
Model 8640B
TM 9-4935-601-14-7&P
CHANGE 13 (Cont'd)
Detector Buffer Amplifier (A26A1)
Transistor Q1 and FET Q2 form a high impedance, unity gain buffer amplifier. Diode CR6 and resistor R19
add a dc offset which compensates for the junction voltage drop of the detector diode to maintain constant %
AM when OUTPUT LEVEL Verner is varied.
Rate Detector (A26A2)
Flip-flops U3A and U3B form a rate detector to turn off the RF level drive to the meter circuits whenever the
pulse repetition rate falls below 20 Hz. Below 20 Hz rates, the output leveling system cannot accurately control
the output amplitude. The flip-flops are arranged as retriggerable monostable (one-shot) multivibrators with
timing elements R25 and C10, and R28 and C11. A low-going output from U2A triggers U3A and the Q output
of U3A goes low for 50 ms. If the repetition rate of the incoming pulses is higher, than 20 Hz, U3A retriggers
and the Q output remains low. In the absence of pulses from the Q output of U3A, the Q output of U3B is low,
transistor Q7 is off and the meter operates normally. For pulse repetition rates less than 20 Hz, U3B is
periodically triggered by the Q output of U3A. The Q output of U3B goes high for 100 ms (or longer if U3B is
retriggered by U3A) and turns on Q7 which disables the meter drive amplifier output, and the meter reads zero.
Thus the meter is turned off for low rate pulses. When not in the pulse modulation mode, the output of inverter
U2B is low; the output of U2C is high and A26A1Q4 and Q3 are held on; the output of U2D is high and the
modulator is held in its normal on mode; and Q7 is held off.
Service Sheet 13 (Troubleshooting):
Add the following note after Initial Control Settings.
NOTE
If pulse burst amplitude is too high for low-duty cycle pulses, check A26A1Q3, Q2, C6, and
interconnecting lines for dc current leakage.
Change the RF Amplifier, Pulse Switching and Step Attenuator Troubleshooting table as follows:
DETECTOR BUFFER
As above
≈
-3 Vdc at TP1 (DET)
Check Q1, Q2, and asso-
AMPLIFIER (A26A1)
ciated circuitry
BW CONTROL
Initial conditions and
Same signal level on both
(A26A1)
settings except set AM to
sides of C4 and C5
- - - -
INT
Set RANGE to
Signal level differs across
Check C5, Q7, Q9 and
4-8 MHz
C5 (i.e., no signal at
associated circuitry
Q7-e)
Set RANGE to
Signal level differs across
Check C4, Q6, Q8, VR2
1-2 MHz
C4 (i.e., no signal at
and associated circuitry
Q6-e)
Set AM to PULSE
Signal level differs across
Check Q5-9, VR1, and
C6 (i.e., no signal at
associated circuitry
Q5-e)
7-38
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