Northern Airborne Technology 251 - Manual (Page 11)

Model 251 Passenger Speaker Amplifier
SM251 Installation and Operation Manual
Section 2 Rev: 1.00 Issue 2 Page 2-3
ENG-FORM: 805-0120.DOT
CONFIDENTIAL AND PROPRIETARY TO NORTHERN AIRBORNE TECHNOLOGY LTD.
2.4.4 Mechanical Installation
The Model 251 is designed to be mounted in virtually any pressurized or unpressurized location within an
aircraft fuselage. The unit can be mounted either vertically or horizontally (with the heatsink facing up)
using four (4) #8 mounting screws. The mounting location must provide adequate ventilation to allow heat
dissipation. No shock or vibration isolators are required.
The Model 251 must be mounted to a clean metal surface which is electrically bonded to the aircraft
ground plane. The unit is finished with a coating which prevents corrosion. This film is electrically
conductive and should not be removed for electrical bonding.
2.4.5 Electrical Installation
All inputs and operating modes are electronically switched. Ground signals, supplied by manual switches
in the cockpit, enable the electronic switches in the Model 251. The circuitry is arranged to give audio
priority to paging, briefer, and chimes; the PA MIC, BRIEF, and Seat Belt/No Smoking chime switch
signals mute the Stereo Left, Stereo Right, ADF 1, ADF 2, PSC, and TV audio inputs. PA, briefer, and
chimes will automatically enable speakers even if the speaker switch is off. Microphone bias current is
available from the Model 251 for the PA MIC microphone input.
Electronic attenuators are employed to control paging volume and master receiver (ADF 1, ADF 2, PSC,
and TV) volume. Each attenuator has approximately 30 dB (minimum) of range and is controlled by a
remote volume potentiometer. The potentiometers required are zero to 10,000 ohms, linear taper;
maximum resistance provides maximum volume. The audio source (stereo tape cassette player, etc.) for
the Stereo Left and Right inputs should have its own external volume control attenuator for the Stereo
Input to Speaker and Monitor Output audio channels. The left and right stereo inputs are fully floating
(differential type).
The chime or ringer levels heard in the passenger speakers may be adjusted (see section 2.5).
The control switches shown will switch low current loads and should have a DC rating of 28 Vdc.
The paging and receiver volume control pots required are linear taper, zero to 10,000 ohms, 1/2 watt or
greater. Electrically, the pots are wired to provide maximum resistance to the Model 251 for full volume
(fully clockwise rotation).
Speaker enable inputs are provided to allow remote ON/OFF switching for the left and right speakers.
A remote electronic speaker mute control is also provided. The mute control can be connected to the
aircraft’s Weight-On-Wheels switch to lower speaker volume upon landing when lower cabin ambient
noise levels are present and less speaker volume is needed. The speaker mute control drops speaker
level by approximately 1/2 (-6 dB) when grounded.
Left and right stereo monitor outputs are provided for connecting to a cabin headphone amplifier or
headphone audio distribution system. The monitor outputs are line-level (1 Vrms adjustable) outputs that
carry the same audio signals as the speaker outputs. Unlike the speaker outputs, the monitor outputs
cannot be deselected or muted independently of the normal input switching circuits.