SMT Bandito Info/Build

[Silver Dollar]

The falsing seems to be caused by a 100hz oscillation on the signals coming out of my OP07's
(replacement for the LM308's). Funny though when it's there I get more depth (silver to 12")
when it goes away the sound is solid but only 8" on silver. I saw it on both OP07's but only one
output of the LM358 (using a TL072 as my LM358 got damaged).

[Silver Dollar]

I put a low frequency signal through the RX input and followed it all the way through
everything looks good! I'm thinking the Disc and GEB are not in the right place, not
sure why? I tried another opamp for the LF353 and new caps. Maybe the 393 is bad?

It works now and better than before. A good signal is heard when a target is in normal
range but there is lots of falsing and chatter...

[Silver Dollar]

I lowered the TX level fed to the comparitors and tweaked the frequency to 14.6khz
but now the Disc and GEB seem to be off by 90 degrees or 180 or something...

[Silver Dollar]

Oops one last mod. I found adding the diode to the bottom of the 4024 diodes gives about 1/2" more air test.
So here's the schematic for all mods;



Playing with the TGSL is weird. The Disc didn't work yesterday it was just widening and narrowing so I removed
the LF353 and put a socket. I used a TL072 and same failure. Well today I put the LM353 and it's sort of working.
It gets a null in the center at one end of the Disc and 3 pulses at the other but it slides back and forth like it should.
I took some pictures and just noticed my TX is 13.7 khz. I'll have to tweak that and maybe lower the TX level somehow
to kill those extra pulses....

[Silver Dollar]

I didn't have 620k resistors so had to use 681k. The 22n caps should change to match
the frequency we want out of the bandpass filters. So Tried 1Meg, 470K, 604K and 681K
and found the 604K gave the best response.

I tried a few values for the cap but 22n was best.

Also the 100n in the Threshold amp has been found to work better as 22n. With these mods I
get 9" on gold and 12" on Silver in air test. Here's a modified Schematic;

[Silver Dollar]

So I tried a couple input mods to try and increase depth.

The first replaces one of the 5.1K input resistors with a cap for DC coupling and the 220K to
ground with 5.1K for input impedance matching. This failed to gain and actually lost depth!
No responce to silver and gold at 6" with this setup.

The second worked quite well. I removed the 220k to ground and shorted the 5.1K input
resistor thus allowing all the signal from the coil to get into the opamp. This netted
8 1/2" on gold and 9 1/2" on silver. As always this might not mean anything in the ground
but could gain a little...

[Silver Dollar]

Did a little testing with the Bandito II and various resistors for R1 in the oscillator;

1K-----10.1Khz TX-----10.7V P/P-----Gold 5 1/2"-----Silver 6"

1.3K-----10.2Khz TX-----10V P/P-----Gold 6"-----Silver 7 1/2"

2K-----10.3Khz TX-----8.8V P/P-----Gold 6 1/2"-----Silver 7 1/2"

2.49K-----10.3Khz TX-----8V P/P-----Gold 7 1/2"-----Silver 8 1/2"

These are all air tests not sure in the ground.....

[Silver Dollar]

Well I'm confused now, he says "Loop field strength is not related directly to the loop voltage" then says
"The sensitivity was increased by increasing the loop voltage about 2.5 times"?

Also if you have an oscillator and 2 coils, one with less turns and one with more, the oscillator will put
less current into the one with more turns as the resistance is greater. I guess that's what I was trying
to say. Same current more turns is better but same circuit more turns = less current so less power in coil.

It is more complicated as a special oscillator like an H Bridge will build up current in the coil and trap it there
so will draw say 4 ma but have 150 ma in the coil where a regular oscillator might draw 2 ma and have 30 ma
in the coil.

The big question is how much is enough current? It seems a depth equal to your coil diameter is about right
(although some companies can make a coil that goes 1 - 2" deeper than that).

The stock 5 pin Tesoro goes 6" with 30 ma and it takes 64x to go twice the depth so we should shoot for
around 2 amps into the coil to match the better MD's on the market at 12"? The H Bridge does around 150 ma
so it's a start.

I found with the stock oscillator on the New Bandito I used 1.2K for R1 and got 11V P/P but some distortion, I
tried various values and found 2.49K gives no distortion but only 8.5V P/P. Will the extra 2.5V gain you anything?
I'm not sure. Is the distortion bad? I don't know. We need some further testing to see.

(1.3K = 10.4V P/P
2.0K = 9.2V P/P)

(Original Bandito reads 1.8K incircuit and has 7.8V P/P)


I do believe the Minelab style coil will go deeper but will you give up better disc in trashy areas? Need to test more.
I also see some variation day to day even with the factory unit so it may be hard to determine what is better....

The newer 4 pin Tesoro's are very similar to the Minelab coil specs so I believe there is some merit to using those...

[Koala]

I read it at times not divide but I could be wrong http://compass-metal-detector-forum.548136.n2.nabble.com/Coil-field-strength-by-George-Payne-td6185238.html

[Silver Dollar]

Well I thought the measure of a TX coil is ampere turns, so 1 amp with 1 turn is 1 amp turn, but 1 amp with 10 turns is 1/10 amp turn.
So for TX more turns equals less power. That being said I have read that a MD with variable TX power will sometimes get better depth
with less power. The problem is magnetism of course. A low power will drain off and possibly allow you to see behind it where a high power
will saturate the field and not allow further object id.

July 23? How about next week? They have to mine the ore to make it? Reminds me of the time Amazon backordered something and finally
sent it 6 months later! I didn't need it by the time I finally got it...

[Koala]

That back ordered cap is going to cost you as you have to pay shipping again. Hope everything gets in
fast!

Hope not email they sent me just says it will ship as soon as it's in stock E.T.A. JUL 23 if they do I will have to cancel and ebay the last part.

Been reading up on coils. Seems like a lower power TX but with more turns equals the same as using more power with less turns. So the Bandido has got the same ground penetration for less battery when compared to my IDX.

[Silver Dollar]

I couldn't get the TGSL going. There seems to be a problem with the Disc. It gets wider
and narrower as you adjust the pot. I checked the Resistors and changed the Cap but
still acting weird. Maybe my LF353 is bad?

I checked the Original Bandito with big Silver today and got 8"! By ground balancing the
new one I got 8 - 9"! So they are about the same.

[Silver Dollar]

Thanks for the tip on the low batteries. I checked the TX and negative supply but didn't check each
chip. I had the GEB and Disc parts off the board and the 4024 was reconfigured. It basically works
but something isn't right. I work on it some more tonight.

It completely rejects a screwdriver, drywall screw and bolt with nut and washer. I'm not sure what I
have that is iron?

I got a 10 turn pot in yesterday which I think I ordered after you so those China guys are shipping faster!

That backordered cap is going to cost you as you have to pay shipping again. Hope everything gets in
fast!

[Koala]

You results seem to confirm what is said on the findmall.com forum that the II umax is slightly deeper but the original has sharper sound

Strange as you would expect gold and aluminum to be affected first. I take it the iron reject was good. Looking at some video of homemade Bandidos and they appear slightly better at large iron than the IDX

http://jb-ms.com/Carlsforum/Dec24-2006/1161873823.html does confirm that too positive ground balance will reduce detection range on silver air test.

Your problem with you TGSL sounds exactly the same as when the batteries are low on my IDX. Close to the coil beeps as normal but further away sound like chatter sort of a pop and spit type sound. But it still work fine in all metal mode.

Annoyingly I am still waiting parts. It's the parts I ordered first from China that I am waiting for. Solder wick,solder and the ground potentiometer. One from China has arrived today the connectors. Look to be of high quality amazing that 5 sets or 10 pieces equal one switcraft connector in price.


One SMT capacitor went on back order from Mouser no big deal but the estimated delivery is mid July got them to ship the rest and they are here.

Start working on some coils next week

Hope you can sort you TGSL as a direct comparison would be good. I will build the same size coil as I did for the IDX.

[Silver Dollar]

Might be a ground balance thing. I was reading the TGSL threads again and with a bad ground
balance you will reject silver. I didn't balance the new board properly. I just got my 10 turn
GEB pot yesterday so will have to connect it up.

I just remembered I disconnected the PP function too while messing around so that might be
the problem with the chatters (not the 39pf)...

I worked on the TGSL too and found it works at close range (2-3") but just ticks at deeper
ranges. Not sure what's causing that....