Hi Mick,
Found some time for detailed answers to your questions....
1. How much does this unit weigh? <b>4.3 pounds</b>
2. What is "gain". Is it sensitivity adjustment? <b>Yes</b> Frequency is only part of the equation in determining sensitivity to small gold. The DFX has a 15 kHz mode, but the MXT is better on small gold than the DFX. This is because it has a very high gain circuit. The Gain control allows for more power than on most machines but if set too high will cause instability and possible circuit overload. The idea is to give you the power if you can use it but in many places you simply will not be able to run at full Gain. In a nutshell set it as high as you can while getting stable operation (minimal falsing) with a smooth threshold.
The Gain on the MXT is more complicated than the Sensitivity control on most detectors. From the Engineering Report:
"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 loudness 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."
3. Does it have a pinpoint or do you just use the middle? This is something that I really struggled with on the tracker4 and why I love the X-Terra so much. <b>The MXT has a pinpoint mode - just squeeze the trigger switch. It incorporates a depth reading.</b> In practice I almost never use the pinpoint mode unless coin detecting, where I like to use the depth reading as a discrimination aid. For nugget detecting it is just eyeball the beep.
4. Do the VDI numbers go up in ones or in groups of numbers?The picture on the control indicate groups of 10 and they're bunched up in places. <b>Single digit numeric range from -95 to 95 or 191 separate VDI numbers.</b> In general negative numbers are ferrous, positive non-ferrous, but some small non-ferrous may run into the low negative numbers. This detailed VDI range can give you great accuracy when you learn it, but the numbers tend to move on each pass. Basically you mentally average the numbers on multiple passes, and look for numbers that cluster. Strong individual readings or clusters around a single VDI number indicate a good target. VDI numbers that jump all over the place indicate trash. Not a rule, just a generality. The VDI number incorporates a strength indicator that tells you how confident the machine is in the reading.
Machines like the X-Terra 50 lump about 6 MXT VDI numbers into one segment, giving you more VDI stability but less resolution. An MXT might show you numbers that jump around in a 3-4 VDI number range that would all show as a single number on the X-Terra. This also means you can tell and aluminum item from a gold item with the MXT that the X-Terra would call the same. My CZ-70 divides the MXT 0-95 non-ferrous range into 7 segments, so about 13 MXT numbers get lumped into a single CZ category. This makes my CZ APPEAR to have very solid target locks but it also means you have much less idea about what it really is under the coil. VDI stability is a trade between target resolution and the desire to get a solid lock. people like solid locks as it appears that the machine is more certain of its target, and therefore more accurate. But it is a false sense of security. If my CZ gets a solid lock on a beavertail and calls it a nickel how is that more accurate?
5. What is the S.A.T. control? <b>S.A.T. or the Self Adjusting Threshold setting is a unique White's feature that allows you to set the rate at which the threshold sound resets itself.</b> This in effect adjusts the recovery speed of the machine but also affects the stability in bad ground. In general the higher the setting, the more stable the machine, but at a slight loss of sensitivity. The SAT is used in conjunction with the Gain as instability at higher gain levels may be offset by setting a higher SAT level. Jimmy Sierra likens setting the Gain and SAT as like adding salt and pepper to a soup. You need to adjust them both to get it just right. This control gives the MXT more ability to adjust to bad ground conditions than most VLF detectors.
6. How does it handle heavily mineralised ground? ie compared to a minelab Eureka gold.Have a friend near Mudgee,that has about 40 acres that is covered in quartz and ironstone(the hill on it, is full of diggings from the old gold rush days)and presume that no V.L.F. detector will operate in these conditions. <b>In theory the Eureka can handle more varies ground conditions than the MXT due to its adjustable frequencies and other ground compensation settings.</b> In actual use, I've never found a location where a Eureka worked better than an MXT. I can't speak for others on that, however, and maybe someone else can quote specific areas where the Eureka may have done better. My real answer here is that if the ground is that bad, get a PI unit.
7. At 14kHz how stable is the detector when coin shooting? Read a number comments about it being a bit noisy. Is it only a minor irritation? <b>The MXT is a very good coin detector.</b>Gold detectors want to hit the tiniest item possible. Coin detectors want to hit coin size targets while eliminating trash. Items smaller than a dime would normally be trash items when coin detecting, and so designing a machine that picks up all the tiny stuff would make for a "noisy" coin detector. But in purposefully making machines less sensitive to tiny stuff you lose the ability to have a good nugget detector. It is a distinct and purposeful trade-off. You would not want to use a GMT to coin detect as there is such a thing as too much sensitivity to small items. You also would not want to use most coin detectors, like the Explorer, for nugget detecting, as they are not designed to hit tiny stuff. For a reason. The MXT is hitting the middle ground. It is as hot as White's could make it on gold, while still being usable as a coin detector. But it gets more spurious signals from trash items than a normal coin detector and so is considered to be "noisy". This does not affect its performance and in fact the MXT is one of the better coin detectors. The often unmentioned fact is that if you just turn the Gain down a hair and reduce the threshold a bit you can turn the MXT into a silent search detector. But people tend to do the opposite in seeking max performance and so they are making the machine more noisy than it really needs to be!
8. How does this detector compare to the Explorer II in the discrimination department, or expecting too much of the MXT? <b>Very well indeed.</b> The Explorer is considered by many to be the best machine on the market for giving an accurate id on a silver target at max depth. I would personally make the same case for the MXT as a VLF gold detector. But I have also found that in very mineralized soil the MXT holds its own against machines like the Explorer on silver coins due to its superior ground balance circuit. The MXT does have numbers that jump around more due to the high resolution VDI system. Units that lump more numbers into a single target segment give a false sense of target certainty that many find comforting. The MXT exposes the reality that target VDI numbers vary with sweep speed, target centering, and height of the coil above the ground. If your sweep varies at all you will tend to get numbers that move a bit. The weaker the target, the more the variation. On the DFX these multiple VDI numbers are accumulated and averaged on the display. On the MXT you must mentally average the numbers as the display is renewed on every sweep. I believe a proficient operator that truly understands how the MXT display works and what it is telling them gets a better idea of what a target is than on most machines, but it does take experience.
Personally, while I respect what the Explorer can do, I find it to feel heavier than it is due to its being somewhat nose heavy and for good performance it must be operated much slower than what I am used to. The MXT has a much better feel on the arm and can cover ground a lot quicker. In areas where targets remain that both can hit, an MXT can out-produce an Explorer just by covering more ground. But for hammered areas where two dimes remain at absolute max depth the Explorer is probably the machine to use to go get those last two dimes. Me, I'll just go find a better spot.
Hope this helps. Be sure and review the MXT owners manual at http://www.whiteselectronics.com/manuals/2002_Models/MXT%20Manual.pdf (www.whiteselectronics.com/manuals/2002_Models/MXT%20Manual.pdf) for more details and for some real insights read the Engineering Report at http://www.whiteselectronics.com/mxtengineerw.php (www.whiteselectronics.com/mxtengineerw.php). And as noted in my last post, try http://bb.bbboy.net/alaskagoldforum-viewthread?forum=2&thread=22&postnum=0 (Chris Ralph's MXT FAQ @ bb.bbboy.net/alaskagoldforum-viewthread?forum=2&thread=22&postnum=0) and http://bb.bbboy.net/alaskagoldforum-viewthread?forum=2&thread=680&postnum=0 (MXT Gold Nugget VDI Secrets by yours truly @ bb.bbboy.net/alaskagoldforum-viewthread?forum=2&thread=680&postnum=0) and http://bb.bbboy.net/alaskagoldforum-viewthread?forum=2&thread=759&postnum=0 (MXT vs GMT, also by me @ bb.bbboy.net/alaskagoldforum-viewthread?forum=2&thread=759&postnum=0).
My final comment is this - no machine is perfect and all represent some kind of compromise in reaching desired engineering goals. But I have personally found that if gold is a high priority and yet the machine is also intended for coin, jewelry, relics, and more that the MXT represents what may be the best metal detector on the market today. There are better "gold only" units but none with the versatility of the MXT.
Steve Herschbach
Steve's Mining Journal
