The Fisher VLF-660 is strictly all metals, fully static ("non-motion") with manual ground balance and audio threshold, double-D searchcoil, a slightly hotted up version of the earlier "441". It does work: we still manufacture industrial valve & box locators that are basically a repackaging of that same circuit, using that same searchcoil.
The original 400 series circuit was designed by a contract engineer Jim Jones, a Brit (or possibly Aussie, my memory is a little hazy on that detail), in the early-mid 1970's. I met the guy once, tall and lanky, a rather physically commanding presence, and very smart.
The 660 redo of the 441 was mostly a Jim Lewellen (Fisher CEO) and Louis Ciuffo (manufacturing manager) project approximately 1981, possibly '82, with some user input from Irwin Lee, at that time a gold prospector and metal detector dealer in Turlock, Calif. Air hots on coins about 8, maybe 9 inches, not too shabby for the time but nothing spectacular either. Some gold was found with it, but by that time higher frequency machines with autotune were already making it technologically obsolete for gold prospecting.
It's still a good unit for industrial use (finding property markers, sprinkler heads, buried manhole covers, etc.). In the hobby/consumer market, it was probably the last of the breed to be manufactured by a major US company. The consumer market went to discriminators and motion circuits and never looked back.
* * * * * COMMENT ON THE VAGARIES OF HISTORY * * * * * *
Somewhere about 1974, Fisher (having been bought by COHU) hoping to recover from being spun off from the marine communications division, hired this contract engineer Jim Jones to design them a new product. Their old TR stuff was regarded as pretty good by TR standards, but other major manufacturers were switching to ground balanced VLF technology. VLF synchronously demodulated induction balance was a huge technological advance. Continuing to improve TR technology wasn't just a dead end, it was a real quick dead end. (Oddly enough, TR still has its place, it just happens to be a little tiny place.)
Turns out that Engineer Mr. Jones was very good. In fairly short order, he developed a completely new metal detector platform that was competitive by the standards of that day. It used a tuned receiver coil together with an active "bucker system" that kept the loop nulled and made most of the phase errors of the tuned receiver coil disappear. I don't believe it was copied from anything, I'm pretty sure this was Jones genius at work. Here's the historical outcomes.
Later 1970's, Fisher hired Jack Gifford, who tacked a discriminator onto the Jones circuit and the result was the 555, one of the better VLF-TR discriminators of that era. I don't want to make it sound like Jack didn't do much, back then this kind of stuff was rather new and to do a major rework of an existing but recent design and to get it right you had to know what the heck you were doing. Not long after that, Jack left, and in (I believe) 1981 he formed his own company Tesoro, becoming one of the industry's legendary great engineers. To my knowledge none of the Tesoro circuit platforms used the Jones "bucker system".
I came to work for Fisher in early 1981. Motion discriminators were beginning to reinvent the hobby/consumer metal detector market, and Fisher was desperately in need of something to answer that challenge. In May 1982 we started shipping the 1260-X. It revolutionized the industry. Its 2nd derivative motion discrimination scheme became the foundation of most motion discriminationators to this very day. The mechanical package designed very well but by "seat of the pants" rather than scientifically, decades later became the foundation of the scientifically ergonomically engineered T2/F75 mechanical configuration, to this day the only metal detector mechanical platform with published ergonomic specifications. The details of the circuitry were completely different from those of Jones except in one respect: the 1260-X used a tuned receiver coil, and achieved phase stability through the use of Jones-type "buckers".
For several years, I developed multifrequency machines using Jones-style "buckers" to both achieve phase stability, and to create nulls at one frequency in order to extract the other as a remainder. I built multifrequency prototypes that did stuff that nothing on the market does even today, and even got a patent on an aspect of it.
For all their advantages, the buckers raised circuit complexity and cost. And they were inherently noisy: getting more than 8 inches in an air test was almost impossible. I had to scrap the Jones system and master the alternatives that our competitors were already getting very good at. The buckerless 1265-X kicked the 1260's keister out of the league, and that was the end of the Jones "bucker system" in consumer metal detectors. The lessons learned from the 1265-X made the later CZ's possible. The 1200 series is gone except for the 1280 Aquanaut which is a specialty nitch item: the CZ platform (originally the CZ-6) is still with us, circuitry almost unchanged 23 years later as the CZ-3d and the CZ-21.
Meanwhile, back in Fisher's industrial market, the story of Mr. Jones played out very differently. The 441 formula (low frequency, ground balanced, almost no features, not a lot of sensitivity, simple to use, need to find iron real good and therefore no discrimination, often need to detect large deep objects therefore fully static (non-motion) operation) dialed the number of our industrial market-- and with some repackaging and revisions to manufacturing methods, it still does.
It's a tribute to Jones' genius that here we are 40 years later, we're talking electronics fer gawwdsakes,where everything is supposed to be obsolete in 18 months! still selling his creation to customers because they tell us it's what they want. ......Yeah, if we started out from scratch now knowing what we know now, we wouldn't do it that way. But what Jones created was so dialed-in to the needs of this particular customer base, that it's almost always made more sense to keep making the same thing than to spend a bunch of money trying to fix production problems that don't really amount to a hill of beans.
And that's my tribute to Jim Jones, an engineer who was for one brief moment a player in the metal detector industry, not even an employee, but whose impact was huge and we're still benefitting from his engineering expertise 40 years later.
