Terra TX 760D User manual

OPERATION/INSTALLATION MANUAL
Trimble
2105 Donley
Austin, Texas 78758
[phone]
PUBLICATION NUMBER 82619
PRINTED IN U.S.A.
TX 760D
VHF COMMUNICATION TRANSCEIVER

OPERATION/INSTALLATION MANUAL
Trimble
2105 Donley
Austin, Texas 78758
[phone]
DECEMBER 16, 1996
PUBLICATION NUMBER 82619
REVISION B
PRINTED IN U.S.A.
TX 760D
VHF COMMUNICATIONS TRANSCEIVER
T-1
Title Page

Page A
Revision A
November 25, 1996
Terra by Trimble TX 760DVHF Communication Transceiver
LIST OF EFFECTIVE PAGES
Insert latest change pages and destroy superceded pages.
Page Description Page no. Date
Title Page T-1 November 25, 1996
List of Effective Pages A November 25, 1996
Record of Revisions B November 25, 1996
Table of Contents (Main) TC-1 - TC-2 November 25, 1996
Section I - Introduction 1-1 - 1-4 November 25, 1996
Section II - Installation 2-1 - 2-10 November 25, 1996
Section III - Installation Drawings 3-1 - 3-14 November 25, 1996
Section IV - Operation 4-1 - 4-6 November 25, 1996
Section V - Warranty 5-1 - 5-2 November 25, 1996
Section VI - Installation Bulletins/Notes 6-1 -6-2 November 25, 1996
Dates of Change pages are:
Revision...........A .........November 25, 1996

Page B
Revision A
November 25, 1996
Terra by Trimble TX 760DVHF Communication Transceiver
Assigned To (Job Title) Location
RECORD OF REVISIONS
Rev Revision Insertion Rev Revision Insertion
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No. Date Date No. Date Date

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Revision A
November 25, 1996 TC-1
TX 760D VHF COMMUNICATIONS TRANSCEIVER
OPERATION/INSTALLATION MANUAL
TABLE OF CONTENTS
SECTION I ...............................................................................................................................1-1
1. INTRODUCTION..........................................................................................1-1
1.1 SCOPE ..............................................................................................1-1
1.2 DESCRIPTION .................................................................................1-1
1.3 SPECIFICATIONS ...........................................................................1-2
1.4 EQUIPMENT SUPPLIED ................................................................1-3
1.5 ADDITIONAL EQUIPMENT REQUIRED .....................................1-4
1.6 LICENSE REQUIREMENTS...........................................................1-4
SECTION II..............................................................................................................................2-1
2. INSTALLATION .......................................................................................... 2-1
2.1 GENERAL ....................................................................................... 2-1
2.2 PREPARATION FOR USE ..............................................................2-1
2.3 GENERAL INSTALLATION INSTRUCTIONS
FOR AVIONICS .............................................................................. 2-1
2.4 INSTALLATION..............................................................................2-5
2.5 ANTENNA INSTALLATION .........................................................2-9
2.6 OPERATIONAL CHECKOUT ........................................................2-9
2.7 FINAL INSPECTION .......................................................................2-7
SECTION III ............................................................................................................................3-1
3. INSTALLATION DRAWINGS ....................................................................3-1
SECTION IV ........................................................................................................................... 4-1
4. OPERATION .................................................................................................4-1
4.1 SCOPE ..............................................................................................4-1
4.2 OPERATION ....................................................................................4-1
4.3 FREQUENCY MODE ......................................................................4-1
4.4 NORMAL OPERATION ..................................................................4-1
4.5 FREQUENCY TRANSFER .............................................................4-2
4.6 COMM TUNING KNOB OPERATING SEQUENCE ....................4-2
4.7 MEM (MEMORY) OPERATING SEQUENCE ..............................4-2
4.8 TO PROGRAM A NEW FREQUENCY INTO MEMORY ............4-3
4.9 DISPLAY BLANKING OPERATION.............................................4-3
4.10 VOX INTERCOM OPERATION.....................................................4-4
4.11 EMERGENCY OPERATION ..........................................................4-4
4.12 BLIND OPERATION .......................................................................4-5
4.13 FAILURE MODE OPERATION......................................................4-5
4.14 SAFEGUARDS OPERATION .........................................................4-5

TC-2
Revision A
November 25, 1996
SECTION V .............................................................................................................................5-1
5. WARRANTY................................................................................................ 5-1
5.1 LIMITED WARRANTY OF TERRA BY TRIMBLE PRODUCTS5-1
SECTION VI ............................................................................................................................6-1
6. INSTALLATION BULLETINS/NOTES ......................................................6-1
6.1 INSTALLATION BULLETINS .......................................................6-1
6.2 INSTALLATION NOTES ................................................................6-2

Revision A
November 25, 1996 1-1
SECTION I
1. INTRODUCTION
1.1 SCOPE
This manual provides installation and operation instructions for the Terra by Trimble TX 760D
VHF Communications Transceiver manufactured by Trimble of Austin, Texas.
1.2 DESCRIPTION
TheTerra by Trimble TX 760D is a small, lightweight, all solid-state, amplitude modulated, 5
watt VHF transceiver. The TX 760D provides simplex communication on any one of 760
channels with 25 KHz spacing. Featuring a planar gas discharge digital display, the TX 760D
provides active and standby frequency indication, memory, and knob operated frequency
selection.
The TX 760D features a digital frequency synthesizer that employs one crystal for generating
the frequencies required for transmit and receive functions on all 760 channels. Other
features include automatic voice leveling on transmit, crystal monolithic I.F. filters, integrated
circuit I.F. amplifiers, and internal two place VOX intercom capability. Use of a low noise
MOSFET RF amplifier, automatic squelch circuit with manual override, solid-state antenna
switching,switchingpowersupplywithoverloadprotection,andmicroprocessorcontrolled
frequency selection and monitoring make the TX 760D a reliable answer to today’s
communicationrequirements.
The transceiver can be mounted in two different configurations; in a single tray or in a dual
tray. All power and control connections are made through a 15 pin “D” type connector
installed on the rear of the mounting tray. A BNC chassis connector is used for antenna
connections. The TX 760D is secured by means of a positive locking cam, accessible through
the front panel, using a 7/64" Allen wrench.
Weighing only 1.50 lb. (0.680 kg), the TX 760D measures 11.45" long, 3.125" wide, and 1.625"
high (29.08 cm x 7.9375 cm x 4.1275 cm). Panel cutout is a 3.03" x 1.54" (7.6962 cm x
3.9116 cm) and overall length of the unit is 13.05" (33.147 cm) including mounting tray.
The TX 760D is very power efficient, requiring only 325 mA of standby current and 2.5 A
during transmit. Operation at 27.5 VDC is possible by utilization of an optional Terra by
Trimble MLC 28-5 power converter. The TX 760D is designed to operate with standard
aircraft microphones, headsets, speakers, auxiliary audio inputs and 50 ohm broad-band
communication antenna systems.
1.3 SPECIFICATIONS

1-2
Revision A
November 25, 1996
The following are pertinent specifications for the FCC type-accepted Terra by Trimble Model
TX 760D transceiver.
1.3.1 MECHANICAL SPECIFICATIONS
Mounting: Panelmounted usingmountingtraysuppliedwithunit
OverallDimensions: 13.05"long,3.125"wide,1.625"high(33.14cmlong,7.9375
cm wide, 4.1275 cm high)
MountingDimensions: 11.45"behindpanel,
3.125"wide, 1.625"high (29.08cm behindpanel, 7.9375cm
wide,4.1275cm high)
PanelCutout: 3.03" x 1.54" (7.692 cm x 3.9116 cm)
Weight: 1.50 pounds (0.680 kg)
Connectors: Antenna input/output; BNC Interconnect; 15 pin “D” type
1.3.2 POWER REQUIREMENTS
The Terra by Trimble TX 760D transceiver requires 13.75 VDC +10%, -20% input at
2.5ampsmaximumloadcondition.
1.3.3 ENVIRONMENTAL SPECIFICATIONS
MaximumOperatingConditions
Altitude: To 30,000 feet (9,150 meters)
Humidity: To 95% at 50 C (122 F)
Temperature
Range: -20 C (-4 F) to +55 C (+131 F)
Vibration: 0.5 G from 5 to 500 Hz
1.3.4 ELECTRICAL SPECIFICATIONS
(All specifications at 13.75 VDC nominal)
RECEIVER
Frequency Range: 118.00 to 136.975 MHz
Number of Channels: 760
ChannelSpacing: 25 KHz
Sensitivity: Less than 2 µV for 6dBm signal plus noise to noise ratio.
Selectivity: Typically -6dBm at 14 KHz and -60 dBm at 40 KHz.
SquelchSensitivity: Automatic: 2 to 5 µV
Manual: 25 to 150 µV
AutomaticGain
Control(AGC): Audio flat within 3dB for 3 µV to 30,000 µV input.
Undesired Response: Greater than 60dBm down.

Revision A
November 25, 1996 1-3
Adjacent Channel
Rejection: Greater than 60dBm.
AuxiliaryInputs: 3 inputswith minimum of 30dBm isolation. One volt input at
1 Khz will produce one volt output across a 3.2 ohm resistor
connected to the speaker terminals.
Incidental
Radiation: Certified per FCC Part 15 requirements.
TRANSMITTER
Frequency Range: 118.00 to 136.975 MHz
Number of Channels: 760
FrequencyStability: +0.002% per FCC TypeAcceptance.
Power Output: 5 watts nominal, unmodulated carrier.
Modulation: Automatic audio leveling for a minimum of 80% and a
maximum of 95%.
EmmisionType: 6K00A3E
OutputTermination: 50 nominal.
SpuriousResponses: Greater than 50 dBm below carrier level.
Sidetone: Up to 25 milliwatts into 600 headphones.
FCCApproval: Type accepted per FCC Part 87.
1.4 EQUIPMENT SUPPLIED
The equipment supplied will depend upon the “system” you have ordered. The part numbers
and descriptions listed below indicate the equipment supplied with the TX 760D only. If a
system other than those listed below was ordered, additional equipment will have been
suppliedand listed in another manual.
0990-0405-10 TX 760D, SingleSystem
1. 1ea Comm Transceiver, Terra by Trimble TX 760D
P/N1900-0361-00
2. 1ea MountingTray,Single
P/N1900-0357-10
3. 1ea InstallationKit
P/N1901-2671-10
4. 1ea Operation/InstallationManual
TX 760D P/N 82619
5. 1ea Warranty Card
6. 1ea FCC Form 404
P/N1900-0607-00
7. 1ea FCC Form 406
P/N1900-0608-00

1-4
Revision A
November 25, 1996
1.4.1 OPTIONAL EQUIPMENT
Noise-cancelingMicrophone,Carbon,Push-to-talk
Terra by Trimble P/N 0900-0151-01
Voltage Converter 27.50 to 13.75 V
Model MLC 28-5 Terra by Trimble P/N 0900-3219-12
Push-to-talkSwitch
Terra by Trimble P/N 0900-0203-01
Headset with Microphone
Terra by Trimble P/N 0900-0201-11
Ext. VOX Kit, Pot. and Knobs
Terra by Trimble P/N 1901-5322-00
1.5 ADDITIONAL EQUIPMENT REQUIRED
1. Sufficient RG 58A/U 50 co-axial antenna cable to reach from the transceiver to the
antenna. BNC co-axial fittings as required for the particular installation. If cable
length exceeds 20 feet RG8 co-axial cable is recommended
2. MIL 22759 or equivalent wire sufficient to make the harness.
3. Microphone and headphone jacks as required.
4. Circuit breaker rated at 4 amps.
5. 50 ohm communications antenna rated at top speed of the aircraft.
1.6 LICENSE REQUIREMENTS
The operator of the Terra by Trimble TX 760D transceiver is required by the Federal
Communications Commission to hold a restricted radio-telephone operator’s permit, or a
higher class of license. A U.S. citizen may obtain the operator permit from the nearest FCC
office.
The transceiver, when installed in an aircraft, requires an Aircraft Radio Station License.
This license may be obtained by filing an FCC Form 404. The transceiver should be identified
on the form as CCXTX760D under Type Acceptance Number. If a copy of the filed Form
404 is kept in the aircraft, the TX 760D may be operated up to 30 days while awaiting the
formallicense.
For Ground Station operation, Form 406 should be filed as above.

Revision A
November 25, 1996 2-1
SECTION II
2. INSTALLATION
2.1 GENERAL
This section contains all necessary installation instructions and check-out procedures for the
Terra by Trimble TX 760D VHF Communications Transceiver. For the installer with little or
no experience of installing avionics, Section 2.3 provides important information. Trimble
recommends reading this section before continuing with the installation.
2.2 PREPARATION FOR USE
Every precaution has been taken to protect the TX 760D during shipment. Upon receipt of
the equipment, perform the following inspections:
1. Remove the unit from the shipping container and visually inspect for damage.
2. Check controls and switches to determine if they may have been damaged.
3. Make sure that all hardware and connectors listed in Section I, under “Equipment
Supplied” are present.
If the unit is damaged, a claim must be filed with the carrier. The carrier assumes title of the
unit when accepted for shipment. Do NOT return the unit to Trimble or its representatives.
It is suggested that the package be retained for inspection by the carrier in the case of damage
or for future use should it be necessary to ship the unit for service or to transfer it to another
location.
2.3 GENERAL INSTALLATION INSTRUCTIONS FOR AVIONICS
The following paragraphs contain pertinent hints, advice, and guidance intended for use by
installers of avionics equipment. These have been drafted to address common problems
encountered during the installation process. Specific questions may be addressed to Trimble
for technical assistance by calling 1-800-487-4662 and requesting Technical Assistance.
2.3.1 COAXIAL CABLES AND CONNECTORS
Improper installation of coaxial cables and connectors create many of the problems
encountered during avionics installations. Refer to Figure 3-1 for guidance of typical
BNC connector assembly. Refer to Figure 3-2 for instructions about connecting coax
cable to the BNC connector(s) mounted at the rear of Terra by Trimble mounting
trays. Problems to avoid include twisted, chafed, or pinched cables, sharp bends in
cables, open or shorted center conductors or shield braid, and improper grounding of
shields. Also, proper termination of antenna coaxial cables at the antenna should be
carefully checked. After installing connectors, pull firmly to ensure good mechanical
bonding (particularly if you use crimp-on connectors) and use your ohmmeter to
insure good electrical connection with no shorting. Be sure that coax lengths and
types follow the avionics manufacturer’s recommendations.

2-2
Revision A
November 25, 1996
2.3.2 ANTENNAS
The three cardinal points for antenna installation are location, mounting and
electrical characteristics.
Antenna Location:
Select a location that provides an adequate ground plane, a continuous unbroken
surface of metal. At VHF frequencies, a minimum of a 42 inch diameter circle with
the antenna at center is recommended. Provide an area where shadowing of the
antenna will not occur (eg. blocking of the signal by other parts of the aircraft).
Remember that at VHF frequencies and above, direct line of sight signals are
required from ground stations to the antenna for maximum effective range. Consider
that close proximity of an antenna to other antennas or noise sources may create
interference problems. Read carefully and follow closely the recommendations of
the antenna manufacturer about antenna location.
It is particularly important for VHF communication antennas that the 42 inch
diameter ground plane area be provided for each antenna, and the ground plane
surface must be relatively flat. Serious problems are often encountered in aircraft
which include composite materials or fabric surfaces and in many cases ground plane
should be added if normal communication range is to be achieved.
The effect of a limited or restricted ground plane is a severe reduction of the antenna
gain radiation pattern in the direction of the restriction. For example, if an antenna is
mounted less than 21 inches from the edge of a ground plane surface, then both the
range of reception and transmission to that direction will be reduced in direct
correlation with how much closer than 21 inches that antenna is mounted from the
edge.
Another undesirable antenna placement occurs when a VHF communication antenna
is placed within 21 inches from a second VHF communication antenna, or within 21
inches of the metallic vertical edge of the tail assembly, etc. The effect is reduction
of antenna gain radiation pattern to the direction of the other antenna or the vertical
edge. The 21 inch separation is an absolute minimum!
Antenna Mounting:
Proper attachment of any antenna to the aircraft surface is of primary importance.
Carefully clean all paint and corrosion off the mounting surface and apply an anti-
corrosion treatment. Do the same to the inside surface if a backing or nut plate is to
be used. Mount the antenna securely per the antenna manufacturer’s
recommendations and check carefully to insure good electrical bonding. Do not
mount any antenna to doors, hatches, inspection plates or other moveable surfaces
because proper bonding to the aircraft surface cannot be accomplished. After proper
mounting and bonding is achieved, a bead of high quality RTV around the junction
of the antenna base and the aircraft surface will prevent water seepage and corrosion
from forming between the antenna base and aircraft surface.

Revision A
November 25, 1996 2-3
2.3.2 ANTENNAS (CONTINUED)
Antenna Electrical Characteristics:
Use the antenna supplied with the avionics equipment, or if not supplied, the
antenna(s) recommended by the avionics manufacturer. If no specific antenna is
recommended, choose an antenna which provides good service for the frequency
range and the service conditions of the aircraft (eg. speed, altitude, etc.). If a signal
splitter is to be used, compare the specifications carefully to the application. Many
installation problems are encountered with improper application of signal splitters!
If in doubt, test the power and frequency characteristics of the splitter on the bench to
be sure.
Simple antenna problems such as shorting or open circuit problems can be
determined by use of an ohmmeter, although certain types of antennas may require
use of ramp test equipment to check radiation. But, by far, the most common antenna
problems are diagnosed to be coax or connectors, antenna bonding, or signal splitters.
Never allow an aircraft antenna to be painted.
2.3.3 WIRING AND HARNESSING
Construct the installation wiring harness carefully from the avionics manufacturer’s
wiring diagram. Be extremely careful to note recommended wire sizes, the need for
shielded wiring (if any), and decide upon any optional wiring to be included.
Measure carefully and plan the harness layout to avoid interference of the cable
harness with existing avionics, instruments or controls.
Remove the connector plates from the rear of the trays. Connect all wires to the
proper pin of each connector, checking as you go to insure that no loose strands
cause shorting to adjacent pins or to ground surfaces. We recommend tubing be
placed over each soldered pin connection to prevent wire strands from touching
adjacent connections. Be particularly careful with the shield braids of shielded
wires. Do not expose any more of the conductor than is absolutely necessary and
keep the braid connection as short as possible. Remember that on shielded wires
only one ground point is recommended. Follow manufacturer’s recommendation
about where the ground point should be located. After completing all connections,
check wiring with an ohmmeter again to ascertain that all connections are as
desired and that no undesired shorting to ground or other pins has occurred.
Visually double check to see that braids on shields are not creating shorting, that no
insulator melting has occurred during soldering and pull firmly on all connections to
insure good mechanical bond.
Install the harness and connectors/connector plates in the aircraft with very loose
dress only.
Solder all connections to power and ground and install panel components/controls
and safety devices (eg. fuses or breakers). It is desirable at this point to insert all
equipment in trays and perform preliminary check-out. Following a satisfactory
check-out, and with all equipment in the properly installed location, complete the
final dressing and routing of the harness and secure in place.

2-4
Revision A
November 25, 1996
2.3.3 WIRING AND HARNESSING (CONTINUED)
NOTE:
It is extremely important that units should be installed in trays while final
dress and bundling of the harness is accomplished to assure proper alignment
of connectors between tray and unit. Failure to do this may cause problems
when unit is initially inserted into tray due to misalignment of connectors!
The final step is to perform a complete check of all avionics operations and insure
that free movement of all cockpit controls is available.
2.3.4 NOISE AND INTERFERENCE
The typical airframe is a small and imperfect platform for providing all of the
antenna ground planes and power sources and inter-wiring required for avionics
operations, particularly for a low noise and interference free expectation. This
subject is far too broad and complex to address in detail in a few paragraphs.
However, an approach to categorizing and defining the problem can be outlined.
Noise and/or interference is usually heard in the audio systems, although it may also
be detected as an interference to indicator operation. Unless a strong suspicion of the
exact source is suspected, it is best to begin a process of elimination, in the following
order:
1. Power Source: Check for low voltage when the avionics load is applied. A
high resistance battery cell in the A/C can cause numerous problems. View
the avionics power line at the avionics master and at the affected unit for
noise on the power line. If present, try to categorize the frequency (eg.
alternator whine, which may be caused by one or more bad alternator diodes,
or if interference is present only when a communication unit is transmitting,
etc.). If the noise is present or worse at the affected unit than at the avionics
master, investigate the harness for noise coupling between wires. If
necessary, disconnect the affected unit power from the aircraft power source
and connect to an external power supply or battery.
2. Power Ground: View the power ground line at the avionics master and the
affected unit. If noise is discerned at the master source, ground strapping
may be corroded or partially broken. If only at unit, a larger wire size or
wire re-routing may be required.
3. Interference: Both noise and interference may be either conducted or
radiated, and in some cases electromagnetically coupled between units. If it
is determined that the noise or interference is eliminated whenever another
avionics unit is not transmitting, first investigate the radiated alternative.
Insure complete and proper bonding of antennas to the aircraft surface, and
check the coaxial cable and all shield connections and connectors. Review
the manufacturer’s recommendations for antenna separations. Be aware that
antenna radiation directly to conductors at the rear of mounting trays or units
may occur if shields are stripped too far back from the connector or are
improperly grounded. Disconnecting the interfering antenna and substituting
an external dummy load may assist diagnosis.

Revision A
November 25, 1996 2-5
2.3.4 NOISE AND INTERFERENCE (CONTINUED)
Conducted interference usually occurs through paths which are shared by the
avionics equipment such as power lines, ground points, audio equipment, or
induced interference between adjacent wires or harnesses. Review the
manufacturer’s recommendations for shielded wiring and ground points, and
for separation of specific wires. Measure ground points for a small but
perceptible resistance to true ground and view power lines with an
oscilloscope, turning each unit on and off to detect changes. Recheck
common or adjacent connections to jacks, plugs, or shared equipment such as
power converters, breakers, or audio panels.
4. Compromise: In some cases noise or interference may be subdued but not
eliminated. With the inefficient and imperfect platform provided by the
aircraft for antennas and power source, etc, complete elimination of the
problem may be very expensive or impossible (eg. if there is simply not
enough space to provide ground plane or antenna separation as
recommended). Or, the aircraft strobe noise is audible but not objectionable,
etc. These problems should be discussed early and thoroughly with the
customer.
2.3.5 SUMMARY
The paragraphs above are not intended to be highly technical, completely thorough,
or extensive, but serve as a reminder for certain precautionary or follow-up
procedures for general avionics installations. Trimble is prepared to assist at any
point with additional information, hints, or literature. Simply call 1-800-487-4662
and ask for technical assistance.
2.4 INSTALLATION
Installation of any equipment in an aircraft requires that the work be performed by a
Certificated Radio Repair Station with appropriate ratings. The installing agency must
complete an FAA Form 337 and compute a new weight and balance for the aircraft for
insertion in the Aircraft Flight Manual. Unless the mechanic is the designated inspector for a
Certificated Radio Repair Station or holds an Inspection Authorization, the work is subject to
inspection and approval by an FAA inspector or the holder of an Inspection Authorization.

2-6
Revision A
November 25, 1996
2.4 INSTALLATION (CONTINUED)
The following items and suggestions should be considered prior to installation or the Terra
by Trimble TX 760D Transceiver:
1. Discuss the location of this transceiver with your customer, preferably in a
position that provides ease of operation. The location of the TX 760D in
relation to the pilot’s viewing angle is very important. Due to the small size
of the TX 760D, the vertical viewing angle for the display should not exceed
30 degrees.
2. Avoid installing the TX 760D near heat sources. If unavoidable, insure
additional cooling is provided.
3. Insure that adequate clearance exists behind the panel for connectors, cable
bends, and additional cooling if required. A minimum of 14" (35.56 cm) is
recommended.
4. Refer to Terra by Trimble TX 760D outline drawings for panel cut-out
details and mounting dimensions as well as pertinent notes.
5. Install the sleeve assembly in the instrument panel. Insure compliance with
standards set forth in FAA Aircraft Inspection and Repair Document AC
43.13-2A.
NOTE:
Ensure that the sleeve is installed with the largest opening toward the top.
6. The 15 pin “D” connector assembly supplied with the Terra by Trimble TX
760D must be wired correctly to the aircraft avionics systems or severe
damage may result to the Terra by Trimble TX 760D or other systems. The
interconnecting wiring for the 15 pin “D” connector is shown in Figure 3-6.
Comply with standards set forth in FAA Aircraft Inspection and Repair
Document AC 43.13-1A Section 7 and other pertinent FAR’s as required.
Insure mil-spec wire is utilized for all interconnects.
NOTE:
It is recommended that the TX 760D be installed and locked into the
mounting sleeve before tying and securing the wiring harness. This will
ensure proper alignment of the “floating” connectors to facilitate easy
removal and installation of the TX 760D.
CAUTION!
The operating voltage of this Terra by Trimble TX 760D is 13.75 V nominal
for direct connection to an aircraft 12-14 volt electrical system. If the
aircraft electrical system supply is 24-28 volt, it is necessary to employ a
converter to reduce the 28 volts to 13.75 volts for the Terra by Trimble TX
760D. Trimble can supply an optional converter specifically designed for
this purpose. It is Terra by Trimble P/N 0900-3219-12, Model MLC 28-5.

Revision A
November 25, 1996 2-7
NOTE:
a. If a loudspeaker is not used it will be necessary to connect a 15 ohm
3 watt resistor across the speaker terminals (pin 12 and pin 15) of the
15 pin “D” connector. Failure of the output circuit could occur if
this load is not installed.
b. If the installation is a retrofit of a TX 720, care must be taken to
properly rewire the connector or damage and incorrect operation will
result.
7. Should the customer desire the VOX Intercom feature, it is recommended
that a relay be used in the manner described on the Interconnect Diagram,
Figure 3-6. If this is not done, the co-pilot’s microphone will be “hot” during
transmit and both mic audio signals will be transmitted simultaneously.
8. Another option for use with the VOX Intercom feature is an external VOX
Intercom squelch adjustment. This requires a potentiometer (100 k , 1/4 W)
to be installed as per Note #5 on Figure 3-6. When using this option on
Units, the Vox Intercom squelch adjustment on the TX 760D must be turned
fully CCW and shorting jumper SJ601 must be removed for proper
operation. SJ601 can be located on drawing Figure 3-4.
9. Yet another option for use with the VOX Intercom feature is an external
VOX Intercom volume adjustment. This requires a potentiometer (10 k , 1/4
W, Audio taper) to be installed as per note #6 on Figure 3-6. When using
this option, the VOX Intercom volume adjustment on the TX 760D must be
fully CW and shorting jumper SJ602 removed for proper operation. SJ602
can be located using component locator Figure 3-4.
10. Install the TX 760D in the mounting sleeve assembly. Use caution as the
rear connectors mate. After the float mounts are positioned correctly, (a little
twisting at the front panel may be necessary), the unit will fully engage the
connectors with light to medium pressure on the front panel.
CAUTION:
DO NOT PUSH UNIT INTO SLEEVE WITH THUMBS ON LENS AS
DISPLAY DAMAGE MAY RESULT!
Insert a 7/64" Allen wrench in the front panel hole and engage the locking screw.
Turn clockwise until the cam has engaged itself in the sleeve and is moderately
tightened. Use caution to prevent stripping the threads on the locking cam or screw.
To remove the unit from the mounting sleeve, insert the 7/64" Allen wrench in the
locking screw and turn counterclockwise. The cam will move the unit outward about
1/4" and disengage the connectors. The unit may now be pulled out of the mounting
sleeve by hand.

2-8
Revision A
November 25, 1996
2.4 INSTALLATION (Continued)
11. The connector is mounted from the front side of the sleeve with two Phillips head
screws and lock washers. As viewed from the front of the unit, pin 1 of the
connector is top right and pin 15 is bottom left.
12. The following items are preset at the factory to nominal levels, but individual pilot
desires and installations may require field adjustments. For these adjustments, access
holes have been provided in the covers of the TX 760D. Figure 3-3 shows the
locations of these access holes.
A. Sidetone Volume
If the installation requires more (or less) volume into the headphones during
transmit the sidetone volume adjustment is used. Turning clockwise will
increase the volume and turning counterclockwise will decrease the volume.
B. VOX Intercom Squelch
If the VOX Intercom is breaking on ambient noise or is not breaking soon
enough the VOX Intercom squelch adjustment is used. Turning
counterclockwise will allow the squelch to break at lower microphone levels
and turning clockwise will require more microphone audio to break squelch.
NOTE:
If using the External VOX Intercom squelch adjust option this adjustment
must be turned fully CCW and shorting jumper SJ601 removed for proper
operation of the External VOX squelch adjustment.
C. Intercom Volume
If the installation requires more (or less) intercom volume into the
headphones the Intercom Volume adjustment is used. Turning
counterclockwise will decrease the headphone volume level and turning
clockwise will increase the headphone volume level.
NOTE:
If using the external VOX Intercom volume adjust option, this adjustment
must be turned fully CW and shorting jumper SJ602 removed for proper
operation of the external VOX Intercom volume adjustment. See
Component Locator, Figure 3-4 for location of SJ601 and SJ602.