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 TM 5-6675-250-34
Test
RED TEST PINS
Signal
Jack
Test Component
OHMS SCALE
FROM TO
+18 VDC A regulated dc voltage measured with
g
Pendulum return
0 to 1
X1
F
X
respect to pin X.
B + return
0 to 1
X1
M
X
DC: 18 0.2 Vdc
Servo excitation return
0 to 1
X1
V
X
Ripple: 80 mV peak to peak (maximum)
Chassis ground
h
0 to 1
X1
X
Measured with a dc voltmeter and an oscilloscope.
h
Chassis ground (no. 5 GND) This line is connected to
b. Theodolite Illumination. With the breakout box
ground in the ECU and supplies the return path for the
connected between the GRU and the ECU, connect
theodolite illumination circuit. Refer to resistance check
power to the system and place the MODE SELECT
switch to THEO ILLUM and THEO BRT rotated
Theo illumination (high side) A dc voltage varied by the
j
THEO BRT control with a maximum value equal to
fully CW. Perform the following to determine the
the source voltage; measured with a dc voltmeter with
operability of the circuit.
respect to pin h.
(1) Monitor jacks h and j for a reading equal to
the source voltage using a dc voltmeter.
(2) Close switches h and j.
a. Resistance Check. With the ECU in the power
(3) Verify that the theodolite illumination in the
off condition, connect the breakout box between the
GRU is controlled through the ECU (rotation of
GRU and ECU (figure 2-96. Place all switches on the
THEO BRT control and MODE SELECT switch) in
breakout box in the OFF position. Using an
THEO ILLUM position.
ohmmeter, check for the following resistances (10
(4) Monitor jacks h and j on breakout box to
percent unless otherwise indicated).
verify that voltage varies when THEO BRT is
(1) Gyroscopic reference unit (GRU)
rotated.
c. Constant Signals. Lines requiring signals
BLACK TEST JACKS
during all power-on operations are indicated in the
FROM
TO
OHMS SCALE Tested Component
following paragraphs.
50 to
X100
Uncage lamp
D
M
(1) Place the MODE SELECT switch in the
300
E
Time totalizer meter
500
X100
F
SELF TEST position.
G
Gyro motor
15
F
X1
(2) Monitor the following lines at the ECU red
H(+)
Open
M
X100
test jacks (indicated on the breakout box): K, Y, Z,
*J
Tilt switch
M
Open
X100
c, d, e, and g (fig. 2-5).
**J
Tilt switch
0 to 5
X1
M
Uncage switch
(3) Close the following switches: F, J, K, M, V,
K
L
Open
X100
Caging solenoid
10
N
M
X1
X, Y, X, c, d, e, g, and h.
Tach/servo motor
V
65
P
X2
(4) Monitor the lines specified in step (2).
reference windings
(5) Unsatisfactory readings in step (2) indicate a
NOTE
malfunction in the ECU; unsatisfactory readings in
Meter leads may have to be reversed to obtain
step (4) indicate a malfunction in the GRU. The
proper readings.
signals specified in step (2) are needed in all power-
Servo motor control
S
R
90
X1
on operations, they should be checked when a failure
windings
occurs in any mode of operation. Through the signal
5 to 50 X1
S(+)
T
Speed changer solenoid
suppression diode
summary (para 2-15) and the system diagrams (fig.
T(+)
S
210
X100
Speed changer solenoid
U
S
90
X1
Servo motor control
ponents of subcircuits.
winding
d. Variable Signals. Signals required in some
W
e
850
X100
Tach output winding
modes of operation but not in others are listed in the
Y
Z
300
X100
Bias coil
a
3000
b
Torquer coil
X100
failure analysis chart (table 2-2). The chart shows
c
Pick-off excitation coils
F
6000
X100
which signals must be present for each mode of
Torquer reference coil
d
F
420
X100
operation to function correctly, The symptom
Theo lamps
h
50 to
X1
j
column in the chart shows what indications can be
200
seen should a failure occur. Do the following to
*GRU in vertical position
isolate failures to the GRU or the ECU.
*GRU in horizontal position
(1) Check signals in the operating modes shown
in table 2-2, following the sequence shown from left
(2) Electronic control unit.
to right.
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