SMT Bandito Info/Build

[Silver Dollar]

I got the widescan coil today. It is 7" and has a centertapped RX. A funny thing though the
RX coil measures 6.1 mh and 17 ohms but the TX measures 6.1 mh and 27 ohms! It almost
seems they swapped the TX and RX but the centertap is on RX...

It also doesn't perform as well as the Brown 8" concentric. On coins the Deepsearch does 6"
where the Brown Concentric does 7". I thought the DD coils were supposed to go deeper?

The null acts just like my homemade centertapped coil. With just TX connected a scope
across the RX reads 4.7V p/p but connecting the shield to ground gives no signal at all!
By the way there is no connection at all from the shield to RX or TX in the coil. The center
of the RX is tied to TX ground. The wire seems a bit cheezy and is very short. I'd like to
get inside it and rewire it at least and maybe lift the centertap? I started running an exacto
around it but it might be a bit difficult to cut all the way through...

[Koala]

[ATTACH]30554[/ATTACH]

I got the widescan coil today. It is 7" and has a centertapped RX. A funny thing though the
RX coil measures 6.1 mh and 17 ohms but the TX measures 6.1 mh and 27 ohms! It almost
seems they swapped the TX and RX but the centertap is on RX...

It also doesn't perform as well as the Brown 8" concentric. On coins the Deepsearch does 6"
where the Brown Concentric does 7". I thought the DD coils were supposed to go deeper?

The null acts just like my homemade centertapped coil. With just TX connected a scope
across the RX reads 4.7V p/p but connecting the shield to ground gives no signal at all!
By the way there is no connection at all from the shield to RX or TX in the coil. The center
of the RX is tied to TX ground. The wire seems a bit cheezy and is very short. I'd like to
get inside it and rewire it at least and maybe lift the centertap? I started running an exacto
around it but it might be a bit difficult to cut all the way through...

I thought a concentric were deeper. DD have the same depth ish down the center of the coil where as the concentric is only deep at the center and drops off quickly.


Resistance of you new coil do appear odd

[Silver Dollar]

I did a couple of Mods to the Original Bandito today. I will take it to the beach
this weekend and wanted it to be "tuned up" for the job.

I use a cap substitution box to set the RX cap to the best value. The cap is
actually 2 caps stock. A 10 nf is on the regular PC board so I removed that
and substituted some in it's place. The original measured 9.45n and the new
one is 11.1n. That gained me maybe 1/2" in the air, it doesn't make a huge
difference but maybe some marginal targets will be heard!

I also replaced the threshold pot on the back of the unit with fixed resistors.
That sets the amount of volume change with depth and I don't imagine ever
adjusting it.

I found the cap that sets the length of the tune on battery check beep too.
It was a 10uf on the back side of the sub straight and I changed it to 1uf
so now a nice short beep to let you know it's on but not announce it too much!

[Silver Dollar]

Here's one more tweak. The 100n cap at the last LM358 determines the size pulse
that gets through to the speaker. I have reduced it to 22n in the past but here find
that a 47n + 22n (69n) gives the best response. I can discern targets to approximately
10 inches with this mod. Not sure how it will work in the ground but I believe you can
reduce the sens if it proves to be a problem...



(it's the one circled in orange)

It is very difficult to remove parts from this substrate so be careful if you attempt this mod...

[Silver Dollar]

I'm trying to figure out the filters just after the input opamp and it seems to be designed wrong.
The classic bandpass has a highpass set lower than the lowpass (say 1 khz to 30 khz) but the
bandito has the highpass higher than the lowpass (723 hz to 79 hz) somewhere I had a blurb
about how the 4066 switch affects this but cannot find it right now.

Does anyone have any ideas about why they set the filters like this?

[Silver Dollar]

Oh i found that quote from Davor "It has 20k in series with the bilateral switch that is 50% time open (thus behaving as a 40k resistor) and a 220n capacitor, giving a -3dB point at 18Hz"

I had good results on the SMT Bandito changing that cap to 20n = 180hz but not so good on the real Bandito, it seemed to allow more noise through...

[Silver Dollar]

Well I opened up that Tesoro coil today. It is actually called Deep Search and it looks like an
omega possibly. Not sure why centertapped or why TX resistance is so much. Here's some pics;













The graphic paint reads 10k per inch in spots (bottom cover) and 5k per inch in other spots (top cover)...

[Koala]

Thanks for taking the time to photo the inside. I am interested in all Tesoro original coils as there seem to be a lack of data on the internet. Which are center tapped build configuration Et cetera all add to my knowledge.

[Koala]

At last the final parts from Mouser have shown up today. Was getting a bit worried.

Still trying to make decent lightweight coils. Windings are easy having problems getting the stable shield that's immune to knocks and touching the ground. It's no good having 40 cm on the bench but the coil is unusable in the field.

[Silver Dollar]

Well I set mine up to use the tracker coil. It's light and shielded and works great!
My attempts at coil making have not gone well. I tried a TGSL coil with CT RX and
no shields but it is a bit noisy. I used epoxy to fill it and its a bit heavy too!

The other coil I attempted was a minelab style with screen shielding and I couldn't
get it to work stable. I have it hot melt glued but it can move enough to mess up the
the null so I'm still not happy with it. So I went back to purchasing factory coils for now.

My latest is a Fisher 440 11" coil. It has a CT TX of 12.6mh and RX of 12.6mh. I'm working
on a TX oscillator for it now...

[Silver Dollar]

I built up the original oscillator for the 440 today and it works good!
At 10 khz it puts out a 17.8v P/P signal and uses 4.5ma. The only
problem is it uses the -V so the center tapped TX can be grounded
so I have to find 4.5ma's to feed it.

I also found some MAX667 LOD's today, they can be adjusted so you
can run the Bandito on 6V to gain a bit of depth. I know I lost depth
going from 8 to 5v on the TGSL (so I'll use one there too to get 7v).

Actually I might be able to go to 7v on bandito too as I replaced the
2262's with some chopper amps that can use more voltage....

[Koala]

Too many projects on the go an this got shelved.


Had a need a detector with a small coil for some trash area I want to detect. As this has a true no motion all metal mode I have decided to finish it of.


All gone together well turns on. Discrimination,threshold and All metal mode work in auto mode. In manual mode the detector all I get is a high threshold tone and moving near any of the potentiometers makes is alter. Earthed the potentiometer cases made no difference.

Whole thing might need to be in a metal case to calm it down??


Done a quick Google. Can't find the original specs for the 5.75" concentric coil

[Koala]

Well. Not sure whats going on.

If I switch the soldering iron off and rest the tip against the battery negative terminal the detector works fine in all modes. Works even if I pull the mains cord out of the back of the iron.


At least I don't have to start to check for miss placed components as most of the SMD are unmarked



Still looking for the original specs for a Tesoro 5.75" concentric coil.

[Koala]

Well generally impressed with this detector.


Made a 5.75" concentric coil (now being glued)


All metal and discrimination both working well (some of the time)


Getting up to 8" on a Canadian dim or UK silver six pence


However having a few problems with the switches /reliability


Auto / Manual both work in All Metal Mode. In auto the tone decreases if I hold a target still but in manual the tone stays loud as expected. But in auto I can get a nice threshold tone but in manual I never get a threshold tone but lowering the threshold does lower the distance.


In discrimination mode the distance I get alters when I switch it on it could be anywhere from 4" to 8". One time the distance came back just by lowering the threshold down and back up. Other times it requires a few goes with the retune


Would not be good to be out on a field only detecting at 50%. There would be no way of knowing.

[Koala]

I have a problem for sure but can't pin it down.

When in discrimination mode if I set the dice to 0 then the depth will be poor. If I slowly turn the disc up there is a crack from the speaker an the detection depth will be correct and the discrimination also seems to work fine.

Also at the other end discrimination full there is a sharp cut off.

First and last 1/4 is useless but the middle 1/2 is working fine.


Also discrimination affects threshold tone ?? when set to 0 the tone wobbles up and down but if I turn it up then it settles down.


Ground balance is in the middle of its adjustment so hopping the coil I made is okay.



Not 100 % on the switches. Connected as per Carl's schematics but have seen different connections posted.