Increasing sensitivity of the GD-48

[Gervm]

If you use this old good detector it is easy to increase sensitivity on 10 or even 20%. For this you need only very good nooling search coil and change value of one resistor. Look to the circuit. Preamlifier on Q2 and Q3 is made by standart scheme with AC feedback on C8, R6. So amplification = R6/R7. Now calculate. Amplif = R6/R7=12000om/1200om=10. If you increase R6 you will get more amplif. For examle take R6=18kom. Now amplification = 18000/1200=15. E.t.c. But note that you have to reduce residual signal in RX by moving L2 and tuning C3, may be changing value of the C10. I didn`t have original GD-48 but I`ve made several....

[Geo]

If you use this old good detector it is easy to increase sensitivity on 10 or even 20%. For this you need only very good nooling search coil and change value of one resistor. Look to the circuit. Preamlifier on Q2 and Q3 is made by standart scheme with AC feedback on C8, R6. So amplification = R6/R7. Now calculate. Amplif = R6/R7=12000om/1200om=10. If you increase R6 you will get more amplif. For examle take R6=18kom. Now amplification = 18000/1200=15. E.t.c. But note that you have to reduce residual signal in RX by moving L2 and tuning C3, may be changing value of the C10. I didn`t have original GD-48 but I`ve made several....

Gervm, Thanks .
Any idea for GD348??

Regards

[Gervm]

Principle the same for any IB detector. I remember the preamlifier of the GD348 contains filter at the entry (RC filter) and repeater on the second transistor. I need analis how to do it better. Better if we`ll do it without retuning of the working point of the first transistor. I need free time... But we can easily take one useful thing from GD348 for GD48. It is separation power suply of generator from power suply receiver part. One resistor will do good job.

[Gervm]

So will be better

[Gervm]

If somebody can, please send me fotos of original insides GD-48 (coil, cable and box). I want to examine with screening.

[Gervm]

You can also simplify circuit of the GD-48 .... or make compensator more complex. Look to the USpatent, that I rewrite for you (now it is better for reading). What does it look like?.....

[Gervm]

Mechanical construction of the search head also can be modified.

[Geo]

Fantastic work
But why?? this schematic??, why you did not make a modern amd more sensitive detcector??

Regards

[Gervm]

Interesting question... I have made more than 20 diferrent detectors. These were PI IB OR BFO and even one proton magnetometer. Some of them were enough modern and contained a lot of parts. But I`ve found that for me the best principle is IB. I say more. In field simple IB devices work good ... not worse than expensive and modern. My friends have modern brends and I can compare them in work.
So I prefer enough simple devices as Whites Classic 1 or more simple non momtion detectors. And I find things at the same place after my friends whith expensive md. In this case more important quality of head coils, quality of parts and quality of assembly. For example good maden old static device (author Flind) can detect side of my car at distance 3,5 meters! Try to do it with ACE. Simple detectors have less weight and they are very economical - one battery works very long .

[Melbeta]

HeathKit GD-348 detector schematic....

Here is the HeathKit GD-348 schematic, with some comments on it as to its weaknesses, take a look at it, and see if you can recommend any improvements or enhancements.
Melbeta

[Gervm]

Red marks on the circuit concern to the stage of threshold tuning. This part of circuit must be stable for stable threshold. For increasing sensitivity we can try rise amplification of preceding stages but we`ll have to make very good nooling of search head for decreasing residual signal. Let`s look to the circuit. After RX coil we see RF amplifire (Q103, Q104) with filter (C109, C110, R109), diod detector with filter (C112, C113, R116) and low frequency amplifire (Q201, Q202). Better to increase amplification for both stages but the first contains the filter and need breadboarding. Insted of low freqency amlif I would use not inversing OPAMP with low consumption. In this case very easy realise good amplif controlled by one resistor. If you want I can draw... At free time I`ll analise first stage of amplif. Insted of R210 and zener diod better to use integrated stabilizer.
P.S. for good nooling manufacturer used even little pieces of foil glued to the RX coil.

[Geo]

Here is the HeathKit GD-348 schematic, with some comments on it as to its weaknesses, take a look at it, and see if you can recommend any improvements or enhancements.
Melbeta

You can replace the zener with a 7805 and the circuit becomes very stable

[Gervm]

May try 78L06 - in original zener diod provides 5,6-5,7V, 7805 and 78L05 - 5V, 78L06 - 6V. But stabilisers of 78LXX series have less quiescent consumtion, less voltage drop, less dimensions. May be something more modern.

[Melbeta]

Be very interested to see it...

Red marks on the circuit concern to the stage of threshold tuning. This part of circuit must be stable for stable threshold. For increasing sensitivity we can try rise amplification of preceding stages but we`ll have to make very good nooling of search head for decreasing residual signal. Let`s look to the circuit. After RX coil we see RF amplifire (Q103, Q104) with filter (C109, C110, R109), diod detector with filter (C112, C113, R116) and low frequency amplifire (Q201, Q202). Better to increase amplification for both stages but the first contains the filter and need breadboarding. Insted of low freqency amlif I would use not inversing OPAMP with low consumption. In this case very easy realise good amplif controlled by one resistor. If you want I can draw... At free time I`ll analise first stage of amplif. Insted of R210 and zener diod better to use integrated stabilizer.
P.S. for good nooling manufacturer used even little pieces of foil glued to the RX coil.

When you get some time, would like to see what you come up with. Geo expressed an interest, so I went looking for it, I have looked in the past, but finally found the schematic. Seems it was only posted in one place. But I am interested as well. Thanks...
Melbeta

[Geo]

May try 78L06 - in original zener diod provides 5,6-5,7V, 7805 and 78L05 - 5V, 78L06 - 6V. But stabilisers of 78LXX series have less quiescent consumtion, less voltage drop, less dimensions. May be something more modern.

There is not reason for something more modern.
I tried it before 2 years with 7805 and it was OK.