TK-90
24
■
From Detection Circuit to Speaker Output
The detected audio signal is input into the multiplexer
(IC452). The multiplexer, depending on the mode at the time,
switches the detection circuit output, and the signal is ampli-
fied and filtered at the multiple returning amplifier circuit
(IC453). It is then sent by the analog switch IC (IC551) to the
speaker output route and VGS-1, etc.
IC551 output is passed through the mute switch (Q460)
and is input into the electric volume (IC555). IC555 changes
the level, depending on the speaker output. The output from
IC555 is passed through the mute circuit (Q523) and the au-
dio frequency characteristics switch (Q521), and is then
power-amplified at the audio power amplifier IC (IC521),
passed through the pin-jack (J1), is sent from CN236 to the
Display unit, and is output from the built-in speaker.
■
Audio Frequency Switching by Mode
The switch circuit (Q521) is added so that while in SSB
mode, the high-pitch range can be suppressed to prevent
exhausting the listener’s ears, and while in AM mode, the
high-pitch range sound can be expressed. Therefore, the
sound in AM mode and in other modes are different.
AM
SSB
CW
FSK
IC452
IC453
IC551
IC555
IC521
AF AMP
Display
SP
Q521
Q523
Q460
Except AM
Q521: ON
AM
Q521: OFF
CIRCUIT DESCRIPTION /
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Transmission Circuit
■
From Microphone Terminal to Output for Modulation
This transceiver’s transmission section operates with the
frequencies consisting of double super heterodyne, trans-
mission first IF: 10.695MHz and transmission second IF:
73.095MHz.
The Audio signal (AF signal) input from the microphone
terminal is relayed at the Display unit (X54-356) and input into
the TX-RX unit (X57-721).
The AF signal input into the TX-RX unit is amplified by the
MIC AMP IC (IC605), and the level is adjusted by the DA con-
verter (IC554) according to the microphone gain set in the
firmware. The AF signal is then split as a signal for modula-
tion to the 1st AF ALC amplifier (IC602) and a microphone
signal for VOX operation to the VOX AMP (IC604).
The first AF ALC amplifier detects its output with D605
and D606, and restricts the input signal with Q605 and Q606.
This prevents SSB over-modulation and restricts the VGS-1
(option) recording input level.
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