straight in the battery compartment, but is not large enough to get stuck.

 

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It’s important to insulate the dummy battery such that the conductor will make good contact with the batteries at one end, and with the battery case terminals at the other end, without coming into contact with the inside of the battery case. Take a close look at the pictures above, and notice how the insulation extends right to the end of wooden dowel, and how the copper strip is carefully trimmed to stay away from the edges of the dummy battery. The most critical part is where the strip is bent at each end. Make sure there’s enough insulation there to keep the strip from touching the walls of the battery compartment, even if the dummy battery wiggles a bit. It doesn’t matter so much if you insert the batteries first and then insert the dummy battery, but if you put the dummy battery in first and its conductor comes into contact with the battery case, the batteries will be shorted. That can be dangerous. I recommend insulating the dummy battery properly, and then putting the batteries in the battery case first, just to be extra safe.

The copper strap material that I used was left over from grounding my ham shack. You should be able to substitute any light-gauge copper or brass sheet or strip material from a hobby or hardware store. If you have access to an engine lathe, you might want to make a fancier dummy battery out of brass or copper rod (don’t forget to insulate it, though!), or even a slug of nylon, bakelite, etc. with a copper or brass rod through the center.

 

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To use the dummy battery, remove the cap from the battery compartment, insert a pair of Eveready 523 cells negative-end first, then insert the dummy battery. Replace the cap, and you’re ready to go. It’s not necessary to use the battery adapter with alkaline cells, but if you want to try using it anyway, note that you put the batteries in the adapter in the opposite direction: positive end first. I recommend inserting the dummy battery first when using the battery adapter.

 


Using the AN/PSS-11

Here are some scanned excerpts from TM 5-6665-202-13, Operator, Organizational and Direct Support Maintenance Manual, Detecting Set, Mine: Aural Indication […] AN/PSS-11:

 

 


Schematic Diagrams

 

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Here are schematic diagrams for the mine detector. Not that they will do you much good, because the modules are potted with epoxy…

 

 


References

  • http://www.monmouth.army.mil/cecom/lrc/tools/spr95.html (link no longer works)
  • http://www.monmouth.army.mil/cecom/lrc/tools/fall97.html (link no longer works)

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