False signals and chatter. Fisher Research Labs F75 SPECIAL EDITION

F75-MANUAL(122811).qx 12/14/12 9:04 AM Page 12

F75

Comprehensive Operating Manual & Guide to Metal Detecting

Controls (continued)

The F75 has two controls on the front panel, MENU and SETTINGS.

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MENU Pushbutton (Red button on right of the front panel)

Push the MENU button to:

1. Step through the menu selections on the display.

With each push of the button, the next menu selection will be highlighted.

The SETTINGS knob then allows you to change values for the highlighted selection.

2. Recall the last setting which you adjusted.

After you have adjusted a setting, an indicator will remain highlighted next to this menu selection. One push of the button will recall that selection and display the stored value.

This recall function is useful for a value you want to adjust frequently, such as the ground balance setting. In order to adjust a stored value with the SETTINGS knob, you must first press the MENU button to reactive the user interface.

SETTINGS Knob (On the left of the front panel)

Rotate the SETTINGS knob to:

1. Change the setting (or value) of the highlighted menu selection you have chosen.

2. Select the operating MODE when the top line of the menu is highlighted.

When used to switch back and forth between the DISCRIMINATION mode and ALL METAL modes, the detector changes modes as soon as the corresponding selection is highlighted.

The All Metal modes are used to detect all metal objects, including small or deep objects.

Use the Discrimination mode to ignore trash metal such as nails, foil, or pull-tabs.

NOTE: When the menu selection highlight disappears, the SETTINGS knob is deactivated.

If settings values do not change when the SETTINGS knob is rotated, press the MENU button to reactivate the user interface.

TRIGGER SWITCH (Under the display in front of your hand)

While the trigger is pulled back, metal objects are temporarily detected without the need for searchcoil motion. This aids in pinpointing the exact location of objects which were found while searching in the Discrimination or motion All Metal modes.

When in static all metals mode, pulling the trigger zeroes the audio threshold to the signal level currently preset. If the searchoil is up in the air away from metal, this maneuver corrects for threshold drift due to temperature changes.

When the trigger is pushed forward, FASTGRAB automatic ground balancing is activated. The internal computer measures the magnetic properties of the soil in order to cancel interference from naturally-occurring minerals in the ground. After the detector measures the soil in this manner, the detector then uses this information to control operation in both the All Metal and

Discrimination search modes. FASTGRAB can be used at any time during operation.

F75

Comprehensive Operating Manual & Guide to Metal Detecting

Search Techniques (continued)

False Signals and Chatter

At times the detector may beep when there is nothing there, or it may seem like there is nothing there. There are five major causes for this: electrical interference, nuisance buried objects, ground minerals, hot rocks, and sensitivity set so high that internal circuit noise is audible. The problem can usually be corrected by reducing the sensitivity setting, but sometimes other measures can also be taken.

ELECTRICAL INTERFERENCE

Electrical Interference can be caused by power lines, appliances, computer equipment, cell phones, fluorescent and vapor type lamps, household light dimmers, other nearby metal detectors, electric fences, radio transmitters, and electrical storms. If you get abnormal noise while holding the searchcoil motionless in the air, the cause is either electrical interference or internal circuit noise. If it is electrical interference, by walking around with the metal detector, you can often follow the signal and track it back to the offending device; simply turn the device off, or come back at another time when it may be off. If the interference is from power lines, you might try another time of day.

Interference on power lines is usually caused by something connected to them which may be idle in the evenings or on weekends. If the interference is from a communications or broadcast transmitting antenna, reducing the sensitivity is usually your only recourse.

The F75 allows you to shift operating frequencies to avoid electrical interference. See the

Frequency Shifting section for information about this technique.

NUISANCE BURIED OBJECTS

In some areas there is a lot of metallic trash which produces weak signals. These could include deeply buried objects, little bits and pieces of rusty iron and corroded foil. These items can be detected, but are difficult to pinpoint due to their depth and small size. When you dig and find nothing, it may seem like the machine is beeping at nothing even though there is actually something there. The best solution is usually to reduce sensitivity.

If searching a very trashy area and unwanted signals are a problem, search with the searchcoil 2 inches away from the ground. Trash objects very close to the searchcoil will sometimes not be completely eliminated, even when the discrimination setting should have eliminated the target.

Metal detectors are designed to see one metal object at a time. Where there are two iron objects near each other, the detector can be fooled into thinking that the gap between them is nonferrous metal. This is a common condition where a wooden building has burned or been torn down, and the site is littered with nails. A signal from a nonferrous metal object such as a coin will usually be repeatable, whereas a false positive signal resulting from multiple or oddly shaped iron objects will seem to wander around and even to vanish. Experienced detectorists call these non-repeating signals and usually do not bother digging them since nonrepeating signals are almost always trash.

GROUND MINERALS

Conductive mineral salts usually produce broad signals which will not be mistaken for a metallic object. Common causes are concentrations of mineral fertilizer, spots where evaporation has concentrated naturally occurring mineral salts, residue from de-icing salts, and urine from livestock. Unless dry, “cow pies” can sound off like they are metal. Ocean beaches have salt water—that subject is discussed toward the end of the manual.

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