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In the spring of 1971 I was working at the Aquarium Dive Shop on Atlantic Avenue in Boston . I should have been in college at the time, but I had dropped out for what would have been the second semester of my junior year. I just needed to do something else, and circumstance and my love of diving landed me in that dive shop, working for Wally Westphal, a true character. This was before the Boston waterfront was cleaned up. Before Faneuil Hall and the other developments that make the waterfront the fantastic attraction that it is today. It was pretty seedy back then, a good education for a kid like me who grew up in the suburbs outside of New York City . One day a telephone call came in from a consulting company on Long Island . They were looking for someone with a metal detector for a topside, not underwater, job. I was in the office with Wally when he took the call and I signaled to him that I had a metal detector, so Wally promptly hired me out for the job through the shop. The consultant would be in town the next day. Now it was true that I had a metal detector, but that is as far as it went. I had obtained, from a friend of my father's, a surplus but brand new condition World War II US Army Anti-Tank Mine Detector. I'm not sure, but it was probably the SCR-625 model. This was a big unit with a detection head mounted on the length of a staff, connected to the power/electronics unit that you wore over your shoulder. The unit operated on tubes (follow the link, those who don't know what tubes are/were). I had never even powered it up, much less operated it. It needed a special battery that was somewhere in the 90 - 100V range, which I did not have and which you certainly could not find in the corner convenience store. So I took off from the shop and spent the afternoon retrieving the case with the metal detector and tracking down a suitable battery. Success. The next morning I fired up the mine detector for the first time about a half hour before the consultant was due to show up. I quickly read the operating manual, tuned up the unit, and proceeded to locate the old metal trolley tracks that I could see disappearing under the asphalt just outside the store. The job was at the construction site for the University of Massachusetts Boston . The campus was being built on top of a landfill site, and the underground decomposition of the garbage in the landfall was causing methane buildup in the new buildings. The consultant's company had the job of measuring the rate of methane production over the campus area. To do this they had sunk plastic tubes in the ground at various places, each with a methane collector inside. If I remember right (highly unlikely) the tubes were 3 - 4" in diameter. The tubes were left projecting a foot or two above ground level and the company had made a rough map showing where all the tubes, about 20 of them, were. The problem for them is that as construction proceeded there was lots of dirt being moved around and lots of trucks and heavy machinery going back and forth, and when they went back to collect their samples they couldn't find any of the tubes - they had all been buried! Fortunately for them they had put metal caps on the tubes and they hoped that now they would be able to find them with the aid of a metal detector. Hence their call to Aquarium Dive Shop and their engaging of the world's leading one-minute expert in metal detector operation. The consultant picked me up at the shop and we drove to the UMass construction site. We walked out to the approximate location of the first tube and I, projecting an entirely false air of confidence and competence, went to work. Much to my (hidden) surprise a few minutes of sweeping the detector over the area produced the tell-tale high-pitched whine in the earphones. I swept the head back and forth to get a better idea of the maximum signal and then stepped back. I was the hired hand and the consultant was out there in a shirt and tie, but since I had the big electronics pack on and was carrying the detector, he did the digging. He went to work with a pick and shovel. After he dug down a bit I went back over the area since the distance to the target should have been reduced and I could get a stronger signal. We found the cap and tube in short order and the consultant was thrilled to retrieve his first sample. We repeated this success from tube to tube, and I think in the end we found all but one or two, which easily could have been destroyed by trucks or other equipment. The consultant was very happy with the job and we went for lunch and a beer (still on the clock) before he dropped me back at the shop. He was happy, I was happy, Wally was happy. Now here's where the magic comes in. Some time after that job I fired up the metal detector again for some reason and it didn't work. The indicator needle just jumped all the way to maximum and stayed there. I never could get the unit to work again. I returned to school in the fall and started studying physics. As part of that curriculum I eventually took a course in electronics, and late in the course we were required to do an independent project. I decided to try to convert the metal detector from tubes to transistors. The detector's manual included a full schematic diagram. I examined this in order to figure out where to disconnect things to remove the tubes from the circuit. In going over the circuit I found that the wiring from the circuit in the electronics box to the coils in the detector head was incorrect - two neighboring wires were swapped. The error in the circuit made sense for the failure mode I was seeing, with the meter pegging all the way over. What I don't understand is why the unit operated on the day that I needed it to operate.
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