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Offline giuhalftrackTopic starter
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« on: July 13, 2021, 10:16:20 am »
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Hi it's me again with a treasure hunt for.. a BFO circuit, to make a long story short...

I was reading the treasure hunting manual 7 by karl von mueller, and he talks about a circuit he found in a raytheon booklet of '50 and he built, he dub that "the little monster"

i made 2 videos on my channel about that research and construction, i will test the unit on the field as soon as i can... for now i give you pcb and schematics attached here.

the 2 videos are here:

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http://www.youtube.com/watch?v=sjlXn_jiidM&t=2s


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http://www.youtube.com/watch?v=CJyw6vsN2pQ&t=63s


let me know what you think about that

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Offline nickel_n
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« Reply #1 on: July 13, 2021, 11:26:02 pm »
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good work
let us know how it performs on the ground Smiley

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Offline giuhalftrackTopic starter
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« Reply #2 on: July 14, 2021, 12:14:27 am »
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Quote:Posted by nickel_n
good work
let us know how it performs on the ground Smiley


As soon as i can i will hit a wwii area with trenches... i hope the lager coil and the reduced sensitivity to little objects will be a good starting point to find big iron fragments... the frequency is high in the 1MHz range. I'll keep you informed

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Offline giuhalftrackTopic starter
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« Reply #3 on: July 17, 2021, 12:00:24 pm »
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for those not willing to watch videos i have updated my website too with explanation of the project etc.
I don't see the reason to fill here an enormous post so i link the page.

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http://gfresources.it/eresources/karl_von_mueller_little_monster.html


let me know if someone build this.
thank you

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« Reply #4 on: September 04, 2021, 05:00:34 pm »
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`nice video thank you

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Offline giuhalftrackTopic starter
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« Reply #5 on: September 05, 2021, 04:11:51 am »
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you are welcome, if you want make this unit I store the information plus eagle scheme and pcb here:

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Offline jhjh
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« Reply #6 on: September 05, 2021, 09:09:23 am »
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hiii

i see the ,,,.,frequency response........: 300Hz to 20KHz,,,,,

can make the frequency start ,,under 300hz,,,what coil we need Change,,,and  from where can i get the coil transformer

thank you

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Offline giuhalftrackTopic starter
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« Reply #7 on: September 07, 2021, 11:17:24 am »
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Quote:Posted by jhjh
hiii

i see the ,,,.,frequency response........: 300Hz to 20KHz,,,,,

can make the frequency start ,,under 300hz,,,what coil we need Change,,,and  from where can i get the coil transformer

thank you


the frequency response (you are referring to the data in my website) that you see is the one of the original UTC SSO-3 transformer, i build a transformer (20:1 like the Italian magazine version of this MD) by myself with some empiric data, with 1800 turns for the primary and 90 turns for the secondary (all 0.1mm wire), not sure of the frequency response of my transformer, but it works.

why you want change the frequency response ? this influence only your "ability" to perceive sound but I think it's irrelevant because the difference of the beat frequency (the one you can ear in the headphones) is around 1Khz, reducing this difference will increase sensitivity but increase the risk of sympathetic oscillation (frequency lock) a condition in which the detection is impossible.

in any case i try to answer to your questions:

Q: "...can we make frequency start under 300hz ?...."
A: sure we can design an appropriate transformer that is beyond the scope of this project (at least for me)

Q: what coil we need to change ?
A: the trasformer coils

Q:where can i get the coil transformer ?
A: even if I announce in the video that I propose a substitute for the original part i do not have found a proper substitute, the ones i bought where some audio 1800ohm to 8ohm transformer but they perform poorly. I think will be more convenient redesign a transistor impedance matching.. with the great gain of a silicon transistor will be easy.






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« Last Edit: September 07, 2021, 11:45:50 am by giuhalftrack »
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Offline jhjh
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« Reply #8 on: September 07, 2021, 01:55:48 pm »
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mr giuhalftrack

thank you for explaining,,,

Posted on: September 07, 2021, 01:52:49 pm
i thing low frequencyis good for nugget gold

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« Reply #9 on: September 21, 2021, 11:44:39 am »
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No questions to ask. I was looking at the schematic and it brought back memories of when I was just gettin Into electronics  in 1962. The CK 722 pnp and the Ck721 NPN were the first transistors I worked with. Both were germanium transistors. The leads were very fragile and did not take to being handled too much. Lol. This circuit uses two Colpitts oscillators that are close in frequency and hetrodyned together to creat an itermediate frequency. That is what you hear in the headphones. I built my first one in 1973 but found out that it was not very good for metal detecting because I spent most of my time trying to keep it tuned to the beat frequency. Germanium transistors are very leaky and sensitive to temperature changes. A good substitute is the ubiquitous 2n3906, a silicon transistor.
Thanks for the memories, keep on with your electronics curiosty.

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