COBHAM 251 Installation And Operation Manual

DOCUMENT NUMBER 251-16-1

SEALED

2011.08.16 
17:15:38 

-07'00'

251

PASSENGER SPEAKER AMPLIFIER

1

INSTALLATION AND OPERATION MANUAL

REV "-" Sep 15, 2004

Cobham

6400 Wilkinson Drive

Prescott, AZ 86301-6164

[phone]

Amendment #2 Aug 9, 2011

CONFIDENTIAL AND PROPRIETARY TO COBHAM

251-16-1 Rev. - 251 Passenger Speaker Amplifier Installation and Operation Manual

Cobham

has acquired the assets of Nothern Airborne

Technology Ltd. and dB Systems Inc., and

has reissued this manual in its entirety

Cobham

will be responsible for all future

amendments and revisions.

Aug 9, 2011 Page ii

ENG-FORM: 820-0109.DOT Amendment #2 Aug 9, 2011

CONFIDENTIAL AND PROPRIETARY TO COBHAM

251-16-1 Rev. - 251 Passenger Speaker Amplifier Installation and Operation Manual

Amendment

Zack Pahlman

2011.08.16 
11:01:10 -07'00'

Geoff 
Melton

2011.08.16 
14:45:47 

-07'00'

Aaron 
Voigt 
(Approved
)

2011.08.1
6 
16:23:14 

-07'00'

Periodically Cobham will release manual amendments. In order to maintain the most
accurate and up to date manual these amendments should be carried out immediately
upon receipt and recorded on the following amendment record.

AMENDMENT RECORD

Amend

ment

Date

Amendment Change Check Approve

Number

1 Aug 3/07

2 Aug 9/11 Incorporated ECR11040.

Added 251-[ ] MOD F.

Insert any Amendment Instruction sheets after this page.

Aug 9, 2011 Page iii

ENG-FORM: 820-0109.DOT Amendment #2 Aug 9, 2011

CONFIDENTIAL AND PROPRIETARY TO COBHAM

INSTALL_OPS
MANUAL AMENDMENT

Manual: 251-16-1 Amendment #: 2
Document #: 251-16-1 809-0002 Amendment Date: Aug 9, 2011

The purpose of this amendment is to update the 251 outline drawing and MOD table
per ECR11040.

Amendment Instructions:

1

2

2

Remove Pages Replace With Pages

Title Page Amendment # 1 Title Page Amendment # 2

Page ii Amendment # 1 Page ii Amendment # 2

Page iii Amendment # 1 Page iii Amendment # 2

Page b Amendment #1 Page b Amendment #2
Page c Amendment #1 Page c Amendment #2
Page d Amendment #1 Page d Amendment #2

Remove Drawings Replace With Drawings

251 Rev 1.51 (page 7) 251 Rev 1.70 (page 7)

Add Drawings After Drawings

251-003 810-0 Rev 1.30 Page 15

251-006 810-0 Rev 1.00 251-003 810-0 Rev 1.30

Note: Ensure that all drawings are inserted in the order shown on the latest drawing lists.

2 Update the Amendment Record sheet at the front of the manual.

3 Insert this page into the manual after the Amendment Record sheet (page iii).

Manual Amendment ends after the following amended pages

Amendment # 2 August 9, 2011 Page 1

ENG-FORM: 809-0109.DOT

CONFIDENTIAL AND PROPRIETARY TO COBHAM

INSTALLATION & OPERATION
MANUAL AMENDMENT

Manual: SM251 251 Series Amendment #: 1
Document # SM251\Install_Ops\809-0001 Amendment Date: Aug 03, 2004

The purpose of this amendment is to correct the outline drawing and include the NAT
information.

Amendment Instructions:

1

Pages b – c, rev - Pages b – d, rev -, Amendment #1
Page 7 Page 7, Amendment #1

2

3

4 Update the Amendment Record sheet at the front of the manual.

5 Insert this page into the manual after the Amendment Record sheet (page iii).

Manual Amendment ends after the following amended pages

Remove Pages Replace With Pages

251-16-1 Title Page, rev - SM251 Title Page, rev -

Insert Pages After Page

Pages ii – iii SM251 Title Page

Add Section (to the end of the manual)

4.0 Supplements

Amendment #1 Aug 03, 2004 Page 1

ENG-FORM: 809-0107.DOT

PROPRIETARY AND CONFIDENTIAL TO NORTHERN AIRBORNE TECHNOLOGY LTD.

EQUIPMENT MANUAL 251-16-1 REV -

CHANGE RECORD

DATE CHANGE CHECK APPROVAL

3-11-03

ORIGINAL DOCUMENT RELEASED

AP

ML

251/251-16-1 Amendment #1 Aug 03, 2004

Page a

EQUIPMENT MANUAL 251-16-1 REV -

TABLE OF CONTENTS

Page

1.0 EQUIPMENT OPERATIONS AND INSTALLATION.................................................................1

2.0 SPECIFICATIONS ....................................................................................................................3

3.0 INSTALLATION PROCEDURES, WIRING DIAGRAM, AND LIMITATIONS............................6

3.1 PHYSICAL.................................................................................................................................6

3.2 ELECTRICAL ............................................................................................................................6

Tables

Table A Detail Part Numbers..........................................................................................................c

Table 1 Pinout Listing.....................................................................................................................9

Table 2 Environmental Qualification Form ...................................................................................14

Figures / Drawings

251 Outline, Model 251-[ ], Passenger Speaker Amplifier (Rev 1.70)....................................7

Figure 2 Model 251-[ ] Block Diagram...........................................................................................8

Figure 3 Model 251-[ ] Wiring Diagram .......................................................................................10

Figure 4 Internal Trim Pot Adjustments.........................................................................................13

251/251-16-1 Amendment #2 Aug 9, 2011 Page b

PROPRIETARY AND CONFIDENTIAL TO COBHAM

EQUIPMENT MANUAL 251-16-1 REV -

MODEL 251 DETAIL PART NUMBER PROCEDURES AND LEVELS

The following is a tabulation of all detail part numbers available for the

Model 251 Passenger Speaker Amplifier. A Model 251 with an
associated detail part number is designated as a Model 251-XXX,
where XXX is the applicable three digit detail part number.

Table A. Detail Part Numbers

Detail Part No. Description of Detail

001 Low pass filters adjusted for -3 dB attenuation at 90 Hz on left and

right speaker outputs. High pass filters adjusted for -3 dB attenuation
at 20 Khz on left and right speaker outputs.

-001 calibration is designed for use with 4-inch speakers.

002 Identical to 251 Base Model. Part number 251-002 was created to

prevent a conflict with an existing part number 251 in customer part
system.

003 Identical to 251 Base Model with the following exceptions:

a) Briefing audio is to output through the right speaker only.

b) The receiver inputs and switches are not available.

c) The briefer switch input is externally grounded, disabling the

Stereo Left input and the Stereo Right input.

Installation information can be found in the 251-003 Installation and
Operation Manual Supplement.

006 Identical to 251 Base Model with the following exceptions:

a) The chime tones are routed to the PA sidetone audio output.

b) The chime tones are activated upon the activation of the PA

key.

Installation information can be found in the 251-006 Installation and
Operation Manual Supplement.

251/251-16-1 Amendment #2 Aug 9, 2011 Page c

PROPRIETARY AND CONFIDENTIAL TO COBHAM

EQUIPMENT MANUAL 251-16-1 REV -

SM251 FIGURES

This page lists the drawings necessary for complete maintenance and troubleshooting.
The applicable serial numbers are shown in the right hand column.

Previously only the most current revision of each figure was included in this
manual. For these figures, the rev is listed as "N/A".

Table B. Figures with Applicable Serial Numbers

FIGURE/DWG REV. DESCRIPTION TYPE SERIAL #

251 1.70 Passenger Speaker Amplifier Outline All

2 N/A Passenger Speaker Amplifier Block Diagram All

3 N/A Passenger Speaker Amplifier Wiring Diagram All

4 N/A Passenger Speaker Amplifier Internal Trim Pot Adjustment All

251/251-16-1 Amendment #2 Aug 9, 2011 Page d

PROPRIETARY AND CONFIDENTIAL TO COBHAM

EQUIPMENT MANUAL 251-16-1

EQUIPMENT MANUAL

MODEL 251

PASSENGER SPEAKER AMPLIFIER

1.0 EQUIPMENT OPERATIONS AND INSTALLATION

The Model 251 Passenger Speaker Amplifier is a r emotely controlled, electronic unit that operates
upon command from remote switches and potent iometers in the cockpit or cabin of the aircraft.
The Model 251 provides Seat Belt and No Smoking sign chime t ones, a Cabin Call ringer tone,
ADF, Pilot Select Comm. (PSC), Briefer, and Cabin Paging audio (all in mono) to the cabin
speakers. It also provides audio from an external AM/FM stereo receiver/cassette tape or CD
player (in stereo) to the cabin speakers and auxiliary monitor outputs.

The left and right speaker amplifier channels are each powered through separate internal fuses so
that a failure in one speaker channel will not disable the ot her speaker channel or the remaining
control circuitry. The left and right monitor outputs are powered from a third power supply circuit.

All inputs and operating modes are electronically switched. Ground sig nals, supplied by manual
switches in the cockpit, enable the electr onic switches in the Model 251. The circuit ry is arrang ed
to give audio priority to paging, briefer, and chimes; the PA MIC, BRIEF, and Seat Belt/No
Smoking chime switch signals mute t he 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 f r om 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. T he pots required are zero t o 10,000 ohms, linear
taper, and are electrically arranged to pr ovide maximum resistance for 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).

A musical chime tone is provided to the cabin speakers to alert the passengers of a change in the
Fasten Seat Belt or No Smoking sign status. A Cabin Call ringer tone is also provided to the
cabin for a cockpit to cabin interc om system. The chime and ringer tones do not pass through
either of the electronic attenuators. Internal screwdriver potentiometer adjustments are
accessible, after r e m oval of the Model 251 end plate, to set the chime or ringer levels heard in the
passenger speakers.

Each left and r ight speaker amplifier output rating is 28 Watts int o 2 ohm s ( 30 Watts typical) or 49
Watt s into 1 ohm. Both speak er amplifier outputs are internally protected against open and short
circuits as well as over-temperature (>105°C). A r emote electronic ON/OFF switch for the left and
right speaker outputs is provided. A r em ote 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 V
outputs that carry the same audio signals as the speaker outputs. Unlike the speaker outputs, the

adjustable)

RMS

251/251-16-1

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EQUIPMENT MANUAL 251-16-1

monitor outputs cannot be deselected or muted independently of the normal input switching
circuits.

The operating m odes of the Model 251 are established when either a gr ound (low) signal or a 12
to 32 VDC (high) signal is applied as listed below to the connector pin listed:

Control
Pin No.

Signal Identification Function

P50-16 Low ADF 1 Switch Connects ADF 1 input audio to left
and right speaker and monitor outputs.
P50-15 Low ADF 2 Switch Connects ADF 2 input audio to left
and right speaker and monitor outputs.
P50-14 Low PSC Switch Connects Pilot Select Comm (PSC) input
audio to left and right speaker and
monitor outputs.
P50-13 Low TV Switch Connects TV input audio to left and right
speaker and monitor outputs.
P50-5 Low Stereo Switch Connects stereo left and stereo right
input audio to left and right speaker
and monitor outputs, respectively.
P50-24 Low PA Key Connects PA MIC input audio to PA
sidetone, left and right speaker,
and monitor outputs. Opens ADF, PSC,
TV, and stereo input audio circuits.
P50-25 Low Briefer Switch Connects briefer input audio to PA sidetone,
left and right speaker, and monitor outputs.
Opens ADF, PSC, TV, and stereo
input audio circuits.
P50-31* High No Smoking Chime tone to left and right speaker
and monitor outputs. Opens ADF, TV,
PSC, and stereo inputs.
P50-32* High Seat Belt Chime tone to left and right speaker
and monitor outputs. Opens ADF, TV,
PSC, and stereo inputs.
P50-33 Low Cabin Call Ringer tone to left and right speaker
and monitor outputs.
P50-34 Low Hook Switch Disables ringer tone.
P60-8 Low Speaker Enable Enables left and right speaker outputs.
P60-3 Low Speaker Mute Mutes (lowers volume -6 dB) left and right
speaker outputs.

*An application or

removal of a High signal at pins P50- 31 and P50-32 will provide a chime tone at

left and right speaker outputs regardless of speaker enable status.
NOTE: P50-XX is a pin on the 37-pin connector, P50

P40-XX is a pin on the 25-pin connector, P40
P60-XX is a pin on the 9-pin connector, P60

All internal screwdriver potentiomet er adjustments of the Model 251 are set to factory standards.
If it is necessary to change a setting, refer to Section 4.0 of this manual.

251/251-16-1

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