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Inside the 990XD are two separate detectors
operating simultaneously. An ‘inland’ detector which is factory set to
ignore mineralised inland ground and gives a positive response to all
metals, and a ‘beach/disc’ detector operating at the (adjustable) wet
salt sand exclude level, which gives a negative response to iron. By using
the switches on the front of the 990XD to combine the signals from the
above two detectors, it is possible for the operator to detect all targets
whilst eliminating the effects of the ground while distinguishing between
good and reject targets. Detection is indicated by the loudspeaker
sounding off (located in the rear of the battery compartment), and by the
meter needle moving.
ON/OFF SWITCH AND TUNE CONTROL
This control is a combined on/off switch and rotary tune control. Fully
anticlockwise to the clicked position is power off. To turn the detector
‘on’ rotate the control clockwise from its indented ‘off position.
To ‘tune’ the detector, adjust the control to give the preferred level
of audio (from the internal loudspeaker or headphones). For best
performance the audio should be adjusted to threshold when the sound is
just beginning to break through, since this is when the ear is most
sensitive to small changes, and it is important to maintain this threshold
when searching to achieve maximum depth. The meter gives a visual
indication of the audio level (unless battery check or meter disc is
selected) and will read midscale when tuned at threshold. The tune control
is used in conjunction with the retune button which should be depressed
when making adjustments.
RETUNE PUSH BUTTON
Should the audio level change during searching other than that
resulting from a target, such as from drift occurring from warming up (see
paragraph ‘search head), press (and release) the ‘retune’ button to
recover the threshold sound as set on the tune control. The ‘retune’
button should also be held down when making adjustments to any of the
detector’s controls.
GROUND EXCLUSION SWITCH and DISC MODE
SWITCH
Firstly what is ground exclusion? On some sites mineralisation caused
by iron makes it difficult to operate a detector successfully. The effect
of these minerals is termed ‘ground effect.’ In practice, if the
detector is not correctly adjusted, the signal alters if the search head
is not swept at a constant height above the ground. However by operating
the detector at the correct ‘exclude’ point this ground signal can be
eliminated. The elimination of this signal is known as ground exclusion.
When operated at the inland ground exclude point, all metals give a
positive response which is sometimes called ‘all metal’ mode. What is
discrimination (disc)? Like mineralised ground, metals and wet salt sand
too, have a characteristic exclude point. By operating the detector at the
beach exclude point, iron and small ferrous items produce a negative
response, but valuable targets still produce a positive signal. By
operating at a higher exclude point still, small pieces of silver (aluminium)
paper will also produce a negative (reject) signal.
LEVEL control
The discrimination level of the ‘beach/disc’ detector in the 990XD is
centred at about that of wet salt sand but can be varied from iron reject
through to aluminium foil reject by adjusting the ‘level’ control.
(The ‘level’ control also adjusts the exclude point of ‘meter
disc’ and ‘meter and audio disc’).
INLAND and DISC MODE SWITCH
With ‘inland’ selected the 990XD operates at a fixed level of
ground exclusion. This inland ground exclude point is factory set on the
990XD. All metal targets will produce an increase in audio volume in this
mode regardless of the ‘level’ control setting. The ‘beach/disc’
detector remains operating and can be selected to drive the meter or
control the audio pitch by selection on the ‘disc mode switch’. The
level of discrimination of the beach/disc detector can be adjusted by the
‘level’ control.
The ‘inland’ mode is used on
mineralised inland sites and can be used in conjunction with any one of
the disc modes described below. By selecting ‘meter disc’ mode the
meter needle moves to the right for good targets and to the left for
reject targets. By selecting ‘meter + audio disc’ mode the meter moves
to the right for good targets and to the left for reject targets, and the
audio pitch increases for good targets and decreases for reject targets.
By selecting ‘none’ the meter needle moves to the right for all
targets and the pitch remains constant.
BEACH and DISC MODE SWITCH and LEVEL
control
Due to the varying nature of beach soils (the salinity and wetness)
the 990XD is equipped with an adjustable level of ground exclude in
‘beach’ mode. It is the mode best suited to beach detecting where fine
adjustment of the ‘level’ control is required to exclude wet sand. By
selecting ‘meter disc’ mode the meter needle moves to the right for
good targets and to the left for reject targets. By selecting ‘none’
the meter needle moves to the right for good targets and to the left for
reject targets (same as meter disc since it is in beach mode). Selecting
‘meter + audio disc’ gives a pitch increase to good targets, reject
targets are ignored. The inland detector plays no part when ‘beach’
mode is selected and is switched off. Only use this beach mode on the
beach or at inland sites where there is no mineralisation and where you do
not wish the detector to sound off over iron. It should be apparent from
the above that the ‘level’ control acts primarily as ground exclude
control in ‘beach’ mode, and secondly varies the discrimination point
when operating in ‘meter disc’ or ‘meter and audio disc’ mode.
The End
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