5: Mixer Board
5-6 PRC1099A-MS
potentiometer R83 on the Audio/Filter board, the ALC signal drives the Q11
transmit output signal lower. The lower control limit is set by U2 gain adjust
potentiometer R37.
When ALC voltage from the Audio/Filter board is applied to Q12, it conducts,
causing the Q12 collector and Q11 control gate 2 voltage to drop. The gain of
Q11 decreases causing the Q11 output transmit signal to decrease. Diode D12
provides the fixed reference voltage for gate 1 (pin 4).
The ALC system is normally peak-reading. Capacitor C74 is charged up by
T8 through R54; when ALC is active, C74 discharges through Q12. This
provides a slow released of Q11 gain to smooth the changes in Q11 gain over
time.
5.1.4 AME Option
The AME option circuit injects the 1647 or 1650 kHz BFO (full carrier) signal
from the Synthesizer board directly into the transmit path at double-balanced
mixer U2 as shown in Figure 5-3 on page 5-7 to prevent the carrier being
controlled by the ALC circuit.
The BFO signal is applied to the input of mixer U2 using PIN diode
attenuators D14 and D15 as an on/off switch. In the AME off position, D14 is
forward-biased, shorting the 1650 kHz carrier to ground through R49; D15 is
reverse-biased. This gives approximately 60 dB of attenuation. The 1650 kHz
carrier from the 1650 kHz IF board with the suppressed carrier and either
upper or lower sideband removed is routed to mixer U2
When carrier switch Q20 on the Audio/Filter board is turned on, D14 is
reverse-biased and D15 is forward-biased. Input potentiometer R51 sets the
carrier level.The full 1650 kHz carrier from the Synthesizer board is applied
to mixer U2.
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