Fisher Labs Gemini-3 Operating Manual
Gemini-3
Operating Manual
Deep Search, Two Box Metal and Mineral Detector
F I S H E R R E S E A R C H L A B O R A T O R Y
CONTENTS
About Your Gemini-3 ............................................................... pg. 3
Basic Theory of Operation ....................................................... pg. 4
Depth ......................................................................................... pg. 5
Transmitter .................................................................................pg. 6
Receiver .................................................................................... pg. 7
Searching............................................................................... pg. 8
Narrow Scan Inductive Search ............................................... pg. 8
Set Up -Narrow Scan Inductive Search .............................. pg. 8
Tuning -Narrow Scan Inductive Search ..............................pg. 9
Operation -Narrow Scan Inductive Search ........................ pg. 10
Wide Scan Inductive Search ................................................... pg. 12
Set Up -Wide Scan Inductive Search .................................. pg. 12
Tuning -Wide Scan Inductive Search .................................. pg. 12
Operation -Wide Scan Inductive Search ........................... pg. 13
Tracing ....................................................................................... pg. 14
Inductive Tracing .................................................................. pg. 14
Set Up -Inductive Trace ........................................................ pg. 14
Tuning -Inductive Trace ........................................................ pg. 14
Operation -Inductive Trace ................................................. pg. 15
Conductive Trace ..................................................................... pg. 15
Set Up -Conductive Trace .................................................... pg. 15
Tuning -Conductive Trace ................................................... pg. 16
Operation -Conductive Trace ............................................. pg. 16
Prospecting Tips ........................................................................ pg. 17
Mapping ................................................................................ pg. 17
Locating Placer Deposits ...................................................... pg. 17
Sources of Error...................................................................... pg. 18
Battery Replacement ............................................................... pg. 19
Treasure Hunters’ Code Of Ethics ............................................ pg. 20
Where To Use Your Metal Detector In The U.S. ...................... pg. 21
Specications ...........................................................................pg. 22
Using Headphones ................................................................... pg. 23
SPECIFICATIONS
Using headphones (not supplied) improves battery life, and
prevents the sounds from annoying bystanders.
It also allows you to hear subtle changes in the sound more
clearly, particularly if searching in a noisy location. For safety
reasons, do not use headphones near trafc or where
other dangers are present. This device is to be used with
interconnecting cables/headphone cables shorter than three
meters.
USING HEADPHONES
3
ABOUT YOUR DETECTOR
Two-box metal detectors have long been used by
prospectors, geologists, treasure hunters and public utility
companies to locate large, deep objects, ore vein, pipes
and cables. Fisher Research Laboratory is the recognized
leader in the eld, in fact, they were granted the rst two-box
metal detector patent over 60 years ago.
You might think it would be difcult to improve on a product
that has been in a constant state of development for so many
years. But the fact is, the GEMINI-3 is probably the biggest
leap forward in two-box design since it replaced vacuum
tubes with transistors back in the 1950s.
The new GEMINI-3 goes 10 to 20 percent deeper than its
predecessor. It will trace most pipes and ore veins at least
50 percent farther, and the new crystal controlled circuitry
is rock stable. The old metal-rim antenna is gone and so
are the hard-to-get batteries. The GEMINI-3 is easier to use,
two pounds lighter, & features Voltage Controlled Oscillator
(VCO), audio target response for precision pinpointing.
Your Fisher GEMINI-3 is a high quality, precision instrument
engineered by the oldest and proudest name in metal
detectors. Treat it right, read this instruction manual thoroughly
and practice often. Who knows, with skill, patience and a
little luck, you may nd the BIG ONE.
Drop us a line if you have any questions, comments or
exciting GEMINI-3 stories. In the meantime . . .
Happy hunting,
FISHER RESEARCH LABORATORY
4
BASIC THEORY OF OPERATION
The GEMINI-3 was engineered to detect conductivity
changes in the earth over which it is carried. Hence, a positive
response by the GEMINI-3 indicates the presence of a metal
object, ore vein or mineral that is more conductive than the
surrounding rock or soil.
The function of the GEMINI-3 Transmitter is to send a radio
frequency (RF) signal down to a conductive object and
generate an electromagnetic eld around that object.
When used in any of the Inductive modes, the RF signal
travels through the ground to the object. When used in
the Conductive Trace mode (see page 13), the signal is
transmitted directly through a wire attached to an exposed
portion of the object.
The function of the GEMINI-3 Receiver is to locate the
object by detecting the electromagnetic eld around it and
responding with audio and visual signals.
National Forest and Federal Lands—Metal detecting is allowed
only by special permit acquired from the federal government.
Each area has a district ofce.
Corps of Engineers, Lakes, Shorelines and Lands—Permission has
been granted only on predisturbed sites, such as beaches and
attached swimming areas. New Corps lakes and lands must be
okayed by the main ofce of the Army Corps of Engineers. Each
area has a district ofce.
State Parks and Lands—Some state parks are open to metal
detecting, but some are not. Always check with the park ranger
before attempting to use your detector.
Bureau of Land Management (BLM) Lands—Some areas are
open for metal detecting, and some are not. Always check with
the district ofce.
City or County Park Lands—Most are open to metal detecting
unless notice is given by a sign or city ordinance. When in doubt,
always check with the city’s Parks and Recreation Department.
Public School Grounds—Most are open to metal detecting unless
notice is given by a sign, city ordinance, law enforcement ofcial,
or school employee. You should always check with the school
ofce rst.
Privately Owned Lands (Private Property)—Permission required. It
is always best to have the permission in writing.
Historically Marked Lands or Sites—Metal detecting is not
allowed. Don’t even think about it.
Where To Use Your Metal Detector In The U.S.
5
DEPTH
The GEMINI-3 was designed to locate large, deep objects such
as an iron chest, pipe or an ore vein. It will not detect small coinsized objects. A quart jar full of coins, however, may be detected
one-to-three feet deep. A 55-gallon drum may be detected as
deep as 10 feet. A very large object, like an automobile, may be
detected at 20 feet or more.
The ability of the GEMINI-3 to detect objects at various depths
depends on several factors:
1. Ground Mineralization: The penetrating power of the
GEMINI-3 is adversely affected by the presence of
magnetite (a black iron oxide) and wet conductive
mineral salts. For example, an object that can be
detected at 15 feet in neutral ground may be
detected no deeper than 5 feet in highly mineralized
ground.
2. Size and Depth of Object: A 4-inch diameter target
will produce signal 64 times stronger than a similar
1-inch diameter target at the same depth. An object
1-foot underground will produce a signal 4,000 times
greater than the object 4 feet deep.
3. Length of Time Object is Buried: An object that has
been buried for a long time may be easier to detect
than a newly buried object. Rust and other oxides as
well as mineralization caused by the interaction of
the metal with soil chemicals are good conductors
and aid detection.
4. Shape of an Object: Ring or looped shaped objects
lying at produce the best results. Flat or dish shaped
targets are also easy to detect. Rod shaped objects,
especially when scanned on end, are very difcult
to detect.
5. Operator Skill: Probably the most important factor of
all, practice over known buried targets is essential.
TRANSMITTER
Battery Level Meter
Power Switch
Ground Plate
Jack
Figure 2. Transmitter
Power Switch: Pull out switch turns on power.
Ground Plate Jack: Used with the optional Ground Plate Assembly
for the Conductive Trace Mode.
Battery Level Meter: Gives a visual indication of battery strength.
Replace batteries if below 5 on meter.
6
BATTERY REPLACEMENT
NOTE: As supplied from the factory, the battery pack is
surrounded by a plastic sleeve to retain the batteries during
possible rough handling and shipping. As a precaution, always
reinstall the sleeve after battery replacement.
RECEIVER
Speaker: Provides audible indication of the receiver signal for
balancing and detecting.
Meter: Provides visual indication of the Receiver signal response for
balancing and detecting. Also used as a battery test indicator.
Sensitivity Control: Used to control sensitivity level. The circled “7”
indicates the starting level used for Narrow Scan Inductive Search
Mode.
Headphone Jack: Accepts stereo or monaural headphones and
silences speaker when headphones are plugged in. (Stereo/mono
headphones should be in “stereo” position.)
Power Switch: Pulling switch out turns on receiver.
Meter
Sensitivity
Control
Headphone Jack
Power Switch
Speaker
Figure 3. Receiver
Battery
Test Switch
7
8
SEARCHING
Narrow Scan Inductive Search
This mode of operation is performed by a single operator using
the 3-piece handle. It is the best method for locating small or short
objects (chest, coin-jar, rie, etc.). However, it is also an effective
way to locate pipes, cables and ore veins.
Set Up
1. Assemble the 3-piece handle by inserting each
end piece into the center handle section. Tighten
the two center knobs nger tight until there is no
wobble. Place the transmitter on its back and
insert the end of the handle with the single brass
shaft into the V-shaped slot. Turn the knob until it is
nger tight and the handle is rmly in the slot.
2. Align the receiver panel holes with the upperhandle threaded shafts. Tighten the lower black
knob until it is nger tight.
3. Turn the balance knob (with white arrow) until the
spring is under tension (at least 10 turns).
Figure 4. Narrow Scan
Inductive Search
Figure 5. Assembly for Narrow Scan Inductive Search
PROSPECTING TIPS
Mapping
Locating Placer Deposits
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