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**Carl-NC** · 04-10-2026, 06:42 AM

I think this would be an excellent project. IMO, the MXT was about the pinnacle of single frequency and is still competitive. We now have several all-analog VLF designs but very few digital projects. There are a lot of possibilities, but I would start simple and build on that.

**ivconic** · 04-10-2026, 06:57 PM

Do I remember correctly, that model has mixed mode operation?
That would be a really great project for the forum.
The focus should be shifted to the electronics of the detector itself, to the design, other mechanical and cosmetic things are less important.
How realistic is it to get to the original schematic?

**ivconic** · 04-10-2026, 07:29 PM

**KRinAZ** · 04-10-2026, 08:40 PM

Originally posted by ivconic View Post

Do I remember correctly, that model has mixed mode operation?
That would be a really great project for the forum.
The focus should be shifted to the electronics of the detector itself, to the design, other mechanical and cosmetic things are less important.
How realistic is it to get to the original schematic?

Yes it has 3 modes: Coin and Jewelry mode, Relic mode, and Prospecting (gold nugget) mode, in addition it has in effect 2 "modes" of gain/sensitivity and in effect 2 "modes" threshold/SAT (for Prospecting mode), and quite good discrimination (Jewelry/Coin and Relic modes).
The Jewelry/Coin and the Relic modes focus more on discrimination and target type (type of metal), and also on likely target depth.
The Prospecting mode focus more on detection depth and sensitivity for small gold nuggets, without sacrificing detection of larger and deeper gold nuggets (silver also). In practice I have found that even in Prospecting mode the visual display of probable metal type is quite accurate. It now occurs to me to wonder if those modes have different TX frequencies...hmmm...

I completely agree on the electronics focus, I think this project would ideally accomplish:
Reuse the entire hardware from dead White's GMT (the latest GMT versions's hardware is identical to the MXT except the coils) or MXT detectors.
Reuse MXT (White's Eclipse) coils
Replace the main board and it's housing
Replace the display with a more modern one

I have searched around at various times and found the White's Engineering Report on the MXT, by Dave Johnson if I remember correctly, but not the schematic. I'll keep hunting. My understanding is that Dave Johnson considered the GMT and MXT to be his best work. He also developed the Tesoro Diablo UMax (MicroMax), another legendary VLF detector and an early digital VLF...schematics to it may prove handy as well if they can be found...I am under the impression that he worked out his excellent GB microcode on the UMax while at Tesoro and refined it on the GMT/MXT while at White's.

For my purposes I only use/need Prospecting mode, but I know others will desire the other modes.

I digress in relating this story - that sealed my love for the MXT detector - in 2016 I was hunting the nugget fields near Quartzite, Arizona, USA with my MXT with 6x9 DD coil and encountered a fellow with a MineLab GPX 4500 with 11" round concentric (I believe) coil, who had earlier in the day found a small (sub gram) gold nugget. Out of interest we did an ad-hoc shootout: we buried his nugget in a very hot iron creek bed at increasing depths starting at 5 inches, each set their detector for max detecting of this nugget, and both of our detectors reliably detected the nugget down to 9.5" measured (both gave sketchy but diggable signals at this depth), neither reliably detected at 10 inches. We did the same with a large lead bullet (don't remember the weight) I had found and had similar results down to 13 inches. From this I deduced I would only see an advantage to using a PI in areas with large deep targets.

**ivconic** · 04-10-2026, 08:44 PM

Very good post! Interesting. One can see that you are related to the machine. I am interested in seeing the chematic. But it is very hard to find.

**KRinAZ** · 04-10-2026, 08:47 PM

Originally posted by ivconic View Post

Nice pic, would this be the MXT control board? If so, wonder if it's possible to download the firmware from that PIC controller?...

**KRinAZ** · 04-10-2026, 09:06 PM

I believe this is the complete text of the MXT engineering report:

Whites MXT Engineering Report By Dave Johnson

by David E. Johnson

This Engineering Guide is written to provide dealers and customers greater insight into what kind of product the MXT is, from an engineering perspective. It does not attempt to provide complete information on the features and use of the MXT for that, Please consult the MXT user's manual.

A BIT OF HISTORY
In January 1998, White's decided to develop a true multipurpose metal detector, with the kind of sensitivity it takes to be a real gold prospecting machine, and with computerized ground tracking for ease of use. White's in-house engineering staff was tied up on the project which eventually became the DFX. Therefore, in February White's asked me if I might be interested in taking on a new protect. I had a good track record on gold machines so it seemed like a good fit. I agreed.

A month later at a dealer seminar in Sacramento, California, Jimmy Sierra announced the project, and said if the engineer didn't deliver, the engineer would have to go into hiding in Mexico. I was sitting in the back.

We ran into one problem after another along the way. Jimmy, good chap that he is, didn't sic the thugs on me, though there were periods he was frustrated enough that the thought must have run through his mind. Although the project took longer than we expected we got two products out of it- the GMT and the MXT.

The first major hurdle was to get the basic circuit and software system running, with a firstclass ground tracking system. The system architecture was totally new, not a revision of the existing Goldmasters. In early spring of 1999 an ugly prototype was up and swinging, and Larry Sallee became involved in field-testing. By April the ground tracking system was working so well that sinse that time very few changes have been needed. At that point we knew we had a solid foundation, so work began on the display, discrimination and target ID features.

During the fall of 1999, we decided to tackle the problem of desert heat head-on. A lot of gold prospecting is done in desert heat in full sun. I set up a crude but effective, thermal engineering laboratory, measuring the temperatures reached inside housings of various configurations and colors in full sun. Then began the task of finding an LCD, which would handle the heat. Because the LCD display is an important feature of the MXT, we revisited the whole issue of display. The manufacturers of LCD display had expanded their product offerings. We found a larger one, and changed the mechanical design of the MXT to accommodate it. A FSTN 0160 F was selected, (There are more to chose from nowadays, so we used a bigger one than the GMT in the MXT.)

In early 2000, White's decided to bring out a new Goldmaster based on the work that had already been done, while development of the multipurpose unit continued. So we modified a prototype to work with the Goldmaster search coil at about 50 kHz, and you know the rest of that story- the GMT "tracking Goldmaster" was introduced in early spring of 2001.

As work continued on what eventually came to be called the "MXT", we spent a lot of time on the discrimination and target ID system. There are many different ways to do discrimination and ID, each with its own advantages and disadvantages, which aren't always known until you've had the thing in the field being tested for a while. A lot of work got thrown out as we found deficiencies in what had already been done, and discovered ways to improve things. As the project got closer to production, more people became involved with it, and offered their own ideas to improve it. The reason the MXT is as good as it is, is because of that long process of field-testing and revisions.

While the MXT was still under development, the DFX was introduced. The MXT design was then revised to run at about 14 kHz in order to take advantage of the DFX loops. When it was finally time to call it "good" in June 2002, the MXT went into production quickly and smoothly.

CIRCUlT DESIGN
The circuitry of the MXT is almost identical to the GMT, which has already been on the market for a year and a half and has proven to be rock-solid.The GMT's circuitry broke a lot of new ground. It uses a reactive impedance transformation network to boost transmitter voltage for higher sensitivity. It uses an active transmitter regulator to keep transmitter voltage constant even when the search coil is moved over black sand that would blow an unregulated machine off the air. The differentiator-filter circuits usually found in metal detectors are eliminated. Those functions are now done in software, which is made possible by the use of a high-precision 16-bit A/D converter used in a way that makes it equivalent to 17 1/2 bits. All the controls are digitized, their function actually performed via software rather than in circuitry. The audio system is temperature compensated in software to eliminate threshold drift.

For the MXT, we chose an operating frequency of 13.889 kHz. This is high enough to give good sensitivity to gold, low enough to give good target ID on typical coin, trash, and relic targets, electrically compatible with search coils derived from the DFX and halfway in between power line harmonics to minimize electrical interference.

SOFTWARE
The MXT uses a Microchip PIC 16C76 micro controller, chosen for its low power consumption and its set of features, which was a good match for this application. The software that runs in this chip is based on that in the GMT, but almost all of it is new or has major revisions, except the device drivers and the ground tracking system. Much of the new software is for target ID and discrimination, features that were not present in the GMT. Even the iron probability and VSAT systems in the MXT are new, decpite their apparent similarity to the GMT. The MXT/GMT does as much of the signal processing as possible in software rather than in circuitry, using what we call "low-speed DSP architecture". The demodulated signals are digitized, and processed and analyzed in software. Control positions are also digitized and made part of the data in software. The desired audio signal is computed, and then converted back to voltage using a 12-bit D/A converter. The circuit board communicates with the LCD and trigger switch in the "pod" via a custom-designed serial link.

In the MXT the filters, differentiators, and sample-and-hold functions are performed in software, not in circuitry. This eliminates the problems of channel mismatch and drift, which are often encountered in such circuits.

The discrimination system is a second derivative ("two-filter'') design for quick response over a broad range of sweep speeds. The analysis system for determining what kind of target is present has special features which reduce interference from ground minerals, and which automatically scale target ID confidence according to the mineralization level.

GROUND TRACKING SYSTEM
The ground tracking system comprises two subsystems: a ground analysis engine, and a ground balancing system. The ground analysis engine continuously monitors incoming signaIs to determine whether the signals probably represent ground, or may be something else such as metal targets or electrical interference.

Signals, which seem to be ground only, are put into a data analysis subsystem, which analyzes the data for a number of variables. Then it can be determined what the balance point of the ground matrix is and how fast that balance point is changing. It'd be nice to describe all this in detaiL but we'd rather not teach our competitors how to do it. The ground balance system does the actual balancing of the signals, doing in software somewhat the same job as a ground balance knob does on a manually balanced machine. When the TRAC toggle is in the "ground" or "salt" positions, the ground balancing system follows the output of the ground analysis engine. When the toggle is in the center "lock" position, the ground balance subsystem stops following the output of the ground analysis engine, which is still chugging away in the background continuing to gather ground data.

The ground analysis engine can do a good job of telling the difference between ground matrix and anomalies such as hot rocks and metal targets. In order to tell the difference, it has to see matrix by itself during at least part of the sweep. When you're not in "lock", keep your sweeps broad, and don't loiter over the top of a target when checking it out. Otherwise the analysis engine may lose the ground matrix and start tracking into the target. However, if the target is strong enough to register on the VDI readout, the target ID system will tell the analysis engine to halt, allowing you to check the target without tracking into it. In all three programs, pulling the trigger to pinpoint a target also tells the ground analysis engine to halt. Some users will hunt with the tracking toggle in "lock", occasionally updating the ground balance by flipping into "ground" or "salt" momentarily when they start hearing too much ground noise.

The resolution of the ground balancing system is 1 part in 4,000, and most of that resolution is concentrated in the range where high mineralization occurs. Therefore, the individual resolution steps are below audibility under all conditions.

THE VSAT SYSTEM
The VSAT system on the MXT is similar in a general way to the one on the GMT. The VSAT function is done entirely in software. Up to about 2/3 rotation, the SAT is of the conventional (first derivative or auto tune) kind, giving a "zip" sound on a nugget and a "boing" sound on a negative hot rock (cold rock).

As you approach maximum rotation, the MXT goes into "HyperSAT". HyperSAT is a completely different type of SAT system with different sounds and target responses. The background threshold sound is a little rattier, but nuggets are crisper, the ground is quieter, and negative hot rocks vanish when you slow down your sweep. For all but the most experienced users whose ears are calibrated to hear every little nuance of a regular SAT signal, HyperSAT gives more effective depth in bad ground than normal SAT.

THE DISCRIMINATION CONTROL
The discrimination control does pretty much what you'd expect. Unlike some discriminators, when the control is at zero, there is no discrimination at all-i.e., "true zero discrimination"-and all targets will be detected. Below about 2, the discrimination is based on a combination of both signal phase, and signal strength relative to the strength of ground mineralization. This feature allows the user to get good rejection of shallow iron with minimal loss of deeper targets.

THE GAIN CONTROL
The gain control knob controls two things at once: the preamp circuit gain, and the software gain. The following is a simplified explanation which is not technically correct in all its details, but will serve to give a general picture how the gain control works. As you advance the gain control from 1 to 10, the preamp circuit gain steps through five levels of gain: xl, x2, x4, x8, and x16. On most machines (depending on minor variations in search coil alignment) you can hear a momentary blip as the machine switches from one gain level to the next. The recommended preset (marked by the triangle) corresponds to a preamp gain of x8. In mild ground conditions where there is no electrical interference, you may want to advance the gain control into the crosshatched region. In this region, the signal data in software is multiplied by successively larger numbers, increasing the loudeness of the signals. It is somewhat similar to the "audio boost" function found on some other models of metal detectors. It's particularly useful if you're using the speaker rather than headphones and there's a lot of noise from traffic or wind, or if you're demoing the machine to someone else.

BASIC SENSlTIVITY PERFORMANCE
Since this is a multiple-purpose machine, a U.S. Nickel coin is the most appropriate standard test target. With the gain cranked up, and in the absence of electrical interference, a nickel will typically "air test" beyond a foot using the standard 95O search coil. Your actual "air test" distance will depend on your hearing, the sweep speed, what search coil is used, how much electrical interference is present, and how you have the controls set.

In comparison to other machines in this price range, the MXT is extremely hot on low-conductivity items. On gold, it's right in there with the more popular gold machines, being especially hot on the larger, deeper nuggets. It will compete with all comers on low-conductivity , relics and on nickels. On high-conductivity coins such as quarters and silver dollars, it is still an excellent-performing machine, but there are several other products in the same league for sensitivity.

GROUND TRACKING PERFORMANCE
The ground tracking system is nearly identical to that in the GMT, which is widely regarded as one of the best tracking systems on the market. Compared to most other trackers, the MXT has superior resolution, tracks faster, "jumps" into new ground more quickly, has greater resistance to tracking into targets, and tracks over a wider range of soil conditions. The MXT allows tracking to be inhibited if desired.

DISCRIMINATION PERFORMANCE
All discriminator designs are compromises. Here's how the discriminator in the MXT stacks up against other machines.

AIR TEST "DEPTH": generally well beyond 10 inches, because of high sensitivity, with effective discrimination to within 0-3 inches of the basic air sensitivity of the target. Most discriminators will discriminate in air to within 0-3 inches of the target air depth on most targets, but most don't have the sensitivity of the MXT.

QUICKNESS & TARGET SEPARATION: among the best, because of medium-speed second derivative ("two-filters") design. Initial field reports indicate that the MXT's mixed-mode tone system gives indication of adjacent ferrous/nonferrous targets, superior to that obtainable through discrimination.

IRON REJECTION: Because of its high sensitivity and a slight preference in the software for not losing questionable targets, it'll be a little chattier than some less sensitive machines. Reducing sensitivity by cutting back on gain, or by reducing the threshold control setting to minimum, will help quiet it down when necessary.<

DEPTH IN MINERALIZED GROUND: Although the MXT is a two-filter system, it incdrporates special techniques which reduce ground interference and which reduce the "chopping & popping" which plague most other two-filter machines. This, together with its high basic sensitivity, makes it an excellent machine from the standpoint of discrimination depth.

FAST SWEEPING: Many discriminators tend to lose good target signals, even shallow ones, when quickly sweeping the search coil. The MXT is tolerant of moderate search coil sweeps, that is to say good at both faster and slower search coil paces.

SUMMARIZING: The MXT has the responsivenes and sensitivity of a first-rate 2 filter machine, combined with the discrimination accuracy of a first-rate 4-filter machine.

TARGET I.D., ETC.
With its small medium and large blocks on the target ID screen, the ID system in the MXT bears a superficial resemblance to the "Signagraph" of the Spectrum XLT. It should be realized that the traditional White's Signagraph system is typically (optionally) set to accumulate data over multiple passes over a target, and displays the accumulated average. The MXT displays fresh data on each pass and scales the size of the block according to how strong the signal was relative to the ground conditions on that specific pass over the target. The visual ID system on ths MXT is fast, easy to read, generally more accurate than the discriminator, and gives a visual indication (via block size) of how reliable the identification is. It is going to change the minds of many beeper enthusiasts who previously thought visual ID to be of little practical use outside typical coin shooting.

THE CASCADE OF EFFECTS OF GROUND BALANCE SETTING:
In order to know what the ground balance setting is; flip momentarily to the gold program if you were in another mode.

Electronic ferrite material and most "negative hot rocks" (cold rocks) will usually read in the 75-88 range. Most soils will read somewhat lower. Readings will almost never go below 25 except in salt or moist alkali soils. When readings indicate smaller numbers than 50 you may notice some reduction in sensitivity. Below 35, some rusty iron may give unpredictable responses. Below 25, iron objects may give unpredictable responses and/or may disappear entirely and the sound on nonferrous objects may become slightly more abrupt.

MANY THANKS
To Kenneth White and Alan Holcombe for having sufficient confidence in me to put food on my table through the good times and the rough times on this project. To Jimmy Sierra for having the patience to argue with me about all the stuff that needed arguing about, for being so passionate about the need for this product, and for being willing to compromise when that's what it took to keep the project moving. To Larry and Sue Sallee, for their personal hospitality and for field-testing prototypes. To Keith Zorger, Randy Smith, Mike Brighty who field-tested and helped develop the MXT. To Bob Canaday, for being such a competent technical/engineering liaison, doing a lot of no glorious but necessary work well and managing the project during its sometimes difficult phases. To Rick Maulding, for overseeing the project, for technical contributions to the discriminator and to the salt system, and for committing White's engineering department's finest minds to engineering review during the "slow SAT isn't hot enough" crisis, which led to a major system revision that made the whole machine better. To John Earle and Dan Geyer, for diligently hacking away at problems until they became non-problems. To Steve Howard and Pam Godell of White s. There were other people involved in this project whose contact was primarily or exclusively with White's and not with me. The risk of printing credits is that one may inadvertently omit a name that belongs there; so, if I missed someone whose name belongs on this list, I'm sorry, it was an unintentional oversight.-D.E.J.

**PetraDiamond5** · 04-11-2026, 05:23 AM

There are some features in the Goldmaster GMT that were left out of the MXT in gold prospecting mode that need to be included in a new updated MXT. (Ground Measure, ie ground mineralization strength).

I owned the GMT for more than 12 years before I even knew what GMT stood/stands for (Ground Measure Technology). "What the heck is Ground Measure Technology good for anyways ?".

Same thing with the DFX... there are some features in the DFX that would benifit a new MXT All Pro re-incarnation. Plus we could add some new utra-high-tech features not in any of these three detectors like advanced target seperation technology simular to what we see in the Manticor's 2-D indication mapping system and even beyond what we find in the Maticore.

The 13.9 kHz on the MXT is good for larger peices of gold deep say below 4" but will miss smaller peices of gold above 3" that the GMT will hit.

The GMT with its high 48 kHz frequency will struggle in higher mineralization and simply will eventually not work in hotter soils no matter what dials we turn or settings we select. The newer Goldmaster 24K and GMX were suppose to fix all of that, but like the GMT the 24K puts up a deceint performance in the air, but in highly mineralized soils... just cannot punch deep enough into the mineralized goldfield soils to hit the larger deeper targets we all want.

Simply put the GMT needs more coil energy and a lower frequency (say 24K). The newer Goldmaster 24K needs a lower frequency and the 14" double D coil to punch deeper into higher mineralized soil.

The MXT needs a multi channel system that can run a 6.9 kHz frequency for deeper gold and silver targets, 13.89 kHz for general use and a 24 kHz channel for gold nuggett prospecting.

This may well be outside the limits of the current stock coils including the aftermarket coils created later for the DFX and MXT.

But since there is a limited supply these origional coils... a new rebuild would have to include a new modern DD multi frequency compatable coil system and different sized coils.

...and obviously a new MXT would have a volume control and be waterproof like the MX sport and would have an updated filtering system for mineralized goldfield soils and a new powerhouse ARM processor for better target seperation.

To do it right would require the support of Garrett who ( in my opinion never going to develop a new Whites MXT that would just absolutly cannibalize there entire VLF line including the 24K.

On the DFX, there are two analog ground mineralization filters and 4 additional ground filters in the software that can be turned on or off by the user and this ability was not in the MXT or the GMT.

The MXT and GMT have both analog filtering circuitry and filtering in the software plus a ground balance Auto-Trac feature which even today is still well respected across the industry.

When hunting for gold nuggets in Washington, Oregon and Colorado and anywhere else where the iron mineralization is considerably high... We better have a pulse induction detector on stand by that has both an ability to hit larger deeper peices of gold and also be able to hit smaller peices of gold shallow.

For me... I have a new in the box TDI SL with the two stock coils and the newer Nuggettfinder MK-2 Sadie coil ready to go when the ground gets to hot for my ORIGIONAL LOS BANOS GOLD BUG and the newer Minelab Manticore running in multi-frequency gold mode... which combined are two of my more prized gold finders.

My guess is the newer Minelab SD 2300 will easily upstage the older TDI SL if anybody wants to spend the money... especially or defenatly when working around and in water.

Before I go all in on a total remake and update of the MXT/GMT... I would like to first explore options that will alow the MXT to hit smaller peices of gold shallow with the current stock Whites Eclips double D shooter coil without going all out hog wild on a bunch of big complex modifications for the MXT.

Because at the end of the day...

We still wouldn't get the target seperation of the Manticore, but running the MXT in relic mode for gold does cut out a decient amount of iron trash which is still better than nothing but still not even close to what we all want in a modern gold detector.

Ideally what we all want are vlfs that can run in hotter iron goldfield soils and still hit small gold shallow and also hit deeper gold all on the same pass and also seperate small gold from all of the small junk targets that makes detector prospecting unbearably frustrating and unproductive.

Because highly mineralized soil clobbers the depth performance of vlf detectors (like the GMT) they may never be able to hit deeper gold targets in highly mineralized soil without some currently unknown technology.like lowering the frequency back down to 6.59 kHz.

Right now and for the forseeable future hunting smaller peices of gold shallow and larger deeper peices of gold on the same pass with a vlf is gona be relegated to milder soils like many of the soil types we see in places like Arizona and in Southern California.

The newer Nokta Legend II is suppose to handle hotter soils better than the origional Nokta Legend plus better stone age VDI target seperation to boot.

The Manticore is a huge step forward in the direction we all wish to go for vlfs and to a lessor degree so was the Equinox 800 and 900... but these detectors still fall short of what is possible with todays newer advanced microchips, miniturized electronics and advanced software technology.including neural network computing and fuzzy logic.

Special thanks... not only to David Johnson and everybody at Fisher and Whites, but also all of the briliant engineers worldwide who have collectivly brought us all to where we are today.

"Now let's go find some gold!"

**KRinAZ** · 04-11-2026, 07:30 AM

Originally posted by PetraDiamond5 View Post

There are some features in the Goldmaster GMT that were left out of the MXT in gold prospecting mode that need to be included in a new updated MXT. (Ground Measure, ie ground mineralization strength).

I owned the GMT for more than 12 years before I even knew what GMT stood/stands for (Ground Measure Technology). "What the heck is Ground Measure Technology good for anyways ?".

Same thing with the DFX... there are some features in the DFX that would benifit a new MXT All Pro re-incarnation. Plus we could add some new utra-high-tech features not in any of these three detectors like advanced target seperation technology simular to what we see in the Manticor's 2-D indication mapping system and even beyond what we find in the Maticore.

The 13.9 kHz on the MXT is good for larger peices of gold deep say below 4" but will miss smaller peices of gold above 3" that the GMT will hit.

The GMT with its high 48 kHz frequency will struggle in higher mineralization and simply will eventually not work in hotter soils no matter what dials we turn or settings we select. The newer Goldmaster 24K and GMX were suppose to fix all of that, but like the GMT the 24K puts up a deceint performance in the air, but in highly mineralized soils... just cannot punch deep enough into the mineralized goldfield soils to hit the larger deeper targets we all want.

Simply put the GMT needs more coil energy and a lower frequency (say 24K). The newer Goldmaster 24K needs a lower frequency and the 14" double D coil to punch deeper into higher mineralized soil.

The MXT needs a multi channel system that can run a 6.9 kHz frequency for deeper gold and silver targets, 13.89 kHz for general use and a 24 kHz channel for gold nuggett prospecting.

This may well be outside the limits of the current stock coils including the aftermarket coils created later for the DFX and MXT.

But since there is a limited supply these origional coils... a new rebuild would have to include a new modern DD multi frequency compatable coil system and different sized coils.

...and obviously a new MXT would have a volume control and be waterproof like the MX sport and would have an updated filtering system for mineralized goldfield soils and a new powerhouse ARM processor for better target seperation.

To do it right would require the support of Garrett who ( in my opinion never going to develop a new Whites MXT that would just absolutly cannibalize there entire VLF line including the 24K.

On the DFX, there are two analog ground mineralization filters and 4 additional ground filters in the software that can be turned on or off by the user and this ability was not in the MXT or the GMT.

The MXT and GMT have both analog filtering circuitry and filtering in the software plus a ground balance Auto-Trac feature which even today is still well respected across the industry.

When hunting for gold nuggets in Washington, Oregon and Colorado and anywhere else where the iron mineralization is considerably high... We better have a pulse induction detector on stand by that has both an ability to hit larger deeper peices of gold and also be able to hit smaller peices of gold shallow.

For me... I have a new in the box TDI SL with the two stock coils and the newer Nuggettfinder MK-2 Sadie coil ready to go when the ground gets to hot for my ORIGIONAL LOS BANOS GOLD BUG and the newer Minelab Manticore running in multi-frequency gold mode... which combined are two of my more prized gold finders.

My guess is the newer Minelab SD 2300 will easily upstage the older TDI SL if anybody wants to spend the money... especially or defenatly when working around and in water.

Before I go all in on a total remake and update of the MXT/GMT... I would like to first explore options that will alow the MXT to hit smaller peices of gold shallow with the current stock Whites Eclips double D shooter coil without going all out hog wild on a bunch of big complex modifications for the MXT.

Because at the end of the day...

We still wouldn't get the target seperation of the Manticore, but running the MXT in relic mode for gold does cut out a decient amount of iron trash which is still better than nothing but still not even close to what we all want in a modern gold detector.

Ideally what we all want are vlfs that can run in hotter iron goldfield soils and still hit small gold shallow and also hit deeper gold all on the same pass and also seperate small gold from all of the small junk targets that makes detector prospecting unbearably frustrating and unproductive.

Because highly mineralized soil clobbers the depth performance of vlf detectors (like the GMT) they may never be able to hit deeper gold targets in highly mineralized soil without some currently unknown technology.like lowering the frequency back down to 6.59 kHz.

Right now and for the forseeable future hunting smaller peices of gold shallow and larger deeper peices of gold on the same pass with a vlf is gona be relegated to milder soils like many of the soil types we see in places like Arizona and in Southern California.

The newer Nokta Legend II is suppose to handle hotter soils better than the origional Nokta Legend plus better stone age VDI target seperation to boot.

The Manticore is a huge step forward in the direction we all wish to go for vlfs and to a lessor degree so was the Equinox 800 and 900... but these detectors still fall short of what is possible with todays newer advanced microchips, miniturized electronics and advanced software technology.including neural network computing and fuzzy logic.

Special thanks... not only to David Johnson and everybody at Fisher and Whites, but also all of the briliant engineers worldwide who have collectivly brought us all to where we are today.

"Now let's go find some gold!"

Lots of good thoughts...I think it would be wise to stick with Carl's advice in post 2 - start simple and build from there. Dave Johnson was a brilliant engineer and I think it will be a monumental task to try to recreate what he designed, and even more so to try to build upon it.
Just getting ground balance to work as well as the GMT/MXT will be quite the accomplishment.

I personally would be ecstatic to just build a functional MXT as I think it performs in the field as good as most any VLF, and as my above story illustrates, as good as a MineLab PI up to the MXT's VLF depth limitations.

FWIW the corner of Arizona I live in (generally the north side of the Bradshaw Mountains) has acres and acres of very hot soil and river bed. As in - I cannot simply set the coil to the ground with the default gain setting - if I do I just get the screaming Coil Overload, Turn Down Gain message and tone. No metal bits of any type, just the hot soil. There are outcrops with so much iron that the magnet on my pick sticks to the outcrop! Yet I can detect through and find sub gram little nuggets as well as tiny lead shot, tiny rust fragments, etc... Actually every gold hunting locale in Arizona that I have been to has very hot ground. So the MXT in hot field use performs very well - at least in the hands of someone familiar with it.
This is the area that Tesoro developed the Lobo SuperTraq to handle, specifically the Lynx Creek river bed that is so hot and known for tiny nuggets (as well as the rare lucky larger ones).

The local prospecting shop has a big tub of Lynx Creek river bed material and PVC tubes running horizontal through it so targets can be inserted down the tube, various detectors tested, and various targets detected (attempted anyway). We have spent time comparing detectors in the tub - the detectors that stand out far from the rest are:
Tesoro Lobo Supertraq (very close third best target signal, or maybe a tie with the MXT)
Fisher Gold Bug and II (sketchy target signal probably due to being difficult to ground balance over the tub)
White's GMT (the winner with the clearest target signal)
White's MXT (a close second place, second clearest target signal)
The other detectors we tried gave no diggable target indication and go bonkers with noise and ghosts.

On coils, I have all 3 of the White's DD coils for the MXT - The little 4x6 DD "Shooter", the standard 6x9 DD, and the 8x14 DD "Deep Scan", and so can test with those. I don't have any of the concentrics - the ground is too hot here to use them so I never bothered buying them, though in the past I borrowed them to try out.

**ivconic** · 04-11-2026, 08:28 AM

"...The reason the MXT is as good as it is, is because of that long process of field-testing and revisions..."- from Daves letter.
That's the most significant claim. I like it.

**KRinAZ** · 04-11-2026, 07:50 PM

Originally posted by ivconic View Post

"...The reason the MXT is as good as it is, is because of that long process of field-testing and revisions..."- from Daves letter.
That's the most significant claim. I like it.

I agree!

**KRinAZ** · 04-11-2026, 08:10 PM

Originally posted by ivconic View Post

Thx again for this pic, I've started a couple searches to try to acquire one of these boards and/or a broken MXT for research.
I'm also searching for info on how to download the code from the Microchip PIC16C76 that the MXT uses. So far I see this is the One-Time-Programmable version of the chip, that the programmer and IDE can still be found, and so far I haven't seen any code protections covered in the datasheet, but also don't yet see a method to download the code. Per the datasheet the IDE is used to compile and upload code to the PIC, not clear if the IDE has read capability from the PIC, I'm still reading the datasheet though...

And of course this project could use a more modern controller, in which case the existing code wouldn't be of much use, unless the existing code could be reverse compiled - unlikely but maybe...back in the day my first and fav language was Assembly, then C as a shorthand for Assembly...but that was a long time ago...

**Carl-NC** · 04-12-2026, 01:49 AM

The original MXT board used an OTP chip that was read-protected. It was updated when I worked there to use a later version chip (for the MXT Pro) that could be reprogrammed, but it was still read-protected. It is possible to defeat the read protection and extract the code but it's a complicated process. An easy solution is to send the board to China and one of the hacker houses will send you back the extracted binary, but it costs around $500. Might be able to find someone who will do it cheaper. The binary can be decompiled to assembly language and then maybe AI can clean it up and even convert to C.

The Chinese cloned the GMT which uses almost the exact same circuitry and mostly the same code as the MXT. It's possible the GMT clones are not protected. In any case, they are a lot cheaper to buy and cannabalize. But if you want to extract the code with them most features, it would be the MXT Pro or the MX Sport.

The original MXT was developed by Dave Johnson and all the code was written in assembly language. There were a lot of fudge factors in the code, often with the comment "this seems to work." I had a co-op student convert all the code to C and port it to an STM processor -- that was released as the MX5, and later used in the MX Sport and GM24K models.

**Carl-NC** · 04-12-2026, 01:51 AM

I owned the GMT for more than 12 years before I even knew what GMT stood/stands for (Ground Measure Technology).

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