BFOs

[Tom G]

Hi Carl, I own two BFOs that I bought several years ago second hand, although one is in new condition, the result of a life in a closet. Both models are tuned by means of a variable air capacitor, with one model also inploying a variable ferrite core inductor. My experience with both of these units are that they drift off frequency readily, hence constant retuning. I imagine that this tendency was one of the BFO's shortcomings. None the less, I would like to find a way to reduce the frequency drift. I would appreciate any help that you or anyone else can offer. Tom G

[Carl]

Re: BFOs

>Hi Carl, I own two BFOs that I bought several years ago second hand, although one is in new condition, the result of a life in a closet. Both models are tuned by means of a variable air capacitor, with one model also inploying a variable ferrite core inductor. My experience with both of these units are that they drift off frequency readily, hence constant retuning. I imagine that this tendency was one of the BFO's shortcomings. None the less, I would like to find a way to reduce the frequency drift. I would appreciate any help that you or anyone else can offer. Tom G


Drift is a major nuisance with traditional twin-oscillator BFOs. There are ways to design around this, but I don't think that's what you want to do.


It is possible that some of the fixed caps have deteriorated with age, or perhaps lower quality caps were used in the first place. In such cases, BFOs can be extremely sensitive to thermal changes. Just going from sun to shade can make it drift. You might try replacing critical caps with high-stability ones such as polystyrene.


BTW, what BFOs do you have?


- Carl

[Tom G]

Re: BFOs

Carl,


My used detector is a Garrett "Master Hunter" and the unit that is in new condition is a D-Tex Professional. I forgot to mention that I also built a BFO about 8 years ago and forgot about it, after having cannibalized parts from it over the years. After my post to you I rebuilt it. My home built unit employs both a varible capcitor and inductor. I find it to work as well as the commercial units. One interesting aspect is that it employs a search loop of 3/8" dia. copper tubing in a 27" diameter circle. The directions to build this metal detector, plus others, is in the book "Metal Locators" by Charles D. Rakes. I used his plans for the "Large Loop BFO Locator" with some modifications. Although the BFO is considered by many to be obsolete, I find its reaction to metal vs mineral to be interesting. Tom G

[Sam Scafferi]

Re: BFOs

As Carl mentioned drift in a BFO is an inherent problem. I once owned both of the BFO's that you have. The D-Tex was a super machine in trashy areas, for that era of machine.


Natural drift was a problem, but so was changing mineralization levels in the soil. I don't know if you could do it on a BFO, but I had an A.H. Pro and certain Garrett (non-BFO) machines that I modified with a special little tuner that I mounted on the end of the handle. It was operated by your thumb.


At first we only mounted a Fine Tuning pot. which could instantly keep the machine in perfect tune, with not only drift changes, but also mineralization changes. Then, in later prototypes, we mounted a toggle switch in the box, and this would switch between two pre-set discrimination levels. That way we could hunt in the lower discrimination level, and then if we thought we had a zip-tab up close we could switch over and check it at the higher discrimination level.


I realize there are several machines which do that today, but I and my buddy were the only ones in this area to have such a capability back in the early and middle 70's.

[Tom G]

Re: BFOs

Interesting that you should mention the discrimination modification, I modified my White's 4900 in the same manner. I installed two pots and a three position switch. This allows me to set the discrimination to three different settings. The lowest setting cancels iron, the next setting cancels nickels, and the third setting cancels pull tabs. The following is the theory of operation.


Discrimination Setting:


Iron-If signal is present go to nickel.


Nickel-No signal, dig target. Signal present, go to pull tabs.


Pull Tabs-No signal, do not dig target. Signal is present, dig target.

[Carl]

Re: BFOs

>Carl,


>My used detector is a Garrett "Master Hunter" and the unit that is in new condition is a D-Tex Professional. I forgot to mention that I also built a BFO about 8 years ago and forgot about it, after having cannibalized parts from it over the years. After my post to you I rebuilt it. My home built unit employs both a varible capcitor and inductor. I find it to work as well as the commercial units. One interesting aspect is that it employs a search loop of 3/8" dia. copper tubing in a 27" diameter circle. The directions to build this metal detector, plus others, is in the book "Metal Locators" by Charles D. Rakes. I used his plans for the "Large Loop BFO Locator" with some modifications. Although the BFO is considered by many to be obsolete, I find its reaction to metal vs mineral to be interesting. Tom G


I suppose the copper tubing is your Faraday shield and you shove wire through it?


BTW, I am looking for a copy of Rakes' book. I had one ordered from a used book store ($12), the lady shipped it, then got it back a few days later from USPS. It was in a ziploc bag and shredded and waterlogged. Sigh...


- Carl

[Carl]

Re: BFOs

>As Carl mentioned drift in a BFO is an inherent problem. I once owned both of the BFO's that you have. The D-Tex was a super machine in trashy areas, for that era of machine.


>Natural drift was a problem, but so was changing mineralization levels in the soil. I don't know if you could do it on a BFO, but I had an A.H. Pro and certain Garrett (non-BFO) machines that I modified with a special little tuner that I mounted on the end of the handle. It was operated by your thumb.


>At first we only mounted a Fine Tuning pot. which could instantly keep the machine in perfect tune, with not only drift changes, but also mineralization changes. Then, in later prototypes, we mounted a toggle switch in the box, and this would switch between two pre-set discrimination levels. That way we could hunt in the lower discrimination level, and then if we thought we had a zip-tab up close we could switch over and check it at the higher discrimination level.


>I realize there are several machines which do that today, but I and my buddy were the only ones in this area to have such a capability back in the early and middle 70's.


I have an idea for a "ground-balanced" BFO. It utilizes a dual coil search head, one for ground compensation, and thus would also end up being a motion-type detector. I've seen similar designs in literature but nothing quite like what I'm thinking of. Hope to build it up soon, been too busy with getting this web page going, among other things.


- Carl

[Tom G]

Re: BFOs

According to the book I mentioned, the advantage of useing the single turn of 3/8" copper was to reduce the ground effect on the coil. In this case the ground effect is the capacitance that is formed between the loop and the ground, which in turn causes the frequency to shift. Since it requires a high capacitance for the loop oscillator circuit to oscillate at 600KHz, with such an inductor, any additional capcitance due to ground effect is minor. I realize that is not what you were referring to, in regards to ground balancing.


I intend to use my home made large loop BFO to search for concentrations of oxidized iron in a large area of iron mineralization, also know as gossan. I plan on useing the mineral mode of operation to locate these deposits, although I could just as easily use the metal mode and note the decrease in frequency. Your circuit to deal with the iron mineralization sounds interesting, as this condition presents a real problem when searching for metal. I am searching for residual gold indirectly.

[Tom G]

Re: BFOs

The copper tubing is not a faraday sheild but the coil itself, although in another type of detector the tubing is the faraday sheild. A large capacitance was required to obtain the desired frequency of approximately 600KHz. I hope that you can obtain a copy of the book as it offers designs on a number of different detectors. I plan on building Rake's two box detector next, with some variations.

[Bill Pierce]

Re: BFOs

>As Carl mentioned drift in a BFO is an inherent problem. I once owned both of the BFO's that you have. The D-Tex was a super machine in trashy areas, for that era of machine.


>Natural drift was a problem, but so was changing mineralization levels in the soil. I don't know if you could do it on a BFO, but I had an A.H. Pro and certain Garrett (non-BFO) machines that I modified with a special little tuner that I mounted on the end of the handle. It was operated by your thumb.


>At first we only mounted a Fine Tuning pot. which could instantly keep the machine in perfect tune, with not only drift changes, but also mineralization changes. Then, in later prototypes, we mounted a toggle switch in the box, and this would switch between two pre-set discrimination levels. That way we could hunt in the lower discrimination level, and then if we thought we had a zip-tab up close we could switch over and check it at the higher discrimination level.


>I realize there are several machines which do that today, but I and my buddy were the only ones in this area to have such a capability back in the early and middle 70's.


Sam, Igot one of the originah A>H> probes, they came out with this before, they came out with the metal detector. I thought the AH line of products were excellent for discrimination. I did some testing for them and I'll tell you I sure found a lot of silver coins, and that little travel detector they had was excellent, Boy that was a long timr ago! Bill

[Carl]

Re: Rakes' book

>The directions to build this metal detector, plus others, is in the book "Metal Locators" by Charles D. Rakes.


Finally found a copy of this book on the web. A tad expensive at $20 but I'm tired of looking.


- Carl

[Christian]

Re: Rakes' book

>Finally found a copy of this book on the web. A tad expensive at $20 but I'm tired of looking.


>- Carl


Hi Carl,


nevermind spending 20 bucks on a book ... ha ha I'll have to move my *** and get even more webdesigning jobs as I've been spending so much money on books lately ...


JJ is keeping all of them for my right now ... there are many nice ones among them ... also a A+ copy of the 1914 federal dowsing report ..


BTW: You are doing agreat job with this website ... once I've left some freetime I'll try to add some nice graphics etc ...


Until then hang in there ...


All the best,


Christian

[Carl]

Re: USGS report

>JJ is keeping all of them for my right now ... there are many nice ones among them ... also a A+ copy of the 1914 federal dowsing report ..


While in San Diego I got ahold of the USGS report and copied the 25 pages or so of references. I think it must include every paper ever published on dowsing in the entire history of man. I will eventually put it online, but I have too many other things to do right now.


- Carl

[Sam Scafferi]

Re: Test

>Interesting that you should mention the discrimination modification, I modified my White's 4900 in the same manner. I installed two pots and a three position switch. This allows me to set the discrimination to three different settings. The lowest setting cancels iron, the next setting cancels nickels, and the third setting cancels pull tabs. The following is the theory of operation.


>Discrimination Setting:


>Iron-If signal is present go to nickel.


>Nickel-No signal, dig target. Signal present, go to pull tabs.


>Pull Tabs-No signal, do not dig target. Signal is present, dig target.


"Test"