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9 – Maintenance
DPN 402197 © TSS (International) Ltd Page 3 of 26
9.1.1 Sensing Coils
See drawing number 401105 in sub-section 10
The coil array includes six identical and electrically independent sensing coils, each of
which includes an internal pre-amplifier board. Drawing number 401105 shows the
pre-amplifier board for a single coil. The pre-amplifier board receives power through
the coil connection cable.
CAUTION
Any water entering the coil housing will cause permanent damage to the coil winding
and to the pre-amplifier board.
TSS matches and calibrates the pre-amplifier boards carefully to their specific coil
windings during manufacture. You must not remove these items from the coil hous-
ings, which contain no user-serviceable parts.
To avoid damage to the coils, do not remove the brass end-caps from the coil hous-
ings. You will invalidate the warranty if you open a coil housing for any reason.
Connection between the coil winding and the input to the pre-amplifier is through a 2-
pin Molex connector PL1. Signals from the coil windings arrive at the input to a low-
noise buffer amplifier U1 that provides two functions:
Four different gain settings. TSS establishes these settings during manufacture by
setting link LK1.
Low-pass filtering with a cut-off frequency of 300Hz.
The diode assemblies D1 and D4 provide input protection. The buffered signals from
the coils then arrive at a band-pass filter formed by U2 and U3 to improve noise atten-
uation. This filter arrangement has a pass band from 7.2Hz to 318Hz.
U4 and U6 provide low noise programmable amplification with absolute gain settings
of ×1, ×2, ×4, or ×8 according to the control voltages on pins 1 and 2 of the coil con-
nector PL2.
The output from the amplifier U4 splits, with one half inverted by U5, to provide a dif-
ferential signal output on pins 3 and 4 of PL2. This differential signal passes through a
twisted pair in the coil connection cable to the SEP input.
Power supplies for the pre-amplifier board enter through PL2 pin 5 (+12V) and pin 6
(–12V), with the ground connection on pin 7.
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