Garmin GPS 155 Operating instructions

GPS 155
MAINTENANCE AND REPAIR
MANUAL
GARMIN INTERNATIONAL PART NUMBER: 190-00065-05
1200 E. 151st Street REVISION E, MARCH 26, 1998
OLATHE, KANSAS 66062

TABLE OF CONTENTS
SECTION 1
INTRODUCTION
1.1 GENERAL DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . page 1
1.2 DETAILED DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
1.2.1 Control/Display Unit (CDU) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
1.2.2 Main Board Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
1.2.2.1 Central Processing Unit (CPU) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
1.2.2.2 Power Supply Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
1.2.3 RF/IF Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
1.2.4 Interface Board and Data Cards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
1.2.5 Altitude Decoder Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
SECTION 2
TROUBLESHOOTING
2.1 TROUBLESHOOTING EQUIPMENT REQUIRED . . . . . . . . . . . . . . . . . . . . . . .page 7
2.2 SELF-TEST FAILURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
2.3 INITIAL TROUBLESHOOTING OPERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
2.3.1 Power Supply Check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
2.3.2 Internal Clock Check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
2.4 TROUBLESHOOTING CHART . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
SECTION 3
DISASSEMBLY INSTRUCTIONS
3.1 REMOVAL OF THE COVERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . page 11
3.2 BOARD AND MODULE DISASSEMBLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
3.2.1 Control/Display Unit Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
3.2.2 Altitude Decoder Board Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
3.2.3 Interface Board Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
3.2.4 Main Board Sub-Assembly Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
3.2.5 RF/IF Assembly Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
3.2.6 Battery Pack Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
3.2.7 Power/Dimmer Switch Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
3.2.8 Rotary Switch Assembly Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
3.2.9 Display Board Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
3.3 REASSEMBLING THE UNIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
SECTION 4
SERVICE AND TESTING
4.1 MEMORY BATTERY REPLACEMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 16
4.2 ROM REPLACEMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
4.3 FINAL TEST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
4.3.1 Test Equipment Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
4.3.2 Set-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
4.3.3 Final Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
i

SECTION 5
ILLUSTRATED PARTS LIST
5.1 LIST OF REPLACEABLE PARTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 29
5.2 ASSEMBLY DRAWINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30
LIST OF FIGURES
FIGURE 1 GPS 155 BLOCK DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 2
FIGURE 2 CONTROL/DISPLAY UNIT BLOCK DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . 3
FIGURE 3 MAIN BOARD BLOCK DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
FIGURE 4 RF/IF BLOCK DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
FIGURE 5 MAIN BOARD TESTPOINTS (TOP VIEW) . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
FIGURE 6 MAIN BOARD TESTPOINTS (BOTTOM VIEW) . . . . . . . . . . . . . . . . . . . . . . . 9
FIGURE 7 RF/IF ASSEMBLY REMOVAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
FIGURE 8 ROM TOOL INSERTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
FIGURE 9 ROM REMOVAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
FIGURE 10 J2 REAR CONNECTOR PINOUT DEFINITION . . . . . . . . . . . . . . . . . . . . . . .27
FIGURE 11 J101 REAR CONNECTOR PINOUT DEFINITION . . . . . . . . . . . . . . . . . . . . .28
FIGURE 12 GPS 155 ASSEMBLY DRAWING (View 1) . . . . . . . . . . . . . . . . . . . . . . . . . . .30
FIGURE 13 GPS 155 ASSEMBLY DRAWING (View 2) . . . . . . . . . . . . . . . . . . . . . . . . . . .31
FIGURE 14 GPS 155 ASSEMBLY DRAWING (View 3) . . . . . . . . . . . . . . . . . . . . . . . . . . .32
FIGURE 15 GPS 155 CONNECTOR DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32
ii

SECTION 1
INTRODUCTION
This manual is intended to provide instructions and guidelines for the maintenance and repair of the
GARMIN GPS 155.
WARNING: This unit contains static sensitive devices. Service personnel must insure that proper precau-
tions are taken to prevent damage to this equipment from Electro-Static Discharge (ESD). Disassembly and
repair should only be accomplished in an approved ESD workstation by properly trained and grounded
personnel.
1.1 GENERAL DESCRIPTION
The GPS 155 is a full-featured aviation GPS navigation instrument that offers not only a fixed panel
mounted installation in its rack, but also provides a take-home capability. The GPS 155 utilizes the patent-
ed GARMIN MultitracTM system for tracking up to eight satellites simultaneously. The unit features a GPS
receiver; a front loading NavData card; and a sealed rechargable battery pack. The unit is constructed from
high quality materials and uses the latest techniques in manufacturing technology.
In order to achieve the desired reliability, size and power requirements, surface mount components are
used extensively. Specialized equipment and procedures are required to repair circuit boards having sur-
face-mounted components. GARMIN does not authorize the repair of GPS 155 circuit boards. All circuit
boards and assemblies for the GPS 155 can be economically replaced if necessary.
The following modules and printed circuit board assemblies are available for field replacement:
Display Board Assembly:
Contains the vacuum fluorescent display and display circuit board.
Rotary Switch Assembly:
Contains dual concentric switch and its assembled wiring harness.
Control/Display Unit:
Contains bezel, buttons, lens, keypad board, light pipes, front insert, plus the Display Board
Assembly and Rotary Switch Assembly.
Main Board:
Printed circuit board which contains the microprocessor, LSI, power supply circuitry, and battery charging components.
Altitude Decoder Board:
Printed circuit board which contains the altitude data decoder circuits and interface circuitry.
Interface Board:
Printed circuit board which connects the main board to the NavData® card and the Altitude Decoder
Board.
RF/IF Assembly:
Module which contains the GPS receiver and high precision crystal oscillator.
1

External Battery Pack:
Sealed package containing rechargable cells, fuse and wiring harness.
1.2 DETAILED DESCRIPTION
Internally, the GPS 155 is divided into six printed circuit boards: the main, RF/IF, altitude decoder,
interface, display, and keypad circuit boards. The block diagram in Figure 1 below shows the interrela-
tionships between major circuits and modules within the GPS 155.
Figure 1
GPS 155 Block Diagram
1.2.1 Control/Display Unit (CDU)
The Control/Display Unit is an assembly consisting of ten keycaps, keypad board, light pipes optical
lens, front insert panel, vacuum fluorescent display (VFD) and board with drivers, display board shock
mounting grommets and screws, housed in a die cast bezel with a dual concentric rotary switch and
two knobs. The VFD is a 3 row by 20 column unit with variable intensity. This display provides
excellent day and night visibility from wide viewing angles. The front panel provides function selec-
tion and alphanumeric input to the Main Board Assembly via 10 push-buttons and the dual concentric
rotary knob. There are two LEDs behind each keycap, providing backlighting for night time use.
2

The block diagram in Figure 2 shows the interaction between components on the Control/Display Unit.
Figure 2
Control/Display Unit Block Diagram
1.2.2 Main Board Assembly
The Main Board Assembly contains the CPU, operating system ROM, system memory, discrete I/O, ser-
ial communication drivers/receivers and the power supply. These items are discussed in further detail
below.
1.2.2.1 Central Processing Unit (CPU)
The Main Board is a microprocessor-based computer board. This board contains an Intel 80L186EB
microprocessor running at 16 MHz, a plug-in read only memory chip (ROM) and random access mem-
ory chips (RAM). Data stored in RAM is maintained by a 3 volt lithium battery when the unit is
switched off, and by the regulated 5 volt supply when the unit is powered on. A custom LSI is used to
decode the signals from the GPS satellites. A real time clock IC is used to keep track of the date and
time. Other circuits on the board are used for input/output functions such as controlling the display,
reading the keypad, and controlling the receiver. Discrete input/output lines are provided for CDI
course deviation, CDI to/from flag, CDI NAV flag, Super Flag, external annunciators, OBS course, and
to activate the Approach mode. Serial communication lines for OBI (clock/data/sync), ARINC 429, RS-
422 and RS-232 (two channels) are also included.
3

The block diagram in Figure 3 below shows the interaction between the various Main Board circuits.
Figure 3
Main Board Block Diagram
1.2.2.2 Power Supply Circuits
The power supply section of the Main Board supplies +60 volts for the display drivers, ±12 volts, +15
volts, +5 volts (VCC), on/off circuits, and the battery charger. In addition, the main board contains com-
parator circuits for detecting low NiCad battery level, low memory battery voltage, and external power.
The external power input is through the rear connector (J101, Figure 5) and from the AC adapter through
a DC power jack located near the rear connector. Both of these sources as well as the battery input are
diode isolated to prevent parallel sourcing. The unit is turned on and off by a digital latch circuit operated
by a knob located on the front panel adjacent to the NavData® card opening. The switching circuit is a
fly-back design. The switching regulator operates at a frequency of approximately 40 KHz and uses a high
efficiency switching regulator for the switching device. The high efficiency switching regulator drives a
transformer that is configured in a fly-back mode. The +15 volt output is post-regulated to provide 11.75
to 12.25 volts. The +5 volt output is regulated at 4.8 to 5.2 volts. All other voltages are ±20%.
4

The NiCad battery charger circuit is built around the Integrated Circuit Systems, Inc. ICS1700A rapid
charge controller. This controller uses a pulsed charging system, which is in effect when external power
is supplied through the rear connector. When the unit is off, the batteries are not being charged by the
external supply. When the unit is connected to a wall charger, however, the batteries are trickle charged
at a constant current of 125 mA.
1.2.3 RF/IF Assembly
The RF/IF assembly consists of a dual conversion receiver, a frequency synthesizer, and a high precision
crystal oscillator. The receiver consists of a ceramic RF filter, RF and IF amplifiers, mixers, and an IF fil-
ter. The frequency synthesizer uses the high precision oscillator as a reference frequency for the phase
detector in the synthesizer. The resultant frequency is used in the mixer section to produce the first IF.
After further amplification and mixing with a product of the crystal oscillator, the baseband IF is passed
to the CPU Board for processing. The crystal output is also used for the system clock pulse. The RF/IF
assembly is contained within a shielding fence/cover and connection to the antenna is made via a BNC
connector. The RF/IF supplies +5 volts to the antenna’s preamplifier through this BNC connector.
The block diagram in Figure 4 below shows the interaction between components on the RF/IF Assembly.
Figure 4
RF/IF Block Diagram
1.2.4 Interface Board and Data Cards
The Interface Board provides two-way data transfer capability between the Main Board and a data card.
NavData® cards, available in sizes from 1 to 4 megabytes depending on the content/coverage area,
allow for data transfer to the Main Board only. User data cards, with 768 kilobytes of flash memory
data storage, may be used to save user data/settings from the GPS 155 or restore that information to
the GPS 155. Address and data lines from the Main Board route through the Interface Board’s output
buffers to the data card. A plastic race secures the Interface Board at the front of the unit and serves to
guide the data card onto the 40-pin connector.
NOTE: When servicing the GPS 155, user-defined waypoints, routes, settings, etc., may be saved on a
user data card (010-10032-03) first. In the event these data are lost, they may then be restored from
the user data card before the unit is returned to service. Refer to Appendix A of the GPS 155 Pilot’s
Guide for additional information on user data card operation.
5

1.2.5 Altitude Decoder Board
The Altitude Decoder Board contains the 26-pin rear connector (J2), discrete/parallel inputs and
Gillham/Greycode decoder circuitry. Encoded pressure altitude data from a parallel altimeter device is
received as a 10-bit data word at the 26-pin rear connector. Diode isolation on these input lines pre-
vents damage to the GPS 155. The data is then decoded and output to the Main Board data bus via
the Interface Board.
The Altitude Decoder Board also provides a discrete input for the GPS SEQ switch. When grounded,
this line activates the GPS 155’s HOLD mode. This input is sent to the Main Board data bus via the
Interface Board.
6

SECTION 2
TROUBLESHOOTING
2.1 TROUBLESHOOTING EQUIPMENT REQUIRED
• 0-30 Volt DC / 3-Amp Power Supply
• Digital Multimeter
• 20 MHz Oscilloscope
2.2 SELF-TEST FAILURES
The GPS 155 monitors many of its internal functions and displays a message to alert the user should a
failure occur. The table below shows the recommended repair for each self-test failure message.
MESSAGE RECOMMENDED REPAIR*
Altitude input fail
Verify connections to altitude serializer/encoder
Replace Altitude Decoder Board (see Section 3.2.2)
Replace Interface Board (see Section 3.2.3)
Replace Main Board (see Section 3.2.4)
Battery rqrs service Verify connection of external battery pack to Main Board
Replace external battery pack(see Section 3.2.6)
Course input fail Check ARINC 429 / RS-422 course input connections
Replace Main Board (see Section 3.2.4)
Data card failed Replace NavData® card
Replace Interface Board (see Section 3.2.3)
Data card write fail Replace user data card
Replace Interface Board (see Section 3.2.3)
Fuel/Air input fail Check connections to fuel/air data system
Replace Main Board (see Section 3.2.4)
Memory battery low Replace memory battery (see Section 4.1)
No altitude input
Verify connections to altitude serializer/encoder
Replace Altitude Decoder Board (see Section 3.2.2)
Replace Main Board (see Section 3.2.4)
Osc needs adjustment Replace RF/IF Assembly (see Section 3.2.5)
Replace Main Board (see Section 3.2.4)
RAM failed Replace Main Board (see Section 3.2.4)
Receiver failed Replace Main Board (see Section 3.2.4)
Replace RF/IF Assembly (see Section 3.2.5)
ROM failed Replace ROM (see Section 4.2)
Replace Main Board (see Section 3.2.4)
Stored data lost Replace memory battery (see Section 4.1)
* Where multiple repair actions are listed, they are listed with the most likely failure cause presented first.
7

2.3 INITIAL TROUBLESHOOTING OPERATIONS
The first step in troubleshooting the GPS 155 is to remove the top and bottom covers (see Section 3.1).
Verify that the unit is being properly powered and the internal clock is functional.
NOTE: The NavData® card should be removed before disassembling the unit.
2.3.1 Power Supply Check
1. Check the DC voltage between D805 and ground as shown in Figure 5. The voltage should be between -9.60 and
-14.40 volts. If the voltage is out of the limits specified, replace the Main Board.
2. Check the DC voltage between L800 and ground as shown in Figure 5. The voltage should be
between 4.8 and 5.2 volts. If the voltage is out of the limits specified, replace the Main Board.
3. Check the DC voltage between D804 and ground as shown in Figure 5. The voltage should be
between 48.00 and 72.00 volts. If the voltage is out of the limits specified, replace the Main
Board.
4. Check the DC voltage between I806 pin 1 and ground as shown in Figure 6. The voltage should be between
11.75 and 12.25 volts. If the voltage is out of the limits specified, replace the Main Board.
2.3.2 Internal Clock Check
Check the clock speed by placing an oscilloscope probe at I112 pin 25 as shown in Figure 6.
The frequency should be approximately 16.4 MHz. If the clock is not operational, replace the RF/IF
Board. If the clock still fails to function, replace the Main Board.
Figure 5
Main Board Testpoints (Top View)
8
Rear Connector

Figure 6
Main Board Testpoints (Bottom View)
2.4 TROUBLESHOOTING CHART
PROBLEM RECOMMENDED ACTION
Unit will not power on
Check External Power Connection
Check External Battery Pack(see Section 3.2 & 4.3.2)
Replace Pwr/Dimmer Switch (see Section 3.2.7)
Replace Main Board (see Section 3.2.4)
Unit will not power off Replace Main Board (see Section 3.2.4)
Replace Control/Display Unit(see Section 3.2.1)
Replace Pwr/Dimmer Switch (see Section 3.2.7)
Display has horizontal or vertical lines Replace Display Board (see Section 3.2.9)
Replace Main Board (see Section 3.2.4)
Replace Control/Display Unit(see Section 3.2.1)
Display has missing or dark pixels Replace Display Board (see Section 3.2.9)
Replace Control/Display Unit(see Section 3.2.1)
9

TROUBLESHOOTING CHART (cont.)
Display brightness not adjustable Replace Pwr/Dimmer Switch (see Section 3.2.7)
Replace Main Board (see Section 3.2.4)
Annunciators & Keycaps have no backlighting Replace Control/Display Unit (see Section 3.2.1)
Replace Main Board (see Section 3.2.4)
Keycap backlighting not uniform Replace Control/Display Unit (see Section 3.2.1)
Annunciators do not function Replace Control/ Display Unit (see Section 3.2.1)
Replace Main Board (see Section 3.2.4)
No external annunciator output(s) Check connections and verify operation in Test
(message, arrival, approach) Mode (see Section 4.3.2)
Replace Main Board (see Section 3.2.4)
Keycaps do not function properly Replace Control/Display Unit (see Section 3.2.1)
Replace Main Board (see Section 3.2.4)
Dual concentric (rotary) knobs do
Check wiring harness connection from rotary switch
not function properly
to back of Control/Display Unit
Replace Rotary Switch Assembly (see Section 3.2.8)
Replace Control/Display Unit (see Section 3.2.1)
Approach select switch not
Check connections and verify operation in Test
functioning properly Mode (see Section 4.3.2)
Replace external switch
Replace Main Board (see Section 3.2.4)
OBS select switch (“course select”
Check connections and verify operation in Test
or “hold”) not functioning properly
Mode (see Section 4.3.2)
Replace external switch
Replace Altitude Decoder Board (see Section 3.2.2)
Replace Main Board (see Section 3.2.4)
GPS reception weak or GPS Status Replace Antenna and/or antenna cable
Page shows low signal levels Replace RF/IF Assembly (see Section 3.2.5)
No GPS reception
Check Date/Time Page for correct UTC time1
Check Position Page for close approximate starting
position (within 300 nautical miles)2
Replace Antenna and/or antenna cable
Replace RF/IF Assembly (see Section 3.2.5)
Replace Main Board (see Section 3.2.4)
Unit displays velocity when not Replace RF/IF Assembly (see Section 3.2.5)
moving3
NiCad battery life is short Replace External Battery Pack
or NiCad battery inop Replace Main Board (see Section 3.2.4)
Unit loses memory in spite of new Replace Main Board (see Section 3.2.4)
memory battery
Unit does not detect low battery Replace Main Board (see Section 3.2.4)
10

TROUBLESHOOTING CHART (cont.)
Unit does not detect external Check external power connection
power4Replace Main Board (see Section 3.2.4)
No serial data output Replace Main Board (see Section 3.2.4)
No external CDI output or external Check connections and verify operation in Test
CDI functions improperly Mode (see Section 4.3.2)
Replace Main Board (see Section 3.2.4)
Invalid characters displayed Replace ROM (see Section 4.2)
Replace Main Board (see Section 3.2.4)
1If UTC Time is incorrect, unit may be reset by holding the CLR key down while turning the unit on. The unit must then be
attached to an antenna, allowed to search the sky and collect new almanac data. This process may take 20-30 minutes. CAU-
TION: All user waypoints/settings will be lost if the unit is reset.
2The GPS 155 will eventually perform an AutoLocate™ and lock onto satellites, however this can add an additional 10-15 min-
utes to the normal lock-on time.
3Display of low velocity numbers when not moving can occur when the unit is in 2D mode and should not be considered a
problem. However, this should seldom occur in 3D mode.
4The “Turning Off” message appears at/below approximately 9 VDC external power.
SECTION 3
DISASSEMBLY INSTRUCTIONS
To avoid damaging the GPS 155’s circuit boards and assemblies, the following procedures should be
carefully followed. Assembly drawings are included in Section 5 for reference (Figures 12 through 15).
NOTE: The NavData card should be removed before disassembling the unit.
3.1 REMOVAL OF THE COVERS
1. Remove the two M3 x 5mm screws (211-50227-05) retaining the top cover (115-00037-00).
2. Remove the top cover by sliding it rearward approximately 1/4” until the retaining lip is fully exposed. The cover
can then be lifted off the unit.
3. Carefully turn the unit over. CAUTION: The GPS 155 contains numerous static-sensitive components. Always
observe proper anti-static precautions.
4. Remove the two M3 x 5mm screws (211-50227-05) retaining the bottom cover (115-00038-00).
5. Remove the bottom cover by sliding it rearward about 1/4” until the retaining lip is fully exposed. The cover can
then be lifted off the unit.
NOTE: If further disassembly is required, it is a good idea (at this time) to disengage the battery
pack wire from the main board (see figure 12).
11

3.2 BOARD AND MODULE DISASSEMBLY
3.2.1 Control/Display Unit (011-00127-00) Removal
1. Remove the covers as described above.
2. Disconnect the battery pack wiring harness from the 3-pin locking connector (J800) on the main board
(see figures 5 and 12).
3. Remove the power/dimmer control knob and shaft (430-00010-00) by loosening the M3 x 3mm set screw (211-
00025-00) on the coupling sleeve (125-00021-00). (NOTE: On some early versions of the GPS 155, the shaft
extension is connected directly to the power/dimmer switch without an in-line coupler. The control knob is
glued to this one piece extension and cannot be removed. If you are replacing the CDU on one of these units,
you will need to order a replacement control knob and coupling sleeve as well.)
4. Remove the four M3 x 5mm flat head screws (211-00024-00) which attach the CDU to the chassis and the battery
pack.
5. Carefully remove the CDU from the chassis, pulling the assembly away from the two Main Board connectors
(J100 and J102) and the power/dimmer switch.
3.2.2 Altitude Decoder Board (012-00090-00) Removal
1. Remove the covers as described in Section 3.1.
2. Disconnect the battery pack wiring harness from the 3-pin connector (J800) on the Main Board
(see Figures 5 and 12).
3. Remove the two #4-40 standoffs (211-63234-08) which hold the 26-pin DSUB connector to the chassis.
4. Carefully pull the Altitude Decoder Board toward the front of the unit approximately 1/4” until the 26-pin DSUB
connector is clear of its cutout on the chassis. Lift the Altitude Decoder Board away from the Main Board and
chassis.
5. Remove the ribbon cable connector (J701) from the Altitude Decoder Board.
3.2.3 Interface Board (012-00031-01) Removal
1. Remove the covers as described in Section 3.1.
2. Remove the Control/Display Unit as described in Section 3.2.1.
3. Remove the Altitude Decoder Board as described in Section 3.2.2.
4. Remove the four M1.7 x 5mm flat head screws (211-56201-05) retaining the data card race (145-00056-00) and
Interface Board.
5. Slip the data card race off the Interface Board connector (J700).
6. Carefully pull the Interface Board upward out of the unit, disengaging it from the Main Board
Connector (J107).
12

3.2.4 Main Board Sub-Assembly Removal
The Main Board Sub-Assembly consists of the Main Board (012-00089-00) and the RF/IF Assembly
(012-00091-00). If required, this sub-assembly may be further dismantled by following the steps in
Section 3.2.5.
1. Remove the covers as described in Section 3.1.
2. Remove the Control/Display Unit as described in Section 3.2.1.
3. Remove the Altitude Decoder Board as described in Section 3.2.2.
4. Remove the hex nut securing the BNC connector to the chassis. This nut is 9/16” and may be removed with a
deep 12-point socket.
5. Pull the BNC connector out of its cutout on the chassis.
6. Remove the heat sink clamping bar (115-00049-00, see Figure 12 - Detail A) by removing the M3 x 5mm mounting
screw (211-50227-05).
7. Remove the heat sink insulator (250-00016-00) from regulator I800 (see Figure 12 - Detail A) and save this piece
for later reassembly.
8. Carefully turn the unit over.
9. The power/dimmer switch uses a flexible circuit cable for connection to the Main Board. This flexible circuit
cable is inserted into a ZIF connector (J804). Pull up on the retaining lip of the ZIF connector. Gently pull the
flexible circuit cable out of the ZIF connector. Take care not to crease the flexible circuit cable.
10. Remove the two #4-40 standoffs (211-63204-08) which hold the 37-pin DSUB connector to the chassis.
11. Remove the M3 x 5mm screw (211-50227-05) and the M3 x 10mm screw (211-50227-15) holding the Main Board to
the chassis.
12. Carefully slide the Main Board upward; pulling it away from the power/dimmer switch and the Interface Board
Connector (J702).
3.2.5 RF/IF Assembly (012-00091-00) Removal
1. Remove the covers as described in Section 3.1.
2. Remove the Altitude Decoder Board as described in Section 3.2.2.
3. Remove the hex nut securing the BNC connector to the chassis. This nut is 9/16” and may be removed with a
deep 12-point socket.
4. Pull the BNC connector out of its cutout on the chassis.
5. Remove the four screws (211-50202-15) and spacers (233-00012-00) which secure the RF/IF Assembly to the
Main Board.
6. Carefully pull the RF/IF Assembly upward, disengaging it from the Main Board Connector (J803). (NOTE: With the
mounting screws removed, the cover for the RF/IF Assembly will be loose. Grip the RF/IF Assembly firmly by the
sides, instead of by its cover, when lifting upward.)
13

Figure 7
RF/IF Assembly Removal
3.2.6 Battery Pack (011-00067-00) Removal
1. Remove the covers as described in Section 3.1.
2. Remove the heat sink clamping bar (115-00049-00, see Figure 12-Detail A) by removing the M3 x 5mm mounting
screw (211-50227-05). (Note: Removal of the Altitude Decoder, Interface and Main Boards is not required to
remove the battery pack from the unit.)
3. Remove the heat sink insulator (250-00016-00) from the regulator I800 (see figure 12-Detail A) and save this piece
for later reassembly.
4. Remove the two M3 x 5mm flat head screws (211-00024-00) which attach the CDU to the battery pack.
5. Remove the three M3 x 5mm screws (211-50227-05) securing the battery pack to the chassis. (Note: One screw
is recessed in a hole on the side of the battery pack.)
6. Slide the battery pack cable and connector (P800) through the cutout in the chassis.
3.2.7 Power/Dimmer Switch (011-CS032-00) Removal
1. Remove the covers as described in Section 3.1.
2. Remove the Control/Display Unit as described in Section 3.2.1.
3. Remove the Main Board Sub-Assembly as described in Section 3.2.4.
4. Remove the coupling sleeve (125-00021-00) by unscrewing the pivot pin screw (211-00030-00).
5. Remove the hex nut securing the power/dimmer switch to the chassis. This nut is 7/16” and may be removed
with a deep, thin-walled 12-point socket.
6. Slide the power/dimmer switch out of the mounting hole on the chassis.
14
012-00091-00
012-00091-00

3.2.8 Rotary Switch Assembly (011-CS033-00) Removal
1. Remove the covers as described in Section 3.1
2. Remove the Control/Display Unit as described in Section 3.2.1
3. Remove the two rotary switch knobs by loosening the two M3 x 4mm set screws on the inner knob (430-00002-
00). After removing the inner knob, gently pull the outer knob off of the rotary switch shaft.
4. Disconnect the rotary switch wiring harness from the connector on the Display Board.
5. Remove the hex nut securing the rotary switch to the Control/Display Unit. This nut is 1/2” and may be removed
with a deep 12-point socket.
3.2.9 Display Board (012-00029-00) Removal
1. Remove the covers as described in Section 3.1.
2. Remove the Control/Display Unit as described in Section 3.2.1.
3. Disconnect the rotary switch wiring harness from the connector on the Display Board.
4. Remove the four M2.3 x 6mm screws (211-00028-00) that secure the Display Board to the CDU bezel.
5. Carefully pull the Display Board away from the CDU bezel, disengaging it from the Keypad Board connector.
(NOTE: Retain the four rubber grommets [252-00005-00] for later reassembly.)
3.3 REASSEMBLING THE UNIT
1. Attach the power/dimmer switch to the chassis by reversing the steps followed in Section 3.2.7. The switch is
correctly installed when the contacts and flexible circuit cable are pointed away from the chassis frame.
2. Ensure that the power/dimmer switch is in the “off” position
3. Slide the battery pack connector (P800) and cable into the cutout in the chassis.
4. Mount the battery pack to the chassis using three M3 x 5mm screws (211-50227-05).
5. Attach the RF/IF Assembly to the Main Board by reversing the steps followed in Section 3.2.5.
6. Mount the Main Board Sub-Assembly on the chassis by reversing the steps followed in Section 3.2.4. Be sure
to pull up on the retaining lip of the ZIF connector (J804) before inserting the flexible circuit cable. Also, ensure
that the heat sink insulator (250-00016-00) is installed on regulator I800 before installing the heat sink clamping bar
(see Figure 12 - Detail A).
7. Mount the Interface Board to the chassis by reversing the steps followed in Section 3.2.3.
8. Install the Altitude Decoder Board by reversing the steps followed in Section 3.2.2. Carefully observe proper
orientation and alignment when reconnecting the ribbon cable to the board. Do not connect the battery pack
wiring harness to the Main Board until the Control/Display Unit is installed.
9. Mount the Display Board to the CDU bezel by reversing the steps followed in Section 3.2.9. Be sure to reinstall
the four rubber grommets before mounting the board.
10. Mount the Rotary Switch Assembly to the Control/Display Unit by reversing the steps followed in Section 3.2.8.
The switch is correctly installed when the wiring harness points toward the top of the unit. Torque hex nut to
25±3 in oz.
15

11. Install the Control/Display Unit by reversing the steps followed in Section 3.2.1. Gently slide the CDU onto the
two Main Board connectors (J100 and J102), observing proper pin alignment during the process.
12. Connect the battery pack wiring harness to the 3-pin locking connector (J800) on the Main Board (see figure12).
13. Install the top and bottom covers by reversing the steps followed in Section 3.1.
SECTION 4
SERVICE AND TESTING
4.1 MEMORY BATTERY REPLACEMENT
The GPS 155 utilizes a built-in lithium battery to maintain user waypoints/settings stored in RAM.
This battery has an operational life of up to five years. Should the battery require replacement, as indi-
cated by the “Memory battery low” message, disassembly of the unit will be required to access the bat-
tery clip.
When replacing the memory battery, all user waypoints/settings may be lost. If this occurs the message
“Stored Data Lost” will be displayed. The unit must then be attached to an antenna, allowed to search the
sky and collect new almanac data before it will be usable again. This process may take 20-30 minutes.
1. Remove the covers as described in Section 3.1.
2. Remove the Control/Display Unit as described in Section 3.2.1.
3. Remove the four screws (211-50202-15) and spacers (233-00012-00) which secure the RF/IF Assembly to the
Main Board.
4. Carefully pull the RF/IF Assembly upward, disengaging it from the Main Board Connector (J803) Place the RF/IF
Assembly to the side of the unit. (NOTE: With the mounting screws removed, the cover for the RF/IF Assembly
will be loose. Grip the RF/IF Assembly firmly by the sides, instead of by its cover, when lifting upward.)
5. Remove the old battery by gently pushing it toward the back of the unit and out of the battery clip. A tuning tool,
or similar flat-tipped instrument, may be used to aid removal of the battery, but EXTREME CAUTION should be
exercised to prevent damage to the Main Board.
6. Immediately install a new battery (positive side up) into the battery clip.
7. Secure the RF/IF Assembly to the Main Board using the four screws (211-50202-15) and spacers (233-00012-00).
8. Install the Control/Display Unit by reversing the steps followed in Section 3.2.1. Gently slide the CDU onto the
two Main Board connectors (P501 and P502), observing proper pin alignment during the process.
9. Turn the GPS 155 on.
10. Press ENT to accept “NORMAL” mode operation.
11. Press ENT again to accept the Database Page.
12. Verify that the “Memory battery low” message does not appear on the Message Page by pressing the MSG key.
13. Turn the GPS 155 off and carefully turn the unit over.
14. Verify that the memory battery voltage (from test point to ground, see Figure 6) is at least 2.9 VDC.
15. Install the top and bottom covers by reversing the steps followed in Section 3.1.
16

4.2 ROM REPLACEMENT
If a software update is required, it may be performed by replacing the operating system ROM located
on the Main Board. When replacing the ROM, all user waypoints/settings will be lost. The message
“Stored Data Lost” will be displayed. The unit must then be attached to an antenna, allowed to search
the sky and collect new almanac data before it will be usable again. This process may take 20-30 min-
utes. (A user data card is available to save user waypoints/settings prior to replacing the ROM. Refer
to page 5 of this manual for more information.)
1. Turn the GPS 155 is turned off.
2. Remove the bottom cover as described in Section 3.1.
3. Locate the ROM in the middle of the Main Board, as shown in figure 6.
NOTE: The ROM removal tool (P/N T00-00007-00) is contoured for insertion into either of the two
extraction cavities on the chip carrier socket. Remove the chip by inserting the extraction tool with the
correct angle (see figure 8) and pivoting the tool on its heel against the socket.
4. ROM Extraction Procedure:
a. Orient the socket and tool as shown in figure 9.
CAUTION: Do not attempt to completely extract chip from chip carrier socket on the first extraction. Lift
from one side until the corner is above the socket, then repeat the procedure on the opposite corner.
b. Insert the extraction tool into the socket so that the front angle of the tool is perpendicular to the chip
carrier. NOTE: Insertion of the tool with an incorrect angle will result in damage to the chip carrier
socket. The incorrect angle will bottom under the standoff rather than the bottom of the chip.
c. Lower the extraction tool into the extraction cavity until it bottoms.
17
Figure 8
ROM Tool Insertion