
Model 8640B
TM 9-4935-601-14-7&P
Model 8640B
TM 9-4935-601-14-7&P
SERVICE SHEET 8
PRINCIPLES OF OPERATION
Over-Deviation Detector (A7)
If the FM input signal is too large for the FM circuits to operate properly, the
Over-Deviation Detector lights the REDUCE FM VERNIER annunciator lamp
A6DS1. Integrated circuit U2 is a dual comparator amplifier with wired-OR
outputs. Pin 7 of U2B is at 1.1 Vdc; pin 4 of U2A is at -1.1 Vdc; these two
voltages are the high and low reference voltages. Pins 6 and 3 of U2 are the
common inputs. If the input, which comes from the FM buffer amplifier, is not
b1.1 and -1.1V, the outputs go high (>1V). Integrated circuit U3 is a
hex inverter with open collector outputs. U3A inverts the comparator output.
When U3A goes low, capacitor C13 is discharged; when U3A goes high again,
C13 slowly charges through R76. This effectively increases the duration of the
comparator output when overloading occurs only for short periods. U3B
inverts the output of U3A and drives four parallel inverters U3C to U3F. When
the outputs of the four parallel inverters are low, the display lamp turns on,
which occurs whenever the input to U3B is low.
Peak Deviation Switch (A9)
The Meter Attenuator scales the FM input signal to give the correct reading on
the meter.
The Scale/Annunciator Lamp Control section of the switch lights the proper
scale annunciator lamp (on A6) for a given peak deviation range when the
meter mode selected is FM.
TROUBLESHOOTING
It is assumed that a problem has been isolated to the over-deviation detector,
meter attenuator, or scale/annunciator lamp control circuits as a result of using
the troubleshooting block diagrams.
Test Equipment
Digital Voltmeter ................................................................... HP 3480B/3484A
Oscilloscope..................................................................HP 180A/1801A/1820C
Initial Test Conditions
Bottom cover removed (see Service Sheet F for removal procedure). Extend
A7 FM Shaping Assembly on extender board. Connect AM OUTPUT to FM
INPUT.
Initial Control Settings
AM ............................................................................................................. INT
AUDIO OUTPUT LEVEL ...............................................................................cw
MODULATION ...........................................................................................ccw
MODULATION FREQUENCY ...................................................400 Hz (Fixed)
FM .............................................................................................................. AC
PEAK DEVIATION ...................................................................................5 kHz
PEAK DEVIATION Vernier .........................................................................ccw
RANGE ..........................................................................................0.5 - 1 MHz
SERVICE SHEET 8 (Cont'd)
Over-Deviation and Meter Control Circuits Troubleshooting
Component
Test Conditions and
Normal Indication
If Indication
or Circuit
Control Settings
is Abnormal
OVER-
Initial conditions
REDUCE FM
Replace faulty
DEVIATION
and settings. Ad-
VERNIER lamp
component
DETECTOR
just PEAK DEVI-
unlit and
(A7)
ATION vernier for
1. pins 6, 8, 10,
1.8 Vp-p at U2
12 high
pins 3 and 6.
2. U3B pin 4 low
3. U3A pin 2-high
4. TP4 (FM OVER-
LOAD) low
Adjust PEAK
REDUCE FM
Replace faulty com-
DEVIATION
VERNIER lamp
ponent
vernier for 2.4
lit and
V p-p at U2
1. pins 6, 8, 10,
pins 3 and 6
12 low
2. U3B pin 4 high
3. U3A pin 2 low
4. TP4 (FM OVER-
LOAD) >2 Vp-p
SCALE/
Initial conditions
SCALE lamps light
Check scale lamps
ANNUNCI-
and settings. Set
as follows:
(A6) and switches
ATOR
Meter Function to
(A9)
LAMP
FM and set
CONTROL
PEAK DEVIA-
(A9)
TION as follows:
5 kHz
5
10 kHz
10
20 kHz
3
40 kHz
5
80 kHz
10
160 kHz
3
320 kHz
3
640 kHz
10
1.28 MHz
3
2.56 MHz
3
5.12 MHz
5
Figure 8-28. P.O A6 Annunciator Assembly Component
Locations.
FM Shaping Circuits and Phase Lock Loop Filter (A7, A9)
SERVICE SHEET 7
8-32
Figure 8-29. P/O A9 Peak Deviation and Range Switch Assembly
Component Locations (1 of 2).
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