COBHAM 247 Audio Mixing Amplifier Installation and Operation Manual
The Model 247 Audio Mixing Amplifier is a versatile device designed for mixing various aircraft audio signals, including Aural Warning, Cockpit Voice Recorder, and other audio signals. It features three independent audio mixing circuits with four input channels and one output channel each. Each input channel can accept microphone, headphone, or receiver signals, and the gain can be adjusted for optimal performance. The amplifier's output can drive a wide range of impedances, from 8 ohms to 10K ohms, making it suitable for various applications.
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Installation and Operation Manual
Model 247
Audio Mixing Amplifier
(size the image to allow for optimal viewing).
SM247
SEALED
2012.03
.06
17:59:2
7 -08'00'
ISSUE 2.10
IMPORTANT NOTICE
Chelton Avionics Inc. doing business as Wulfsberg
Electronics Division is responsible for full distribution and revisions of ICA's (Instructions for
Continued Airworthiness).
For inquires regarding the content and currency of this manual contact Wulfsberg Electronics Division,
6400 Wilkinson Drive, Prescott, Arizona, 86301.
Telephone (928) 756-1615 or refer to the following website: www.wulfsberg.com
for more information.
Published on
April 25, 2012
CONFIDENTIAL AND PROPRIETARY TO COBHAM
A
Model 247 Audio Mixing Amplifier
SM247 Installation and Operation Manual
Prepared By:
Chris
Fox
2012.03.
05
17:04:09
-08'00'
Checked By:
Geoff Melton
2012.03.05
17:21:55 -08'00'
Approved By:
Zack Pahlman
2012.03.06
10:54:58
-08'00'
The status of this installation and operation manual is controlled by issue shown on the title page. The status of each section is controlled by revision shown in the footer of each page. All revisions affecting sections of this manual have been incorporated into the latest issue.
ISSUE/REVISION RECORD
Manual Issue
Number
Section
Revision Number
Revision Description Issue Date
2.10 See Manual Issue
Number
See Cover Page
2.00
DCA W12119: Combined all
Sections into one manual as
247\804-0. Added Revs 1.60,1.70,
1.80 and 1.90 drawings for Outline drawing 247.
Rewritten in full NAT format July 31, 2009 Section 1 Rev: 1.00
Section 2 Rev: 1.00
Section 3 Rev: 1.00
N/A NAT Front Cover on dB Manual June 29, 2005
247\804-0 Rev 2.10
ENG-FORM: 800-0115.DOT
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CONFIDENTIAL AND PROPRIETARY TO COBHAM
Section
3.1
3.2
2.1
2.2
2.2.1
2.3
2.4
2.4.1
2.4.2
2.4.3
2.4.4
2.4.5
2.4.6
2.4.7
2.5
2.6
2.7
3.
1.
1.1
1.2
1.3
1.4
1.4.1
1.4.2
1.4.3
1.4.4
2.
Model 247 Audio Mixing Amplifier
SM247 Installation and Operation Manual
Table of Contents
Title
Description
Introduction
Product Description
Design Features
Specifications
Electrical Specifications
Physical Specifications
Environmental Specifications
Product Approval
Installation
Introduction
Unpacking and Inspection
Warranty
Airworthiness Limitations
Installation Procedures
Warnings
Cautions
Cabling and Wiring
Mechanical Installation
Electrical Installation
Pin Assignment
Post Installation Checks
Adjustments
Accessories Required But Not Supplied
Installation Drawings
Operation
Introduction
General Information
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Page iii
2-2
2-2
2-3
2-3
2-3
2-4
2-4
2-5
2-5
2-1
2-1
2-1
2-1
2-1
2-2
3-1
3-1
Page
1-1
1-1
1-1
1-1
1-1
1-2
1-2
1-2
Model 247 Audio Mixing Amplifier
SM247 Installation and Operation Manual
Section 1 Description
1.1 Introduction
Information in this section consists of product description, design features and specifications for the
Model 247 Audio Mixing Amplifier. The Model 247-001 is also included in this manual – any other derivative product information shall be contained in the applicable manual supplement, which may be obtained from COBHAM as required.
Review all notes, warnings and cautions.
1.2 Product Description
The Model 247 Audio Mixing Amplifier contains three electrically independent audio mixing circuits with four input channels and one output channel each.
The Model 247 is designed for use as a multipurpose mixing amplifier for mixing Aural Warning, Cockpit
Voice Recorder or virtually any aircraft audio signals. Each input is adjustable to accept 0.25 Vrms microphone input signals, headphone audio signals from a standard aircraft audio system, or receiver input signals up to 50 mW. Mic bias for each input channel is user selectable. Access for gain adjustment and mic bias selection is made by cover removal.
Outputs from the Model 247 are designed to drive any impedance from 8 ohms up to 10K ohms. Output power is limited by 100 ohm resistors on each output. If additional inputs are needed beyond the four inputs provided for each mixing circuit, an output from one mixer circuit can be fed to the input of another mixer thereby increasing the size of the mixer. The Model 247 can also drive headsets for headset amplifier applications.
The Model 247 gain is established at a maximum of 12 dB with the bias/ground jumpers installed.
However, for input signals which do not require DC BIAS or a 600 ohm input impedance, the bias/ ground jumper for a given input can be removed which will result in an additional 6 dB gain increase (doubling of the gain).
1.3 Design Features
The Model 247 standard configuration is designed for driving non-DC biased (ground referenced) loads.
The Model 247-001 includes bi-polar output capacitors for compatibility with DC biased loads such as DC biased microphone inputs to other equipment.
1.4 Specifications
1.4.1 Electrical Specifications
Power Supply:
Input impedance:
Output impedance:
28 VDC at 150 mA maximum (each circuit).
Maximum current (3 mixer circuits combined) = 450 mA
600
Ω
(15K
Ω
with gnd/bias jumper removed)
100
Ω
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CONFIDENTIAL AND PROPRIETARY TO COBHAM
Frequency Response:
Distortion:
Gain Range:
Output:
Model 247 Audio Mixing Amplifier
SM247 Installation and Operation Manual
Flat within +3dB from 300 to 6,000 Hz
<1% THD+N from 300 to 6,000 Hz
>12 dB
4.5 Vrms (33mW) into 600
Ω
, Outputs are current limited
Mic bias
Note: Model 247-001 includes bi-polar output capacitors for compatibility with DC biased loads such as microphone inputs
18 Vdc at 30 mA (600
Ω
source impedance) individually selectable
1.4.2 Physical Specifications
Height
Depth
Width
Weight
Enclosure
Mounting
41.40 mm (1.63 in) maximum
76.96 mm (3.03 in) maximum
76.96 mm (3.03 in) maximum excluding flanges
102.36 mm (4.03 in) maximum including flanges
0.6 kg (0.42 lbs)
Conversion coated aluminum
Bulkhead Mount: four # 8 screws
1.4.3 Environmental Specifications
Temperature -55
°
to +70
°
C
Altitude -15,000 to 70,000 feet
Qualification of the Model 247 Audio Mixing Amplifier was completed in accordance with
RTCA/DO-160C Env. Cat. [A2F2]-BA(CL)XXXXXXZ(BZ)AAATZ(XXC2)XX
Note: Refer to Environmental Qualification Form located in Section 2 of this Manual for complete details of the environmental categories.
1.4.4 Product Approval
FAA: TSO-C50c (RTCA/DO-214 Class I b, RTCA/DO-160C).
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Page 1-2
Model 247 Audio Mixing Amplifier
SM247 Installation and Operation Manual
Section 2 Installation
2.1 Introduction
Information in this section consists of unpacking and inspection procedures, installation procedures, post-installation checks and installation drawings for the Model 247 Audio Mixing Amplifier.
Review all notes, warnings and cautions.
2.2 Unpacking and Inspection
Unpack the equipment carefully and locate the warranty card. Inspect the unit visually for damage due to shipping and report all such claims immediately to the carrier involved. Check that all items listed below are present before proceeding and report any shortage immediately to your supplier:
- Warranty Card
- Certificate of Conformity or Release Certification
2.2.1 Warranty
This product is warranted for 2 years from date of sale, to be free of defects in workmanship or performance. This warranty covers all materials and labor, but is exclusive of any transport to deliver the defective unit to and from COBHAM or its designated warranty repair center, or any labor to remove or reinstall the defective unit in the aircraft. Contact COBHAM for any questions regarding this warranty, its applicability to your units and/or for return authorization. COBHAM is the final arbitrator concerning warranty administration. Units which have been physically damaged, burned, immersed in water or otherwise abused beyond the scope of normal use will not be considered for warranty.
2.3 Airworthiness Limitations
The Airworthiness Limitations section is FAA approved and specifies inspections and other maintenance required under 14 CFR §§ 43.16 and 91.403, unless an alternative program has been FAA approved.
Maintenance of the Model 247 Audio Mixing Amplifier is ‘on condition’ only. Periodic maintenance of this product is not required. Assembly drawings, parts lists, and schematics are included in this manual for use during troubleshooting and repair.
2.4 Installation Procedures
Installation Notice
This product must be installed in accordance with the installation instructions provided in the latest issue of this Installation and Operation Manual. Check with COBHAM for the lastest issue status of the manual.
All risk associated with installation of this product contrary to these instructions shall be the responsibility of the installing agency.
247\804-0 Rev 2.10
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CONFIDENTIAL AND PROPRIETARY TO COBHAM
Page 2-1
Warnings
Model 247 Audio Mixing Amplifier
SM247 Installation and Operation Manual
2.4.1
WARNING:
High volume settings can cause hearing damage.
Set the headset volume control to the minimum volume setting prior to conducting tests, and slowly increase the headset volume to a comfortable listening level.
2.4.2 Cautions
CAUTION:
Do not bundle any lines from this unit with transmitter coax feed lines. Do not bundle any logic, audio, or DC power lines from this unit with 400 Hz synchro wiring or AC power lines. Do not position this unit next to any device with a strong alternating magnetic field such as an inverter, motor or blower, or significant audio interference will result.
In all installations, use shielded cable exactly as shown, and ground only as indicated. Significant problems may result from not following these guidelines.
Failure to follow the installation and wiring instructions provided in this manual for power and ground connections, including the rating of the circuit breaker, may lead to damage in the power input circuitry of the unit.
2.4.3 Cabling and Wiring
All wire shall be selected in accordance with the original aircraft manufacturer's Maintenance Instructions or AC43.13-1B Change 1, Paragraphs 11-76 through 11-78. Unshielded wire types shall qualify to
MIL-W-22759 as specified in AC43.13-1B Change 1, Paragraphs 11-85, 11-86, and listed in Table 11-11.
For shielded wire applications, use Tefzel MIL-C-27500 shielded wire with solder sleeves (for shield terminations) to make the most compact and easily terminated interconnect. Follow the Interconnect in
Section 2.7 as required.
Allow 3" from the end of the shielded wiring to the shield termination to allow the connector hood to be easily installed. Reference the Interconnect drawing in Section 2.7 for shield termination details.
Note that the hood is a "clamshell" hood, and is installed after the wiring is complete.
Maintain wire segregation and route wiring in accordance with the original aircraft manufacturers
Maintenance Instructions.
Unless otherwise noted, all wiring shall be a minimum of 22 AWG, except power and ground lines, which shall be a minimum of 20 AWG. Reference the Interconnect drawing for additional specifications. Check that the ground connection is clean and well secured, and that it shares no path with any electrically noisy aircraft accessories such as blowers, turn and bank instruments or similar loads. Power to this unit must be supplied from a separate circuit breaker or fuse (fast blow), and not attached to any other circuit breaker without additional protection. Verify that the selected circuit breaker size and wire gauge are adequate for the installation using the techniques specified in AC43.13-1B Change 1, Paragraphs 11-47 through 11-51 and 11-66 through 11-69.
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Page 2-2
Model 247 Audio Mixing Amplifier
SM247 Installation and Operation Manual
2.4.4 Mechanical Installation
The Model 247 is designed to be mounted in virtually any pressurized or unpressurized location within an aircraft fuselage. The unit can be mounted in any axis using four (4) #8 mounting screws. No shock or vibration isolators are required.
The Model 247 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
The Model 247 has 3 separate power and ground inputs for each of its 3 internal mixer circuits. This is designed to allow maximum flexibility for the installer if separate power supplies are used. All three power and ground inputs can also be connected together and supplied from one source. Maximum current draw for each mixer circuit within the Model 247 is 0.5 Amps assuming maximum output voltage into a maximum load of 8 ohms.
2.4.6 Pin Assignment
Pin Assignment
P40-1 Mixer 1, Input 1 HI
P40-3
P40-5
Mixer 1, Input 3 HI
Mixer 1, 28 VDC
P40-7
P40-9
P40-11
P40-13
Mixer 2, Input 2 HI
Mixer 2, Input 4 HI
Mixer 3, Input 1 HI
Mixer 3, Input 3 HI
P40-15
P40-17
P40-19
P40-21
P40-23
P40-25
P40-27
P40-29
Mixer 3, 28 VDC
Mixer 1, Input 2 LO
Mixer 1, Input 4 LO
Mixer 2, Input 1 LO
Mixer 2, Input 3 LO
Mixer 2, Power Return
Mixer 3, Input 2 LO
Mixer 3, Input 4 LO
P40-31
P40-33
P40-35
P40-37
P40-39
P40-41
P40-43
Chassis Ground(shield ground)
Chassis Ground(shield ground)
Mixer 1, Output HI
Chassis Ground(shield ground)
Mixer 2, Output LO
Chassis Ground(shield ground)
Mixer 3, Output LO
Pin Assignment
P40-2 Mixer 1, Input 2 HI
P40-4
P40-6
Mixer 1, Input 4 HI
Mixer 2, Input 1 HI
P40-8
P40-10
P40-12
P40-14
Mixer 2, Input 3 HI
Mixer 2, 28 VDC
Mixer 3, Input 2 HI
Mixer 3, Input 4 HI
P40-16
P40-18
P40-20
P40-22
P40-24
P40-26
P40-28
P40-30
Mixer 1, Input 1 LO
Mixer 1, Input 3 LO
Mixer 1, Power Return
Mixer 2, Input 2 LO
Mixer 2, Input 4 LO
Mixer 3, Input 1 LO
Mixer 3, Input 3 LO
Mixer 3, Power Return
P40-32
P40-34
P40-36
P40-38
P40-40
P40-42
P40-44
Chassis Ground (shield ground)
Mixer 1, Output LO
Chassis Ground (shield ground)
Chassis Ground (shield ground)
Mixer 2, Output HI
Chassis Ground (shield ground)
Mixer 3, Output HI
247\804-0 Rev 2.10
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Page 2-3
Model 247 Audio Mixing Amplifier
SM247 Installation and Operation Manual
2.4.7 Post Installation Checks
2.4.7.1 Voltage/Resistance Checks
Do not attach the Model 247 until the following conditions are met.
Check the following: a) Check pin <5> <10> <15> for +28 Vdc relative to ground. b) Check pins <20> <25> and <30> for ground (less than 0.5
Ω
).
2.4.7.2 Power On Checks
Power up the aircraft’s systems and verify normal operation of all functions of the Model
247.
Upon satisfactory completion of all performance checks, make all required log book entries, electrical load, weight and balance amendments and other documentation as required by your local regulatory agency before releasing the aircraft for service.
2.5 Adjustments
The Model 247 cover must be removed to gain access to the internal adjustments shown in Figure 1
Internal Adjustments below. Refer to Assembly drawing 247-1 in the SM247 Maintenance manual.
To remove the Model 247 cover, remove the 2 screws in the top of the unit. Do not remove screws under the base of the unit. Lift the cover to remove.
Reinstall the cover by following the same steps in reverse order. Tighten all screws securely but do not over-tighten.
CAUTION
The Model 247 Series Audio Mixing Amplifier contains static sensitive devices. Proper ESD handling procedures must be followed to prevent damage to the unit.
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Page 2-4
Model 247 Audio Mixing Amplifier
SM247 Installation and Operation Manual
Figure 1 Internal Adjustments
2.6 Accessories Required But Not Supplied
Installation kit p/n D44SV-IKC (crimp) is required to complete the installation. The kit consists of the following:
Quantity
1
Description
D-sub Socket, Crimp, Locking
NAT Part No.
20-21-044
44 Contacts, Crimp, Female 20-26-014
2.7 Installation Drawings
Model 247
247
247
247
247
1.50
1.60
1.70
1.80
Audio Mixer Amplifier
Audio Mixer Amplifier
Audio Mixer Amplifier
Audio Mixer Amplifier
Outline
Outline
Outline
Outline
247
247\247\521-0
1.90 Audio Mixer Amplifier
247\403-0 1.00 Mixer
1.00 Audio Mixer Amplifier
Outline
Environmental Qual Form
Section 2 ends following the above documents
062711-082411
082511-102711
102811-111311
111411-021312
021412+
All
All
247\804-0 Rev 2.10
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Page 2-5
CONFIDENTIAL AND PROPRIETARY TO COBHAM
Conditions Section Description of Conducted Tests
Fungus resistance
Salt spray
Magnetic effect
Power input
13.0
14.0
15.0
16.0
Category X, no test performed
Category X, no test performed
Class Z
Categories B and Z
Equipment met requirements for Emergency
Electrical System Operation per DO-160C subparagraph 16.5.2.1 b (3)
Category A
Category A
Voltage spike
Audio frequency conducted susceptibility
Induced signal susceptibility
Radio frequency susceptibility
17.0
18.0
19.0
20.0
Change No. 3
Category A
Category T
Emission of radio frequency energy
Lightning induced transient susceptibility
Lightning direct effect
22.0
Change No. 2
23.0
Category XXC2
Category X, no test performed
Icing 24.0 Category X, no test performed
REMARKS:
Model 247-xxx was qualified to the environmental test requirements of RTCA DO-160C by similarity to
Model 250-xxx
- Tests of DO-160C, Sections 4.0 (paragraphs 4.5.1, 4.5.2, and.4.5.3), 5.0, 16.0 and 18.0 were conducted on the Model 250 at dB Systems, Inc. in Redmond, Washington.
- Tests of DO-160C, Sections 4.0 (paragraph 4.6.1), 6.0, 7.0, 8.0, 15.0, 17.0, 19.0, 20.4 (cond. susc.),
21.0, and 22.0 were conducted on the Model 250 at the Eldec Corporation in Lynnwood, Washington.
- Tests of DO-160C, Sections 4.0 (paragraphs 4.6.2, and 4.6.3) and 20.5 (radiated susc.) were conducted on the Model 250 at Allied Signal in Redmond, Washington.
End of Environmental Qualification Form
Rev: 1.00 Aug 4, 2009 Page 2 of 2
ENG-FORM: 521-0102.DOT
CONFIDENTIAL AND PROPRIETARY TO NORTHERN AIRBORNE TECHNOLOGY LTD.
Model 247 Audio Mixing Amplifier
SM247 Installation and Operation Manual
Section 3 Operation
3.1 Introduction
Information in this section consists of the functional and operational procedures for the Model 247 Audio
Mixing Amplifier.
3.2 General Information
The Model 247 Audio Mixing Amplifier has no operator accessible controls. During installation, it may be determined that internal level adjustments are required. Only qualified personnel shall complete internal level adjustments.
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Page 3-1

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Key features
- 3 Electrically Independent Audio Mixing Circuits
- 4 Input Channels Per Circuit
- 1 Output Channel Per Circuit
- Adjustable Inputs
- Selectable Mic Bias
- Gain Adjustment
- Multipurpose Mixing Amplifier