SMT Bandito Info/Build

[Silver Dollar]

I tried many different frequencies by changing the TX cap and RX cap.
The null voltage got too big at some settings but then I lost the ability
to ground balance against ferrite. The ground and disc pulses look OK.

I went back to stock caps and the original Bandito Coil.

I tried messing with the components around the GEB circuit but nothing
changed. It seems ferrite is on the disc now for some reason. I'm not
really sure which components control the start of the disc range (or both)?...

[Silver Dollar]

The null is 4V P/P with 15.2 khz but it still works good! Not sure if I should try to
lower it or change frequency closer to the designed one?

[Silver Dollar]

Tried setting it to 12.5 khz but amplitude was only 5V! I tweaked the 1.2K back to 1.2
(as 2.49K was better at 10khz) now 9.2V but very low depth! It seems a strong TX chokes
the signal somehow. It seemed to run fine at 15Khz so maybe I'll go back to that...

I ended with 15.2 khz at 9.2V P/P and got 9" on silver and gold and 7" - 9" on coins.

I still haven't optimized the RX cap but it looks OK!

[Silver Dollar]

Well I managed to open the Tracker coil and cut the cap out. So now I have a nicely
nulled and shielded coil to play with!

I tested it on the Minelab oscillator and got 5.9 khz at 23V P/P!

On the Bandito with no mods (220n and 20n) it gives 15.5 khz and 9.1v P/P
(with the TGSL caps it was 21.5 khz and 19v P/P)

Hmm the Bandito run seems a bit low. I might have to play with biasing on that
transistor. We might actually want a small TX the originals are usually around 9V.

I'll try to find some values to make the Bandito run at 10 khz and see how it works.
Right now at 15.5 khz it is just a little short at 8" for some coins and 9" on Silver
probably 8" for gold... I was able to null out the ferrite so I could leave it here if
the disc works ok...

[Silver Dollar]

I have no boards for sale just one I got from Silverdog.

I need a good coil. I have one that I superglued that had ty wraps on it. After the glue dried
I couldn't get the ty wraps off. I'll have to try some solvents to see if I can reclaim that one.

I also have the Tracker coil. It's 2.7 mh and 2.4 mh but has a cap in the coil so fixed at 6.6 khz.
If I can get the half's separated I can remove the cap and experiment a bit. It's tough going as
it appears to be PVC glue so you almost have to cut the plastic apart.

I found some plans for an Omega coil for TGSL that might be fun to build. I'm not sure I have enough
wire left though...

[anonymouse]

Hi Dear Silver Dollar
Did you have any PCB for SMT Bandido?(no SMD version,Deep version)
I'll build it
Regards

[Silver Dollar]

Well it seems the coil I built is the problem. I went back to the original and it works fine.
I didn't shield it and used a centertap. I'll have to try it outside to see if it is usable.

I did find one suggestion for the oscillation on the IDX thread. It was suggested a 22k to
ground on the output of the first filter stage will reduce tendency to oscillate. I might try
that....

[Koala]

Have you tried turning the threshold all the way up when in disc mode

[Silver Dollar]

Today 8" and noisy. I see 10 hz oscillation on the filter opamps. I tried changing the front end gain
and not much difference. It does loos range by lowering but doesn't gain by raising the gain. I saw
double the gain on the TGSL and expected it might be useful here...

The pahse from TX to RX seems to change too. Today it is 236 degrees. I saw 40 degrees the other day...

Not sure how to calm this thing down. I added 10 ohm resistors to each power pin on the opamps
then a cap from the pin to ground hoping to isolate and quiet down the power to them. The original
doesn't use any but it is on a substrate with all resistors and etches in it probably to fight chatters...

[Silver Dollar]

This thing keeps changing. Today I added the Threshold pot and now I have 100mv noise at the
last LM393 and I only get 5" on gold...

[Silver Dollar]

Well I tried the servo filter but the board didn't work well. So I tried the LM358
in the original circuit but less depth. So I put the MC33178 back in. Then it
worked as before but with weak sound. I get a bit of chatters too forcing me
to use low sensitivity. The average noise looks OK (40 mv peak to peak at last
LM393) but occasional big hits come through on a random basis.

I thought things seem worst after removing the TX while trying the Minelab
oscillator so maybe the new parts are a bit different? I had noticed a larger TX
at 14.7 khz (TGSL frequency) so changed the caps in the TX to get 14.5 khz (10.5V P/P).
The beeps got a bit high pitched so I had to move the diode on pin 11 of the 4024
to pin 5. But now I get a kind of echo after a target goes by. I did seem to get silver
at 15" though so will persue this setup to see if some depth can be gained.

The gain seemed to come from the input opamp. I saw Davor added a different opamp
to IGSL and balanced the input by putting a cap across the gain resistor to ground so
I did the same. I changed the 39 pf to 15 pf and added one to ground then saw 15"
on silver...

There could be a problem with the ground balance but it looked OK. The Disc is defiantly
shifted as I was able to disc out the gold. A few caps might fix that.

I might try using a 7660 for the negative voltage (as the original Bandito does) mounted
on the back side of the board thus using the ground plane as a shield to try and reduce
noise.

[Silver Dollar]

The original was acting a bit funny too. Seems there was a bit of noise in the air.
I did find pressing on the first filter chip caused a low frequency oscillation and the
sound got stronger! Measuring the gain resistors and one reads right the other reads
low so it seems there is a mechanical problem in that chip. I didn't buy spares either...

I can try a 27L7 or LM358 as I have those in SMT... I wanted to try the original filter
setup anyway. That uses a servo like filter with a 358 and 27L2...

[Silver Dollar]

Well I'm doing some playing around today with different frequencies and found that the higher null
signal gives greater depth?? The detector sounds sick today though a bit quiet and "scrunchy" not
sure how to get a clear tone? I'm looking at the ground signal right on the 100n cap and see big jumps
with the random beeps. Power to the chip bad?

[Silver Dollar]

Well you can get the Bandito to 14 khz by using a .022 instead of .047 in the TX. It showed the best
range before adding a RX cap on gold and coins but not so good using the .015 of the TGSL on RX. Could
need some Disc and GEB work to perform better.

The SMT Bandito draws 15.3 ma when not sounding. Changing the TX from 10 khz to 14 khz didn't change
the draw any but the Peak to Peak value for TX went from 8.5v to 10V...

[Koala]

Hmm I'm looking to tune the coil a bit but am confused. The TX is around 10 khz but the RX calculates to 10.867 khz.
On TGSL the TX vs RX frequency is about 1.5khz difference...

You and me both I read through most forum post I can not find any measurments of the newer Epsilon coils designed for µmax machines. Seem most of the work was done on the Tesoro years ago and interest has been lost in all except for the TGS