
Model 8640B
TM 9-4935-601-14-7&P
SERVICE SHEET 9
PRINCIPLES OF OPERATION
General
When either the AM or FM modulation select switch is set to
INT, the Modulation Oscillator is enabled. The oscillator feeds
a 1000 or 400 Hz signal (selected by the MODULATION
FREQUENCY switch) into the AM or FM modulator circuits and
to the AM or FM front panel OUTPUT ports. The fixed-
frequency modulation oscillator, shown on this Service Sheet,
is the standard oscillator supplied with the generator.
Modulation Oscillator (Al 1)
Amplifier U1 is the gain block. A frequency-selective bridged-
tee network forms a negative feedback path for U1. (This
network is a notch filter with zero phase shift at the minimum of
the notch.) The frequency of oscillation is determined by the
network: C1, C2, and either AllAiR1 and R2 or R3 and R4.
The positive-feedback path is a voltage divider in which the
amount of feedback is determined by the output of a peak
detector. (The amount of feedback automatically adjusts to
maintain oscillation at a constant amplitude.) The voltage
divider consists of R4, R3, CR1, and CR2. Diodes CR1 and
CR2 are in ac parallel and dc series. The ac resistance is
determined by the dc voltage across capacitor C5. At the peak
of each output cycle VR2 and CR3 conduct and replenish the
charge lost from C5. The ac voltage at the output of U1 is
about 5.1 Vrms.
Buffer Amplifiers (A11)
Resistors R5, R6, and R7 lower the oscillator output voltage to
2.3 Vrms at TP5. Resistors R13
and R14 lower the voltage to about 0.84 Vrms at TP3
and TP4. Transistor Q5 drives the FM PEAK
DEVIATION vernier potentiometer; Q4 drives the AM
MODULATION potentiometer; Q1 drives the AM
OUTPUT port; and Q2 drives the FM OUTPUT port.
Signal levels at the two ports are approximately 1 Vrms
into 6002.
TROUBLESHOOTING
It is assumed that a problem has been isolated to the
fixed-frequency modulation oscillator as a result of
using the troubleshooting block diagrams.
Troubleshoot by using the test equipment listed below,
performing the initial test conditions and control
settings, and following the procedures outlined in the
table.
Test Equipment
Digital Voltmeter ................................ HP 3480B/3484A
Oscilloscope .............................HP 180A/1801A/1820C
Initial Test Conditions
Top and bottom covers removed (see Service Sheet F
for removal procedure). Extend All Fixed-Frequency
Modulation Oscillator Assembly on extender board (see
Service Sheet C for removal procedure).
Initial Control Settings
AM ......................................................................... INT
AUDIO OUTPUT LEVEL ...........................................cw
MODULATION FREQUENCY ...........................400 Hz
Fine Frequency Adjustment
The oscillator's frequency can be lowered slightly by
twisting the orange (3), yellow (4), and green (5) wires
together. The wires connect MODULATION
FREQUENCY switch A11A1S1 to the All circuit board.
Over-Deviation Detector and Meter Control Circuits (A6, A7, A9)
SERVICE SHEET 8
Fixed-Frequency Modulation Oscillator Troubleshooting
Component
Test Conditions and
If Indication
or Circuit
Control Settings
Normal Indication
is Abnormal
MODULATION
Initial conditions and
Peak-to-peak voltages
Check appropriate
OSCILLATOR
settings. Then set MOD-
are as shown on
circuit and replace
ASSY (A11)
ULATION FREQUENCY
schematic
faulty component
to 1000 Hz.
Set AM to OFF. Use DVM
DC voltages check good
Replace faulty compon-
to check dc voltages.
ent
8-34
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