SMT Bandito Info/Build

[Silver Dollar]

I found some problem using the 9V for the TX. I think the speaker causes problems on the 9v.
The thing oscillates implying positive feedback. The opamp filters are on separate channels so
should act differently. The only place for a problem is at the TX or RX input. The speaker will
radiate a magnetic field though I tried a 25 ohm speaker from a TV that had metal shields on the
back and no improvement. This CB speaker seems to have a limited bandwidth that somehow
reduces the feedback. I might try removing the transistors feeding the speaker and use an
external amp. Ivonic had problems when adding headphones so there's something funny there.

What is the difference between MPSA13 and 14?

I did notice the original Bandito has loud audio. The SMT one didn't but is close with the new speaker...

[Koala]

wounder if the audio lacks some punch to drive the speakers. I did some experiments with headphones on the IDX and the lower the impedance the greater the depth in all metal mode made no noticeable difference in Disc mode. Always meant to change the mpsa13 for a mpsa14 but never got round to it as I am always using the detector.

[Silver Dollar]

I saw the depth lower with a bigger TX before. 9V seemed to be the sweat spot.
But I now have 10.2V at 12.5 khz and by tuning the RX have gotten a good
signal.

I compared the original Bandito to my SMT one today and the SMT one beats the
original by 1" on each test. Then I tried a few different speakers and gained 1"
by using an old CB radio one. It's louder too! Now I have to make a volume adj
pot!

The final figures;

Silver Dollar 13"
Gold Ring 12"
Coins 9"

But these are only with that "special" speaker. It allows higher Sens somehow.

[Koala]

make sure you don't loose sensitivity to very small objects by increasing the TX voltage. I know golfnut uses this mod to good affect on his IDX. Some tried the same mod but had worse sensitivity to small target. They had no more depth but hit on coin size targets better.

[Silver Dollar]

I tried powering the TX off the input 9v to get more power. The TX went up to 16V peak to peak but
it was unusable due to oscillation! It oscillates at 15 hz. Not sure where that is coming from as I retuned
the filters a bit. It shouldn't be affected by the TX. The RX residual voltage was only 400 mv. I have lots
of decoupling around but it seems some signal is radiating and causing this. Maybe shield the GEB pot
wires?

I have the gains lowered. I am using 500k in both the filter stages. It operates the same as with 1 Meg
and 681K which tells me all my improvements are in the area that oscillates. I'm worried that in real
ground conditions I will be limited further so I need to get a handle on what's causing this oscillation...

[Koala]

Well today it seems to work like always. I think it changes the GEB setting somehow.
After I ground balanced the extra depth was gone!

It's running reasonably good but still has "run on" after a target goes by. It's on the verge
of oscillation. I tried 5K to ground on the outputs of the last filters as suggested in another
thread but it didn't do much. I narrowed the frequency on the first filters and that helped a
bit. I don't have any more ideas on how to tame this beast!

Yep it's hard to get consistent results when testing.

Been working on my Baracuda and a small difference in threshold ruins the results as does the charge in the batteries.


Had inconsistent results when test my IDX until everything was in a metal box. Did find grounding all the pots and coil connector helped a lot but still not so good.

Get a bit more depth out of the IDX by ground balance slightly positive and cranking up the threshold. One video on YouTube shows the Bandido ground balance slightly negative which seems to be confirmed by the link

[Silver Dollar]

Well today it seems to work like always. I think it changes the GEB setting somehow.
After I ground balanced the extra depth was gone!

It's running reasonably good but still has "run on" after a target goes by. It's on the verge
of oscillation. I tried 5K to ground on the outputs of the last filters as suggested in another
thread but it didn't do much. I narrowed the frequency on the first filters and that helped a
bit. I don't have any more ideas on how to tame this beast!

[Silver Dollar]

I had a bit of a breakthrough today. I had just about changed every component on the board
trying for more depth. Well I was looking at the IDX as a possible board for my daughters little
MD. I got a BH coil that has a cap in it so wants to oscillate at 6.5 Khz. I built the IDX oscillator
tonight and found it works quite well on the Bounty Hunter coil.

So anyway I was studying the schematic to see the differences from Bandito/TGSL to IDX and
noticed the cap after the input op amp is 22n on IDX and 220N on the other MD's. I changed it
on the Bandito and Bang 1 - 2" more on all targets. Silver is solid to 13" and cracks to 15". Gold
goes to 12" and the the coins go 1 - 1.5" deeper too!

I'm not sure the function of that cap. It could be a low pass or high pass but changing it sure
makes a difference! The sound is a bit crunchy towards the max depth and haven't tried it in the
ground yet but it appears promising.

[Koala]

Hi

this is the final change for bandido?
now,we can design PCB for it?


Please put your test results here
which is the best?(TGSL,IGSL,IDX-Pro,SMT Bandido)


Thank

All very similar. Each build and coil is slightly different but on average all the same. IDX is probably slightly easier to get going good. and the IGSL slightly better discrimination.

Out of the Tesoro none digital range Monte preferred the Bandido II µmax for trashy areas (Low noise, quick recovery and true no motion) and as a metal detector dealer he had the choose of any detector

Technically there should be a small advantage in the lower noise of SMT not seen in the real world yet

For a first build go for a tried and tested one IDX or IGSL and get a board from silverdog as they are proven to work unless you detect on the beach then you would be better with the Surf PI, Baracuda and the new MiniPulse.

Wish you much success in what ever you build

[anonymouse]

Hi

this is the final change for bandido?
now,we can design PCB for it?


Please put your test results here
which is the best?(TGSL,IGSL,IDX-Pro,SMT Bandido)


Thank

[Silver Dollar]

Well after lots of work I've arrived at the most optimal values for most every part on the board.
I'm using the Tracker Coil (TX 2.7 mh RX 2.4 mh) and have the max depth and stablest config
I could tune. The last mod allows for volume change with depth. You might think your missing
some targets due to low volume but they are not there. There's a wall you hit and no signals
get through it. Motorboating lives at extreme sensitivities and I couldn't find a way to get rid
of it to hear those faint signals. The volume mod stops the tendency to run on after a target
goes by. So I'll clean the board one more time and start working on the front panel and putting
it in a box!

Here's the final schematic. There's some funny values arrived at by lots of experimenting;

[Silver Dollar]

I have a couple of ideas. The first is that the input config has a great effect on the signal
and noise. I changed the gain resistors from 5k and 100k to 1k and 20k to lower the noise
but that might be having a loading effect on the coil. I'm not sure the equivalent noise resistor
for the MC33178. Maybe try 10k and 200k just for giggles? On the TGSL eliminating the 2 resistors
on the RX positive side helped depth a lot but here it hurt it....

The other thing I'd like to try it to eliminate the diode and 1meg from the comparators to the final
opamp just to see what kind of signal I get. It looks like the noise is hiding under that diode so it might
become very noisy. On TGSL they add the 1meg for loud audio but I found reducing the gain of the
last opamp gives very proportional volume.

It seems the whole audio chain is tuned and so it's hard to make huge improvements but easy to hurt
performance. I'd like 1" more on all targets but might not be able to get it....

[Silver Dollar]

I tried to run the TX on + and - 5V to get a larger TX and got 13v P/P but it was pulling the
supply down and sounding continuously. I couldn't even get an idea if it gained any depth as
it was too noisy...

13 khz seems to work the best with the Bandito board and Tracker coil. It's modded a bit and
gets 9" on gold, 10" on silver and 7 - 8" on coins. Time to package it up and take it outside!

Another curious thing, I cut the gain in the first stage in half and when I adjusted the sens for
the edge of chatter I got the exact same depth. I wonder if even lower gain will reduce the
noise so allow a higher sens and be able to see a little deeper?

I also removed the diodes in the second stage with virtually no effect.

The signal getting to the threshold opamp was about 55 ms wide. I'd like to know where that
55 ms is coming from, that's where some changes might have an effect....

[Silver Dollar]

The Bandito is very resistant to any tweaks! I found something funny, if you
remove the 470n's and short the connections it hardly works! I get 2" like that.
I'd imagine the other filters and opamps would do a bit better... Weird.

I did get a setup where it was very linear in volume depending on distance by
changing the 100K on the threshold opamp for 40K but I didn't like it as the
volume gets really low.

The threshold doesn't seem to do much, I'll probably leave it as a fixed resistor

It seems there is a bit more in there but hard to coax it out. I could up the TX
voltage but not sure it will help. I may try lowering the gain on the first stage.
It's counter intuitive but some depth might be gained. You just have to change
the caps too to keep the same overall filter response.

The chatter filter is the big limiter here. I can hear ticks that should sound as a
good signal but that filter stops it...

[Silver Dollar]

Oops I found that I had changed the 39pf on the input opamp for some reason.
Now it's ground balancing fine. I guess the hidden target phase was changed by
making that 15pf instead of 39pf. I thought that was only for high frequency roll off...

I have the Tracker coil running at 13 khz and 7.7V p/p but depth is a bit low a little
more tweaking is in order!