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5. SM40 SURVEILLANCE CENTRE MODULES
5.5
Loudspeaker Surveillance Board - LSB
PCB of 8.0 x 6.0 cm with solder-pin connection points. Used to monitor the secondary side of
the loudspeaker transformer and voicecoil, and for sensing the presence of the 20kHz pilot tone
signal and inside loudspeaker cabinets.
The Loudspeaker Surveillance Board is mounted inside a loudspeaker cabinet in order to monitor the
continuity of the loudspeaker's transformer (secondary ) the loudspeakers voice coil, and the wiring to the
amplifier by checking the presence of the 20kHz pilot tone signal. A quantity of LSB's are placed at strategic
points throughout the distribution system, monitoring the loudspeakers, and sending error information back
to the Central Processor.
The unit, a small circuit board, can easily be mounted inside a loudspeaker cabinet using standard wood
screws and spacing bushes (insulating posts). The LSB is wired in series with the secondary side of the 100V
transformer and the loudspeaker voice coil (see Fig.7). In this position it is able to sense any dramatic
changes in the current drawn by the loudspeaker, at 20kHz.
If the unit senses that no pilot tone signal is present, or that the loudspeaker voice coil is open circuited, a
message is sent (via the system's communication line) to the CPC, which would give visual, or printed,
information about the error. This unit is designed to monitor low voltage applications and is therefore not
suitable for use with amplifier outputs or 100V loudspeaker lines.
In total more than 4000 Loudspeaker Surveillance Boards (LBS's) (and/or Amplifier Surveillace Boards
(ASB's)) can be connected, via 4 Surveillance Switch Cards (SSC's)) (see chapter 12), to the Central
Processor Card CPC. The amount of LSB's used in a system is dependent on the amount of surveillance
coverage required. In principle an LSB may be used to monitor each loudspeaker in every column in the
system, but one LSB per column should be adequate. Because of the wide range of variations possible using
the 100V line principle, both at the amplifier output and the loudspeaker transformer, a sensitivity adjuster
is provided on each LSB. When the LSB has been installed it may be adjusted for loudspeaker optimum
sensitivity. Adjustment may be carried out, either in its installed position, or in a much more convenient
place such as a workplace. All that is required is a 10V dc source and a pilot tone generator (20 Khz).
To adjust the LSB for the optimum sensitivity, in its installed position, place the DIL switch on the LSB
(SW2) to its TEST position, and turn the ADJUST potentiometer (R8) until the LED just begins to
illuminate. On the threshold of the LED illuminating carefully turn the adjust till the LED switches off.
Adjustment is now complete. Place the switch back to its ON position again (see Fig.7).
To adjust the LSB in a more convenient position such as a workplace for instance, connect a +15V dc power
supply between pins 1 (+15V) and pin 2 (Gnd). Now connect a 20 kHz pilot tone signal between pins 4
and 5. Adjust the LSB as described above.
Information can be communicated to the SM40 Surveillance Centre via the DATA contacts on the LSB (see
Fig.5). A single, standard 2 core, 0.75mm screened microphone cable may be used for connection of all the
LSB's (and ASB's), which are linked via the same SSC line, to the CPC. In order for the Central Processor to
"recognise" which LSB is giving the error indication, and in which sequence it receives its communications
data, each unit is given an "address". This address setting is accomplished by setting the DIP switches on the
board (see address settings list in chapter 17).
The 15V power required to drive the LSB (distributed via an SSC), connected to the +15V and contacts, is
provided by the Basic System. All In and Out connections are simply made using 'faston' push connector
tags. The DATA, and +15V tags are duplicated in order to simplify connection of the communications line
and the power supply to the following LSB (or ASB).
LBB 1367
5
35
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