bobinyelm

I am taking the TDI out today and will try underbalancing it just a tad (on the counterclockwise side of absolutely quiet during the ground balancing) to see if small targets sound off better. I am also trying a Bluetooth transmitter attached to the TDI with a pair of low latency Bluetooth headphones to see if it will work to free me from the earphone cord. I would hope there's no EMI generated by the bluetooth transmitter. ---------------------------------------------------------------------------------------------- Results: On the ground balance, located lots of BBs and Bullet fragments, but no really weak targets to see if slightly underbalancing the GB would result in better detecting. The low-cost ($25) Bluetooth transmitter attached to the headphone jack of the TDI was WONDERFUL, as were the $30 lightweight Bluetooth headphones. "Cutting the cord" was great. And the on-the-ear headphones is safer than over-the-ears headphones as far as hearing ambient noises (like rattlesnakes that are fairly common where I hunt). Obviously will work with any non-Bluetooth Metal Detector Headset Bluetooth Adapter 1/4" Headset Jack to 3.5mm Headset Plug Adapter Thanks.

On my Gen 1 TDI, that GB range is VERY wide and poorly defined, meaning when balance is achieved (NO sound when pumping) a setting either side of that perfect balanced center is also quiet before you start getting the tone-up (when the coil is lowered when pumping) as you turn the knob counter clockwise, and going clockwise before you hear the tone-down as you lower the coil when pumping. A hypothetical could be where pumping, the detector goes silent and no longer produces a higher tone as the coil approaches the ground (balanced) at knob position "7.5," but if you keep rotating the control clockwise, it stays silent (balanced) until you reach "8.5", at which point you start hearing the low tone approaching the ground with the coil. That 7.5 to 8.5 position is pretty wide. Maybe if I were in extremely mineralized soil, the balance knob setting becomes more critical/precise- I don't know because I have not operated the TDI in such conditions (Australia seems to have HEAVILY mineralized soil, for instance, much more mineralized than where I've tried the TDI here in Arizona). If I read you right, you have found that the LOWEST GB knob setting (most counter clockwise) that produces silence when balancing makes the detector the most sensitive? And conversely as you rotate the knob clockwise, just as you approach hearing the tone-down when pumping is LESS sensitive? (In the hypothetical example I used above, that would mean using the 7.5 setting, as it is the lesser "barely balanced" setting you describe?) Of course Gen 1 does not have a vernier GB, perhaps because the GB is not to touchy (critical) in finding the quiet GB position. Having not used a SL, I am guessing here. Another question about GB: If in unmineralized soil, running GB OFF results in a single tone response to all metals, but they say it makes the detector more sensitive to deeper targets. The criteria for turning it OFF is that when attempting to ground balance, there is NO position at which you can null out the detector, meaning there is no balance point by varying the knob position w/ the GB switch ON. If you CAN ground balance in GB ON (meaning you have mineralized soil), the manual says you "must" use GB ON. My question is, In MILDLY mineralized soil where you can find a ground balance (twisting the knob does something), but it's not so mineralized to where a VLF cannot be used effectively (indicating not BADLY mineralized ground), will running with GB OFF be productive at all, or more sensitive/deeper than with it ON? I apologize if my questions sound "muddy," but trying to put into words in text that precisely mean what I am getting at is difficult. It's one of those situations where it takes a thousand words in print to say what you could describe in person in about 15 seconds.

Thank you Steve for your excellent explanation of the condition! I know you have a LOT of experience specifically with the TDI from literally years of ueu (as well as most machines around), and since that's what I am using because I cannot justify a more sophisticated PI machine right now, I was wondering how those of us still using that detector can minimize that effect and use it more effectively. Most of us I suspect are running TDI with GB at around 8 and delay of 10us. I know that not using GB can be used in soils that are not badly mineralized for greater sensitivity. In soils that are not highly mineralized (soil that allows effective use of a VLF machine, for instance), it is better to run on GB Off, or to in GB and use less or more than the usual "8" that balances out perfectly in most conditions, ie. purposely running a bit unbalanced)? In other words, from your vast experience, are there settings other than those typically used that can "trick" or "fool" the detector to see things it would ordinarily not see (and mask) by varying adjustments that are favored to get a quite, stable threshold and 'quiet running'?" If running GB, would under balancing, or over balancing it by a, say, 1/2 or 1 unit result in better detection of small gold (at the cost of noisier or less stable operation). Or is the best solution with the TDI just to operate in proper GB and "ALL" conductivity and accept that you're just going to miss certain size/shape/character pieces of gold and call it good until one can afford a more modern and more sophisticated detector? I guess if I could better visualize the chart you provided I wouldn't ask, as the chart you likely contains the answer, but on the other hand maybe there's a practical-compromise setting that summarizes the best overall performance (minimum blind spot) without multiple passes at different varying settings. I am not addressing other PI detectors w/ regard to settings because I haven't used them, but if you have suggestions for other common machine settings to get the most from, I know that insight would be most welcome. Being able to have and use a GPZ7000 would be great to aspire to, as for now it's likely the best of the best and most capable "one-pass" unit on the market, albeit it at a price point relatively few can justify. Thanks, Bob

One commenter to the test on YouTube by Chris Ralph mentioned the Axiom has 2 settings for balancing ground for mineralization and he used only one, so best to wait for Steve to weigh in, or to specifically post in a Garrett Axiom Forum at this site or others? I know when I spoke to Miner John, he was enthusiastic about the Axion, and he was a contributor to Whites for their design and testing, so should be knowledgeable. He suggested the Axiom as a worthy upgrade for my no longer state of the art TDI. I certainly do not want to in any way denigrate the detector given I have not seen or used one. I am completely agnostic, and just trying to learn myself.

I do not own or have ever tried an Axiom, but in a video test by "Chris Ralph, Professional Prospector" of an SDC2300 and the Axiom, both seemed comparable on small gold, but the Axiom was MUCH more affected by hot rocks than he SDC. 'Blind spot' was not addressed in his comparison: (Note he also demonstrates the GM1000 VLF on mineralized soil and hot rocks, but it's the Axiom vs SDC2300 ($600 Cheaper than Axiom) that is operative if his test is representative and ability to ignore hot rocks is important to the user.)

It would seem automatic variable timing/delay could be the easiest to incorporate (if that is a variable that improves more thorough detection), and would preclude multiple passes over the ground, so it makes me wonder why they have not done it. Twisting a knob to select delay is kind of like having a knob like really old cars had on the top of the windshield to move the wiper blade. Now with microchips we have rain sensors that completely sense the rain, determine the intensity, select the speed of the wiper, and the speed/spacing of sweeps, and shuts off when the rain stops. I mean the GPZ 7000 listed for $10,000 (now $8500 it seems). That's a LOT of money; for that price it should have wings and fly. I paid that for a PA18/150 Super Cub I had in Alaska not THAT long ago. You can practically buy/build a Super Computer for that kind of money https://www.asianscientist.com/2017/09/features/build-your-own-supercomputer-1000/ . Software these days can work wonders. The VLF Legend has multiple programs (There have been 11 software versions released, which are user downloadable, as user suggestions are adopted.) with virtually a huge list of user adjustable parameter, including multi simultaneous frequencies, or selectable single frequencies, all for $400 (machine with one coil or $600 with 2 coils, aux lithium battery, bluetooth headset and submersible to 10ft) , and it's not alone. Most new VLFs are amazing, and I cannot believe that PIs couldn't benefit from this kind of magic. Sorry for the rant, but I suspect that more is possible in PIs these days. Not enough competition, or large enough market. I know there are detector designers/engineers that frequent the Forum. I'd love to hear their opinion on that.

Having been well behind the times by only recently purchasing a beautifully kept TDI, I did not discover that, though I did read (recently) posts where people just said they were "interchangeable," though it didn't mention it was one-way compatible until Miner John told me when he mentioned I should buy ML coils going forward rather than TDI coils as when I upgraded to an ML it would save me money and grief.

Is it known what size/shape range it ignores? Or does it depend upon the coil used more than the machine itself? Is it a function of the delay I wonder? It would be interesting if there were a way to "sweep" the delay, so that you could have it rapidly sweep +/- 20% above/below the selected delay setting. I've read that even the difference from 10us (the optimum for small low conductivity gold) to 13us can make a big deal in "seeing" things. Where I hunt in desert, 90% of "junk" is lead pellets or bullets, so stuff like tin foil or pop-tops wouldn't pose a bigger problem. Obviously I am not a detector engineer-just a rhetorical curiosity. Anyone ever detect an area at say 10us, and then again at 13 or 15us to see the differences in detecting ability in the field or a test garden I wonder? More than 50 years ago I was an electronics engineer before changing careers, and back then it would not have been easy to accomplish that (analog and digital microcircuits were pretty new then), but today it would likely be relatively easy. If changing delay slightly would catch more targets close to the nominal ideal delay (without finding mostly junk) it would be interesting (but beyond my obsolete capabilities, unfortunately).

Though I have not yet received my 8X6 Sadie, from all the glowing reports, it's obviously well thought of. Perhaps John's mention that it doesn't work as well on the TDI is more of a comment about the lesser ability of the TDI detector itself, rather than a degradation of the coil's ability when used on the TDI. As Steve mentioned, the TDI is nowhere close to the abilities of a GPX5000. I haven't used a GPX, but I can only imagine the 6000 and 7000 are even better. Interestingly John mentioned a quirk of the GPX6000 (I think he said) that I had heard about with at least certain PI machines. It is the tendency for them to completely ignore certain sizes or shapes of gold nuggets, finding larger, or smaller gold with no problem, but completely ignoring nuggets in the "donut hole" detection sizes. He mentioned he found this after thoroughly going over an area with the 6000 and cleaning it out, only to go over the area with the 7000 and easily turning the previously ignored nuggets up in depths easily exceeded by the 6000-it just was blind to them. I wonder if the TDI exhibits the phenomenon with certain target sizes? I don't have enough experience with it to have a clue.

As to a difference between Razorback Coils marked "TDI" vs "ML," last week I spoke with Miner John who very graciously took a good deal of time educating me on the different coils available for he TDI. One of the things he mentioned was that he has found that because the ML detectors put out considerably more power (more amperage to the coil), they can easily blow out coils evidently using higher gauge wire, or other components in the TDI coils. He therefore did not recommend using TDI coils on a ML detector. On the other hand, ML coils like the Sadie work well on the TDI, but since they are designed for the higher power ML, they won't perform as well on a TDI. Now, I did not specifically ask whether Razorback coils made for ML work as well on TDIs as do the ones specifically made for the TDI. In other words, maybe what allows the ML coils take the additional power (Different wire of lower resistance could mean more turns to have the same coil impedance?), degrades performance in the TDI? I am not stating a fact as far as HOW the ML coils can take more power, or whether ML coils are less effective on TDIs than TDI coils of the same make and size. Just conjecture on my part. But he DID say that MLs can blow out TDI coils. I am not sure whether if a coil blows out, it affects the detector output transistors in a ML since the pulse voltage would likely be much higher without the load of an intact coil. Again, I didn't ask him. He did say that after taking over production of Razorback coils, and after disassembling several, he noted significant design flaws in them, and modified the ones HE produced to eliminate these flaws, so are much better. Though he is OUT of the coil building business for now at least (due to the forest fire that took his place), he suggested obtaining newer, improved Razorback Coils with labels including "Miner John" on them, which I guess he made along side Miner John design coils (not sure if he just stopped selling them as Razorback at some point, and continued production as Miner John, but at least SOME Razorbacks have Miner John on them, as do some improved "Jimmie " mono coils which he manufactured for Jimmy Siera for the TDI)

I don't have any experience with the TDI yet, so I am sure you're correct. In reality, the ability to see 0.5gr targets just would demonstrate extreme sensitivity, though practically, looking for pieces that small, other than indicating the presence of gold in the area, worth about $2 each would be economically unproductive. In spreadsheet presentation of Korelian's data, it's obvious you are on the money with your statement.

Yes, I read that Thread and it's very thorough! I am amazed at he information volunteered here and the other MDing Forums, and much appreciate it! In some cases, it's difficult to make direct comparisons as there are SO MANY different variation of coils. Korelian mentions a 'Razorback Longreach 10X6 Mono, which I haven't found anywhere, while I found a Razorback 8.5X11 Mono TDI locally, which is close size wize, but could be different electrically and perform very differently perhaps. SO many Coils, so little time (and money). I just figure since I am using a "2nd Class machine*," I would need a 1st Class Coil" to get the most out of it to maximize results. I kinda' figure it's like tires on high performance cars- Best tires on a lesser vehicle can trump a superior car with on tires. *Compared to an ATX or Minelab PI Bob PS Based on many recommendations on the Forums, I did the other night purchase a new Sadie 6X8 directly from Australia. I'll be eager to see how it performs when I get it. I can borrow the Razorback 8.5X11 TDI coil to compare, and always sell the Sadia if it isn't as good (Miner John mentioned in an email that the Sadie 6X8 was GREAT on Minelab but not so much on the TDI, but of course there are many variables (soil, mineralization, etc) so I will compare them on he ground here. But thanks and I appreciate your suggestions very much. Bob Korelian Data: 0.5 grain gold coin. Winner is Razorback Longreach 10 x 6 mono at 27cm, second goes to Nugget Finder Sadie at 26cm.

Very cool. I have not seen or used the SL, but I know they are better balanced and of course lighter. On the original TDI, if you have not used one, the ground balance and threshold are not all that sensitive to need a vernier or "fine tune" control, so I guess Whites altered the TDI to create the TDI SL is many ways. Not sure if the original TDI has had mods to make it a better or more sensitive detector as I have not seen any mentioned on the Forums. I've heard of the TDI Pro, TDI Beachmaster, and the TDI 'OZ,' but I know nothing of these sub-models technically. Thanks again, Bob PS I received an email from Miner John, and he no longer makes coils for the TDI, and cautioned that the 6X8 Sadie coil is GREAT for the Minelab machines, but performs rather poorly on the TDI. I don't know how that compares to your results. So many variables in such tests. He recommended a Miner John 12X8 Folded Mono MJD for the TDI if I could find one used on the Internet as being superior to others available. (Not sure what MJD means, though).

Thanks, I edited my post to credit the OP as well as directly link any viewers to the thread in which he kindly posted his results. My failure to observe this courtesy was inadvertent, though obviously discourteous to the gentleman. Bob

Maybe that is over optimistic on my part, though I reviewed some TDI tests of a gent in Australia who tested about a dozen different coils, and one of the tests he did involved a 1 grain target at least, as you say your SL will do reliably. I didn't know whether a 1/2 gr target would register proportionally weaker and at reduced depth, or not at all. As I (should have) mentioned, I have VERY little time with the machine so far because I live where every trip to where I prospect is a 1+hr drive directly through Phoenix, so until I get a good 4X4 w/ and overland tent on it so I can spend multi-day outings, I am doing as much research as I can about the capabilities of the detector. This (below) is a chart for a 1 grain nugget with various coils. I HAVE the Jimmy 6X10 1/2 Dual Field Mono Coil which claims about 3" depth, while the 6X8 Nugget Finder "Sadie" 6X8 saw the target at about 6", which is quite an improvement. Since a lot of sub-grain items I was previously finding were at 2" depth (mostly tiny shards of lead), I was thinking the best coil I could find would provide a useful improvement over my VLF even taking into account in easy ground a VLF is more sensitive to small targets. What coil are you using that you find decent and useful with 1 grain targets, and at what depth can you detect them? Thanks- Bob BTW, Do you have the 16.8v battery mod, or maybe also the electronic component modification? I understand those mods make he SL about equal to the original TDI. I hear/see a TDI "OZ" claimed to be higher power, but I don't know that for a fact. I guess those were exclusively sold in OZ, or maybe just got a sticker put on them there by the distributor? According to karelian who posted his test results on the TDI w/ varios coils at link: https://tinyurl.com/25vv2yt8 Thanks to him for the useful information!