COBHAM 251 Operating instructions

Amendment #2 Aug 9, 2011

CONFIDENTIAL AND PROPRIETARY TO COBHAM

DOCUMENT NUMBER 251-16-1

251

PASSENGER SPEAKER AMPLIFIER

1

INSTALLATION AND OPERATION MANUAL

REV "-" Sep 15, 2004

Cobham

6400 Wilkinson Drive

Prescott, AZ 86301-6164

(928) 708-1500

SEALED

2011.08.16 

17:15:38 

-07'00'

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

Aug 9, 2011 Page ii

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

CONFIDENTIAL AND PROPRIETARY TO COBHAM

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.

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

Aug 9, 2011 Page iii

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

CONFIDENTIAL AND PROPRIETARY TO COBHAM

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

Number

Amendment

Date

Amendment Change Check Approve

1 Aug 3/07

2 Aug 9/11 Incorporated ECR11040.

Added 251-[ ] MOD F.

Insert any Amendment Instruction sheets after this page.

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

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16:23:14 

-07'00'

INSTALL_OPS

MANUAL AMENDMENT

Manual: 251-16-1 Amendment #: 2

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

Amendment # 2 August 9, 2011 Page 1

ENG-FORM: 809-0109.DOT

CONFIDENTIAL AND PROPRIETARY TO COBHAM

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

per ECR11040.

Amendment Instructions:

Remove Pages Replace With Pages

1

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

2

Remove Drawings Replace With Drawings

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

2

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

INSTALLATION & OPERATION

MANUAL AMENDMENT

Manual: SM251 251 Series Amendment #: 1

Document # SM251\Install_Ops\809-0001 Amendment Date: Aug 03, 2004

Amendment #1 Aug 03, 2004 Page 1

ENG-FORM: 809-0107.DOT

PROPRIETARY AND CONFIDENTIAL TO NORTHERN AIRBORNE TECHNOLOGY LTD.

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

information.

Amendment Instructions:

Remove Pages Replace With Pages

1

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

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

Page 7 Page 7, Amendment #1

Insert Pages After Page

2

Pages ii – iii SM251 Title Page

Add Section (to the end of the manual)

3

4.0 Supplements

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

EQUIPMENT MANUAL 251-16-1 REV -

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

Page a

CHANGE RECORD

DATE CHANGE CHECK APPROVAL

3-11-03

ORIGINAL DOCUMENT RELEASED

AP

ML

EQUIPMENT MANUAL 251-16-1 REV -

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

PROPRIETARY AND CONFIDENTIAL TO COBHAM

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

EQUIPMENT MANUAL 251-16-1 REV -

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

PROPRIETARY AND CONFIDENTIAL TO COBHAM

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.

EQUIPMENT MANUAL 251-16-1 REV -

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

PROPRIETARY AND CONFIDENTIAL TO COBHAM

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

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251/251-16-1

Page 1

EQUIPMENT MANUAL

MODEL 251

PASSENGER SPEAKER AMPLIFIER

1.0 EQUIPMENT OPERATIONS AND INSTALLATION

The Model 251 Passenger Speaker Amplifier is a remotely controlled, electronic unit that operates

upon command from remote switches and potentiometers in the cockpit or cabin of the aircraft.

The Model 251 provides Seat Belt and No Smoking sign chime tones, 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 other 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 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 pots required are zero to 10,000 ohms, linear

taper, and are electrically arranged to provide 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 intercom system. The chime and ringer tones do not pass through

either of the electronic attenuators. Internal screwdriver potentiometer adjustments are

accessible, after removal of the Model 251 end plate, to set the chime or ringer levels heard in the

passenger speakers.

Each left and right speaker amplifier output rating is 28 Watts into 2 ohms (30 Watts typical) or 49

Watts into 1 ohm. Both speaker amplifier outputs are internally protected against open and short

circuits as well as over-temperature (>105°C). A remote electronic ON/OFF switch for the left and

right speaker outputs is provided. 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 V

RMS

adjustable)

outputs that carry the same audio signals as the speaker outputs. Unlike the speaker outputs, the

EQUIPMENT MANUAL 251-16-1

251/251-16-1

Page 2

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

circuits.

The operating modes of the Model 251 are established when either a ground (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 potentiometer 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.

EQUIPMENT MANUAL 251-16-1

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The amplifiers are adjusted for acceptable listening output levels into the proper loads with ADF

receiver inputs of 7.75 V

RMS

, stereo audio inputs of 0.45 V

RMS

, PSC, TV, and Briefer audio inputs

of 1.00 V

RMS

and PA MIC input of 0.25 V

RMS

. With the volume controls set to maximum volume

position, these inputs will provide 7.5 V

RMS

(28 watts) into 2 ohms at each speaker output. The

ADF receiver input levels are achieved when receivers are set for 100 milliwatts into 600 ohms. If

levels are greatly exceeded at any of the inputs, the audio will become distorted due to peak

clipping and audio bleedthrough will occur. However, the amplifiers will not be damaged.

2.0 SPECIFICATIONS

Input specifications are expressed in volts RMS, rather than milliwatts, to avoid any

misunderstanding.

INPUTS

:

ADF: Two individual, switched audio inputs for ADF1 and ADF2.

Input level is 7.75 V

RMS

.

PSC: One switched audio input compatible with the Pilot Select COMM output of

dB Systems cockpit audio control amplifiers (such as Models 438 and 700), which

provides a 1.00 V

RMS

signal produced from a summation of selected COMM,

DME and MKR receivers. The PSC input can also be used for TV audio or other

similar audio.

TV: One switched audio input for TV (or other device). Input level is 1.00 V

RMS

.

STEREO

LEFT: One switched differential audio input for the left audio channel from an

AM/FM Stereo receiver/tape or CD deck (or other device). Input level is maximum

0.45 V

RMS

(at maximum source volume) and audio LO is not common to that

of other inputs.

STEREO

RIGHT: Same as stereo left, switched by the same control input as stereo left.

BRIEF: One switched audio input for automated cabin briefer audio. Input level is

1.00 V

RMS

.

PA MIC: The microphone input used for cabin paging audio supplies bias current and is

compatible with carbon and amplified dynamic microphones. Input level is

0.25 V

RMS

.

PAGING

VOLUME: Controls level of PA MIC and brief audio inputs to the PA sidetone and left and

right speaker outputs. Volume control range is 30 dB (minimum) controlled by

a 10,000-ohm, 1/2-Watt, linear pot. Maximum volume is with maximum resistance.

RECEIVER

VOLUME: Controls level of ADF, PSC and TV audio inputs to the left and right speaker

outputs. Volume control range is 30 dB (minimum) controlled by a 10,000-ohm,

1/2-Watt, linear pot. Maximum volume is with maximum resistance.

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Page 4

OUTPUTS

:

RIGHT

SPEAKER: The right speaker amplifier is designed for speech and music signals.

Rated power specification is the maximum power the amplifier can

continuously deliver into the maximum rated load without clipping.

Rated Power

Load

49 W 1 ohm (40 Watts typical)

28 W 2 ohms (30 Watts typical)

15 W 4 ohms (18 Watts typical)

LEFT

SPEAKER: Same as right speaker specification

RIGHT

MONITOR: This output will deliver up to 26 milliwatts into 600 ohms continuously.

Factory calibration is 1.6 mW (1 V

RMS

).

LEFT

MONITOR: Same as right monitor specification

PA SDTN: This output will deliver 6.7 milliwatts (2 V

RMS

) into 600 ohms continuously.

Rated and continuous power ratings are the same.

CHIME

TONE: Generated and supplied to left and right speaker and monitor outputs when

12 to 32 VDC is applied to or removed from connector pins P50-31 or P50-32.

A 790 Hz tone followed by a 610 Hz tone (2-tone, HI/LO) is factory set for 3.5 VPP

(at speaker outputs), adjustable from 1.0 to 6.9 VP-P and is also available at

monitor outputs.

RINGER

TONE: Generated and supplied to left and right speaker and monitor outputs

when a ground signal is applied to connector pin P50-33. Ringer tone is

700 +

75 Hz tone, ON for 1.8 +0.1 second, then OFF for 3.2 +0.1

second, repeating. Factory set for 1.5 VP-P (700 Hz component)

(at speaker outputs), adjustable from 0 to 5.2 VPP.

FREQUENCY

RESPONSE: Upper and lower frequency limits are adjustable. Stereo, ADF, TV, and

PSC inputs to speaker and monitor outputs are flat within 3 dB from 40 Hz to

20,000 Hz. All other inputs to speaker, monitor, or PA sidetone outputs flat within

3 dB from 300 Hz to 6,000 Hz.

HARMONIC

DISTORTION

(THD+N): Less than 0.30 percent distortion, for stereo and less than 1% for ADF, TV

and, PSC to all outputs (40 Hz to 20 Khz). Less than 3% for PA and briefer to

all outputs (300 Hz to 6 Khz).

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Page 5

ISOLATION BETWEEN

CHANNELS: 60 dB, minimum.

OUTPUT NOISE: Greater than 75 dB below rated, all outputs.

POWER SUPPLY: 28 VDC, 5 ampere maximum.

OPERATING TEMPERATURE: From -55 to +70°C. Thermal fold back of the speaker

outputs occurs if the heatsink temperature reaches 105°C.

OPERATING ALTITUDE: To 70,000 feet.

QUALIFIED TO:

FAR PART 21, TSO-C50c [A2F2]-BA(CL)XXXXXXZ(BZ)AAATZ(XXC2)XX

RTCA DO-160C: Refer to Table 3, Environmental Qualification Form,

for test category information.

EQUIPMENT MANUAL 251-16-1

251/251-16-1

Page 6

3.0 INSTALLATION PROCEDURES, WIRING DIAGRAM, AND LIMITATIONS

3.1 PHYSICAL

Installation dimensions are shown on the Model 251 outline on Figure 1. The amplifier should be

mounted to a metal surface and it is necessary to ground the case to the metal aircraft structure.

The case is finished with an electrically conductive film so it is not necessary to remove the film for

electrical bonding.

The Model 251 may be mounted in any position. No shock or vibration isolators are required.

3.2 ELECTRICAL

Connections to the Model 251 are made through three "D" subminiature connectors. The mating

connectors are manufactured by Positronics Industries, Inc. and the part numbers for the

complete connector assemblies are:

Connector

Positronics Part No.

P40 (25 pin) SD25F10JVLO

P50 (37 pin) SD37F10JVLO

P60 (9 pin) SD9F10JVLO

Numerical pin identifications are shown in this section. The wiring diagram is shown on Figure 2.

Shielded wire must be used where indicated on the diagrams. AWG 22-24 size wire should be

used for all connections except those to the input power, power ground, and speaker amplifier

outputs where AWG 20 size should be used. Multiple power, ground, and speaker output pins are

provided to allow parallel connection of wiring. This reduces voltage drops (losses) across the

wiring and provides the best audio performance.

The control switches shown will switch low current loads and should have a DC rating of 28 VDC.

Typical switches for this application include Alco MTA series.

B

Prescott,AZ

Chelton Avionics, Inc

dba Wulfsberg Electronics Division

Amendment #8 Page 7

EQUIPMENT MANUAL 251-16-1

251/251-16-1

Page 8

Figure 2. Block Diagram, Model 250-XXX

Passenger Speaker Amplifier

EQUIPMENT MANUAL 251-16-1

251/251-16-1

Page 9

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).

Table 1. Pin Identification

P40-1 (25-pin connector):

1 Left Spkr Out HI

2 Left Spkr Out HI

3 Left Spkr Out LO

4 Left Spkr Out LO

5 Left Spkr Out LO

6 28 VDC

7 28 VDC

8 28 VDC

9 Right Spkr Out LO

10 Right Spkr Out LO

11 Right Spkr Out LO

12 Right Spkr Out HI

13 Right Spkr Out HI

14 Left Spkr Out HI

15 No connection

16 No connection

17 Power Ground

18 Power Ground

19 Power Ground

20 Power Ground

21 28 VDC

22 Chassis Ground

23 No connection

24 No connection

25 Right Spkr Out HI

P50-1 (37-pin connector):

1 Audio Common (except stereo)

2 PA MIC HI

3 Paging Volume

4 Briefer Audio HI

5 Stereo Switch

6 Left Input HI

7 Left Input LO

8 Right Input LO

9 Right Input HI

10 No Connection

11 Receiver Volume Control

12 No connection

13 TV Switch

14 PSC Switch

15 ADF 2 Switch

16 ADF 1 Switch

17 ADF 1 Audio HI

18 TV Audio HI

19 PSC Audio HI

20 Audio Common (except stereo)

21 Audio Common (except stereo)

22 Audio Common (except stereo)

23 No connection

24 PA Key

25 Briefer Switch

26 No connection

27 Audio Common (except stereo)

28 Audio Common (except stereo)

29 Audio Common (except stereo)

30 No connection

31 Smoking Sign Switch

32 Seat Belt Sign Switch

33 Cabin Call Switch

34 Hook Switch

35 PA Sidetone HI

36 Audio Common (except stereo)

37 ADF 2 Audio HI

P60 (9-pin connector):

1 Right Monitor Output HI

2 Right Monitor Output LO

3 Speaker Mute

4 Left Monitor Output LO

5 Left Monitor Output HI

8 Speaker Enable

6, 7, 9/No Connection

EQUIPMENT MANUAL 251-16-1

251/251-16-1

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Figure 3. Wiring Diagram

EQUIPMENT MANUAL 251-16-1

251/251-16-1

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Figure 3. Wiring Diagram (continued)

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COBHAM 251 Operating instructions

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