The Crossbow Classic PI

[Qiaozhi]

It turns out that the Detech SEF coil is rather unusual. Initially it was assumed that the coil was a DD type, which was quite reasonable as Detech advertise it as such. However, the cover of the box says:
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The double elliptical coil with Symmetric Electro-magnetic Field
is the best performance coil.
Compared to the Double D coils it has the following advantages:
- better ground balance
- more stable operation
- deeper in the ground
- better pinpointing
- more accurate target identification
-----------------------------------------------------------------------------
So clearly it's not a DD coil after all.

The coil plug wiring is as follows:
L1 263uH (6R6) coil between pins 1 and 2
L2 706uH (7R0) between pins 2 and 3
pins 1 and 5 connected together
pins 3 and 4 connected together

Obviously, with pins 1 and 5 shorted together, the coil plug wiring is connecting the coil as a mono with L1 and L2 in series. Effectively this means the detector "sees" a mono coil with 439uH, requiring a 330R damping resistor between pins 4 and 5.
What's interesting is that a mono coil is sensitive from edge to edge, whereas the SEF coil is sensitive across the overlapping section of the two loops. Hence it reacts like a DD coil, but with the advantages of a mono.

In a way, this setup is similar to a White's Dual Field, but in a Butterfly (DD) format. Although, in this case, it was possible to critically damp the coil with one resistor.

[Mega B]

I own and use one of the 'original' Crossbow machines and is No13 out of the original batch that had been made,still in exactly the same condition as when it was made,wont ever sell it for sure as its such a rare beast.

The coils that i use are the stock 10.5'' Mono and the Garrett SeaHunter Mono coils that George has mentioned about ie 14x10 and 8'' Mono's,the smaller 8'' coil stays on the machine almost all the time.The larger Garrett coil does go deep for sure,i have a spare new Detech 18'' Mono from a GPX that is sitting around gathering dust so i may and try and see if that works on the Crossbow without altering anything,the main reason being is that i also use that 18'' coil on my TDI Pro which does work reasonably well.

The Crossbow is a very rare machine and the original ones very rarely come onto the market,the original batch run was for just 12 machines but No13 which i have was a spare machine that George built for me.Ideal machines for deep relic or hoard hunting but if you you are looking for fine gold jewellery or nugget hunting then not the best machine for the job,i also lucky enough too own a Mirage Pulse for fine gold jewellery and a TDI pro for deep hoard hunting.

[Qiaozhi]

Also, I did try and use the SEF coil as separate TX and RX loops, but it was hopeless. The loops are not balanced (unlike the Garrett Infinium coil) and the sample delay was at 35us. Detection was abysmal, with coins only being detected at 3" to 4". Clearly, it's intended to be connected in mono mode, which incidentally removed the need for an adapter cable.

[6666]

So clearly it's not a DD coil after all.

The coil plug wiring is as follows:
L1 263uH (6R6) coil between pins 1 and 2
L2 706uH (7R0) between pins 2 and 3
pins 1 and 5 connected together
pins 3 and 4 connected together

Obviously, with pins 1 and 5 shorted together, the coil plug wiring is connecting the coil as a mono with L1 and L2 in series. Effectively this means the detector "sees" a mono coil with 439uH, requiring a 330R damping resistor between pins 4 and 5.

Thanks for this surprising info, glad you got it to work , interesting that this butter fly coil configuration gives 439uH with those two coils in series.
Given that it is such a large coil, and in the short amount of testing, did you see any advantage over a real round mono ?

[6666]

TowySalmon would you mind measuring the outside dimension of one of the coils ellipses please
as per attachment you will see the arrows point to the measurement I'm asking about thanks.

[TowySalmon]

W=192mm, or as far as I can see, 7 9/16". The measurements were made with coil cover on so please take the thickness of that into consideration. Thickness of plastic on spare cover is 1mm, so without cover it would be 190mm.

[6666]

W=192mm, or as far as I can see, 7 9/16". The measurements were made with coil cover on so please take the thickness of that into consideration. Thickness of plastic on spare cover is 1mm, so without cover it would be 190mm.

190mm, great, thanks for the info !

[Carlos]

Can someone comment me the times in uS the times in the delay and in the sample are adjusted ?

[Carlos]

http://www.geotech1.com/forums/attac...9&d=1507986237

[Qiaozhi]

Can someone comment me the times in uS the times in the delay and in the sample are adjusted ?

Are you building a Crossbow clone?

[Carlos]

No. I build the sandbanks modified with the lm318 and I adjust the time of the two monostable with fixed values of resistances. as the operation is the same I want to regulate the times of these monostable. Currently the values of the delay I adjust it in 35 uS and the sample in 25 uS. the frequency at 400 Hz. and the pulse time at 120 uS

[Carlos]

here photos

[kt315]

feeling you did not set damping resistor. or something wrong with a coil. do you see R7 on the board?

[Qiaozhi]

No. I build the sandbanks modified with the lm318 and I adjust the time of the two monostable with fixed values of resistances. as the operation is the same I want to regulate the times of these monostable. Currently the values of the delay I adjust it in 35 uS and the sample in 25 uS. the frequency at 400 Hz. and the pulse time at 120 uS

The Crossbow has an internal minimum sample delay setting of 25us (or lower if necessary), and can be adjusted by an external control up to 125us. The sample pulse width can be adjusted between 3.3us and 103us. The TX pulse rate is 1000pps with a pulse width of 120us. The EFE sample pulse delay is 400us.

[Carlos]

R7 in the crossbow schematic ? near the U4 ic?