Abell TH308G, Руководство по обслуживанию

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ABELL TH-308G VHF Service Manual
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by checking whether the input voltage is higher or lower than the preset value.
To output sounds from the speaker, IC302 sends a high signal to the AFCC lines and
turns IC102 on through Q107, Q109, Q106 and Q105.
6
）
Receive signaling
QT/DQT
300Hz and higher audio frequencies of the output signal from IF IC are cut by
IC101A and IC101D. IC302 determines whether the QT or DQT matches the preset
value, and controls the AFCC and the speaker output sounds according to the squelch
results.
3
．
PLL frequency synthesizer
The PLL circuit generates the first local oscillator signal for reception and the RF
signal for transmission.
1
）
PLL
The frequency step of the PLL circuit is 5 or 6.25 kHz. A 12.8 MHz reference
oscillator signal is divided at IC200 by a fixed counter to produce the 5 or 6.25 kHz
reference frequency. The voltage controlled oscillator (VCO) output signal is buffer
amplified by Q204, then divided in IC200 by a dual-module programmable counter. The
divided signal is compared in phase with the 5 or 6.25 kHz reference signal in the
phase comparator in IC200. The output signal from the phase comparator is filtered
through a low-pass filter and though the VCO to control the oscillator frequency.
2
）
VOC
The target frequency is produced by Q207(RX) and Q208(TX) centered
oscillator. The reference voltage is put on to varactor D202, D207, D205 and D206
to correct the VCO frequency. RXC and TX control the power to switch transmit or
receive.
3
）
UNLOCKED DETECOR
If a pulse signal appears at the LOCK pin of IC200, an unlocked condition occurs,
and the DC voltage obtained form D200, R219 and C224 causes the voltage applied to
the LOCK pin of the microprocessor to go low. When the microprocessor detects this
condition, the transmitter is disabled and the error indicating alarm occurs, ignoring the
push-to-talk switch input signal.
If the LOCK pin on IC200 is low electricity level ， the detector will be loosed lock and
get DC from D200 and R219
，
the voltage generated by C224 to the lock foot of MPU
will be decreased simultaneity. When MPU detected this situation, it can not transmit
and ignoring the push-to-talk switch input signal.
4. Transmitter