Technical description
4-6 HF SSB Transceiver 9323/9360/9390/9780 Technical Service Manual
LPF and first mixer
1 04-02972 and 04-03135 Sheet 1
The output from the HPF is fed direct, or via the RF amplifier, to a 29 MHz LPF
(C59, L24, C60/61, L25, C62, & L26) and then via transformer T2 to the balanced
input of the first mixer IC4 pins 12 and 13.
Mixer IC4 is a combined amplifier/balanced mixer with a gain of approximately
20 dB. The received signal is mixed with the output of the Voltage Controlled
Oscillator (VCO1), then applied to pin 5. This operates between 45.250 MHz and
75 MHz to produce a balanced IF output (pins 3 and 14) centred at 45 MHz.
DC is applied to the mixer IC4 pins 3 and 14 via the filtered "B" rail and the
centre tap of the mixer output transformer T3. In Receive mode, V2 is conducting
and supplies the filtered "B" rail to IC4 pin 4 and 11, via R34 and R36
respectively.
45 MHz band-pass filter
1 04-02972 and 04-03135 Sheet 1
The 45 MHz IF signal is filtered through a 15 kHz BPF (centred on 45 MHz)
consisting of T3, Z1, L30, Z2 and T4. This removes unwanted signals produced by
the first mixer. The filtered output is applied via coupling capacitors C84 and C85
to the balanced input of the second mixer IC7 pins 12 and 13.
Second mixer
1 04-02972 and 04-03135 Sheet 1
Mixer IC7 is a combined amplifier/balanced mixer and identical to mixer IC4.
The 45 MHz signal is mixed with the output of the second Voltage Controlled
Oscillator (VCO2), then applied to pin 5. This operates between 44.5435 MHz
and 44.5455 MHz to produce a second IF centred at 455 kHz.
DC to the mixer output is provided by the filtered "B" rail, which is connected to
the centre tap of the 455 kHz output transformer T201 (Sheet 2). It is then fed to
the mixer output transformer T3. In Receive mode when V6 is conducting, the "B"
rail provides DC via R45 to IC7 pin 4 and bias via R47 to pin 11.
Noise limiter
1 04-02972 and 04-03135
The 455 kHz output of IC7 pins 3 and 14 is applied via C201, R201, C202 and
R202 to the balanced input of a high gain amplifier IC201 pins 4 and 6.
The balanced outputs at pins 1 and 8 are connected to a tuned auto transformer
T202 (455 kHz). The output of T202 is applied to the base of V201, which is an
active rectifier. Noise bursts produce positive going pulses at the collector of
V201 and trigger, via V202, the monostable flip-flop IC6A/B. The pulse width is
determined by C208 and R210 (nominally 250 µs).
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