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F75-MANUAL(122811).qx 12/14/12 9:04 AM Page 17
F75
Comprehensive Operating Manual & Guide to Metal Detecting
Capabilities And Limitations
DEPTH
The F75 can detect U.S. coins to a depth of up to 15-16 inches (37-40cm) under good conditions. Large objects (55 gallon drums, manhole covers, etc.) can be detected to a depth of up to several feet (1-2 meters).
Electrical interference from power lines and from electrical appliances and electronic equipment can reduce detection depth, or cause audible interference, making it necessary for the user to reduce the sensitivity setting. Soils with large amounts of iron or salt minerals may also reduce detection depth or necessitate a reduction in the sensitivity setting.
TARGET IDENTIFICATION
The F75 identifies the probable type of metal object by measuring its effective electrical conductivity, which is displayed as a number from 0 to 99 on the LCD screen. The effective
electrical conductivity of an object depends on its metallic composition, size, shape, and orientation relative to the searchcoil. Since coins are minted to tightly controlled specifications, they can be accurately identified. Identification of pull-tabs and foil is less consistent because these kinds of targets come in wide variety. In general, smaller objects, and objects made from lower conductivity alloys such as iron, bronze, brass, lead, pewter, and zinc will read lower on the effective conductivity scale. Larger objects and objects made from higher conductivity alloys such as silver, copper, and aluminum, will tend to read higher. The notable exceptions are gold, which usually reads low because it is rarely found in large pieces, and zinc pennies, which read moderately high because of their size and shape. Although nails and other iron and steel objects will usually give low readings, ring-shaped pieces of iron (for instance steel washers and harness rings) will usually produce medium to high readings. Flat pieces of iron or steel, such as can lids, will occasionally do the same.
Most targets can be identified accurately in air to a distance of about 10 inches. The minerals in many soils will cause identification to be less accurate. In most soils, effective target identification can be had to a depth of at least 8 inches.
REQUIREMENT FOR MOTION: PINPOINT FEATURE
As with other modern metal detectors, the F75’s searchcoil must be kept in motion in order to both detect and identify targets. The All Metal modes are more forgiving of sweep speed variation than is the Discrimination mode.
The trigger-activated PinPoint feature continues to detect metal if searchcoil motion stops over the target. The PinPoint feature is used primarily to pinpoint the exact location of a target so that it can be retrieved with a minimum of digging, and does not provide target identification.
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Comprehensive Operating Manual & Guide to Metal Detecting
Motion All Metal Mode
The Motion All Metal mode is more sensitive and offers better feel than the Discrimination mode, and is used to find all metal objects present in the ground. The searchcoil must be in motion for objects to be detected. This is a single filter search mode similar to the “fast autotune”, “SAT”, or “P4” modes found in other detectors you might already be familiar with.
F75
THRESHOLD: Adjustable from -9 to +9. For maximum ability to hear the weakest signals, adjust this background noise level high enough so that it is barely audible while the detector is in use in the field. To eliminate the weakest signals, adjust into the negative region, which will allow the machine to run silently if the Sensitivity is not set too high.
The threshold level changes slightly with each detent step on the SETTINGS knob. Each number on the numeric readout corresponds to five steps.
AUDIO PITCH This control allows you to change the range of frequencies that you hear.
Values range from -9 to 9; 0 is default. Negative numbers lower the frequency of the tone you hear; positive numbers raise the frequency. This feature is intended to make the tone more pleasant to your ear; choose your personal preference. Users who have suffered some hearing loss, (including the natural loss of ability to detect high and low frequencies with age) may find this control helpful.
SENSITIVITY This controls the signal gain, and is adjustable from 1 to 99. In the presence of electrical interference, high ground mineralization, or variable ground mineralization, operation will usually be too noisy (wobbly and erratic sound) if the sensitivity is set too high.
At settings above 90, the internal circuit noise of the machine will probably be audible.
The sensitivity level setting is largely a matter of personal preference. However, if you cannot hear at least some noise, the smallest or deepest objects will not be detected.
The sensitivity system has two stages, 1-29 (low gain), and 30-99 (high gain). As you decrease sensitivity and cross the transition from 30 to 29, you may notice that the background noise increases. Despite this increase as you decrease sensitivity to the lower setting, 29, the sensitivity is in fact lower under 30, and the machine is less susceptible to overload from large targets, ground minerals, and salt water. You may need to adjust to a setting of 29 or less to silence electrical interference, or to prevent overload on highly mineralized ground or in salt water. You may also notice a shift in the ground balance setting over difficult ground conditions as you cross this transition.
MANUAL GROUND BALANCE
Manual Ground Balance can only be performed while in an All Metal mode, but the resultant setting will carry over if you change into Discrimination mode. See the previous section on
Ground Balancing for instructions on how to use this feature.
We suggest that you manually ground balance using only the MOTION All Metal Mode. The result will be more accurate than when using STAT All Metal Mode.
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F75
Comprehensive Operating Manual & Guide to Metal Detecting
Ground Balancing (continued) the setting.
The goal is to eliminate the sound as the coil is being pumped over the ground. In some soils, the sound is not completely eliminated.
If the ground balance adjustment is incorrect, there will be a difference in the sound as the searchcoil is either moving toward or away from the ground. It sounds like you are either
pulling the sound out of the ground, or pushing the sound into the ground.
• If the sound is louder as you raise the searchcoil, increase the ground balance setting.
• If the sound is louder as you lower the searchcoil, reduce the ground balance setting.
NOTE: Experienced users often prefer to adjust the ground balance to get a weak but audible response when lowering the searchcoil. This is called adjusting for positive response.
Positive and Negative Response
The purpose of ground balancing is to adjust the metal detector to ignore ground minerals. If the setting is incorrect, ground minerals will give either a positive or a negative response, depending on which direction the adjustment is off.
POSITIVE RESPONSE
If the G.B. setting is too high a number, the response of minerals will be positive. This means that when the searchcoil is lowered to the ground in PinPoint, Stat, or Motion All Metal mode, the sound will get louder as the searchcoil approaches the ground. The sound will grow quieter as the searchcoil is raised. What, if anything, you will hear in discrimination mode depends on the discrimination setting.
When searching in an All Metal mode, if ground balance is properly set to cancel the ground, and you sweep over a positive hot rock, the rock will give a “zip” sound similar to that of a metal object.
NEGATIVE RESPONSE
If the G.B. setting is too low a number, the response of minerals will be negative. When the searchcoil is lowered to the ground in PinPoint, Stat, or Motion All Metal mode, the machine will be silent. The machine will sound off as the searchcoil is lifted away from the ground.
What, if anything, you hear in discrimination mode depends on the discrimination setting.
When searching in Motion All Metal mode, a negative hot rock will produce a “boing” sound after passing over it, making it difficult to know where it is located. It will not have the sound and “feel” of a metal object.
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F75
Comprehensive Operating Manual & Guide to Metal Detecting
Capabilities And Limitations (continued)
GROUND BALANCING
To achieve maximum depth in any detection mode, as well as when using the PinPoint feature, the F75 offers the ability to cancel out ground minerals either by manual adjustment, or automatically using the FASTGRAB feature.
If you do not perform the ground balancing operation, the Discrimination mode will usually still work fairly well, but the ALL METAL modes will not. The Pinpoint feature can be used for pinpointing objects at moderate depth in most soils without prior ground-balancing.
The internal computer will not cancel salt water, so when detecting on wet ocean beaches, ground balancing must be done manually.
DISCRIMINATION
Discrimination refers to a metal detector's ability to ignore metal objects in selected categories, especially iron and aluminum. This makes searching much more pleasant in an area with a lot of metal trash. The F75 offers a wide variety of discrimination features which you can select according to the search conditions and your personal preference.
DEPTH READING
The estimated Depth Reading displayed when pulling the PinPoint toggle is based on the strength of the signal. It is calibrated to coin-sized objects. Smaller objects will read deeper than they actually are, and large objects will read shallower than they actually are.
AIR TESTING
There may be times when you want to test or demonstrate the metal detector without sweeping it over the ground, for instance, if not fully assembled, or if you are indoors. To air test, place the searchcoil in a spot where the detector is stable and more than two feet away from any large masses of metal, including the reinforcing steel usually present in concrete. If you are wearing a wristwatch or jewelry on your hand or arm, remove it. Then, test or demonstrate by waving metal objects over the searchcoil; wave objects briskly, several inches over the top of, and parallel to, the searchcoil.
Ground balancing cannot be tested or demonstrated in air unless you happen to have appropriate specimens of iron minerals available.
SWEEP SPEED
The F75 is noted for its quick response. This gives it unsurpassed ability to locate and identify good targets surrounded by trash. It also permits the user to sweep the searchcoil quickly in order to cover more ground with very little risk of losing targets. In general, if you are searching an area where the desirable targets are more than 8 to 10 inches deep, a faster sweep speed will detect to a greater depth and yield more accurate target IDs.
CHECKING A TARGET
In order to most accurately verify a detected target with most other metal detectors, users will narrow their sweep and loiter over the top of the target. The F75 is different. The F75's quick response and advanced signal sampling system produces the most accurate target IDs with deliberate (shoulder width) sweeps all the way across the target, even if there are other targets nearby. If you check targets using sweep techniques learned on some other detectors, you run the risk of getting less accurate target IDs. You can use the confidence level indicator to improve your technique.
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F75
Comprehensive Operating Manual & Guide to Metal Detecting
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Capabilities And Limitations (continued)
STEEL BOTTLE CAPS & FLAT IRON TRASH
Modern motion-type target ID metal detectors usually have difficulty consistently identifying steel bottle caps and other flat iron trash objects. Double-D searchcoils also have a reputation for having difficulty distinguishing steel bottle caps from coins, and for being unable to eliminate steel bottle caps from detection. If you are searching in an area where there are many steel bottle caps or other flat iron trash targets, minimize the amount of unnecessary digging with the following methods:
1. SEARCH WITH THE bc (BOTTLE CAP) PROCESS using the PROCESS # feature.
This method calculates visual ID differently in order to cause steel bottle caps to read lower on the scale, and to register less consistently. A desirable object such as a coin will usually produce numeric values that are fairly consistent in both directions of sweep. See PROCESS # under the DISCRIMINATION Mode section of this manual.
2. SEARCH WITH THE dP PROCESS. This method calculates visual ID differently in order to cause steel bottle caps to ID lower on the scale and to register less consistently. It also produces more audio clues to the character of the target.
3. LIFT THE SEARCHCOIL. Within 2 inches of a Bi-Axial searchcoil, the crossed magnetic fields of the Double-D construction can produce anomalous responses. If the object feels shallow (strong signal, narrow response, or multiple responses in a single sweep) and is giving consistent high readings like a coin, raise the searchcoil 2 to 3 inches and try again.
A coin will almost always continue to give consistent readings unless it is right next to an iron object. A steel bottle cap that is at least 3 inches away from the searchcoil will usually produce readings that bounce around from medium to low numbers.
4. SWEEP THE REAR OF THE SEARCHCOIL OVER THE CENTER OF THE TARGET,
OR SWEEP RAPIDLY.
a. If the ID# is repeatable in the range of 68 to 72, when passing the center of the searchcoil over the target at a normal speed, then the target is probably a dime or copper penny.
b. If the ID# is not in the range of 68 to 72 then: i. Sweep the back end of the searchcoil over the target. If tones change from high to low, the target is probably a bottle cap.
ii. Sweep the center of the searchcoil rapidly across the target.
1. If tone and ID# drop, it is probably a bottle cap.
2. If a bottle cap, then the faster you sweep, the lower the tone.
5. USE THE CONFIDENCE INDICATOR
Coins will usually produce a high confidence level, whereas steel bottle caps
Sweep back end of searchcoil over suspected bottle cap.
(Low Tone = Bottle Cap) and other trash will usually indicate a low or erratic confidence, even if the ID numbers are consistently in the range of a coin.
Confidence level is useful even if not using the dP or 3b processes.
F75
Comprehensive Operating Manual & Guide to Metal Detecting
Ground Balancing (continued) push the trigger switch to activate FASTGRAB automatic ground balancing. Generally, it is best to first let the computer automatically cancel interference from ground minerals.
However, for gold prospecting, searching on a wet saltwater beach, or searching in an area with so much metal trash that there is no clean ground for the computer to sample, we recommended that you manually ground balance. Manual ground balancing requires a bit of skill, acquired with some practice.
The range of ground balance settings indicated on the display range from 0 to 99; however, each displayed number spans 5 detent steps on the settings knob. The actual internal ground balance settings change with each step; there are a total of 500 different settings. Under some ground conditions you may be able to hear the individual steps in the setting.
The Fe
3
O
4 bar graph on the LCD display indicates the amount of magnetic mineralization. The searchcoil must be in motion to measure Fe
3
O
4
. The most accurate measurement is obtained by pumping the searchcoil, as in the Ground Balancing procedure.
The two-digit G.B. Setting number displayed on the LCD indicates the type of ground mineralization.
Some typical ground mineralization types are:
0 – 10 Wet salt and alkali
5 – 25 Metallic iron. Very few soils in this range. You are probably over metal.
26–39 Very few soils in this range -- occasionally some saltwater beaches
40–75 Red, yellow, and brown iron-bearing clay minerals
75–95 Magnetite and other black iron minerals
When manually ground balancing, try to “feel out” a spot on the ground to make sure there is
G.H. PHASE
90 ground where the GB setting is less than 40. Where the ground reads less than 40, manual ground balancing is required.
To perform the Manual Ground Balancing operation, do the following:
1. Select the MANUAL G.B. function
The legend G.B. SETTING appears.
The present ground balance setting is displayed (0-99).
The message PUMP COIL TO G.B. will appear.
2. Physically pump the searchcoil and detector up and down over the ground.
Lift it about 6 inches above the ground and lower it to within 1 inch of the ground, about once or twice a second.
3. Turn the SETTINGS KNOB to adjust
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