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GTI Technical stuff

gabbyhayes

gabbyhayes

Member
I said I was an electronics teckie in my last post.

I couldn't wait to check the 2500 out from the inside out.

Inside the construction is not notch.

A couple of interesting observations:

Changing the sensitivity does not affect the strength of the transmitter...it remains constant...so I assume you are adjusting the gain of the receiver.
The frequencies are as follows:

1...7248 Hz
2...7230
3...7208
4...7191
5...7163
6...7146
7...7126
8...7109

Give or take a hertz or two. Its opposite from what you would expect. The default is 4 about that 7200Hz

I also found an excellent website where this guy did some exhaustive testing on the GTI 2500 and posted his results....very impressive.
here's the link:
http://www.waynesthisandthat.com/metaldetecting.html#menu

I am also very impressed with how well the 2500 works...on coin mode 90% of what I find is coins. In zero mode I was finding tiny paperclips etc down 8-10 inches.

This detector is one of the best all around detectors made in my opinion
 
Thanks for the frequency chart. How did you come up with it?

The link has to be some of the best reading I have done. I haven't read it all yet but it's just the sort of information I have been looking for.

Thanks a million!

Chris
 
I hooked up a small pickup coil to my freq. counter and changed the freq. on the GTI 2500.

Since the change is actually pretty small I doubt it would make any real difference besides the interference between machines.

Though I suppose you might want to use freq.1 cause it is the highest.

Changing the ground balance also has no effect on either the transmit freq. or amplitude....

Glad you enjoy the site....I know it was exactly what I was looking for when I made my decision on what to purchase.
 
Changing the freq in small increments does a couple of things. It creates a new set of harmonics within the detectors coil field. This allows some items to respond better (at least differently). It also changes the reference freq in the front end and causes the 2500 to be less susceptible to harmonic coupling, or "cross talk" from other detectors, power lines, etc..

The general range around 7khz is a best average freq. for most detecting. A hundred hertz or so either way makes little difference. Actually, silver and copper coins respond better to a lower range, around 3khz, while alloyed gold jewelry do well with a higher one at 15khz or so. Thus, the 7khz fits in between as a best compromise.
As an aside, free gold (unalloyed, as occurs in nature) reponds well to a much higer freq, like 50khz.
None of this is absolute, but the ranges of response I've given are generally fixed, referenced to US coins. This is why many foreign coins, with their myriad alloy compositions, respond differently than one might expect...

You are right about output power - you do not increase the output transmit power of the coil field when you increase the SENS. In truth, you do not "send out" any signal at all with a VLF detector. The only detector that does is a Pulse Induction (PI) type. It is actually a radar unit, in that it transmits a signal pulse, cuts off and then and waits for a response. All of this stuff is based on simple RF physics.

With VLF motion units, on the other hand, you create a constant field around a highly directional loop antenna, your searchcoil. Think of it as a bubble above and below the coil. As items come into this field, or "bubble," they are energized by it and affect it; in the case of metallic objects they create a measurable phase shift response within the field, just like the doppler shift that makes radar detectors work. This is why they require the coil to be in motion in order to work properly. Various alloys create different shifts, so are measurable and at least identifiable in terms of their phase shifting response.

So instead of increasing the amount of power, making the field increase in size, you bump up the gain of the receive circuits to what is really a phase shift within a constant field. Calling the SENS control a "depth" control really does a disservice to the user, and plants the wrong idea in his mind. "GAIN" would be a much better term.
 
If I have to keep printing and saving these great posts I'm going to run out of paper! My Detecting reference material binder is getting plumb full.

Thanks Dahut. (Jabba? Pizza?)

Chris
 
Yeah the minimal frequency shift isn't enough to write home about, just enough for power line interference and crosstalk. I've been after Garrett to come out with a detector with variable, adjustable frequencies where you could switch from low to high frequencies at will.

Bill
 
Nice post gabby, I really like the freq chart, man I always thought the highest setting was the highest freq. I prospect with it alot and now I will set it on 1 as maybe just a hair more sensitive to lower conductive gold nugs.
Also I thought that the freq's woulb be in 10ths of increments like 7.2-7.3-7.4 etc....nice to know. Did you notice how the mother boards were stacked on each other inside the housing?, I thought it was a brilliant design.

Great Post man,
Alan (GA)
 
n/t
 
Thanks for all the responses!!! :cheers:

I'm really happy to be here with so many fellow GTI 2500 owners!!

Just curious, has anyone ever done a survey or taken a count of who owns what on this forum?

I'm looking forward to digging and learning and digging,

Gabby
 
Most of us own several Garretts along with other brands. I have seven Garretts.250/550/1350/1500/2500/Infinium/Scorpion, and one Bounty Hunter. Used to have 13, including an X5 but weeded them out awhile back.

Bill
 
Garrett 250 & 1350
Tesoro Vaquero, Golden and Tiger Shark

This is pared down from previous numbers.
 
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