M A I N T E N A N C E P R A C T I C E S
KEY ITEMS
D E T A I L S T E P S / W O R K I T E M S
Adjust signal generator to 200 kc, unmod¬
ulated and adjust output level to produce a
field strength of 1000 pv per meter at the
fixed loop.
Tu n e r e ce i ve r to 2 0 0 k c an d a ll o w i n di c a t o r
pointer to null. Observe the indicated
bearing.
Turn off receiver and manually set the
fixed loop to aposition 175 degrees from
that producing the observed bearing.
Adjust the signal generator output level to
produce afield strength of 50 pv per meter
at the fixed loop.
T u r n o n t h e r e c e i v e r a n d r e c o r d t h e t i m e
required for the indicator pointer to move
from the original bearing to within 3° of
the new bearing.
(e)
(f)
(g)
(h)
The time required should not
exceed 7seconds.
(i)
NOTE
With the receiver properly tuned, the bearing error
should not exceed ±3 degrees at any point throughout
360 degrees over the range of 100 to 10, 000 nv per
m e t e r l o o p i n p u t .
3. TROUBLESHOOTING
A. GENERAL
The tabulated procedures detailed in this section are designed to isolate malfunctions in the
ADF-T-12B,C System to the level where operational tests, signal tracing, or voltage and resistance
m e a s u r e m e n t s c a n p r o fi t a b l y b e m a d e .
TRANSISTOR TESTING METHODSB.
Many conventional methods of troubleshooting electronic equipment can be destructive where applied to
transistorized equipment. Although transistors can withstand much greater physical abuse than vacuum
tubes, they are particularly sensitive to heat and excessive voltage. Before attempting to service the
ADF-T-12B,C System, maintenance personnel should be come familar with the information contained
herein.
Signal Tracing
Stage-by-stage signal tracing is the most effective method for locating trouble in the system.
This procedure is accomplished by connecting an output meter or oscilloscope across the output
of astage and injecting aknown signal into the input of that stage. Acomparison of the injected
signal with the resultant output signal indicates the operating condition of that stage. Typical
waveforms and RMS voltages existing at various stages of arepresentative receiver and servo
amplifier-indicator are shown in Tables 104 and 105. During signal tracing or when making
measurements, care must be exercised to prevent the test equipment from adversely influencing
the circuits under test. Proper methods of coupling the test equipment during measurement pro¬
cedures are illustrated in Figures 102, 103, 105 and 106.
D-C Voltage Measurements
The voltages encountered in this equipment are less than 28 vdc. Meters with asensitivity of
20, 000 ohms per volt are satisfactory for these voltage measurements. Due to the inherent voltage
variation characteristics of zener diode CR13 (ADF Receiver) and CRl (Servo Amplifier Indicator),
April 1965
(R e v i se d J a n u a r y 1 9 6 8)
(1)
(2)
Page 117
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General