Technical description
4-4 HF SSB Transceiver 9323/9360/9390/9780 Technical Service Manual
Receive/Transmit switching
The main transmit/receive DC switching circuit originates from the latched output
of IC209 pin 6 located on the Rx/Exciter PCB [1 04-02972 and 04-03135
Sheet 2]. IC209 pin 6 is connected to two transistors V209 and V212. V209
governs the AGC function. V212 forms part of the circuit controlling the transmit
and receive mixer circuits.
When the Receive mode is selected, the output of IC209 pin 6 is latched to 0 V,
switching V209 off and allowing the AGC to function normally. V212 is also
switched off, allowing the first NAND gate of IC6/D to be pulled high by resistor
R42. This sets the output of IC6/D pin 11 low, switching on V2 and V6, thus
enabling the receive mixers IC4 and IC7. The output of the second NAND gate
IC6/C will go high, switching V3 and V7 off and inhibiting the transmit mixers
IC5 and IC8.
When the PTT inputs are detected by the microprocessor IC101 [1 04-02976
Sheet 1] it sends a command via the local I
2
C bus to IC209 [1 04-02972 and
04-03135 Sheet 2] to latch the output pin 6 high (+5 V). This causes V209 in the
AGC circuit to conduct, setting the AGC to 0 V and disabling the 455 kHz IF
amplifier. Resistor R255 provides a small DC offset to IC205/A to ensure the
AGC always switches to 0 V.
With IC209 pin 6 latched high, V212 conducts, setting the input of IC6/D pin 13
low. This in turn sets the output of IC6/D pin 11 high, thus switching off V2 and
V6, disabling the receive mixers IC4 and IC7 and providing the DC voltage to
enable the transmit mixers IC5 and IC8.
The noise limiter in Transmit mode is disabled by V7 conducting, thus applying a
DC level via D201 and R203 (see Sheet 2) to the AGC input pin of noise
amplifier IC201 pin 5.
DC from V7 has two other functions. It provides bias via R238 to IC202 pin 5 to
enable the modulator. It also biases V5 (see Sheet 1) on, clamping the base of V4
to 0 V, thus preventing D19 from conducting and attenuating the transmit signal.
Serial data from the microprocessor IC101 [1 04-02976 Sheet 1] is also applied
to IC1 on the PA assembly. When transmit is selected, a ground on pin 12
energises the transmit/receive relay K7. The path is now completed from the PA
output to the antenna.
In Transmit mode, the microprocessor IC101 outputs a low from pin 28. This is
applied to the input of inverter buffer IC207 pin 12 (see Sheet 2). The high at the
output pin 14 is connected to:
• the antenna control connector P202 pin 15
• the optional PCB output P204 pin 12
• the mute squaring amplifier IC307/B via R365 and D307 (see Sheet 3)
The DC applied to IC307/B applies a small offset to disable the squaring amplifier
during transmission.
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