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Hi Eric, there is an article from J.A.Corbyn in Wireless World, March & April 1980, describing a PI with an TX and two noise cancelation RX-coils (900mm diameter!) with much theoretical stuff. The noise of the earth magnetic field is measured and there is also a sofisticated analog ground rejection cirquit, overcoming magnetic viscosity effects. This way he comes to a gain up to 5000 and claims to detect a 50mm piece of brass at 750mm distance! Do you have this article? Besides the unpractical size, do you have further comments? regards Walter
 
Walter,
I hope Eric answers you on this as I've always been tempted to build it. I've included the link for those who haven't seen the article yet
Randy Seden-Simi Valley,CA.
 
Hi Walter and Randy,
Hi Walter,
Yes, I remember the article and still have the original Wireless World with it in. In fact I have all the Wireless World volumes back to 1943 and hundreds of single copies from then back to the mid 1920
 
It ain't me.
Wish I knew what he knows about PI. Started with his article many years ago though. The opposite pulsing was to deal with Magnetic Viscosity effects, as he called them.
"The magnetic properties of soils and rocks are mainly attributable to magnetite and maghaemite. These minerals exhibit a magnetic viscosity effect because their magnetization does not instantaneously follow and applied magnetic field. Magnetic viscosity is qualitatively similar to the effects of a conductor on a metal detector."
Corbyn
In other words it looks like a conductor (coin, gold, goodie) , sounds like something that would need to be dealt with in a good gold detector, but not sure any manufacturer does.
JC
Not Joesph Carr either though I have his books and many of his articles.
 
Hi again,In many ways Corbyns PI was ahead of its time but misunderstood.It actually worked as a gold machine and found a 9oz. nugget in Western Australia.The story is documented with pictures in a chapter in a goldfields book by Mike Wattone? People didnt persevere probably because the machine was large and cumbersome and was mounted on a wooden wheelbarrow thus difficult to navigate in uneven terrain.The large coil size and comparitively long sample pulse delay would have mitigated against finding smaller gold nuggets.It would have required considerable geological knowledge/ luck to actually wheel the machine over a large gold nugget in a limited time frame.I started building a refined version of this machine but shelved the project with the advent of SD technology.Goldfields PIs. should rate a mention in your book Eric? Yours etc. Jim S.
 
Would you mind sharing your updated schematic of Corbyn's PI unit? I wouldn't mind building one that used IC's that could do the job of all those many componets he used back then. It would be kinda neat to have other people step up to the plate and build it at the same time. We could pool our results and make Bruce Candy a bit jealous :) .
Randy Seden
 
Hi JimS,
How is the snow over there? Coming down very early here. Nice to see you again.
Anyone considering Corban's design should read Bruce Candy at 5576624. No, I am not a knocker - believe it or not - I suppose it depends where you are - Aussie is just one iron heap!!!
g.
 
Eric, You mentioned that you developed a ground balence circuit (for the Goldscan) that used the decay period of the ground signal which was sampled and subtracted from the metal signal. Did you have to use a unique pulse width or timing compared to a conventional P.I. search signal, and, could this technique be applied to your newer beach machines? Thanks for this very interesting forum. Can't wait for your book!
Dan G.
 
Hi Dan,
The system I developed in 1982 after a visit to the Australian goldfields was based on sampling the decay curve at two points, integrating and amplifying in separate channels and then subtracting the second signal from the first. The amount of subtraction was adjusted by a pot. In fact, if you look at Candy's first patent for ground balancing, the schematic on the front page does exactly that. Only difference is that I used a subtractive integrator rather than inverting and then summing. Candy's schematic shows a third channel so that you ground balance 1 and 2 then 2 and 3. That way you cover a wider range of object conductivities.
Spoke yesterday to a Goldscan user in Melbourne. He has used it for over 10 years all over Australia and found the ground cancelling very stable and effective.
Eric.
 
Eric, Thanks for your prompt reply to my inquiry. I have been loosing sleep pondering how your ground balance circuit, that you had mentioned earlier, might have functioned. As usual, my sleepy conclusions were way off! I shall get some rest tonight. Thanks again for a great forum! Dan Geyer
 
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