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TM-610 Service Manual
second local oscillator signal (50.4MHz) to become a 450KHz second IF signal. The second IF signal is then fed through a
ceramic filter (N: CF202; W: CF203) to eliminate unwanted signals. The resulting signal then is detected by IC102 and
output from Pin 9 as audio signals.
2.4 Audio Amplifier
The audio signal obtained from IC102 is amplified and filtered by IC402 before being amplified by IC401 (the received
signalling is separated and input into CPU for decoding). The processed signalling passes through Q406 (AF MUTE) and
is amplified by IC405. The amplified signal is fed to K301 controlling the volume and Q511 controlling SP MUTE on the
front panel before entering audio amplifier (IC511). The output audio signal drives the speaker.
2.5 Squelch Open
Part of the audio signals output from IC102 passes through IC102 Pin8 and is amplified by IC102 again, then is filtered
and rectified to produce an SQL level. The SQL level is compared with the existing reference level in CPU (IC502) to
generate a level to control AF MUTE and SP MUTE, which determines whether to output sounds from the speaker by
controlling Q406 and Q511.
3. Transmitter Circuit
The transmitter circuit is composed of the MIC circuit, modulation circuit, RF driver, final-stage power amplification circuit
and APC circuit.
Figure 2 Transmitter Circuit
3.1 MIC and Modulation Circuit
The audio signal from MIC is amplified by IC405 before being pre-emphasized and encoded by IC401. The output audio
signal is added into signalling and then is fed through VCO for modulation.
3.2 RF Driver and Final-Stage Power Amplification Circuit
The Tx-RF signal output from Q703 in the VCO circuit is amplified by Q101, drivers Q110 and Q109. The amplified signal
is then fed to IC101 (final-stage amplifier) and passes through LPF before reaching the antenna.
3.3 APC
The APC circuit stabilizes the output power at a predetermined level. D101 and D102 transform the signal from the
detector to DC voltage, which is then compared with the reference voltage from CPU in IC104 to output a DC voltage. The
DC voltage controls the output power by controlling the grid of IC101.
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