Tesoro Lobo news

[marcellotronic]

Excuse me, the trace on channel one is the collector of 2N2907 of TX (Q1).

[porkluvr]

Seeing the transmitter waveform is good but it would be helpful to see the rx channel output (U7-1) and compare that with the DISC ZCD output (u5-7). By your using a non-standard Lobo coil is very likely causing your rx signal to be out of optimal phase for the disc Zero Crossing Detector to work correctly.

Also, your time base setting is ambiguous; the pointer of the knob is hidden making it difficult to detemine your tx frequency. Help us out here?

[marcellotronic]

Hi, the pointer of the knob of Time is 10us.

U7 pin1 RX



U6-1 GB min


U6-1 GB max


U5-7 DISC min


U5-7 DISC max


search head 9x8 silver umax


thank's

[porkluvr]

What exactly is wrong with the DISC operation? I should have asked you that at the beginning but I won't wait for your answer since all I have is a guess, anyway.

Why don't you try replacing C7 in your preamp with 10nF. That would delay the rx signal ~40 degrees relative to what you have now (using 10pF according to your schematic). You may also need to re-adjust your ground balance trimmer pot to accommodate the new phase delay.

[marcellotronic]

The DISC is not a good because for 3/4 of a turn both iron and copper rings, then stopped ringing for both.
I'll try replacing C7.
Someone has images of oscilloscope waveforms?
thank's

[porkluvr]

I've had to rethink what I told you before. It's hard to keep experiments under control when there are so many variables as there are here (and it gets harder as the day goes on so I would like to try and wrap this up soon).

Using LTspice and comparing your circuit with the "original" circuit (as I understand it) I don't think my idea about replacing C7 with 10nF is good. Try 4.7nF instead. It took me a long time to figure that out, so I hope that value is alright.




Alternatively, leave C7 alone and increase rx tank C5 from 6.8nF to 8.2nF. Do one or the other, but not both and I cannot tell you which would be better, if either. I won't even guess.

Another thing I suggest is replace R10 (from your schematic) with 470kΩ ~ 1MΩ, or else remove it completely. By increasing (or removing) that resistor you will get much better linearity from your discrimination control. You do get a wider range by having 100k, but it is stretched out at the CCW end and scrunched up at the CW end. That is not good, imho, but the choice of value to use is ultimately yours.

Look at my simulations where I have simulated sweeping the discriminate control. The traces areas follows: red is the discriminate control at 0%, light blue 25%, purple 50% , yellow 75%, dark green 100%. You can see in the bottom simulation how the discriminate control is non-linear using the 100k resistor. I removed the 100k resistor in the top simulation. You can see the increased linearity (but with a decreased control range).

I have two plot panes in each simulation display. The -only- component affecting the differences between the two top plot panes (discriminate control sweep) is the 100k resistor.

There are other differences in the two simulations, which reflect my reconciling your circuit's rx phase with the 'original' circuit's rx phase. The bottom plot panes show how the GB and rx traces align, given the recommended change to the rx tank capacitor but I included the bottom two panes mainly for the purpose of providing a fixed reference for the discriminate traces that are displayed in the top panes.

Goodnight.

[djjmilan]

I build Lobo and I want to use a coil of my Tejon 10x12 DD. Can anyone advise me with those capacities RX, TX circuits. According FOMA schematic Tx runs only 18kHz can not achieve 17.5khz. Rx does not oscillate. 10x12 DD coil TX 4.4ohm,RX 62ohm. I have no instruments for measuring the inductance. Thank you for your reply.

[Derx]

please help me find lt-spice macromodel for CD4060 14-stage counter ?

[Kev]

How,
Here's a cut down Lobo come Super Diablo that uses Gold Bug coils.
Its sensitivity approaches the GB Pro, but it has a lumpier threshold, and of course no Disc.
It will GB to heavily minerialised pieces of schist and still see a sub gram nugget underneath it, in fact in some respects it's better than in air(?).
I've yet to test this in the goldfields, but it surely works well on the bench.
It goes well with headphones, but could do with a compressor or simple limiter for large near surface targets.
I'll likely be tweaking some of the filter values once I get to test it in the real world.

Cheers
Kev.

[Kev]

Howdy,
I tried the Lobo Lite on the goldfields and was very impressed that such a simple design can work so well. I didn't find any gold at this spot, but going by the number of small lead pellets and other bits of rubbish I found, it would've had I put my coil over any.

I'm not going to do any further tweaking at this stage, other than install it into a pelican case and change the batteries to a much smaller capacity given that the current drawn is less than 25 mAh. That in itself was a surprise since it develops such a strong clean sine wave with the Gold Bug elliptical that I used I expected it to consume 50 or 60 mAH.

The coil connection is important and will only work this way
Coil connector
Pin 1 to Gnd
Pin 2 to RX
Pin 3 to Gnd
Pin 4 to TX

When I get some 68N caps I will try one of those on the TX tank, it may even increase sensitivity a bit more, not sure though, I didn't have a suitable device to try.

All the chips are decoupled at the supply pins by 10uF and 100nF caps, and the upper and lower PCB sides have gnd planes, all of which I believe helps with the stability of this unit.

The fact that it doesn't have disc is not an issue on the goldfields, as we have found our Gold Bug Pro will sometimes indicate a nugget with rusted ironsand inclusions as an iron target. So no matter who you are or what you are using, if you use disc you are leaving gold behind.

This unit will make an excellent light weight exploration unit when tramping into the back country. In a water tight Pelican case and running on so little battery it will be a supper long running bedrock sniffer.

Some pics.

[amtech2005]

Excellent Work Kev !! Very clean circuit board too , from vendor or did you etch yourself ?

Regards.............Eugene

[Kev]

Excellent Work Kev !! Very clean circuit board too , from vendor or did you etch yourself ?

Regards.............Eugene

Thanks Eugene very nice of you to say.
I used Kinsten, positive presensitised boards they are very nice to work with, and don't age in the refrigerator like some.

Cheers
Kev.

[Altra]

Beautiful build Kev! It looks really solid and field ready. How did the ground balance work in your location?

[Kev]

Beautiful build Kev! It looks really solid and field ready. How did the ground balance work in your location?

Thanks Altra,

I quickly knocked it together for a recent trip, but ended up using my GPX4500 most of the time. Since it performed so well I will spend some time making it lighter and more compact.

The GB works very well. During the build I set the ten turn pot to mid position and adjusted the trimmer so that a heavily iron impregnated piece of schist was just balanced out. When I got to the goldfields I only needed to adjust the 10 turn about half a turn from this point to balance, and after that it only needed minor adjustments of a quarter turn or so. This ground is very mild going by Oz standards as schist derived alluvium often is, but I'm sure the GB would handle quite hot ground once adjusted correctly.

Cheers
Kev

[Davor]

I kind of wanted to hear this first-hand. I have my own thoughts on how to go about a LF rig to detect very small gold and have a good GB too, and this is a heads-up to go through. Thanks.