SMT Bandito Info/Build

[Silver Dollar]

The bandito I have is interesting. It's a rev B board and has these components on a substrate;
LM308
LF353
LM358
LM393
27L2
7660
4024
4016

OFF the substrate is a LM2930-5 and a circuit with a LF442 in a dip package.


The GEB has a 100k trimmer to ground then a 100k 10 turn with a fixed 100k across it.
The sens and Disc pots are 100k.


Maybe something like this? I'm not sure where the 442 is though;


The negative voltage on this board is -4.4V. The batteries are paralleled with reverse diodes on the plus side.

Here's a picture

[Silver Dollar]

The TX on that Bandito is pretty wimpy;
10.4 khz
3.72v
7.78v Peak to Peak

Looks like RX has 0.01uf plus maybe one on the pcboard too.
TX has 0.015 uf plus 0.015 uf...

I also got the parts for the SMT board from Mouser today. It's almost done!



I did get some parts so can make it 99% to the schematic.

[Silver Dollar]

Here's a better schematic for the original Bandito. I fixed the pin numbers and changed to the 7660
for the negative voltage.

[Koala]

Noticed TR6 is a 2n2222 instead of 2n3904 used elsewhere

2n3904 is a higher speed and lower noise device is there any reason that the 2n2222 was used other than availability ?

[Silver Dollar]

Oops the transistors have not been verified yet. I need a scope to see the
markings as they coated the board with clear spray and it's hard to read.

The TX is 2907 and one other is too.

I'm finding some funny readings too. There are no 4.7uf caps and the 358 resistors
seem to be 100k and 1k. The caps are 100n and 10n?

The resistors on the 308's appear to be 840k (1meg?) and 10K (instead of 47K) but the
caps are definitely 470n but 10K and 470n isn't right....

The lower string is the values I found in circuit.

[Silver Dollar]

Well I've found the 442 is used for the pinpoint function (and it has a 20 meg like a mod I showed) so what is the 27l2 for?
I assumed the 27L2 was CMOS so would hold the ground longer before bleeding off....

[Silver Dollar]

Well I fired up my Bandito II SMT today. The first problem was a tiny TX (600mv) I had installed a 12K instead of 1.2K
(their wrong in my bag). I got that solved but now I do not have any negative voltage. It reads 0.5V positive. Hmm
I'll have to do some troubleshooting... I wonder if I loaded a 3906 in that Spot that was mislabeled?

I saw some distortion on the TX sine wave so kept increasing the value till it went away. I ended up with 2.49K and the
wave is bigger than the old Bandito.

[Silver Dollar]

Well the negative voltage problem turned out to be the 100uf electrolytics that I
scavenged from some old equipment. It turns out sometimes the marked end is
positive! After getting that straightened out I had 2.9V for the negative supply.
I also was getting around 9" in air for a gold ring. I took the capacitive multiplier
out and now have 4.0V! The air test is now 10" but there is some instability. I
jumped the sensitivity to the negative supply so maybe I'll put it back to ground
and see what happens...

[Koala]

Sounds Like good progress

Was surprised that the original had less than 8V pp but works as expected.

How does the new SMT board compare to the original in air tests

Reading through one of your old posts are you saying that the RX and TX capacitors are half on the PCB and half in the coil ? not hear anyone mention this before.

[Silver Dollar]

Ya I'm thinking the small TX might help with seeing good targets in trashy areas.
The original TX was 7.8v p/p and the new board is 8.5v p/p.

The RX caps are .015 and .015 off the substrate but on the mounting board.

I didn't do a baseline of the original board with the current setup and target. I cut the
coil connector off as the stock wire was 8' long and I'm switching out the connectors
so all my units can use each others coils. So I can't test the original board right now.
I hope to finish the bracket for the original unit tomorrow then I can close up the cable
and use it on both units.

It seems like the new board is a tad less air test distance. I'm thinking the schematic is
not quite right or they changed it on later boards. For one thing the 4.7uf caps on the
first filter stage are not present on the original board. That would make it a no motion
unit but it should have lots of falsing. I'm not sure I will ever have the complete schematic
as it is difficult to measure things on that substrate...

[Silver Dollar]

I was thinking about the 27L2's on the bandito and had a look at some other schematics when I found this one;



I'm pretty sure that's how the original Bandito I have is configured. I remember reading 2.xx megs and 3xxk around
the 27L2 and there are .47's there too.

Should look something like this;



Anyone know how to calculate the filter frequencies for this type of filter? I usually do that but cannot find this configuration
on any calc sites....

I also found some unsoldered pins on my Bandito II so I hope that fixes my glitching. I used the microscope at work. That makes quick
work to spot mistakes!

[Silver Dollar]

So I whipped up this tiny transmitter (oops I noticed a jumper missing). It's an Hbridge
designed by Davor. Unfortunately you cannot use it with a stock Tesoro coil.

It puts a lot of current into the coil for not much battery use and fixes some other
problems with harmonics and such. I'll have to tickle the value a bit to get it to 10khz
and I'm not sure if it will work with the 6mh coil or a 1mh Minelab style is needed.

[Koala]

I am waiting for parts

Only have the board and the toggle switches so far. Everything is ordered quite a large parts list.

What are your thoughts on the Bandido both the original and Silverdog one before any mods and how do they compare with your TGSL

[mschmahl]

And if you use original 4-Pin tesoro coils with RX: 15,04 mH,66,9 Ohm and TX: 0,929 mH, 4,3 Ohm? These are for 14-18 khz.

[Silver Dollar]

The 4 pin is the right inductance's but this oscillator requires the TX to not be tied to ground.

The Minelab oscillator would work with the 4 pin and a standard Tesoro coil and get you some
additional current into the TX. The big question is will it make any difference? It appears the Deux
has different power levels available and most people say there's no difference other than the battery
not lasting as long.

It might be a big field charges the target too much and it doesn't decay fast enough to see it...

I have the original Bandtio and the SMD one with no mods so far that I just got the connectors on
so can now compare. My TGSL was reworked several times for various mods and now I have put it
back to stock but it doesn't work! I probably missed a cut etch somewhere. I'll try to get it going and
do a back to back comparison (maybe this weekend).