SMT Bandito Info/Build

[Koala]

Big Silver---- 5-5 1/2"--- 1" Solid---- 2 - 4" Intermittantly

Does not like silver ?

[Silver Dollar]

Well it looks like the 39pf might not have been making a good connection.
So today I tested the original Bandito to the SMT one using the original
coil hung from the ceiling. I couldn't try the TGSL as it's still not working
(maybe later on tonight I'll work in it).

They were pretty close. The original sounded louder and here's the results;

Original SMT
Gold Ring ----6-7" -------7" Solid ----8" Intermittantly
Big Silver---- 5-5 1/2"--- 1" Solid---- 2 - 4" Intermittantly
Nickel --------6" ----------6 - 6 1/2 ---7" Gone
Bottle Cap ---6 1/2" -----6 - 6 1/2"
Penny --------6" ---------6"
Quarter ------5 - 6"------6"
Dime ---------4 - 5" ------5 - 6"

[Silver Dollar]

So there's defiantly a problem with my Bandito similar to the problem I had with TGSL.
The RX input is a differential amp but is fed from a single ended coil. I get a sound
when I probe to the grounded side of the opamp and see a 400mv signal out of
the first opamp. I didn't check on the original Bandito but I'm using it's coil...



I see a ring down on the output of the LM393 and am not sure if it's going through the
power rails or standard path. I can hear targets but also get an oscillating tone. I did
try to isolate the 393 using 10 ohm resistors and a 4.7uf cap on the plus and minus supplys.

On the TGSL I shorted out the 2 resistors on the plus input of the opamp and grounded the
4.7K/5.1K on the other side and that helped cure it. Maybe a cap across the 200k to ground?

I'm not sure about the 39pf cap. I measured all resistors and caps incircuit but the pf value caps
would not read...

[Silver Dollar]

I'm not sure this is a problem as I didn't use a Sens or Threshold pot. I didn't think they were
necessary unless you wanted more depth.

So here's the inputs to the 393's;




The fixed input to compare to;


And the chattering output;

[Silver Dollar]

Well the new 4 pin coils are 1mh TX and 10 or 12 mh RX somewhat similar to the Minelab coil.
They require a different oscillator but put out a lot more field. Though I'm not sure it gains you much...

[Koala]

Well I just measured and there's no centertap. Both TX and RX have one end grounded.
So maybe my RX wires are swapped?

Thanks for the info on front panels. I hadn't seen that thread (I thought I've read everything
on here).

Maybe there is a difference between the uMax coils. Tesoro only list two types of coil 4 pin Delta and 5 pin Epsilon.

Found this


Resistance looks quite different looks more like a normal coil with a finer wire and more turns on the RX compared to the TX

[Silver Dollar]

Well I just measured and there's no centertap. Both TX and RX have one end grounded.
So maybe my RX wires are swapped?

Thanks for the info on front panels. I hadn't seen that thread (I thought I've read everything
on here).

[Koala]

Anyone know a good free software package to design front panels? I've tried a couple but they
were tough to use. They either are CAD packages that need a long time to learn or are drawing
programs that have no built in designs. I just need something to make an oval panel and some
tick marks in a circle around each knob...

Have you seen this thread

[Koala]

I am confused with the center tap wiring

Found two diagrams but they are not the same

Very subtle difference one shows the center tap connected to both the TX ground and the cable shield.

This does not seem right to me as would cause a loop

Also both are missing the coil shield Tesoro use a conductive plastic would this not be connected to the cable shield then back to the board

[Silver Dollar]

It seems the instability is related to grounding the RX. I had similar trouble with my
TGSL and could not run it as a balanced input but had to ground one side of the RX.
I'm using the Tesoro coil now and the original Bandito works and is wired the same way
so I don't understand why the problem on the new board?

I just figured out the caps in the second filter were wrong on the original Bandito.
Here's a corrected schematic;

[Silver Dollar]

Well I was getting ready to put the Bandito in a case but it has developed an instability.
If I probe to the input of the first filter it stabilizes. Must be an open somewhere near there?

I probed around a bit on the original Bandito too. I found the resistors on the outputs of the
comparators that compare metal and disc are 2k. It also uses back to back diodes on the final
stage. Here's a corrected schematic;

[Silver Dollar]

Well I tested the Tracker coil with the Minelab oscillator and HBridge oscillator.
I get 6 khz with the Minelab and 6.6 khz with the HBridge. Hmm to get up to
10 khz we need to reduce the capacitance and it's inside the coil housing...

[Silver Dollar]

So my thoughts turn to packaging. I plan to use my old Tracker stick and housing but
not sure if I can use the coil. It has 2.4 mh TX and RX and a cap in the coil for TX to
run at 6.6 khz or so.

I don't know of any oscillator that would work with this setup. Maybe the IDX one?

I could also make a TGSL one or Minelab style and use the Minelab oscillator (as it
puts more current into the coil).

I made a metal plate for the control box and have to decide on what controls to include.

The so called "Sensitivity" control is really a threshold for the comparators. Some say it's
not too useful so maybe leave it as an internal trim pot? It's possible having 2 trims could
get you a more sensitive device too...

The threshold controls volume and with a simple mod can go from the stock setting to a
pure binary mode so could be useful to include.

The Disc is necessary and GEB is good to have too. So maybe Disc, GEB, Volume and a switch
for Disc/All Metal/Retune would be good? Oh a Tone control might be a good one too but needs
a SPDT switch to toggle the 4024 output but maybe we can make it work...

Anyone know a good free software package to design front panels? I've tried a couple but they
were tough to use. They either are CAD packages that need a long time to learn or are drawing
programs that have no built in designs. I just need something to make an oval panel and some
tick marks in a circle around each knob...

[Silver Dollar]

Her's a dual oscillator setup. You need a 4 pole double throw switch as I'm not sure
what can still be connected when you switch over. Trying to save the 4ma the other
oscillator will use too. Not sure if I will use it as there will be a lot of wires...

[Silver Dollar]

So I've been working on a big mod. I call it the Big Bandito and Tone Tune.
Here's the schematic;


It does a balanced TX and RX and has tunable dual tones via a level select able 4024 output.

Here's the first part I call it the sleeping H bridge as the transistors are laying down;


Here's the bottom. I'm using a special technique where some parts are through hole
(transistors) and some are SMT so as to make a tiny pcb.


It oscillates at 10 khz and has about 9V amplitude but that is hiding the fact that
there is around 150 ma in the coil! It only draws around 4 ma too!


The dual tone mod allows for all high tones, all low tones or switch from low to high at some amplitude...