TDI Pro Oz Vernier Ground Balance Benefit in hot soils?

This is a question for those with both a Pro Oz and an SL like Karelian & Ridgerunner.

I have often wondered, Does the fine GB control makes a significant diff or not to being able to properly GB the machine in hot ground?

The reason I wonder this is because I notice that with my SL in mild soil that the GB null point is not so much a point as a small range and that as you move away from that point the effect is gradual eg for a GB 8 null point, 7.25 is Screaming High Tone, 7.50 High tone, 7.75 sl chattery, 8.0 silent. 8.25 sl chattery, 8.50 low tone, 8.75 Honking low tone.

This contrasts with really hot soil where IF I can get it balanced at all it is a knife edge ie 7.9 Screaming High, 8.0 sl chattery, 8.1 Honking Low. Under these conditions the machine is then much more sensitive to variations in sweep speed, variation in height above ground etc producing lots of exciting noises as I move about. I can sort of see thru the spurious signals but it is less than ideal.

So I am wondering, ok I can see that a vernier fine GB should help in finding that knife edge null point in the first place but after both are ground balanced is the PRO Oz noticeably quieter that the SL in hot ground or not?

[video=480,360]https://youtu.be/34cr0HDIseM[/video]

Just an example of the Tdi Pro being ground balanced with the Sadie coil in the beginning of the video. Much easier and faster than the Tdi SL. The ground at this site is very heavily mineralized and full of iron stone and hot rocks. Really nasty ground. Turn the volume up a bit to clearly hear the difference in ground noise between the Low and All positions on conductivity. SL users can also get some indication on how the machines compares with the Pro with regards to ground noise. Same with the bigger Digger coil. A bit of a camera sweep to show what the ground looks like and the area I was operating in.

The Pro is much easier to ground balance compared to the SL but once balanced both machines are roughly equal, perhaps a slight advantage to the Pro in really nasty ground. Hope this answers you question XLOOX .

As with most Manual ground balancing Detectors, Most Pros will not try to set it perfect But they will try to set the GB slightly on the positive side, This hots up the machine making it slightly more sensitive to smaller targets and adding a fraction more depth,

If a machine only has Auto GB and a Lock / Fixed position you can still do this when pumping the Coil by stopping the coil a few inches off the ground on the down stroke and locking the GB,

Hope that helps.

J.

Hi Ridgrunner, thanks for that. You bring up another point I have never been clear on - running with a GB offset in ALL mode.

I can understand running an offset GB if you are in HIGH/LOW since the threshold is still silent but when you say "Set the GB slightly on the positive side" do you mean that if the GB null point is say 8 then you set the machine to 8.25 or thereabouts and still run in ALL?

If you do that don't you just get a continuous loud low sounds when sweeping? Sure a small high tone target will still cut thru but how do you know that each of the hundreds of low tone blurts isn't a huge nugget ?

Mate the only time I would not want to hear a steady threshold was if I was using the TDI or Coin or Jewellery hunting in an Iron filled site because I would only want to here the low or high conductors, Apart from that I would always want to hear the softest of thresholds, with the volume up high and the threshold very very low, But never nulled

As for the GB you want to GB the machine and then tweak it slightly so it just starts to rise as you get the coil closer to the ground,

With the TDI SL it has what I call the smoothing effect when the GB is switched ON, The TDI Pro has this but it only does it when the GB is turned OFF, you can see that in Action with some Guys TDI Beach detecting Tips on youtube, But the TDI SL does that when the GB is switched ON so even though you off set the GB the TDI SL will settle down giving you a smooth threshold as long as you keep the coil at the same height all the time that way you can run it a bit hotter and some of the those phantom signals will be gone,

But if you off set the GB by a larger amount say from 8 to 9 some targets that are low conductors will come through as high conductors and vice versa, This is not a quick thing to learn but it is worth playing with in the back yard so you can understand and then put it to use out in the Bush or on the beach,

Hope that helps.

J.

Neat! - I am going to play in the paddock this weekend and see what I can learn from putting that into practice...

I was reading thru the SPP threads and came across an intriguing post by Old Hand in Sept 2014 talking about replacing the GB knob with a vernier dial to improve finesse of GB adjustment. I contacted him and got the details & ordered one off ebay. Not cheap in that what started as a USD35 item ended up costing AUD100 delivered inc customs but after using it I think its worth it.

The Philmore S38 36mm vernier dial is used for ham radio and other fine tuning applications. There is an internal planetary gear system that connects the knob with the shaft. It says its 8:1 but seems like 6:1 to me ie 3 full turns (1080deg ) of the knob moves the GB shaft 180 deg. It really is a totally cool bit of mech in an increasingly elec world.

So whereas before I was struggling to move the GB 1deg - just a smidgin without overshooting against the friction & hysteresis of the pot & O-ring, now I can easily turn the knob 8 degrees to achieve the same result or even finer control and really nail that balance point, or apply a tiny offset. Sold !

To install it the old plastic GB knob simply gets prised off and the vernier dial slips on top of the GB pot shaft. It is held onto the GB pot shaft with a screw. The vernier dial body then needs to be mounted to stop it rotating when the knob is turned. Old Timer mounted his on a piece of aluminium held by the pot mounting nut but just to be different I riveted an alu tab to the MD shaft and mounted it off that.

The only thing to be aware of is that since the total movement of the vernier shaft is only 180 deg, but the GB pot shaft 0-11 is something like 300 deg travel, you wont cover the full GB range with this particular dial. Not a big deal for me. I mounted it so it works between 5.5 & 11 as I have never had soil GB less than 6 and mostly 8-9.

If you really need a much lower GB at some point, for say the beach, then you can loosen the vernier shaft screw, rotate the GB shaft half a turn with a scribe and retighten the screw to give you say 0-7.5 . Or just take it off with 2 screws & slip the old plastic knob back on for the day.

At this point, with 16V & now a vernier GB, my SL is edging closer to a Pro Oz.

Using it in the field, it works very well and definitely helps get a more precise GB when the ground/coil combo is making it hard to quieten the beast. So thanks heaps to Old Hand for a great idea.

A simple and reversible solution to ground balancing in hot ground with a Tdi Sl. If prospecting is your main game then this is certainly going to make life easier.
I'm going to pass this one on to others, Old Hand and XLOOX full credit.

Alternative:
I replaced the GB 17mm Pot with a 24mm Pot of same value and the 17mm audio threshold Pot with a 10 turn Pot.
Had the 5 turn Pot as GB Pot but found there was to much turning, like the 1 turn 24mm Pot better.



Correction the 10 turn Vernier with lock ..... is a 10 turn Pot rotation counter with graduated scale.

Hi Nesral,

I am not clear on how a 1 turn 24mm pot makes it easier to fine tune GB vs a 17mm 1 turn pot. Wouldn't they be the same Ohm/degree ?

The 5 turn pot sounds quite good actually but my soldering/desoldering skills are at a level such that I don't mind blowing up a Xtal radio kit or a veroboard door chime but I'd be too afraid to touch the electronics in my SL anyway

The track on a 24mm Lin. pot is half time longer than the track on a 17mm Lin. pot, resulting in an expanded graduated resistance.

Also possible is to use a 24mm anti-log taper pot that will expand the resistance of the last half of the track even more, good when prospecting hot ground, less so when GB is used in mild ground.

A larger knob on the GB pot also help with fine tuning.

The ADS Deepseeker have a 5 turn 10k ohm pot.

Thanks for the GB pot info Nesral.

Being a skip reader I just noticed your comment re 10 turn pot for audio threshold.

I assume that was to get a finer adjustment for a whisper threshold ?

How do you like the 10 turn 50 k Linear pot - does it do what you want well ?

It was a test, it's a 10 turn 50k Ohm pot with a 50k Ohm resistor in series = 100k Ohm.

It is now fitted with a proper 10 turn 100k Ohm pot.

Yes ... for spot on audio threshold.

Well Nesral I have always found the SL threshold adjustment to be a bit coarser than ideal since just a smidgen either way made it too quiet or too loud for my personal liking. I like a nice happy low level hum, just audible above any background noise of dry undergrowth, streams, wind etc.

I decided to be brave, open the box & copy your idea, so I changed the Threshold pot from 1 turn to a 10 turn one, to allow much finer adjustment of threshold volume. Having now done the mod, I can say that it is an improvement well worth doing to get a nice sensitive audio level & to help with faint targets. It is also straightforward to do, as long as you can solder.

Since I had the box apart, I also took the opportunity to replace the GB pot with a 10 turn one. The 3 turn vernier knob I showed in a previous post works perfectly fine and you definitely dont need both GB mods but the 10 turn pot gives 13x finer GB control than standard pot and is a lot cheaper to do than the vernier knob.

I am also happy with the GB pot mod as it is even better than the vernier knob at finding a spot on GB or setting a very slight offset. For me it didnt result in excessive twiddling as 95% of the time I am on ground that balances between 7.5 & 9 GB and after initial GB it normally only takes 0.05-0.2 GB (15-60 degrees) movement to tweak it as the ground changes.

For anyone looking to do the mods, to open the magic box remove the 4 screws on the sides & the 4 screws on top just leave the 2 screws immediately above & below the speaker in place. The top then lifts off with fairly short wires still connecting the pots to the circuit board on the bottom piece, so dont yank. You can gently unclip the cable plugs from the circuit board if you want.

The pots required are a 100k Ohm Linear for the Threshold & a 10k Ohm Linear for the GB. Cost is about $12 for each pot

Both pots are 10 turn with turn counters & crucially they cannot extend more than 19mm below their mount else they will interfere with components on the board. Obviously they cant be super massive diameter either but 16mm upto 25mm nominal would be fine.

I used 6.3mm dia shaft pots same shaft dia as original pots, but for some reason the threaded portion was wider than orig pots so I needed to drill out the pot holes a little with a 9.5mm drill.

The original GB pot also has an off switch, but unlike most detectors, the SL gives max depth with GB switched on, so a switched pot isnt really needed. Instead I just joined the 2 green wires on the GB pot together so GB is always on.

As you can see from the before photo, for the Threshold pot the red wire goes to pin 1, white to pin 2 & grey to pin 3 but for the GB pot the grey wire goes to pin 1, white to pin 2 and red to pin 3.

You might notice in the photos that there is no coil shaft attached to the box. Since the top was off the box I also took the opportunity to drill out the shaft rivets so I can later make a bracket to easily swap from shaft to chest/hip mount, just to see if either of those options are my cup of tea.

Overall these 2 mods make an easy to use detector even more user friendly and sensitive.