Abell A720T, Service Manual

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A720T (VHF) Service Manual V1.0
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mode), adopt voltage-controlled oscillator to control oscillation frequency. In receiving mode, Q607 conduct to
provide Q601 power supply. In transmitting mode, Q607 cut off Q611 to conduct and provide Q602 power supply.
(Q607 cut off or conduct is determined by output high/low level of micro processor. Micro processor(U300 and 56
th
pin) output high level in receiving mode, output low level in transmitting mode)output of Q601 and Q602 will be
amplified by Q605 and sent to buffer amplifier.
5.4.2 Unlocked Detector
Lose lock if there is low level on LLP(10 th pin of U601). Then get DC voltage from R634, if the low level detected by
micro processor(53
rd
pin of U300), transmitting will be unable regardless of communication switch.
5.5 Transmitter
5.5.1 RF Audio
FM signal from microphone will be amplified when through U307(A), and will be processed by IDC pre-emphasised
circuit when through another U301 and amplified. Then part(higher than 3KHz) of the signal will be filtered out when
going through inner low-pass filter of U301. Then start FM after going trough amplifier and VCO. Audio deviation
is controlled by the inner output electric level of audio filer U301. Check Chart 6 followed
5.5.2 QT/DQT/4FSK Coding
Required signal for MOD/QT/DQT/4FSK coding is produced by U301 and U300, which is being adjusted according
to reference frequency of PLL all the way. Since reference oscillator cannot adjust any frequencies out of loop
frequency, FM will be at another side of VCO in another path. Check Charter 6 followed.
5.5.3 Wide/Narrow Band Switch
We can use programming software to enable wide/narrow band to set that of various channels. Wide/narrow band data
will be realized by inner output of U300. Check Charter 6 followed.
MIC
Q323
SW
MIC_MUTE
U300
DCTC
AMP
MIC
U307
PREEMPHASIS IDC
U301
(SPLAT TER FILT ER)
LPF
U301
BUFFER
D601,Q602
VCO
X600
REFERENCE
OSC 19.2M
(TCXO)
Chart 6 Transmit Audio QT /DQT/DTMF
LPF
R350,R349,C378.C385
DT MF
CT C-TCXO
CPU
U301