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I'm attaching the manual (downloaded from Fisher's site). (Edit:  see Steve's link in his response below.)  On page 9, bullet point 4. it says:  Still holding the GEMINI-3 parallel to the ground, slowly turn the balance knob (with arrow) on the three-piece handle counterclockwise until you get silence (null) and zero meter reading.

My question is this:  is the knob actually doing an electronic adjustment (such as being attached to a variable resistor or capacitor) or is it just re-aligning / repositioning the transmitter/receiver/attachment rod to optimal location?

 

 

 

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Please link to manuals in the database instead of uploading duplicate copies - thanks!

You are aligning / repositioning the transmitter/receiver/attachment rod to optimal location. Two box detectors are a large coil, a transmit loop and a receive loop. When you make a search coil the two loops are positioned to find the ”null”. You are doing the same thing here.

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13 hours ago, Steve Herschbach said:

When you make a search coil the two loops are positioned to find the ”null”. You are doing the same thing here.

That's a subtlety I didn't appreciate.  A majority of the used Fisher 2-box (Gemini family) detectors I see on Ebay are missing the connecting rod.  Maybe you can buy one of those parts from First Texas?  But making your own seems more complicated than simply drilling a few holes in an aluminum extrusion.  Induction balancing is a lot harder than it sounds.

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The Gemini can be used without the rod as described in the manual. The Gemini is sold as the TW 6 locator for the utility market and comes without the rod.

The rod not needed per se and you do not need the adjustment even if you use a rod. It just puts the finest tune possible on the setup if used that way. The rods are sold separately, part number 202727. Making rods would be nothing more than drilling holes in aluminum rod. The adjustment setting is just a screw that tilts the box, getting the tilt mechanism right without seeing one first might be challenging. I recall the end attached to the box just spins somehow while the screw part is threaded through the rod but it’s been a long time since I operated one of these so it’s kind of vague. The inductive coupler and line trace function works great.

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1 hour ago, Steve Herschbach said:

The rod not needed per se and you do not need the adjustment even if you use a rod. It just puts the finest tune possible on the setup if used that way.

Thanks for the detailed explanation, Steve.  I was thinking the length of the rod would also be critical, but apparently not.

 

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You are creating a virtual search coil with the box spacing. Two people can hold the boxes 10 feet apart for instance to create a 10 foot or 20 feet apart to create a 20 foot coil. Again, refer to the manual. The rod set is creating a 4 foot coil but you could make a 6 foot coil if you wanted. Like all coils, the larger the coil the deeper you go, but the larger the item needed to trigger a signal. Smaller configurations are more sensitive to smaller items but don’t go as deep.

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From a previous thread:

These "two box detectors" can be used as an integrated unit connected by a rod. These detectors are in effect a very large coil and can only detect objects about the size of your fist or larger, but at great depths depending on ground mineralization. These are induction balance detectors and a large coil pulse induction model may work better in severe soils, especially on objects that are small enough to be borderline targets for this type of detector.

However, you can also use the transmitter and receiver separately to create a larger search field, that can be used to look for large hidden bodies like large veins or ore deposits. This is what Fisher calls a Wide Scan Inductive Search on page 12 of the Fisher Gemini-3 Operating Manual:

Wide Scan Inductive Search

This mode of operation is the preferred method for searching a large area quickly. Two operators are required and the handle assembly is not used.

The Wide Scan Inductive Search mode is practical only when searching for ore veins, pipes or cable 40 feet long or more. If the  operators are too close together, “direct air coupling” will result, meaning the receiver is detecting the Transmitter signal directly through the air instead of through a buried conductor.

fisher-gemini-3-wide-scan-inductive-search.jpg
Figure 7 - Wide Scan Inductive Search

Set Up

Two operators line up at least 20 feet apart, one with the receiver and one with the transmitter, parallel to the assumed direction of the buried conductor. The transmitter and receiver should be in line with each other, facing the same direction.

Tuning

1. Set the transmitter: Power: ON

2. Set the receiver: Power: ON Sensitivity: ZERO

3. Slowly increase the SENSITIVITY control until the audio signal is heard, then reduce SENSITIVITY until the audio just disappears. This is the point at which “air coupling” has been eliminated.

Operating

1. Keeping the receiver and transmitter sections in line, the two operators can now walk their predetermined search pattern. The SENSITIVITY control should be checked periodically to ensure that the receiver is tuned just below the “air coupling” threshold.

2. If both operators cross the same conductive body (pipe, cable, ore vein, etc.) at approximately the same time, the receiver tone and meter will rise to indicate its presence. The receiver operator should alert the transmitter operator that they have detected a conductive object.

3. The receiver operator should then hold his position while the transmitter operator moves back and forth for the strongest receiver response. At this point, the transmitter operator should stop and place the instrument on the ground with the handle grip on top.

4. The receiver operator can then pinpoint the buried object by moving the receiver back and forth in line with the transmitter. The object should be directly beneath the point of maximum response.

5. The receiver operator may then trace the signal along the length of the unseen object as described in the Inductive Trace section (page 12).

The methodology above describes looking for a pipe or buried fuel tank. You can however find a very detailed expansion of the subject as regards prospecting on pages 46 - 52 of the Handbook of Geophysical Prospecting Methods for the Alaskan Prospector under the heading "Electromagnetic Methods". This publication is an older one but this type of detecting is even older, dating back to the original two box Fisher Metalloscope of the 1930's. The booklet is written for the practical layman prospector and so describes several geophysical prospecting methods in a relatively understandable manner.

See also the discussion starting on page 4 of Geophysics For Mineral Exploration - A Manual For Prospectors for more up-to-date information on the subject.

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