Model 8640B
TM 9-4935-601-14-7&P
CHANGE 13 (Cont'd)
Service Sheet 13 (Principles of Operation):
Change Principles of Operation to read as follows:
General
The RF signal from the RF Filters is amplified by RF Output Amplifier A26U1. The amplifier is a sealed
microcircuit that plugs into the rear of the AGC detector board. The amplifier drives resistor A26U1R1 which
sets the amplifier output impedance. The output is ac coupled through A26U1C2 and drives the Output
Attenuator. The step attenuator consists of resistive attenuator sections which are switched in and out by cam
driven microswitches. The attenuator steps are 10 dB with an impedance of 5092. Capacitor A1A2C1 ac
couples the RF output to the RF OUTPUT jack J1.
AGC Detector (A26U1)
The AGC detector (CR1) detects the negative peaks of the RF signal from the Output Amplifier. The detector
output is summed with the positive AGC reference voltage in the Summing Amplifier (Service Sheet 12).
Detector diode CR1 conducts whenever the RF amplifier output is one diode junction voltage drop below the
voltage across C3. The capacitor is then negatively charged until the amplifier voltage rises, at which time
CR1 shuts off. C3 then slowly discharges through resistors A26A1R22 and R23 until another negative peak
recharges it.
AM Bandwidth Control (A26A1)
In the 2-4 and 4-8 MHz (or LO BAND 2) frequency ranges, capacitor C5 is switched in parallel with A26U1C3
by Q7. Transistor Q7 is a switch which operates in the inverted mode (i.e., the emitter functions as a collector
and the collector as an emitter). The added capacitance of C5 reduces the amount of capacitor discharge
between RF voltage peaks on the lower frequency bands, but limits the AM bandwidth. Capacitor C4 is also
switched in for the 0.5-1 and 1-2 MHz (or LO BAND 1) ranges by Q6.
In the pulse modulation mode Q5, Q8, and Q9 are switched on. This switches out C4 and C5 and switches C6
in. Switching of Q5-9 is multiplexed onto one line by transistors A26A2Q8 and Q9. A26A2Q8 is a switchable
current source. In LOW BAND 2 it generates just enough current to turn on the collector-base junction of Q7.
In LOW BAND 1 the current increases enough to turn on both Q6 and Q7 (because the voltage drop across R4
is enough to turn on Zener diode VR2). When the PULSE CODE line is low, A26A2Q9 is on which turns on
Q5, Q8 and Q9 through Zener diode VR1.
Sample and Hold (A26A1)
The Schmitt Trigger (A26A2) and Q4 bias Q3 (normally biased on) off between pulses, which prevents C6 from
discharging. (If C6 were to discharge between pulse bursts, the Modulator would be driven to maximum output
when the next pulse arrived.)
Schmitt Trigger (A26A2)
The Schmitt Trigger formed by U1 and U2A converts the pulse input voltage into 0 to 5V pulses. When the
PULSE CODE line is low, the Schmitt Trigger output is enabled at U2C. Resistors R20 and R21 set the trigger
reference at about 0.5 Vdc. When the input to U1 is above the reference, the output of U2A is low. When the
input goes below the reference, the output of U2A goes high (+5V). Resistor R23 adds a small amount of
hysteresis to the reference voltage.
In the pulse modulation mode, NAND gate U2C inverts the trigger output and switches transistors A26A1Q4
and Q3 on when the input pulse is high, or off when the input pulse is low. Thus the charge on capacitor
A26A1C6 is stored between pulses. Similarly, NAND gate U2D inverts the trigger output and switches the
Modulator Driver Amplifier A26A4 (Service Sheet 12).
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