
MODEL 8640B
TM 9-4935-601-14-7&P
RF Amplifier, Pulse Switching, and Step Attenuator
(A1, A26A1, A26A2)
SERVICE SHEET 13
SERVICE SHEET 14
PRINCIPLES OF OPERATION
AM Offset (A26A2)
The AM Offset Amplifier establishes the AGC reference for the output leveling system and
superimposes on it the AM modulation signal. The modulation signal is coupled into the
amplifier through slide switch A13S2C and MODULATION potentiometer R2. The amplifier
input stage is the differential transistor pair Q1A and Q1B. Transistor Q2 is a constant current
source for the emitters of Q1. Transistors Q3 and Q4 form a second differential amplifier
stage. Transistor Q5 is a common emitter output stage. Resistors R16, R19, and R8 form a
resistive feedback divider. The ac voltage gain (-+2) is adjusted by R19. Transistor Q6 is a
constant current source. The collector current of Q6 causes a 2V drop across R16 which
offsets the amplifier output by +2 Vdc and establishes the AGC reference. Capacitors C5,
C6, and C7 frequency compensate the amplifier. The amplifier output drives OUTPUT
LEVEL vernier A1R1 (Service Sheet 16).
TROUBLESHOOTING
It is assumed that a problem has been isolated to the AM preamplifier 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 cover removed (see Service Sheet F for removal procedure), A26 AM/AGC and RF Amplifier Assembly casting top
cover removed, and A26A2 AM Offset and Pulse Switching Assembly extended for service (see Service Sheet E for
procedures).
Initial Control Settings
Meter Function ............................................................................................. AM
AM................................................................................................................ INT
MODULATION .......................................................................................... 100%
MODULATION FREQUENCY................................................................1000 Hz
AM Preamplifier Troubleshooting
Component
Test Conditions and
Normal Indication
If Indication
or Circuit
Control Settings
is Abnormal
AM PREAMPLIFIER
Initial conditions
≈
2 Vp-p at TP1 (AM IN)
Check input switching
(A26A2)
and settings
≈
4 Vp-p and +2 Vdc
Check Q1-Q6 and
at TP3 (AM OUT)
associated circuitry
8-46
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